Thin Films on Glass
Springer-Verlag Berlin Heidelberg GmbH
Schott Series on Glass and Glass Ceramics
Science, Technology, and Applications
Low Thermal Expansion Glass Ceramics
ISBN 3-540-58598-2
Fibre Optics and Glass Integrated Optics
ISBN 3-540-58595-8
The Properties of Optical Glass
ISBN 3-540-58357-2
Thin Films on Glass
ISBN 3-540-58597-4
Electrochemistry of Glasses and Glass Melts,
Including Glass Electrodes
ISBN 3-540-58608-3
Surface Analysis of Glasses and Glass
Ceramics, and Coatings
ISBN 3-540-58609-1
Analysis of the Composition and Structure
of Glass and Glass Ceramics
ISBN 3-540-58610-5
Mathematical Simulation in Glass Technology
ISBN 3-540-43204-3
Hans Bach
Dieter Krause
Editors
Thin Films on Glass
With 217 Figures
and 46 Tables
Springer
Editors
Dr. Hans Bach
Prof. Dr. Dieter Krause
Schott Glas
HattenbergstraBe 10
D-55122 Mainz, Germany
Library of Congress Cataloging-in-Publication Data
Thin films on glass / Hans Bach, Dieter Krause, editors. p. cm. -- (Schott series on glass and glass ceramics)
Includes bibliographical references and index.
ISBN 978-3-642-08205-4
ISBN 978-3-662-03475-0 (eBook)
DOI 10.1007/978-3-662-03475-0
1. Thinfilm
devices--Design and construction. 2. Glass coatings. 3. Dielectric films. 4. Optical coatings. 5. Coating processes.
1. Bach, Hans, 1930- .II.Krause,Dieter, 1933- .II.Series.
TK7872.T55T4561997
621.3815'2--dc21
97-29134
CIP
2nd Corrected Printing 2003
Ist Edition 1997
ISBN 978-3-642-08205-4
This work is subject to copyright. AlI rights are reserved, whether the whole or part of the material is
concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting.
reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or
parts thereof is permitted on1y under the provisions of the German Copyright Law of September 9, 1965, in
its current version, and permission for use must always be obtained from Springer-Verlag.
Violations are liable for prosecution under the German Copyright Law.
@
Springer-Verlag Berlin Heidelberg 2003
Originally published by Springer-Verlag Berlin Heidelberg New York in 2003
Softcover reprint of the hardcover lst edition 2003
The use of designations, trademarks, etc. in this publication does not imply, even in the absence of a specific
statement, that such names are exempt from the relevant protective laws and regulations and therefore free
for general use. The following trademarks used in this book are registered trademarks of Schott Glaswerke
or of Schott Group companies, respectively: Amiran, Calorex, Conturan, Fiolax, Irox, Mirogard, Schott BK 7,
Schott Type 1plus, Tempax. The following trademarks are registered trademarks of Cari Zeiss or of Cari Zeiss
Group campanies, respectively: Carat, Carat Filter, Clarlet Cool Blue, Claret ET, Claret Gradal Top, Claret Hart,
Claret Super ET, Punktal ET, Punktal SL Cool Blue, Punktal SL Super ET, Punktal Super ET, Super ET, Super
Filter ET, Umbra Gold ET, Umbra Punktal. Other trademarks mentioned in this book (e.g., Ludox, Nimonic,
Stellite) are registered trademarks of other companies.
7)pesetting: Computer to film from editors data
Production: LE- TE.X Jelonek, Schmidt & Vtlckler GbR, Leipzig
Printed on acid-free paper 56/31411YL 5432 10
Foreword
This book, entitled Thin Films on Glass, is one of a series reporting on
research and development activities on products and processes conducted by
the Schott Group.
The scientifically founded development of new products and technical processes has traditionally been of vital importance to Schott and has always
been performed on a scale determined by the prospects for application of our
special glasses. Since the reconstruction of the Schott Glaswerke in Mainz,
the scale has increased enormously. The range of expert knowledge required
could never have been supplied by Schott alone. It is also a tradition in our
company to cultivate collaboration with customers, universities, and research
institutes. Publications in numerous technical journals, which since 1969 we
have edited to a regular schedule as Forschungsberichte - 'research reports'
- describe the results of these cooperations. They contain up-to-date information on various topics for the expert but are not suited as survey material
for those whose standpoint is more remote.
This is the point where we would like to place our series, to stimulate the
exchange of thoughts, so that we can consider from different points of view
the possibilities offered by those incredibly versatile materials, glass and glass
ceramics. We would like to share the knowledge won through our research
and development at Schott in cooperation with the users of our materials
with scientists and engineers, interested customers and friends, and with the
employees of our firm.
Though the results documented in the volumes of the Schott Series are of
course oriented to the tasks and targets of a company, we believe that readers
can nevertheless - or just for that very reason - find demanding challenges for
the development of process engineering, the characterization of measurement
practice, and for applied research. Besides realizability, the profitability of
solutions to customers' problems always plays a decisive role.
The first comprehensive presentation of research findings after the reconstruction ofthe factory in Mainz was edited by Prof. Dr. Dr. h.c. Erich Schott
in 1959. It was entitled Beitriige zur angewandten Glasforschung - 'contributions to applied glass research' (Wissenschaftliche Verlagsgesellschaft m.b.H.,
Stuttgart 1959). Since then, there has been an extraordinary worldwide increase in the application of glass and glass ceramic materials. Glass fibres and
VI
Foreword
components manufactured from them for use in lighting and traffic engineering or in telecommunications, high-purity and highly homogeneous glasses
for masks and projection lenses in electronics, or glass ceramics with zero expansion in astronomy and in household appliance technology are only some
examples. In many of these fields Schott has made essential contributions.
Due to the breadth and complexity of the Schott activities, it takes several
volumes to describe the company's research and development results. Otherwise it would be impossible to do full justice to the fundamental research
work and technological innovation that is indispensable for product development, and to give an appropriate description of the methods of measurement
and analysis needed for the development and manufacture of new products.
Apart from Thin Films on Glass, five volumes, entitled The Properties
of Optical Glass, Low Thermal Expansion Glass Ceramics, Analysis of the
Composition and Structure of Glass and Glass Ceramics, Electrochemistry
of Glasses and Glass Melts, Including Glass Electrodes, and Mathematical
Simulation in Glass Technology have already been published. Another two
volumes, entitled Surface Analysis of Glasses and Glass Ceramics, and Coatings and Fibre Optics and Glass Integrated Optics, are in preparation and
will be published in the next few years. Glasses for various applications in
industry and science and their properties are being considered, and melting
and processing technologies described.
With the presentation ~ in part detailed ~ of the work required for the
development of successful products, Schott employees are giving all their interested colleagues working in the field of science and technology an insight
into the special experiences and successes in material science, material development, and the application of materials at Schott. Contributions from
scientists and engineers who work at universities and other research institutes and who played an essential role in Schott developments complete the
survey of what has been achieved and prove the usefulness of the collaborations mentioned above.
In all the volumes of the series the fundamental issues from chemistry,
physics, and engineering are dealt with, or at least studies are cited that enable or assist the reader to work his or her own way into the topics treated.
Thus, the series may serve to fill gaps between the basic knowledge imparted
by textbooks on material science and the product descriptions published by
Schott. We see this as the best way to enable all our potential business partners who are not already familiar with glass and glass ceramics to compare
these materials with alternatives on a thoroughly scientific basis. We hope
that this will lead to intense technical discussions and collaborations on new
fields of applications of our materials and products, to our mutual advantage.
Every volume of the Schott Series will begin with a chapter providing
a general idea of the current problems, results, and trends relating to the
subjects treated. These introductory chapters and the reviews of the basic
principles are intended for readers dealing for the first time with the special
Foreword
VII
properties of glass and glass ceramic materials and their surface treatment in
engineering, science, and education.
Many of our German clients are accustomed to reading scientific and
technical publications in English, and most of our foreign customers are better
conversant with English than with German. We therefore decided to publish
the Schott Series in English.
The publication of the Schott Series has been substantially supported by
Springer-Verlag. We would like to express our thanks to Dr. H. K. V. Lotsch
and Dr. H. J. K6lsch for advice and assistance in this project.
The investment of resources by Schott and its employees to produce the
Schott Series is, as already stated, necessary for the interdisciplinary dialogue and collaboration that are traditional at Schott. A model we still find
exemplary today of a fruitful dialogue between fundamental research, glass
research, and glass manufacture was achieved in the collaboration between
Ernst Abbe, Otto Schott, and Carl Zeiss. It resulted in the manufacturing
of optical microscopes that realized in practice the maximum theoretically
achievable resolution. It was especially such experiences that shaped the formulation of the founding statute of the Carl Zeiss Foundation, and the initiative for the Schott Series is in accord with the commitment expressed in
the founding statute "to promote methodical scientific studies" .
Mainz, April 2003
Dieter Krause
Vice President R&D (retd.)
Preface to the Second Corrected Printing
The second printing has been corrected and supplemented with three new
sections: "Glass Ceramic Reflectors with Schott PI Coating®", "Coatings on
Plastics with the PICVD Technology", and "Optical Multilayers for UltraNarrow Bandpass Filters Fabricated by PICVD". These contributions represent fields of recent activities. Several corrections and additions have been
made, wherever it was necessary.
We thank the authors for reading, correcting and updating their contributions, Mrs. Karin Langner-Bahmann for processing all the figures, and Mrs.
Wiltrud Witan for revising the English. We also thank the Springer-Verlag
for supporting this edition.
April 2003
Hans Bach, Dieter Krause
Preface to the First Edition
In a glass company, the core technologies are glass melting and hot or cold
forming. Several branch technologies are necessary to finish the products or to
add value by modifying the bare glass. One branch technology widely needed
and used is thin-film coating for several quite different purposes.
The main aim of Thin Films on Glass is to describe the research, development, and scientific and technical background of thin films for selected products as well as the specific processes used at Schott. The book is conceived as
a monograph. However, the individual chapters have been written by different or several authors, who are themselves active in the corresponding fields
of research or product and process development or manufacturing. Thus, the
reader is given direct access to the expertise of these authors, some of whom
are employees of our subsidiaries Deutsche Spezialglas AG, Grunenplan;
AVER-SaG Glaswerke, Bad Gandersheim; or of our sister company Carl
Zeiss, Oberkochen. Some authors, such as Prof. Dr. A. Thelen, Frankfurt/M.,
acted as consultants; others are employees of companies engaged with Schott
as partners in development, such as the Institut fur Mikromechanik, Mainz,
or of companies that are either business partners or potential customers, such
as F. Hoffmann-La Roche AG, Basel, Switzerland.
To give the reader an idea of the extraordinarily broad range of coating
materials, processes, applications, and products, the volume opens with a
general survey of the subject 'thin films'. As a result of the high diversification and specialization prevailing in this field, however, the range of more
substantially discussed topics had to be restricted to the narrower field of
coatings on glass with special emphasis on optical coatings. Here, new developments, limitations, the current state of the art, and general trends are
described.
For more than six decades, Schott has significantly contributed to the
development and production technologies of thin films on glass. The subsequent chapters treat in detail the design strategies, coating technologies, and
the characterization and application of thin films. In addition to outstanding
functionality of the products, economical and ecological manufacturing has
always been a major target of the Schott Group. Consequently, two processes
are used at Schott: dip coating within the sol-gel route and plasma impulse
chemical vapour deposition. Both have proved to be powerful but remained
XII
Preface to the First Edition
singularities in the worldwide efforts in the field of coating. This book aims
to impart a deeper understanding of these Schott-specific developments. We
have therefore explained some basic tools in detail without striving, however, for completeness by reproducing the excellent comprehensive literature
available.
In Chap. 2 the basic modern design tools for optical coatings are described.
In an open design contest, sophisticated solutions have been presented. The
progress in this field triggered many initiatives for technical improvements to
reproducibly realize such new designs.
Chapter 3 gives a short overview of major coating technologies. For many
years, physical vapour deposition and sol-gel (dip) coating have been the major technologies within the Schott Group. The breakthrough is a merit of the
late Prof. Dr. Hubert Schroder, who in the early 1960s provided the scientific
foundations for a production process in cooperation with Dr. Helmut Dislich
(chemistry) and Dr. Hans Bach (surface and thin-film analysis). At the end
of the 1980s, chemical vapour deposition was ready for application in production thanks to the personal commitment of Dr. Johannes Segner, Schott
Glaswerke, and Godehard Kaffrell, AUER-SOG. As a kind of supplement,
thermal coating processes have been included, which Schott does not apply
for the coating of glass but for the coating of moulding or pressing tools that
are temporarily in contact with molten glass.
Chapter 4 is dedicated to the methods and results of thin-film characterization. The frequently observed strong deviations between the material
properties of thin films and those of a bulk sample with nominally identical composition are one reason for poor process stability and reproducibility,
making strict process control very difficult. Moreover, the small materials
volume of a thin film and its interaction with the ambient due to the large
surface~to-volume ratio necessitate the application of sophisticated characterization methods to yield reliable information. Such methods require expensive
equipment and skilled personnel to run and evaluate the measurements. Unexpected results often took us by surprise and the permanent availability of
these characterization methods has always proved highly advantageous because it enabled immediate trouble-shooting and the realization of process
improvements on a solid basis of knowledge instead of assumptions.
In Chap. 5 we pay tribute to the pioneering work of Dr. Walter Geffcken,
achieved under difficult conditions in that turbulent period, the 1930s and
1940s. He invented various antireflection coatings and realized early production processes for AR coatings on optical lenses. Ever since then, the coating
of glass has been a field of competence of the Schott Group, a fact which is
illustrated by some selected topics.
Finally, in Chap. 6, selected modern coated products from the Schott
group are discussed. Here the successful teamwork between development,
marketing, and production is reflected by the list of contributing authors.
Preface to the First Edition
XIII
We were fortunate in also winning employees of (potential) Schott customers
to write contributions to various sections.
In summary, all the information given in this book shows the successful
transfer of research results via product and process development into production. In most cases this is no straightforward procedure, but often requires
strong feedback and many fresh attempts at meeting a moving target. The
literature cited should help the interested reader or teacher in finding access
to more detailed presentations. Thus the content of this volume is placed between classical textbooks on materials science or engineering and the product
information on coating materials, equipment and coated products.
We wish, above all, to express our thanks to all the authors of this book
for their steady and pleasing cooperation. We have received further valuable
help from many colleagues whom we would like to thank for advice, critical
reading of the manuscript, and technical support in dealing with computer
hardware and software and the layout of the cover photograph.
We are also indebted to several employees and collaborators at SpringerVerlag, especially to Dr. Victoria Wicks and Dr. Angela Lahee for copyediting this volume, to Jacqueline Lenz for the coordination of the publishing
process, to Peter StraBer as the responsible production editor, and to Kurt
Mattes for converting the manuscript into Springer TEX. We are grateful to
Dr. Hans J. K6lsch for many helpful discussions in the early phases of this
volume and in planning the Schott Series in general.
Very special thanks go to Wiltrud Witan, M.A., and Karin LangnerBahmann, Schott Glaswerke, for all the translations and corrections of the
manuscripts which were submitted either in German or English, for the creation of numerous computer graphics and images from often very raw data or
poor originals, and for their enthusiasm in doing all the hard work necessary
to prepare manuscripts ready for printing.
August 1997
Hans Bach, Dieter Krause
Contents
1. Overview - Thin Films on Glass:
an Established Technology .. . . . . . . . . . . . . . . . . . . . . . . . . .
Burkhard Danielzik, Martin Heming, Dieter Krause, Alfred Thelen
1.1 Introduction: Why Surface Coating? . . . . . . . . . . . . . . . . . . . . . .
1.2 Coating Materials ......................................
1.3 Thin-Film Forming Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 Fabrication Issues for Coatings . . . . . . . . . . . . . . . . . . . . . . . . . ..
1.5 Product and Overall Process Design ......................
1.6 Today's Situation and Trends . . . . . . . . . . . . . . . . . . . . . . . . . . ..
References .................................................
1
1
2
8
11
13
17
19
2. Design Strategies for Thin Film Optical Coatings 23
Alfred Thelen
2.1 Optical Thin Film Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.2 Exact Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.2.1 Equivalent Layers ................................
2.2.2 Simulation of a Single Layer by a Multilayer. . . . . . . ..
2.2.3 Chebyshev Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.2.4 Effective Interfaces ...............................
2.2.5 Buffer Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.2.6 Absentee Layers ................................. ,
2.3 Approximate Methods Based on Starting Designs. . . . . . . . . ..
2.4 Numerical Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.5 Results of Recent Design Contests. . . . . . . . . . . . . . . . . . . . . . ..
2.5.1 Berlin Contest 1991. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.5.2 Tucson Contest 1995. . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
2.6 Design Strategies for the Different Deposition Technologies ..
2.7 Conclusion............................................
References .................................................
24
25
25
29
30
32
33
34
35
37
40
40
41
45
47
48
XVI
Contents
3. Coating Technologies ................................. 51
3.1
Physical Vapour Deposition
Ulrich Jeschkowski, Hansjorg Niederwald. . . . . . . . . . . . . . . . ..
3.1.1 Non-Reactive Evaporation. . . . . . . . . . . . . . . . . . . . . . . ..
3.1.2 Reactive Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.1.3 Energy-Enhanced Evaporation. . . . . . . . . . . . . . . . . . . ..
3.1.4 Sputtering.......................................
3.2 Chemical Vapour Deposition
Wolfgang Mohl. . ... . ... .. ..... . .... . . ... . .. . . ... . . . . . ..
3.2.1 Techniques of Chemical Vapour Deposition. . . . . . . . ..
3.2.2 Peculiarities of the Various Techniques. . . . . . . . . . . . ..
3.2.3 Variation of Processing Conditions and Properties. . ..
3.3 Sol-Gel Coating Processes
Wolfram Beier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.3.1 Sol-Gel Chemistry ...............................
3.3.2 Sol-Gel Fractals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.3.3 Dip Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.3.4 Spin Coating ....................................
3.3.5 Heat Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
3.4 Thermal Coating Processes
Joachim Disam, Dirk Gohlke, Katharina Lubbers. . . . . . . . . ..
3.4.1 Processes and Materials. . . . . . . . . . . . . . . . . . . . . . . . . ..
3.4.2 Applications.....................................
References .................................................
51
51
53
53
57
59
59
61
63
66
66
73
75
77
80
83
83
89
92
4. Properties and Characterization
of Dielectric Thin Films .............................. 99
4.1
Surfaces of Substrate Glasses
Klaus Bange . ..........................................
4.2 Macroscopic Properties of Thin Films
Klaus Bange, Clemens Ottermann ........................
4.2.1 Density .........................................
4.2.2 Optical Properties of Coatings .....................
4.2.3 Electrical Conductivity ............................
4.2.4 Mechanical Properties ............................
4.3 Microscopic Properties
Klaus Bange . ..........................................
4.3.1 Composition .....................................
4.3.2 Oxidation State ..................................
4.3.3 Structure of Oxide Films ..........................
101
104
104
106
113
114
125
125
128
130
Contents
XVII
4.4 Examples of the Characterization of Thin Film Materials . . ..
4.4.1 Titanium Oxide
Olaf Anderson, Klaus Bange, Clemens Ottermann ....
4.4.2 Silicon Oxides
Olaf Anderson, Clemens Ottermann ................
4.4.3 Tantalum Oxide Layers
Klaus Bange . ....................................
4.4.4 Nickel Oxide and Hydrous Nickel Oxide
Klaus Bange . ....................................
4.4.5 Tungsten Oxide
Klaus Bange . ....................................
4.5 Properties of Multilayer Systems
Clemens Ottermann ....................................
References .................................................
135
135
159
171
175
189
201
203
5. Developments at Schott: Selected Topics .......... 225
5.1
The Pioneering Contributions of W. Geffcken to the
Field of Optical Coatings from 1935 to 1945
Alfred Thelen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 How Thin Films Came to Schott ...................
5.1.2 Multilayer Antireflection Coatings ..................
5.1.3 Theory of Periodic Multilayers .....................
5.1.4 Other Contributions ..............................
5.1.5 Conclusions ......................................
5.2 Interference Filters
Ulrich Jeschkowski . .....................................
5.2.1 Coating Technology ...............................
5.2.2 Computer-Aided Design and Manufacturing .........
5.2.3 Products ........................................
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
Dieter Krause. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Fundamentals of the PICVD Process. . . . . . . . . . . . . . . .
5.3.2 Impact of the Environment
on the Optical Performance of Thin Films . . . . . . . . . . .
5.3.3 Flip-Flop Layers and the "Design-to-Go" Concept ....
5.3.4 Multilayer Stacks and Rugate Filter ................
5.3.5 Summary ........................................
225
226
227
230
236
236
237
237
239
240
243
243
247
251
253
258
XVIII Contents
5.4 Electrochromic Devices
Klaus Bange, Friedrich G.K. Baucke . .....................
5.4.1 The Layer Components of Electrochromic Devices ....
5.4.2 Typical Examples of Electrochromic Devices .........
5.5 Electron-Sensitive Coatings
Frank- Thomas Lentes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1 Physical/Chemical Principles for Generating
Optical Extinction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.2 Structural Aspects of Ag-Containing
Electron-Sensitive Layers . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.3 Experimental Determination of Properties ...........
5.5.4 Modelling of the Generation and the Stability
of Ag Colloids ...................................
References .................................................
259
261
264
270
272
276
278
285
289
6. Products ................................................ 295
6.1
The Principle of Interference Filters
Klaus-Dieter Loosen . ...................................
6.1.1 Spectral Specification of Interference Filters .........
6.1.2 All-Dielectric Filters ..............................
6.1.3 Metal Dielectric Filters. . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.4 Induced-Transmission Filters .......................
6.2 A Universal Transducer for Optical Interface Analytics:
Transducer Design and Concepts for an Economical
Mass Production
Burkhard Danielzik, Wolfgang Ehrfeld, Christof Fattinger,
Martin Heming, Holger Lowe, Andreas Michel, Frank Michel,
Norbert Oranth, Jurgen Spinke ...........................
6.2.1 Optical Transducer ...............................
6.2.2 Materials and Processes ...........................
6.3 Laser Coatings
Wolfgang Rupp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.1 Lasers ..........................................
6.3.2 Laser Applications ................................
6.3.3 Carl Zeiss Lasers .................................
6.3.4 Requirements on Laser Coatings ...................
6.4 Cold-Light Reflectors
Lars Bewig, Thomas Kupper, Roland Langfeld . .............
6.4.1 Requirements and Design ..........................
6.4.2 Applications .....................................
6.4.3 Processes ........................................
6.4.4 Glass-Ceramic Reflectors with SCHOTT PI Coating®
Thomas Kupper, Christoph Moelle, Lars Bewig . ......
295
296
299
308
310
311
313
320
335
335
336
338
341
344
344
346
346
348
Contents
6.5
6.6
6.7
6.8
6.9
6.10
XIX
Automotive Rear-View Mirrors
Falko v. Unger ......................................... 353
6.5.1 Specifications for Automotive Mirrors ............... 354
6.5.2 Manufacturing Process ............................ 356
Large Area Sol~Gel Dip Coatings
Eckart K. Hussmann . ................................... 359
6.6.1 Historical Background ............................ 359
6.6.2 Sol~Gel Dip Coating Process ....................... 360
6.6.3 Dip Coating Facility .............................. 361
6.6.4 Influence of Process Parameters on Properties
of the Coatings .................................. 363
6.6.5 Accuracy of the Dip Coating Process ............... 365
6.6.6 Solar Control Coatings: Calorex® .................. 365
6.6.7 Antireflection Coatings ............................ 370
6.6.8 Dichroic Coatings ................................ 372
Schott Type I Plus® Containers for
Pharmaceutical Packaging
Marten Walther . ....................................... 373
6.7.1 Selection of Coating Technology .................... 374
6.7.2 Layer Structure .................................. 375
6.7.3 Process Control .................................. 375
6.7.4 Characterization ................................. 377
6.7.5 Formation of Glass Particles ....................... 379
Ophthalmic Coatings
Michael Witzany ....................................... 380
6.8.1 Market Review ................................... 380
6.8.2 Layer Systems ............................... : ... 381
6.8.3 Processes ........................................ 382
6.8.4 Requirements on Ophthalmic Coatings .............. 386
6.8.5 Production of Eyeglasses at Carl Zeiss .............. 388
IR-Reflecting Multilayer Films for Energy-Efficient Lamps
Hrabanus Hack, Torsten Holdmann ....................... 389
6.9.1 The Principle .................................... 389
6.9.2 Materials ........................................ 390
6.9.3 Deposition Processes .............................. 391
6.9.4 IR-Reflecting Coating by PICVD ................... 391
6.9.5 Possible Lamp Configurations ...................... 392
Coatings on Plastics with the PICVD Technology
Markus Kuhr, Stefan Bauer, Uwe Rothhaar, Detlef Wolff . ... 393
6.10.1 General Experimental Procedure ................... 393
6.10.2 Substrate Cleaning ............................... 394
6.10.3 Preconditioning ~ Interaction of Plasma
with the PMMA Surface .......................... 395
XX
Contents
6.10.4 Adhesion of the Layer System ......................
6.10.5 Scratch Resistance ................................
6.11 Optical Multilayers for Ultra-Narrow Bandpass Filters
Fabricated by PICVD
Stefan Bauer, Lutz Klippe, Uwe Rothhaar, Markus Kuhr ....
6.11.1 Experimental Procedure ...........................
6.11.2 Results and Discussion ............................
References .................................................
399
401
407
409
410
413
List of Contributors . ...................................... 421
Sources of Figures and Tables ........................... 425
Index ........................................................ 427
1. Overview - Thin Films on Glass:
an Established Technology
Burkhard Danielzik, Martin Heming, Dieter Krause, Alfred Thelen
1.1 Introduction: Why Surface Coating?
In the real world every solid-state object has a surface. This surface is a
discontinuity in the properties of the bulk materials and has been the target
for modifications (artistic decoration and/or functional improvement) since
the earliest times of mankind.
Today, modern products offer ever more new functions, requiring more
and more materials with unusual properties that often cannot be found in
simple bulk substances. Combinations of the characteristics of different materials are required. They can be achieved in various ways: Binding of different
materials in composites can be done by macroscopic joining and structuring (e.g., glass-fibre-reinforced polymers or SiC-fibre-reinforced glasses); on
a microscopic scale methods such as mixing (e.g., glass ceramics or other
types of nanocomposites including glass as one component), surface coating
and many others are suited. These improved materials and products often
open completely new areas of application for the basic materials. The multifunctionality of technical products made from these improved materials,
especially concerning their surface properties, is an increasingly important
criterion in determining their value and price. This trend is perceptible in
many branches of industry, also in the glass industry.
Following the title of this book, and taking special interest in the problems
of glass as a substrate material, we focus sharply on the surface coating of
glass for major applications, which often means improving various properties
while maintaining the high transparency in the visible wavelength range as
one of the most important properties. Therefore, we exclude many substrate
materials such as metals, stone, wood, and fabrics, and we exclude many
technologies such as galvanic coatings, painting techniques, enamels and so
on.
Surface coating of glass with different types of films is one of the technologies that occupy a key position in the material and product development
with a view to improving various properties of the glass surface.
Transparent electrodes are produced by the deposition of tin oxide or
indium-tin oxide (ITO) coatings on glass, which provide electrical conductance to the otherwise highly insulating glass substrate without impairing the
2
1. Overview - Thin Films on Glass: an Established Technology
excellent optical transparency of glass [1.1J. The surfaces of chemically sensitive glass types are stabilized with silica coatings to overcome corrosion in
different aggressive environments. Silica coatings are also applied as blocking
barriers to avoid the diffusion of glass components into a neighbouring reservoir. Moreover, thin films increase the strength of glass in high-temperature
applications. Metal coatings convert the almost perfectly polished mirror base
of otherwise low-reflecting glass into a superior high-reflecting mirror [1.2J.
Modern deposition techniques with innovative choices of the deposition conditions enable the production of thin-film membranes, sensors and films for
ion storage. By the use of electrochromic materials, multilayer systems can
be produced that allow an electrically controlled, continuous variation of the
otherwise fixed transmittance, reflectance or absorptance of the glass [1.3J.
Traditionally of technical significance is the ability of optical coatings to
add interference effects to the surface: Optical interference films can change
the transmittance, reflectance and absorptance of a surface in a prescribed
manner. Examples are antireflection coatings [1.4], which lower the often unacceptably high surface reflection of glass; low-e coatings [1.5], which reduce
the heat losses of architectural glass windows; and cold light reflector coatings [1.6], which take the infrared portion out of the electromagnetic radiation
emitted by incandescent lamps.
For optical interference to occur, at least two coherent light waves must
interact. In the case of a single thin film, the two waves are generated at
the substrate-film and the film-air interface. If the two waves subtract, the
reflectance is reduced (single-layer antireflection coating); if they add, the
reflectance is increased (single-layer beam splitter). The constant phase difference over the useful area requires a precise control of the optical thickness
nd of the film.
Yet for most technical applications the action of a single interference film
yields insufficient results for transmission magnitude, transition sharpness
and bandwidth. Therefore many layers have to be used. For some applications
the number of layers may exceed 100 (e.g., infrared filters). In addition to
the interference properties, the mechanical, electrical and chemical functions
are important. Coatings playa key role in all the areas listed in Table 1.1.
1.2 Coating Materials
The properties of the coating material determine the achievable performance
of the manufactured product. Table 1.2 shows as an example the desired
properties for typical optical coatings [1. 7J.
One might assume that a coating consists of layers of well-known materials. This is far from the case in reality: Thin films of coated materials
generally have neither the same physical properties nor the same chemistry
as the respective bulk material. Deposition processes which create films far
away from thermodynamic equilibrium (e.g., at low substrate temperature or
1.2 Coating Materials
3
Table 1.1. Applications and functions of coatings
Product group
Special product-specific functions of coatings
Displays:
Antireflection coatings for cathode ray tube contrast
enhancement, transparent electrodes for radio frequency interference shielding and liquid crystal displays, liquid crystal light valves for projection television and head-up displays
Antireflection coatings for camera lenses, camera mirrors, filters, colour film processors, enlarging heads,
professional and amateur television cameras
Antireflection coatings for safety and aesthetics, antiscratch coatings for ophthalmic glasses, sun protection
and photochromic glasses, laser goggles, welding goggles; electro chromic films
Energy-saving coatings for regenerative lamps, coldlight reflectors for shop window display lights, movie
projectors, stage lighting, dental/surgical lighting;
coloured coatings for traffic/railway signal lights
Interference coatings applied as antireflection coatings,
beam dividers, mirrors, filters, variable filters, narrow band Raman spectroscopy filters, fluorescence microscopy filters, laser end mirrors, laser windows, highpower polarizers, laser gyro coatings
Different types of coatings for high reflectors for photcopiers, compact disks, optical data disks, magnetooptical data disks, optical read/write heads
Conductive coatings for burglar alarms, interference
flakes for anticounterfeiting inks for documents, licences and bank notes
Energy control films for architectural glazings, antireflection coatings for shop windows, "smart" windows
Reflective coatings for rear-view mirrors; scratchresistant coatings for plastic windows and plastic headlights, heat-reflecting windows
Interference and antireflection coatings on semiconductor laser end faces, narrow band filters, wavelength division multiplexers, waveguiding films; integrated filters
Heat-reflecting coatings for oven windows, oven wall
heater reflectors, optical pot heaters
Photography / video:
Eyeglasses:
Lighting:
Instruments / lasers:
Photocopiers /
data storage:
Security:
Buildings:
Automobiles:
Optical communication /
integrated optics:
Home appliances:
Jewellery and art:
Flexible substrate:
Interference and iridescent colour effects for art and
jewellery
Different interference and conductive films for architectural window films, cathode ray tube contrast enhancement films, touchscreens, inexpensive solar reflectors,
iridescent packaging, lightweight mirrors
Pharmaceutical packaging: Transparent diffusion barriers; UV-absorbing coatings
Containers:
Coatings for reduced friction, improved scratch resistance and strength
4
1. Overview - Thin Films on Glass: an Established Technology
Table 1.2. Properties of thin dielectric films
Property
Desired features
Refractive index
Transmission
Scattering
Geometrical thickness
Stress
Adherence
Hardness
Temperature stability
Insolubility
Resistance to laser radiation
Structural defects
defined, homogeneous, reproducible
high, extinction coefficient < 10- 4
low, < 10- 4 for a ),,/4 film
defined, reproducible
low, defined, reproducible
high, at least MIL C 675
high, like glass, at least MIL C 675
-200 to +400°C
at least MIL C 675
as high as possible
as few as possible
Note: Military standards and specifications (MIL) of the US Department of Defense
are commonly used for coated products, due to the lack of international standards.
with high energy particle bombardment) and the high surface-to-volume ratio
dramatically change the physical properties (e.g., index of refraction, extinction coefficient, homogeneity, density, hardness, internal stress, adhesion to
the substrate) and the chemical characteristics (e.g., purity, stoichiometry,
crystal structure, reactivity, gas permeability, solubility). These differences
between bulk and thin-film material depend strongly on the type of deposition process and its process parameters (deposition temperature, deposition
rate, gas pressure, substrate geometry and preconditioning, preparation of
the coating material, post-deposition treatment, etc.).
How large these changes can be has been demonstrated for the optical
constants of Ti0 2 coatings [1.8]. In Fig. 1.1 the extinction coefficient k is
plotted as a function of the refractive index n at a wavelength of 550 nm for
films prepared by different processes and at different institutions. Extinction
coefficients ranging from about 0 to > 25 X 10- 4 and refractive indices ranging
from < 2.2 to > 2.6 were observed.
Ti0 2 is very often deposited with an X-ray amorphous structure; see
Fig. 1.2a. Thermal post-deposition treatments in most cases change the crystal structure of the film, as shown in Fig. 1.2b,c, where the formation of
anatase and rutile structures was observed. On float glass substrates - but
not on silica substrates - the brookite structure can also be observed, depending strongly on the time-temperature programme. An Na concentration,
which is indiffused from the substrate and temporarily observed in the Ti0 2
film, has been shown to be essential for this phase transformation.
In Sect. 4.1, many other results of the characterization of Ti0 2 films
are reported; Fig. 4. 13a-f, for instance, shows the strongly differing crystal
structure. Table 1.3 summarizes some results on density, refractive index and
composition of films deposited by different techniques. One clearly sees the
large variability of the density of the films depending on their crystalline
1.2
Coating Materials
5
30,----------------------------------,
•
"fo
><
~
•
20
C
•
<I>
'u
!E:
<I>
o
u
§ 10
t5c:
~
UJ
.
•
• •••
• • • • • ••
2.2
2.3
2.4
2.5
•
2.6
Refractive index n
Fig. 1.1. Plot of extinction coefficient k against refractive index n measured at
550 nm for a number of independent depositions of Ti02 using a variety of tech-
niques
structure and porosity. An immediate consequence is the variation of the refractive index, which in a first order approximation is directly proportional
to the density. A quantitative analysis of the composition always shows some
deviations from the ideal stoichiometry of the Ti0 2 as well as various impurities which are indicative of the precursor materials and the deposition
process used. This result shows that it is meaningless to "know" material
properties without knowing and describing the deposition process in detail.
We have used Ti0 2 to demonstrate the wide range of property changes
induced by different deposition techniques or by the choice of process parameters. There are of course many other materials and properties to discuss in
the variety of applications of coated products. Special segments of these fields
are covered in numerous books and a useful overview and selection is given
Table 1.3. Properties of nominal Ti02 coatings deposited with different techniques
(which are explained in Chap. 4)
Preparation
method
Density
gcm- 3
Sol-Gel: DC
SC
Spray
PVD:
RE
lAD
SP
IP
CVD:
PICVD
2.88-3.26
2.88-3.46
3.46-3.84
3.65-3.92
3.07-3.84
Refractive index
at A = 550 nm
2.23-2.30
2.23-2.30
2.19-2.21
2.21-2.45
2.30-2.52
2.42-2.53
2.43-2.56
2.44-2.51
Composition
Ti01.9o Clo.o8 CaO.Ol
Ti01. 75 Clo.24 NaO.1O
Ti02.o8 Sio. o3
Ti02.o4
Ti02.04 Aro.o5
Ti01.99
Ti01.98 Clo.3o
Ho.o4
Sio.o5Ho.6o
HO.1O
CO.1O
Ho.25
TaO.OO3 Ho.o5
MOo.oo2 Hom
Ho.o5
C O.25
6
1. Overview - Thin Films on Glass: an Established Technology
1000
Ul
a.
~
900
o
~ 400
C~
8
100
o +--..----,r----r--.....--.--.---I
20
30
40
50
60
Angle 2 e/deg
70
80
90
10000.,...--:---------------, b)
6400
8.
o
'"Q;
~
§
o
3600
1600
()
400
o +---r---.---.---r---r---r----I
20
30
40
50
60
Angle 2 eIdeg
70
80
90
10000..------------------------, c)
fr
6400
3600
~
C 1600
'"Q;
5
()
400
o +--..----r--.--r--r---r----I
20
30
40
50
60
Angle 2 e fdeg
70
80
90
Fig. 1.2. X-ray diffraction patterns of 90-nm-thick Ti02 films prepared by sol-gel
dip coating on silica substrates. (a) As deposited and dried at ::; 200°: amorphous
structure. (b) After annealing for 1 h at 900 0 : crystallization into anatase structure,
starting at about 400 o. (c) After annealing for 1 h at II 00 0: crystallization into
rutile structure, starting at about 900 0
1.2 Coating Materials
7
by Ohring [1.9]. Schott has been developing and producing coatings to modify, improve and protect glass surfaces for optical, electrical, mechanical, and
chemical applications. These coatings will be the main focus of this book,
whereas others (e.g., films for high mechanical stability of metals such as
nitrates, carbides, diamond-like carbon, etc.) are mentioned only in passing.
For our purposes, oxides (e.g., Si0 2, Ti0 2, Ta205, Ab03, Zr02, Sn02,
Zn203, ZnO) are particularly important because they enable the production
of so-called "hard" coatings that resist thermal and chemical loads encountered in the application. Often, doping or mixing are means to extend the
property spectrum. Another group of coating materials are the fluorides (e.g.,
MgF2' ThF 4 , AIF 3), which have low absorption in the visible and extended
UV and IR transmission. Sulphides (e.g., ZnS as high-index material or for IR
applications) and several metals (e.g., Ag, Au, AI, Pt as reflector materials,
Cr and Ni as absorbers for photomasks or neutral density filters and as nucleation agents for other metal depositions) are also used. For electrochromic
coatings, materials such as W0 3, V0 2, NiOxHy are used.
An increasing number of organic materials, for example fluoropolymers,
polyanalynes, and viologens, are used for special purposes. They are often deposited by plasma polymerization processes [1.10,11]. Some of the polymers
are also mixed with inorganic materials to create films with unusual properties, which are used in applications requiring soft, deformable and chemically
dense coatings.
From the above description of the importance of the materials knowhow for deposition processes, one can distinguish two different approaches, a
"technical" one and a "scientific" one (where these terms are used in their
purest sense). Both approaches have their justification. They are in most
cases alternately applied and progress in one field tends to stimulate progress
in the other.
The technical approach is dominant in industrial development projects.
It is based on systematic improvements by using the already known dependencies of properties and process parameters to reach the specification by
statistical design of the experiments.
The scientific approach is characteristic of research-oriented programmes.
It integrates materials science and mathematical modelling of processes, materials and product behaviour into the development project to identify and
understand dependencies and to optimize the result on an extended knowhow basis. The scientific approach is often more time-consuming than the
technical approach, but offers additional improvement potential.
Whether the technical or the scientific approach is preferable will depend
on the actual task. At Schott, both routes have been pursued intensely, as
will be shown in Chaps. 3-5.
8
1. Overview - Thin Films on Glass: an Established Technology
1.3 Thin-Film Forming Processes
There are two fundamentally different approaches to forming thin films on
glass: subtractive and additive processes.
The subtractive leaching of multicomponent glass surfaces was the first
patented technique for forming antireflective coatings [1.12]. These films are
formed as a laterally homogeneous but porous structure of the networkforming components after removal of the highly mobile network modifiers.
In most cases the pore volume is filled with water from the ambient atmosphere. The refractive index n ~ 1.45 is close to that of silica. However, the
chemical and mechanical stability of these coatings is rather limited, which
explains why they lost their attractiveness for technical applications under
normal conditions as soon as other processes were developed.
Nevertheless, one exotic application is known in high-power-resistant coatings for laser components [1.13], which were used in a clean-room environment. With a buffered neutral solution process at elevated temperature for
up to 90 h, a single-layer antireflection coating on BK 7 glass with a residual reflectivity of < 0.1% at a specified wavelength (1.064I-lm) was produced
on large and highly polished laser components. Even in a clean-room environment, the reflectivity grew within 200 days to 1% but could be reduced
again by cleaning. The damage threshold of 12 J cm- 2 was about twice the
threshold of conventional PVD coatings.
The glow discharge cleaning process, which is often applied in PVD processes, also removes components from the glass matrix, thus forming a subtractive low-index film with very slow thickness growth. But due to different
side effects such as colour-centre formation, ion implantation and relaxation
phenomena (compaction), a technical application has never been realized.
The additive processes are much more versatile because almost all materials can be used on nearly all substrates in order to produce the desired
properties. Within certain limits they also enable the production of multilayer
stacks and remarkable coating thicknesses up to the millimetre range.
For each application, the substrate and coating material(s), a suitable
process type and its parameters, which are responsible for the product quality, allow a huge variety of different combinations. All these combinations
have their specific advantages and disadvantages, i.e., there exists no "ideal"
process. An excellent overview of coatings on glass is given by Pulker [1.14].
The classical processes for the deposition of optical coatings are highvacuum thermal evaporation [1.15] and dipping [1.16]. Over the last fifteen years, sputtering became an interesting alternative for large-area coatings [1.17] and ultimate-quality coatings [1.18]. Recently, chemical vapour deposition [1.19-24] has emerged as an alternative for high-quality large-scale
production.
High-vacuum thermal evaporation can produce simple and complex multilayers that fulfil most of the requirements listed in Table 1.4. Most optical
coatings are produced this way. It is a flexible process that can accommodate
1.3 Thin-Film Forming Processes
9
Table 1.4. Requirements to be met in optical coating deposition processes
Requirement
Explanation
Hardness and durability
The deposition process should not downgrade the inherent hardness and durability of the coating materials (oil contamination, residual gas, bad temperature
distribution, etc.).
Vacuum-deposited coatings often have a pronounced
columnar structure. This makes them vulnerable to
moisture penetration. Wet-dry shifts and water erosion occur.
Optical interference coatings need a thickness and refractive index tolerance of < 1%.
The ability of a process to achieve large area deposition is an important economic factor.
Generally, optical coatings have an optical thickness
of about ),,/4, where ).. is the wavelength of the incident light. For some design techniques (flip-flop
[1.31]) very thin layers, and for protective purposes
very thick layers, are needed.
The substrates for optical coatings can have many
shapes: planar, spherical, lenticular, etc.
Optical coatings can have very complex structures
(e.g., infrared filters) with many layers (up to 1000)
and several different coating materials (up to 5).
Plastic substrates are generally soft. The coating
must spring back without damage after large deformations.
Stability
Precision
Large area
Very thin/very thick layers
Substrate shape
Complexity
Resilience
most shapes and sizes. There is one major drawback though: Evaporated
coatings are not very stable and suffer from wet-dry shift. The reason is
their poor microstructure [1.25]. It can be overcome by introducing ionization into the process [1.26,27]. But this adds costs to a process which, due
to its high-vacuum requirement (0.005-0.05 Pa), is already very expensive.
Dipping, as well as spinning and spraying, have been used as thin-film deposition processes from the very beginning of thin-film technology. Remarkable results can be obtained [1.16,28]. Yet they never became methods for
general optical interference film preparation but remained "niche" processes
for products of limited sales volume, for the fast and easy deposition of multicomponent or very thick films. These processes need wet-chemical processing
and in most cases apply high temperature loads to the substrate, which often
makes them incompatible with other technologies in the manufacturing of a
product.
Sputtering has replaced high-vacuum thermal evaporation in two areas:
ultimate-quality coatings and large-area coatings. Two very different sputtering processes are used for the two types of products: Ultimate-quality
10
1. Overview - Thin Films on Glass: an Established Technology
coatings are produced by ion beam sputtering, which is a very slow process
with rates < 0.1 nms- 1 [1.29,30]; large-area coatings are produced by reactive dc magnetron sputtering, which is almost as fast as evaporation but has
problems with absorption and becomes clumsy when the number of layers is
high. Sputtered coatings have less of a problem with microstructure because
the operating pressure is higher (0.05- 1 Pa) and the surface mobility of the
material that is being coated is higher (lO eV versus 0.1 eV for evaporation).
The main advantage of the CVD processes is that they produce coatings
of superior quality at lower equipment cost. Both versions mentioned, lowpressure chemical vapour deposition (LPCVD) and plasma impulse chemical
vapour deposition (PICVD) , use a vacuum of only 10- 150Pa. The mean free
path of the precursor molecules at these pressures is only 0.5-0.005 mm. The
coating is not "line-of-sight" and there is no self-shadowing (which is the
main cause of the poor microstructure of evaporated films) [1.25]. The higher
pressure also translates into less expensive equipment and lower operating
costs.
The disadvantages of the two processes are: high substrate temperature
(200- 1200 °C, depending on the material) for the LPCVD process and limited
substrate size « 20 cm in diameter, depending on the wavelength of the
microwave energy) for the PICVD process. The PICVD process is further
limited to non-conducting substrates and coatings.
In Table 1.4 we list the general requirements for optical coating deposition
processes. In Fig. 1.3 we show the performance of the major current produc-
PICVD
LPCVD
Sputtering
Energetic evaporation
Evaporation
Q)
u
c:
~
.u;
Q)
0::
Fig. 1.3. Performance ratings of current and emerging optical coating production
processes
1.4 Fabrication Issues for Coatings
11
tion processes according to Table 1.4. A rating of 1-10 is used. The ratings
for evaporation, energetic evaporation and sputtering are based On the authors' personal experience. The ratings for the CVD processes (LPCVD and
PICVD) are solely based on the authors' evaluation of the process conditions
as presented in literature [1.20-24].
1.4 Fabrication Issues for Coatings
There are three main types of "classical" coating machines in optical coating
production: batch coaters, load-lock coaters and in-line coaters.
• In the batch coater [1.32]' substrate loading and pretreatment, coatingmaterial deposition and coated-substrate post-treatment are done sequentially in the same vacuum chamber. This receptacle is cycled from atmospheric pressure to high vacuum for every batch of substrates. Also,
coating-material sources are warmed up, used and cooled for every batch
and generally for each layer. For the deposition of multilayer systems, several sources of material are employed.
• In the load-lock coater [1.33], substrate loading and pretreatment is done in
one chamber, coating-material deposition in a second and coated-substrate
post-treatment in a third chamber. The substrate carrier moves from chamber to chamber through gate valves. Only the pretreatment and posttreatment chambers are exposed to atmospheric pressure for each batch.
The coating-material deposition chamber is exposed to atmospheric pressure only for maintenance purposes (reloading of coating material, cleaning). In the coating-material deposition chamber, coating-material sources
are warmed up, used and cooled for every batch and generally for each
layer as in a batch coater.
• In the in-line coater [1.34], the substrate carrier moves continuously from
the loading station through pretreatment chambers, deposition chambers and post-treatment chambers to the unloading station. The coatingmaterial sources run continuously at a constant rate. The thicknesses are
determined by the deposition rate times exposure length divided by the
substrate velocity. For each layer, a separate deposition chamber is required.
Some modern, emerging technologies use advanced "single-piece" schemes
that don't really fit into the categories described above. In PICVD coating,
for instance, every substrate defines its Own small reactor and the production machinery comprises an array of these reactors. Another example is the
metallic coating of compact disks, where a high-speed load-lock and pumping
system allows an in-line coating with a cycle time of some seconds, exactly
matched to the polycarbonate substrate production process.
In the past, coating fabrication was a rather distributed activity. The glass
substrate was finished and then sent out in batches for coating. Batch coating
12
1.
Overview - Thin Films on Glass: an Established Technology
processes were the logical consequence. For modern mass products this is no
longer true: Glass finishing and coating are integrated and the optical coating
becomes part of the composite material coated glass. Single-piece coaters are
better suited in this case because:
• the time between substrate finishing and coating can be made very short
(this is important because longer times may induce surface contamination, chemical reactions and aging; packaging and transportation are major
causes of surface defects);
• production flow is optimal and allows various process steps to be eliminated; and
• substrate identification is easier to maintain.
For obvious reasons, the issue of coating costs is rarely addressed in the
literature and is handled as proprietary knowledge of the manufacturing company. In general, the determination of production costs follows state-of-theart calculating procedures [1.35], taking into account:
• fixed costs
- depreciation of equipment and interest rate on used-up capital,
- fixed personnel,
- maintenance / replacement parts / tools,
- technical infrastructure to run the production / clean-room area /buildings and ground;
• variable costs
- coating materials (e.g., sputtering targets, CVD precursors),
- substrates,
- manufacturing and sales personnel,
- resources (e.g., energy, gases, deionized water)
as well as throughput,· yield, process stability and up-time of equipment.
Pretreatment and post-treatment have to be handled as integral parts of the
overall manufacturing process.
A standardized scheme for the evaluation of production costs is wellestablished in the semiconductor industry; it is termed "cost-of-ownership"
model [1.36]. For the coating of glass, the cost-evaluation issue is more complex and strongly depends on the application. Thus a detailed discussion
would be far beyond the scope of this introduction. However, some considerations are independent of detailed calculations and hold for most applications
and technologies:
• Coating is a capital-intensive technique. As a rule of the thumb, for every
currency unit of sales the same amount of money has to be invested for
equipment.
• The production costs are dominated by fixed costs; thus issues such as
process yield, up-time of equipment, and optimized use of capacity are of
utmost importance for economic success.
1.5 Product and Overall Process Design
13
• Therefore, a skilled staff of personnel and a solid knowledge of the process capability are important prerequisites to achieve highly competitive
production costs. These qualification and experience issues tend to be underestimated, and such "soft facts" may induce major uncertainties in the
cost evaluation.
• In addition to the need to meet the manufacturing-cost targets, marketing
- and sales have to keep pace. In order to buffer externally determined market influences, a well-balanced product mix is very helpful for continuous
capacity use.
The substrates for coating are often semi-finished or almost-finished parts;
thus the yield has a very strong influence on the production costs. The substrate itself may be expensive due to its functionality, material, or pre-coating
processing; examples are integrated-optics components, expensive optical materials (special glass), highly polished substrates and custom-designed substrates (ophthalmics). These substrates are typically coated in batch systems.
On the other hand, the substrate can become expensive if large areas have to
be the continuously coated. Such large-area coatings (e.g., for architectural
applications), with a quality area in the range of square metres in combination with specified defects (types and numbers) per plate, place tough
requirements on defect control.
1.5 Product and Overall Process Design
In the previous sections we have discussed the properties of thin-film coatingmaterials, the advantages and limitations of various deposition processes and
the tailoring of material properties by selection of deposition technique and
process parameters.
With respect to the product design and overall process design, the following additional issues must be taken into consideration:
•
•
•
•
the selection of the substrate material,
preconditioning,
coating design,
postprocessing.
These topics will be discussed using examples of coating products mentioned above.
For display applications, antireflection (AR) coatings are often required in
combination with a grey colour in transmission in order to provide contrast
enhancement. The desired optical properties can be achieved either by a
"standard" AR coating on a tinted substrate, or by a modified design on
a transparent substrate. Using a transparent substrate has the advantage
of giving higher flexibility to the manufacturer of the product because the
tint provided by a coating allows a tailoring according to customer demands
14
1. Overview - Thin Films on Glass: an Established Technology
without needing to keep the various tinted substrate glasses in stock. On the
other hand, it requires a higher degree of coating process capability.
A common product from the optical instrumentation field are bandpass
filters having a high transmission only between a low- and a high-wavelength
cut-off. The characteristic transmission can be achieved with optical interference coatings by one of two approaches: a sequential "double stack" of
a low-pass and high-pass design, or a "single stack" with only low-pass or
high-pass function, where the missing wavelength pass is provided by an optical filter glass used as substrate. Besides optical performance and economic
aspects, the selection between the above approaches will depend on the durability requirements for the filter in terms of thermal, chemical, environmental
and long-term stability.
The surface condition of the substrate is crucial to the coating properties,
particularly to the adhesion of the coating. With glass substrates, besides
contamination control, surface aging and differences in the glass composition
of the near-surface region compared to the bulk material must be given special
attention. Typical contamination effects are dust and dirt from shipping or
storage, and sample holder or finger prints from handling. Additionally, thin
hydrocarbon films are a commonly encountered problem. They can stem from
oil-based pumping units, from softening agents in plastic packaging materials,
or even from the ambient atmosphere. Surface aging, mainly caused by water
interacting with the glass surface, is strongly promoted by high temperatures
and results in surface corrosion [1.37]. Further corrosion effects can be due
to the acidic substances from spacer paper used for packaging.
The chemical surface composition of the glass material almost always
differs from that of the bulk material. This is intrinsically due to the fact
that glass is a multicomponent material that comprises atoms and ions of very
different properties. Therefore every surface processing can cause deviations
from the bulk composition (e.g., preferential evaporation during hot forming,
ion exchange and leaching - especially of alkaline ions - during polishing and
cleaning). An important example is soda-lime glass produced by the float
process, where the chemical surface compositions of the "fire side" and the
"tin side" differ significantly.
Nearly all coating processes require a cleaning of the substrate surface.
This is commonly performed by wet-chemical processing or by plasma processing [1.14]. Since the early 1990s, the wet-chemical cleaning procedures
have strictly been redesigned to all-water based processing, avoiding chloroor fluorocarbonate chemistry. For optical glasses, the cleaning recipes are optimized with respect to the chemical composition. Standard materials such as
soda-lime and borosilicate glass can be cleaned by detergents readily available from the chemical industry. Whereas the basic cleaning mechanisms are
standard state-of-the-art, the detailed cleaning process parameters are mostly
regarded as proprietary knowledge because they can be crucial to the quality and economics of the coating product. Plasma cleaning is employed in
1.5 Product and Overall Process Design
15
almost every PVD or plasma-based CVD coating process, where a glow discharge in an oxygen or argon atmosphere is commonly used. Because this glow
discharge can remove only very thin residue layers, it is mostly used in combination with a wet-chemical cleaning. For deposition processes that don't
allow this integration of a cleaning plasma into the deposition process, the
plasma cleaning can be performed using a separate apparatus ("dry-cleaning
systems"). On the subject of cleaning, coatings deposited without cleaning
play a special role. In such cases, the coating process is integrated into the
production line (e.g., low-e coatings produced by spray coating of tin oxide
on the hot end of the float line, friction-reducing tin oxide layers on container glass sprayed in hot-forming line, and sputtered metallic aluminium
coatings directly deposited onto injection-moulded polycarbonate compact
disk substrates).
The design of optical interference coatings by physical modelling and calculation has been an important capability since the early days of optical
coatings in the 1940s, and the availability of skilled experts was a key success
factor for the manufacturer. The state-of-the-art in optical coating design and
trends are reported in detail in Chap. 2. Here it is worth mentioning that the
availability of powerful computer hardware and software has changed the role
of design capabilities during the recent years. With modern computer design
tools, even scientists or engineers with only average experience can calculate
a coating design that is competitive or identical to state-of-the-art solutions.
This revolution in design strategy can be attributed to algorithms such as
Tikhonrarov's "needle method", which requires no starting design for optimization [1.38]. Whereas it seems that the design skills can be waived for the
pure calculation, the experience and know-how in designs and coatings are
still of crucial importance when it comes to the fabrication of the coating.
Often the synthesized designs employ very thin layers [1.31] that can be deposited by a few techniques only (e.g., flip-flop design made by PICVD). Thus
the strategy to change the numerically optimized design to a design optimized
for fabrication requires simultaneous knowledge of deposition techniques and
processes. The challenge has not really changed, but tools such as needle algorithms, sophisticated process control and readjustment have significantly
improved the quality and economy.
Although coatings add performance and value to the substrate surface,
they are typically semi-finished products and subject to further processing.
This postprocessing has to be taken into consideration when coating design,
material and process are selected. Examples of simple post-processing are cutting to end format and framing of optical filters or ophthalmic coatings, sometimes including edge/corner work. But many coatings are subject to much
severer stresses imposed during post-processing (e.g., thermal toughening of
architectural coatings, thermal bending of automotive mirrors to spherical or
aspherical shape). These post-processing stresses apply typically for products
that, for economical reasons, are coated in dimensions larger than end format.
16
1. Overview - Thin Films on Glass: an Established Technology
Additionally, for some products well-adjusted post-processing properties are
important, for example the etchability of ITO transparent electrodes for flat
panel display applications.
The issues discussed in this section dearly demonstrate that the optimization of the overall fabrication process of a coating product can be of
prime importance to its success. Long-established experience and intimate
knowledge of materials and deposition processes are key factors for successful overall process optimization. For this reason there is no general answer
to the often encountered question regarding the best coating process. Owing
to the broad variety of coating products, applications and fabrication processes, no ideal coating process will ever be identified. A systematic selection
and optimization of materials and deposition processes within the scope of a
manufacturer's own experience can be superior to using the "most advanced
machinery" .
All efforts in developing and fabricating coated products have to be focused towards meeting the customers' demands in an economically viable
manner. Table 1.5 summarizes the broad range of requirements for coated
products.
Table 1.5. General requirements for coated glass products
Type of properties
Property
Substrate
Choice of material, range of substrate formats, surface
quality: flatness, cleanliness
Film
• Optical properties:
• "Active" and non-linear
optical properties:
• Electrical properties:
• Mechanical properties:
• Chemical properties:
• Stability:
Process
• Ecological properties:
• Costs:
• Post-processing
properties:
(n, d) for single and multiple layers, optical reflectivity or transmission, colour (ClE), waveguide losses,
especially uniformity of properties
Adjustable optical reflectivity, transmission, colour,
etc., due to electrical control, high intensity of illumination, or other external forces
Sheet resistance
Scratch resistance, fracture strength, elastic stiffness
Diffusion barrier for ions, gas or water; hydrophobic
or hydrophilic surface properties
Adhesion, resistance against temperature, humidity,
light (UV), chemical and environmental attack, mechanical stability
Environmental safety of materials, recycling of coating / coated substrate
Target cost for product or product group
Etchability, solubility, adhesion of resins or additional
coatings, resistance to thermal loads for substrate
bending or toughening
1.6 Today's Situation and Trends
17
1.6 Today's Situation and Trends
Nowadays, nearly all types of coatings are technologically available, but not
all of them are also economically viable. Further development of coated products will be driven by new demands on their function and the accepted market prices, on the one hand, and by technological progress leading to higher
productivity, improved quality, or new material and process features, on the
other hand.
The broad range of products and applications includes high-volume commodity products, where the applied coating technologies have to meet tough
quality and cost targets, as well as low-volume speciality products, where
availability is more important than cost and even a "single-piece technology"
can be worth a lot.
Coated products may also be categorized according to the functional role
of the coating as "basic coating products" and "added value products" .
In basic coating products the coating is a prerequisite for an application.
Among them are high-volume applications such as automotive rear-view mirrors, cold-light reflectors and low-e glazings in architecture. There the quality
requirements are often met by the standard manufacturing processes and further development is due to the cost pressure and focused on the improvement
of economics. For basic coating products with medium and low production
volumes, the quality improvement is often the motivation for development;
examples are laser mirrors, wavelength-selective filters and components for
optical communication technology and integrated optics.
In added value products the coating by definition is not essential to the
basic function ofthe product. The demand for additional performance is often
stimulated by secondary arguments such as increased safety, comfort, aesthetics or ecology; examples are ophthalmic coatings on eyeglasses, antireflection
coatings on architectural glazings or displays, and energy-saving coatings in
lighting. Many of these would have the potential to grow to high volume if
the economics of production were improved. Thus added value products are
subject to the same cost pressure as high-volume basic coating products. But
on the other hand the market is susceptible to fashion trends and innovations of any kind, so that in the future the use of uncoated material will be
"non-standard" for many applications.
In Fig. 1.4 the situation is analysed in a so-called "Puttick grid" [1.39].
The complexity of a product is plotted against the uncertainty of the demand.
In Fig. 1.4a the four segments are allocated to four different types of products:
basic standard products, diversified and complex standard products, products
designed to meet the needs of individual customers and highly complex and
expensive products. As a consequence ofthe major criteria (see Fig. l.4b) for
each segment, companies must focus their business strategy and behaviour
on some characteristic features in order to be economically successful.
Some coated products are evaluated in Fig. l.4c with the criteria of
Fig. l.4b from the viewpoint of a coating manufacturer, who in most cases
18
1. Overview - Thin Films o n Glass: an Established Technology
Complexity of product
a)
Diversification + Power I
diversified complex
or standard products
..c:
..c:
Problem-Solving Abilitx!
(Feasibility)
~
.9>
highly complex and
expensive products
~"O
high
low
c: c:
rom
t:E
~al
basic standard products
c:"O
:>0
products designed to
individual needs
Prod ctivitv + Qualitv !
Elel!i!:!ilitx + S~eed !
~
..2
Complexity of product
1. Power to define and to market innovations
2. Market vision
3. Use of standard components
4. Catalogue products with many variants for
defined lifetime
5. Creativity
..c:
..c:
.9>
1. Reliability of delivery in spite of difficult
manufacture
2. Incomplete specification
3. Many components or contributions necessary
4. Design changes during manufacture (long time)
5. Cost and productivity of minor importance
high
low
1. Cost most important
2. High productivity. few variants only
3. High quality assurance
4. Reliable availability
5. Production just-in-time and on stock
~
..2
1. Customer service and availability most
important
2. Short design and production times
3. Limited product life
4. Demand by chance only
5. Cost of minor importance
b)
~'"
c: c
~~
alal
U'"
c:_
:>0
Complexity of product
c)
..c:
..c:
.9>
low
Eyeglass
with ASs +AR 7
ADI' rugate filter
ADI' flip-flop filter
Automotive EC a mirror
Automotive rearMDI 2 filter
view mirror
I
IRC3
Display
CD"
Arch . AR7
CLR 5
..2
Glazing : Auto + Arch. low-e
, AII-dielectric interference
interference
~'"
c c:
.~ ~
al al
ADI' standard filter
U"O
c_
:>0
~mAR'
2 Metal-<lielectric
high
Design-to-go
technique
3lnfrared-reflective coating
SCold-light reflector
• Compact disk
6Antiscratch
7Antireflective
8Electrochromic
Fig. 1.4. Coating product port folio. (a) Product types a nd charact eristic features
of a compa ny's orientation in order to be s uccessful in a competitive environment;
(b) major criteria in segments; (c) evaluation of products with coated components
from t he viewpoint o f acoating manufacturer
References
19
is a producer of semi-finished components and not the system manager. This
situation is clearly reflected by the placement of the different product types
and is no surprise.
Current development trends focus on the optimization of fabrication techniques; some areas of process development are:
• integration of the coating process into the manufacturing line of the substrate: cold-light reflectors, AS (= antiscratch) and AR (= antireflection)
coatings for eyeglasses, electrically conductive films on float glass and flat
panel displays, etc.;
• reduction of fabrication cost by increasing the throughput, substrate dimension, deposition rate, up-time, operating pressure, etc.;
• systematic process optimization by mathematical modelling of the deposition process to improve the depth of knowledge and to support decision
making;
• ultra-fast computer control of the deposition process with the option of online design corrections in the event of deviations from the ideal deposition
programme;
• modular technologies that allow flexible adjustments to market-driven capacity needs without quantum leaps in investment costs (this is an interesting aspect also for high-volume and low-volume production lines);
• extraordinary process stability to improve the product quality and to reduce the variety of applied coating materials through flip-flop designs of
ultrathin sublayers which behave with an averaged value of their optical
thickness sum over the operating wavelength in optical film stacks [1.31],
• improvement of the film quality by applying reactive techniques and energetic particle bombardment to the growing film, thus reducing the porosity
and eliminating the coating's sensitivity to environmental impacts.
All existing technologies show substantial improvements in several of these
areas. If a product becomes highly attractive to a growing market (e.g., architecture, information displays or lighting), its position generally shifts to
the left or lower left region of the Puttick grid shown in Fig. 1.4. For such
products, CVD technologies in particular have a strong potential for further
improvements.
References
1.1 J.L. Vossen: "Transparent conducting films", in Physics of Thin Films, Vol. 9,
ed. by G. Hass, M.H. Francombe, R.W. Hoffman (Academic Press, New
York 1977) pp. 1-71
1.2 G. Hass: "Mirror coatings", in Applied Optics and Optical Engineering, Vol. 3,
ed. by R. Kingslake (Academic Press, New York 1965) pp. 309-330
1.3 K. Bange, T. Gambke, G. Sparschuh: "Optically active thin film coatings",
in Handbook of Optical Properties, Vol. 1, Thin Films for Optical Coatings,
ed. by R.E. Hummel, K.H. Guenther (CRC, Boca Raton 1995) pp. 105-134
20
1. Overview - Thin Films on Glass: an Established Technology
1.4 A. Musset, A. Thelen: "Multilayer antirefiection coatings", in Progress in
Optics, Vol. 8, ed. by E. Wolf (North-Holland, Amsterdam 1970) pp. 203237
1.5 P.H. Berning: "Principles of design of architectural coatings" , Appl. Opt. 22,
4127-4141 (1983)
1.6 W.D. Dachselt, G. Kienel: "GroBtechnische Herstellung von Kaltlichtspiegeln
durch Aufdampfen im Hochvakuum", Vakuum-Technik 28, 239-243 (1979)
1. 7 E. Ritter: "Properties of optical film materials" , Appl. Opt. 20, 21-25 (1981)
1.8 J.M. Bennet, E. Pelletier, G. Albrand, J.P. Borgogno, B. Lazarides,
C.K Carniglia, RA. Schmell, T.H. Allen, T. Tuttle-Hart, KH. Guenther,
A. Saxer: "Comparison of the properties of titanium oxide films prepared by
various techniques", Appl. Opt. 28, 3303-3317 (1988)
1.9 M. Ohring: The Materials Science of Thin Films (Academic Press, San
Diego 1992)
1.10 H. Yasuda: Plasma Polymerization (Academic Press, New York 1985)
1.11 R d'Agostino (Ed.): Plasma Deposition, Treatment and Etching of Polymers
(Academic Press, San Diego 1990)
1.12 H.D. Taylor: "A method of increasing the brilliancy of the images formed by
lenses", British Patent 29561 (1904)
1.13 E.K Hufimann, R Schnabel: "Recent developments in antirefiective interference layer-systems", Proc. SPIE 400, 107-114 (1983)
1.14 H.K Pulker: Coatings on Glass (Elsevier, Amsterdam 1987)
1.15 H. Frey, G. Kienel: Dunnschichttechnologie (VDI-Verlag, Diisseldorf 1987)
pp. 16-42
1.16 H. Schroeder: "Oxide layers deposited from organic solutions", in Physics
of Thin Films, Vol. 5, ed. by G. Hass, RE. Thun (Academic Press, New
York 1969) pp. 87-141
1.17 M. Scherer, P. Wirz: "Reactive high rate d.c. sputtering of oxides", Thin
Solid Films 119, 203-209 (1984)
1.18 A. Kalb: "Neutral ion beam sputter deposition of high-quality optical films",
Optics News, 13-17 (August 1986)
1.19 K. Enke: "Plasma CVD: Verfahrensbeschreibung und Anwendungen", Jahrbuch Oberfiiichentechnik, Vol. 45 (ACM Plasma Beschichtungstechnologie
GmbH, D-63768 Hosbach 1989) pp. 2-24
1.20 D.Z. Rogers: "Manufacture of optical interference coatings by low pressure
chemical vapor deposition", Proc. SPIE 1168, 19-24 (1989)
1.21 D.Z. Rogers, R.P. Shimshock: "Low pressure chemical vapor deposition of
emissivity modification coatings on complex shapes", Proc. SPIE 1307, 524532 (1990)
1.22 J. Otto, V. Paquet, RTh. Kersten, H.W. Etzkorn, RM. Brusasco, J. Britten,
J.H. Campell, J.B. Thorsness: "Radio frequency and microwave plasma for
optical thin film deposition", Proc. SPIE 1323, 39-50 (1990)
1.23 J. Segner: "Plasma impulse chemical vapor deposition - a novel technique for
the production of high power laser mirrors", Mat. Sci. Eng. A 140, 733-740
(1991)
1.24 J. Segner, M. Heming, H. Hochhaus, J. Otto, R Langfeld: "Optical coatings
by plasma impulse CVD" , EOS and SPIE Int. Symp. Optical Systems Design,
Sept. 14-18, 1992, paper 1782-12 (Technical University Berlin, New Physics
Building, Berlin)
1.25 A.G. Dirks, H.J. Leamy: "Columnar microstructure in vapor-deposited thin
films", Thin Solid Films 47, 219-233 (1977)
References
21
1.26 H.A. Macleod: "Ion-assisted deposition of thin films", Proc. Int. Symp.
Trends and New Applications in Thin Films (Societe Francaise du Vide,
Rue du Renard, 75004 Paris, France 1987) pp. 43-52
1.27 H.A. Macleod: "Thin films evolve from black art to exact science" , Photonics
Spectra, 103-104 (January 1993)
1.28 E.K. Hufimann, H.J. Arfsten, H.-U. Heusler, P.H. Roehlen, H.J. Piehlke: "Antireflective coatings on very large substrates by the dip coating process", in
Optical Interference Coatings, Technical Digest Series, Vol. 6, post conference
ed. (Opt. Soc. Am., Washington, DC 1988) pp. 335-338
1.29 H. Demiryont, J.R Sites, K. Geib: "Effects of oxygen content on the optical
properties of tantalum oxide films deposited by ion-beam sputtering", Appl.
Opt. 24, 490-495 (1985)
1.30 1. Kessler, J. Muller, G. Steiniger: "First results for optical layers sputtered
with an rf ion source", in Proc. Int. Symp. Trends and New Applications
in Thin Films (Societe Francaise du Vide, Rue du Renard, 75004 Paris,
France 1987) pp. 605-608
1.31 W.H. Southwell: "Coating design using very thin high- and low-index layers" ,
Appl. Opt. 24, 457-460 (1985)
1.32 RF. IIIsley: "Apparatus for vacuum coating", US Patent 3128205, issued
April 7, 1964
1.33 H.K. Pulker: Coatings on Glass (Elsevier, Amsterdam 1984) p. 213
1.34 1. Seddon: "Opportunities in thin films to meet energy needs", Proc.
SPIE 111, 8-14 (1977)
1.35 C.-C. Freidank: Kostenrechnung (Oldenbourg, Munchen 1992)
1.36 R Jairath, A. Jain, R.D. Tolles, M.J. Hampden-Smith, T.T. Kodas: "Introduction/manufacturing issues in CVD processes", in The Chemistry of Metal
CVD, ed. by T.T. Kodas, M.J. Hampden-Smith (VCH, Weinheim 1994)
pp.33-37
1.37 P.F. Duffer: "Glass reactivity and its potential impact on coating processes",
in SVC 39th Annual Technical Con/. Proc., ed. by V. Harwood Mattox,
D.M. Mattox (Soc. of Vacuum Coaters, Albuquerque, NM 1996) pp. 174182
1.38 A.V. Tikhonrarov, M.K. Thubetskov: "Thin film coating design using second
order optimization methods", Proc. SPIE 1782, 156-164 (1992)
1.39 W. Zoller, PA Consulting Group, Frankfurt/M., Germany: Private communication
2. Design Strategies
for Thin Film Optical Coatings
Alfred Thelen
Introduction
It appears to be a natural evolution that every new process calls for design
activities to exploit its unique features which in turn call for adequate control
to implement the precision and quality prescribed by the designer (Fig. 2.1).
As a technology develops, the priority shifts among these three aspects.
In optical coating technology we have gone through one complete cycle, see
Table 2.1. Currently (since approximately 1985) process aspects have top
priority again: The dominant evaporation technology is being challenged by
sputtering and chemical vapour deposition technologies. As a consequence,
new design methods no longer have the low priority they had during the period 1975- 1985 when production aspects (optical coatings left the laboratory
and became an industrial process!) had first priority. If history repeats itself,
design activities will have first priority again in a few years.
As a link between process and control, a specific design is limited to available materials, processes, production precision and quality control. Will the
design methods developed for evaporation processes be good enough for the
upcoming sputtering and plasma-enhanced chemical vapour deposition processes? What are the differences between evaporation, sputtering and plasmaassisted chemical vapour deposition from a designer's point of view?
These questions will be addressed after a review of the major current
design methods.
Process
(materials & sources)
Control
(production & quality)
Design
(products & applications)
Fig. 2.1. Sequence of different aspects of optical coating
technology
24
2. Design Strategies for Thin Film Optical Coatings
Table 2.1. Relative priority of the major aspects of optical coating technology in the last 50 years
Time period
Process
Design
Control
1945-1965
1965-1975
1975-1985
1985-
1st priority
3rd
2nd
1st
2nd
1st
3rd
2nd
3rd
2nd
1st
3rd
2.1 Optical Thin Film Theory
The primary elements of thin film interference are the reflections and the
spacings of the interfaces. For nonabsorbing films these can be related to the
indices of refraction n and the optical thicknesses nd of the individual films.
The reflectance R of an m-Iayer nonabsorbing optical coating is given by
the formula
where no is the refractive index of the entrance medium (usually air), and ns
is the refractive index of the exit medium (often glass). M ll , M 12 , M 21 , and
M22 are the elements of the matrix
M
=
(M ·M) = g
II 1
12
iM21 M22
m
(
,/..~)
cos 'I'
nv
i sin cPv cos cP
'
(2.2)
where nv is the refractive index, cPv = (2n / ).,)nvdv cos Cl: v the phase thickness,
)., the wavelength, d v the physical thickness of the 11th layer, Cl: v the angle of
the light inside the 11th layer and m is the total number of layers. M is called
the characteristic matrix of the multilayer.
Equations (2.1) and (2.2) allow straightforward determination of the spectral reflectance of a design. Yet it is the inverse problem, the determination
of the design formula for a given spectral reflectance, often called synthesis,
which is the problem of design.
Because (2.1) and (2.2) cannot be reversed in full generality we are left
with only two ways to solve the problem [2.1]:
(1) find an exact solution to a simplified model (e.g., periodic arrangement
of the layers, equal optical thickness of all layers, etc.),
(2) find an approximate solution to a realistic model (numerical computer
design).
Designs based on exact solutions are easier to produce because they are
more transparent. Yet designs based on approximate solutions allow tighter
2.2 Exact Methods
25
specifications. As a consequence the two types of method are normally combined: A starting design derived by an exact method is further improved by
computer optimization. The degree of involvement of each method depends
on the type of design problem, the preference of the designer and the experience of the manufacturer, or, in other words, on the design strategy used.
2.2 Exact Methods
Most designs in existence today were derived by exact methods. These designs
have the tremendous advantage of having unambiguous physical significance,
and the effect of layer parameters on the performance of the whole layer system is readily understood. For most systems the layer thicknesses are regular,
and the refractive indices are known, which facilitates their manufacture [2.2].
2.2.1 Equivalent Layers
The method of equivalent layers is based on the discovery by Herpin [2.3] that
each symmetrical combination of thin films can mathematically be treated
as a homogeneous single film with a calculable (but artificial) equivalent
index of refraction and equivalent optical thickness. The application of this
finding to thin film design is due to Epstein [2.4]. He recognized that with
the equivalent layers concept a complex multilayer, constructed by repeating
the same symmetrical layer combination several times, can be considered as
a thick single thin film with the properties of the multilayer reflected in the
(artificial) dispersion of the equivalent index and the equivalent thickness:
The complexity of a multilayer is reduced to a highly dispersive single film!
Matrix theory allows an easy access to equivalent layers [2.5]. According
to (2.2), the characteristic matrix of a multilayer
M =
(.M iM12)
ll
(2.3)
IM21 M22
is the product of the characteristic matrices of the individual layers
M - (
m-
cos<pm
isinp", )
nm
inm sin <Pm cos <Pm
.
(2.4)
The two matrices (2.3) and (2.4) can be made to be equal when Mll = M 22 .
Equation (2.1) is invariant upon exchanging entrance (no) and exit (ns) media
when Mll = M 22 . We can conclude that the multilayer must be symmetric in
this case, or, in other words, only symmetric multilayers have an equivalent
index.
We set nm = N and <Pm = r. Then, M12 = (l/N)sinr and M21 =
N sin r. From this we obtain
26
2. Design Strategies for Thin Film Optical Coatings
equivalent index N
=
equivalent thickness
+J Z~~ ,
r = arccos Ml1
(2.5)
.
(2.6)
Depending on the sign of M2dM12 , we can distinguish two cases:
(a) M12 and M21 have equal signs (+ or -). Then
N
= J M21 = real.
M12
The determinant of matrix (2.4) equals one. Because det(AB) = det Adet B,
M also must have the determinant 1, or Ml1M22 + M12M21 = 1. Combining
this with Ml1 = M22 we obtain
cosr = Ml1 =
Jl -IM12M211 < 1 .
We conclude cos r and sin r remain trigonometric functions.
(b) M12 and M21 have unequal signs. Then
N
.
.
= JM21
- - = Imagmary
,
M12
and
cosr
= Ml1 = Jl + IM12M21 I > 1 .
Now cos r and sin r are no longer trigonometric functions. They become the
hyperbolic functions cosh r and sinh r.
The application of the equivalent layer concept to the quarter-wave stack,
for example H L H L ... H (where H, L stand for quarter-wave thick layers
with high, low refractive indices), leads to layers half as thick as a quarterwave layer, so-called eighth-wave layers:
HLHLHLH
= H/2 (H/2 L H/2)3 H/2 .
The outer H/2 layers are normally dropped unless they perform a specific
function (for example as matching layer to the next layer system, or the
outer media).
As an example we show in Fig. 2.2 the equivalent index N for the threelayer combinations H/2 L H/2 and L/2 H L/2 with nH = 2.35 and nL = 1.45.
Values are only given in the regions where N is real.
The distinction between cases (a) and (b) above is of particular importance for periodic multilayers composed of symmetrical periods. Let us construct a periodic multilayer by repeating a basic sequence ABA p times:
(ABAh (ABAh (ABA)J ... (ABA)p ,
where A, B stand for quarter-wave thick layers with indices nA, nB. In terms
of the equivalent index NAB A and the equivalent thickness rABA, the char-
2.2 Exact Methods
/1
First stop band
5
4
3
N
2
o
1\
U2HU2 1
7\11
~-, 1
Third stop band
~T
\
\
"-
/ 1//
I \~
/1
--
H/2LH/2'I I
o
27
2
'I
3
4
f..O If..
Fig. 2.2. Equivalent index N of the three-layer combinations H/2 L H/2 and L/2
H L/2 with nH = 2.35 and nL = 1.45. In quarter-wave stacks, the second stop band
is suppressed
acteristic matrix of the periodic multilayer takes on the following form , depending on whether N ABA is real or not:
(a) NABA is real.
M = (
COSprABA
i(ljNABA ) sinprABA )
iNABA sin prABA
cos prABA
.
(2.7)
If we increase the number of periods p, the elements of the characteristic matrix oscillate between the same high and low value. If, by proper dimensioning,
we assure that the high and low values of the elements of the characteristic
matrix result in high and acceptable transmittance values, we can use as
many periods as we want without substantially reducing the transmittance.
The wavelength region where this situation exists is called pass band.
(b) NABA is imaginary. Now
(2.8)
If we now increase the number of periods p , the matrix elements no longer
oscillate but, due to the exponential nature of the hyperbolic functions cosh
and sinh, continuously increase in value. The continuous increase of all the
matrix elements leads to a continuous decrease in transmittance. We now
have a situation where the transmittance decreases whenever we add another
period. The region where this situation exists is called stop band (see hatched
areas in Fig. 2.2).
28
2. Design Strategies for Thin Film Optical Coatings
The design of an edge filter, bandpass filter, or narrow bandpass filter is
now a matter of:
• selecting two or more coating materials which have a refractive index spread
as wide as possible,
• calculating the equivalent index of a few simple symmetric combinations,
• positioning the stop band portions of the equivalent index where a stop
band is desired ,
• repeating the symmetric combinations until the desired rejection level is
reached ,
• matching the equivalent index in pass bands to the entrance and exit media
by inserting internal antireflection coatings.
Figure 2.3 shows a short wavelength pass edge filter which was designed
with the equivalent layer H/2 L H/2 described in Fig. 2.2. The dashed curve
shows the straightforward application of the theory. In comparison to the previously used quarter-wave stack (dotted curve) a considerable improvement
is accomplished. Yet , due to the large number of periods, the first secondary
transmittance minimum after the edge is still unacceptably high. If three periods next to the substrate are shifted 5% to shorter wavelength, this minimum
is essentially eliminated (solid line, method of shifted periods [2.5]).
The effectiveness of eighth waves in the reduction of secondary reflectance
peaks in the pass band (the so-called ripple) was recognized prior to Epstein [2.4] by Geffcken [2 .6].
100
80
e::
:\~
\ i \"\ \.,./ . . . . .
:: 0\.!
60
""'Shifted periods
.\::i ..\j "'Ei9ht~I waves
t-
40
.; : "
I
500
600
:\J
.....
-
' Quarter-wave stack
20
o
400
700
Wavelength Inm
Fig. 2.3. Transmittance of the three long wavelength passes 1.0 I (HL)14 H I 1.52
(dotted curve, "quarter-wave stack"), 1.0 I (H/2 L H/2)15 I 1.52 (dashed curve,
"eighth waves"), and 1.0 I (H/2 L H/2)12 0.95(H/2 L H/2)3 I 1.52 (solid curve,
"shifted periods") , with nH = 2.35, nL = 1.45 and AD = 400nm
2.2 Exact Methods
29
Equivalent layers work very well in the design of edge filters, wide and
narrow band pass filters, and minus filters. They do not work well in the
design of antireflection coatings and neutral beam splitters.
2.2.2 Simulation of a Single Layer by a Multilayer
The choice of materials that can be deposited as hard and environmentally
stable coatings is very limited. In addition, the smaller the number of materials in a design, the easier it is to manufacture the coating. The ideal number
is obviously two. On the other hand, the higher the number of materials
the easier it is to design high performance coatings (e.g., with Chebyshev
synthesis [2.7]).
It turns out that the simulation of a material of a certain refractive index by a multilayer composed of materials with higher and lower refractive
indices is possible. In fact, the simulation often is associated with additional
advantages such as easier adaptation to changing refractive indices of the
substrate or compensation of unwanted dispersion.
In 1964, Rock [2.8] filed a patent for a four-layer two-material antireflection coating, which was based on a three-layer three-material quarterhalf-quarter coating by Thelen [2.9]. He used a graphical technique for the
determination of the thicknesses. Yet a four-layer two-material antireflection
coating was already discussed in a patent filed in 1942 by Geffcken [2.10].
He gave the following formula to determine the thicknesses of the simulating
layers
sin( ~) _ nA + nx nx - nB
sin( 471:;B) - nA - nx nx + nB
(2.9)
with dA, nA and dB, nB being the optical thicknesses and refractive indices
of the simulating layers, nx and d the refracting index and optical thickness
of the layer to be simulated, and m the number of simulating layers.
He started with the quarter-half-quarter design:
IlL HH M 1 1.515
with
nL
= 1.453,
nH
= 2.472,
nM
= 1.79, AO = 520 nm .
He used (2.9) to determine a four-layer system to replace the M layer. It
turned out to be: 0.115 H 0.430L 0.230H 0.215L. He neglected the last layer
because nA differs little from ns = 1.515. He combined the last of the substituting layers with the second layer of the original design because they had
the same refractive index. He obtained
IlL 2.115H 0.43L 0.230H 11.515 .
Figure 2.4 compares the reflectance of the designs.
Today, the most common way of substitution is with equivalent layers. A
survey of all methods was given by Herrmann [2.11].
30
2. Design Strategies for Thin Film Optical Coatings
1.0
\
\
\
\
0.8
0.6
/'
1/
1/
\
\
\
~
0::
II
\
\
0.4
\
\
\
0.2
\x 'K-'/v/
r-.....
',//
o
400
_/
500
600
700
Wavelength
Fig. 2.4. Reflectance of 1 I L 2.1l5H 0.431L 0.231H I 1.515 with nL = 1.453,
nH = 2.472, >'0 = 520 nm, solid curve (Geffcken [2.10]), and 1 I L 2H 0.2431L
0.214H I 1.52 with nL = 1.38, nH = 2.08, >'0 = 515 nm, dashed curve (Rock [2.8])
2.2.3 Chebyshev Synthesis
When all the m layers of a multilayer have equal phase thickness ¢, the
characteristic matrix of the multilayer has the form
M
= (
+ a2 cosm- 2 ¢ +...
)
i sin ¢(al cos m- 1 ¢ + a3 cosm- 3 ¢ + ... )
i sin ¢(b 1 cos m- 1 ¢ + b3 cosm- 3 ¢ + ... )
,
bo cos m ¢ + b2 cosm- 2 ¢ + .. .
ao cosm ¢
(2.10)
where the coefficients ao, a1, . .. and bo, bl , . .. are only functions of the
refractive indices of the individual layers. This can be verified by setting
¢ = ¢l = ¢2 = ... = ¢m and carrying out the matrix multiplication of (2.2).
If we insert the elements of the characteristic matrix (2.10) into the formula for the reflectance (2.1), we obtain
1
R = 1 - T = 1 - ------;,----,------;:----:;--------;,-----;:-----Co cos 2m ¢ + Cl cos 2m - l ¢
_
P(cos¢)
- C + P(cos¢) .
+ C2 cos2m- 2 ¢ + .. , + C2m
(2.11)
Now, the coefficients Co, Cl,"" C2m are functions of the refractive indices of
the films, the incident medium and the substrate. C is a constant and P
stands for "polynomial of". There is one type of polynomial, the Chebyshev
polynomial (see Fig. 2.5), which has the most desirable property that all secondary maxima in the pass band, the so-called ripple, have equal magnitude.
When these polynomials are used in (2.11), the polynomial synthesis becomes
2.2 Exact Methods
31
9
~
-
"T12 (x) = 2048x 12_ 6144x 10 + 6912x 8
- 3548x 6 + 840x 4 - 72x 2 + 1
o
-1
(\ / \
\/ \/
-1
f--f---
I \ / 1\ /\
\/ \/ \/ \J
-0.5
o
0.5
x
Fig. 2.5. Characteristic of the Chebyshev polynomial T12(X). Note that the maxima
in the pass band all have equal magnitudes (equal ripple character)
Chebyshev synthesis. There are great difficulties, though: The desideratum is
given in terms of the energy reflectance which is the square of the magnitude
of the amplitude reflectance vector - the phase of the amplitude reflectance
vector remains undetermined. Also, the refractive indices can only assume
positive real values.
For the corresponding case of synthesis of microwave transformers these
problems have been addressed and solved. Based on classical mathematical
studies by Richards [2.12], Riblet [2.13] found the necessary and sufficient
conditions for "physically realizable" (i.e., no negative refractive indices) solutions and an elegant synthesis procedure: The selected polynomial of degree
2m is subjected to a complex transformation, the 2m roots are determined,
the m roots in the right half plane are discarded, and a new polynomial of
degree m is constructed from the m remaining roots. A reverse matrix multiplication scheme is used to calculate the refractive indices from the characteristic matrix established by the new polynomial of degree m. A BASIC
program for Riblet's procedure was described by Thelen [2.14].
Straightforward application of Riblet's procedure leads to antireflection
coatings with the refractive indices of the layers steadily increasing from no
to ns. They can be converted to band pass filters with alternating high and
low refractive indices because the magnitude of the reflectance of a Fresnel
reflector is invariant under replacement of n2/nl by ndn2
nv - nV-l
nv + nv-l
~-1
1_
~+1
1+
nl/_l
nv-l
nv-l
nv-l
nv
nv
nV-l
nv
nV-l
nv
As a consequence, we can convert the sequence
_
1
+ 1
nV-l - nv
nV-l + nv
(2.12)
32
2. Design Strategies for Thin Film Optical Coatings
to
In order to compensate for the minus sign in (2.12) we increase the thickness of all layers from >'0/4 to >'0/2.
Figure 2.6 shows a Chebyshev edge filter with 35 layers, a ripple of 0.2%,
and a rejection level of 0.0002%. The refractive indices are ratioed to fit
within the available index range of 1.46-2.35.
Chebyshev designs are, due to the equal ripple feature, always superior to
other designs. But when the availability of coating materials is limited, difficulties exist in the translation to real physical conditions. The designer has
little control over the resulting refractive index sequence. Often the designs
are downgraded below equivalent layer designs in the adaptation process.
Chebyshev synthesis is the ideal method for the design of antireflection
coatings. It also does well with minus, edge, and band pass filters, provided
enough coating materials are available (e.g., sputtering [2.15]).
2.2.4 Effective Interfaces
The method of effective interfaces [2.16J can be summarized as follows: An
optical coating to be optimized is split into three parts: a selected spacer layer
and the two outer subsystems. The reflectance curves of the two subsystems
(or two optical coatings to be combined) are determined and plotted. In
regions where the sum of the phases of the reflectance of the subsystems
100
r---,-----~7T~~~~~~--rr_r_,~\~
1"/1/\'\'\,/\\ / \
',1/1/ \ I \ ' \
,
80
~
:I
60
J
\
/
20
99.8
99.6
99.4
99.2
I
I
)1
400
/
100%
100 times magnified
!
40
o
"-
\
I
o
.....
~
\1\0.'\
/
500
600
Wavelength
700
99
Fig. 2.6. Reflectance of 1.7635111.914441.739472.001691.658022.100011.58511
2.18559 1.53221 2.24731 1.49852 2.28676 1.47856 2.31017 1.46735 2.32300 1.46171
2.32865 1.46 2.32865 1.46171 2.32300 1.46735 2.31017 1.47856 2.28676 1.49852
2.247311.532212.185591.58511 2.100011.658022.001691.739471.9144411.76351,
all layers are A/4 thick at Ao = 400 nrn
2.2 Exact Methods
33
inside the spacer layer plus the phase thickness of the space layer are around
7r, the total transmittance T through the system is given by
with Tl and T2 being the reflectances of the two outer subsystems for light
coming from the inside of the layer.
For Tl = T2, one has T = 100% (R = 0), and for Tl very much different
from T2, then T = 0%. The design method now consists in modifying the two
reflectance curves Tl and T2 and the phase thickness of the spacer layer until
the desired result is reached.
In Fig. 2.7 we show how the derivation of the antireflection coating 1 I L
HH M I 1.52 is accomplished by this method.
The method of effective interfaces works well with antireflection coatings,
band pass filters, combinations of two known designs, and whenever the effect
of critical layers needs to be investigated.
2.2.5 Buffer Layers
If, inside a layer of a multilayer, the reflectance for a given wavelength to
one side equals zero then thickness changes of this layer do not cause any
variations in the transmittance and reflectance of the total multilayer. A
layer where this condition exists is called a buffer layer [2.17].
Buffer layers allow the transmittance of a multilayer to be kept constant in
one plane of polarization while changing it in the other [2.18]. In the design of
Fig. 2.8, taken from Knittl, the fourth layer is a buffer layer. The reflectance
5
1.00
1.38 - - - - Subsystem 1
4
\
2
V 7'
2
r1
1.62 _ _ _ _ _ Subsystem 2 /
\
\
3
o
2.08
\
1.52
\
\
/
\ r2
\
\
\
,----"
Fig. 2.7. Reflectance of 1
curve labelled Tl), and 2.08
nH = 2.08, and nM = 1.62
/
/
/
t---
/ /
'-
0.5
/
/
/
/
1.0
1.5
I L HH M I 1.52 (solid curve), 2.08 I L I 1 (dotted
I M I 1.52 (dotted curve labelled T2), with nL = 1.38,
34
2. Design Strategies for Thin Film Optical Coatings
100
80
60
c:::
40
20
o
500
550
600
Wavelength Inm
650
700
Fig. 2.8. Reflectance of the design 1 I 0.778H L H x1.011L 0.333H L 0.622H L
0.33H 1.55L 0.311H L 0.622 0.311H I 1.52 with nL = 1.38, nH = 2.35, >'0 = 600 nm,
incidence and match angle = 45°. The value of x varies from 0 to 1.1 in steps of 0.1
of 1.38 I O.333H L O.622H L O.333H 1.55L O.311H L O.622H L O.311H I 1.52
is zero at A = 587 nm in the parallel plane of polarization and at 646 nm
in the perpendicular plane of polarization. As a consequence, the reflectance
changes neither in the parallel plane at 587 nm nor in the perpendicular
plane at 646 nm when the thickness of the buffer layer is varied from 0 to
110%. Buffer layers can also be very useful in the design of antireflection
coatings [2.19].
2.2.6 Absentee Layers
The transmittance of a design does not change when a half-wave-thick layer is
added or deleted. This, of course, is only true at the exact half-wave position.
For example, the following six designs all have equal transmittance (and
reflectance) at the half-wave position:
H L H L H LL H L H L
H L H L HH L H L
H L H LL H L
HLHHL
HLL
H
Half-wave layers are often added to a design when the reflectance at a certain
wavelength point should remain constant, but the curvature be reversed.
2.3 Approximate Methods Based on Starting Designs
35
Combinations of quarter-wave and half-wave films are the backbone of
thin film design, especially for antireflection coatings, neutral beam splitters
and band pass filters.
2.3 Approximate Methods Based on Starting Designs
Designs that were derived by exact methods can generally be improved by
using numerical methods. Baumeister [2.20,21] was the first to do so. He used
successive approximations and first partial derivatives of thickness to match
a design to a specific reflectance curve. These methods have the tremendous
advantage that they are no longer based on a simplified model. Absorption
and dispersion can be incorporated.
The result of the methods based on starting designs depends on three
factors:
• the quality of the starting design,
• the type of merit function,
• the mathematical method used.
It is obvious that the closer the starting design is to the desired curve,
the better the result. Generally, a merit function is of the type
(2.13)
where Wi is the relative weight, Ri the reflectance, and Si the target at the
ith wavelength. k is the exponent of the method (k = 1 is used in linear
programming and k = 2 is the least squares fit [2.22]). Tang [2.2] points out,
though, that this merit function does not work for non-polarizing designs. He
proposed a merit function with two sets of weights: one for the polarization
effect and one for the reflectance.
The use of many mathematical methods has been reported in literature
and extensive comparisons have been made [2.23,24]. Computer optimization is available to the designer through commercial software. A survey was
published by Baumeister [2.25].
In Table 2.2 and Fig. 2.9 we show the result of applying the commercial
program TFCalc 2.9 [2.26] to the shifted periods design of Fig. 2.3. A gradient algorithm was used for the refining process. The exponent of the merit
function was 16. Targets were spaced 2 nm apart from 350 to 750 nm. A gap
from 466 to 478 nm was provided for the transition from stop to pass band.
The refining took 500 passes (1 h on a 486iDX2 66 MHz personal computer)
before it stabilized at the result shown.
Due to the complicated and multimodal structure of the merit function,
optimization and refining generally lead to local minima only [2.2]. For this
36
2. Design Strategies for Thin Film Optical Coatings
Table 2.2. Edge filter design derived by refining the design labelled "shifted periods" in Fig. 2.3 (physical thicknesses in nm)
Air (1.0)
Ti02
Si02
Ti02
Si0 2
Ti0 2
Si02
Ti02
Si02
Ti0 2
Si02
Ti02
Si02
Ti02
Si02
Ti02
21.11
55.31
42.25
63.12
34.64
69.60
42.72
63.18
37.48
72.02
42.10
63.16
38.62
72.85
41.68
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
100
r\ I\{\V
/\
IVI!
99
~
97
"-./
--------
I
'-
\
~ Unrefined
/ \
/
I /
/
\ /
1/
\
I
I
I
./
-
/\
r\
I
I
\
\
\
500
Si02
Ti02
Si0 2
Ti02
Si02
Ti02
Si02
Ti0 2
Si02
Ti02
Si0 2
Ti02
Si02
Ti0 2
Si0 2
Ti02
Glass (1.52)
'-Refined
II
400
62.74
39.04
73.76
41.32
61.30
39.31
75.29
40.46
57.72
40.13
78.47
36.32
46.87
47.73
102.82
4.141
!\~
\
\
I
I
~
\
I
I
I
I
I
96
95
/
i\ I! \
/ I 1/ \
/ I I
98
I-
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
600
I
\
J
700
Wavelength Inm
Fig. 2.9. Comparison of the edge filter of Fig. 2.3 labelled "shifted periods" before
and after refining
reason, a comprehensive search is often incorporated [2.27]. This is a systematic global search in the multidimensional construction parameter space.
This method can only be used for designs with a relatively small number
of layers, like antireflection coatings and neutral beam splitters. Its success
depends very much on the density of the refractive index/optical thickness
grid.
The result of a simplified exhaustive search is shown in Fig. 2.10 [2.28].
In order to take advantage of the inherent symmetry of equally thick layers
2.4 Numerical Synthesis
37
1.0
0.8
0.6
0::
0.4
0.2
o
I
I,
I
I
I
I
I
I
I
1/
1/
! ~---~ ..:::.-~ ~/
,_/
V
400
"-
500
600
/
/
700
Wavelength Inm
Fig. 2.10. Reflectance of the antireflection coating 1 I 1.38 1.98 2.08 1.62 1.38 1.44
I 1.52, all layers .\j4 thick at AO = 480nm (solid curve). For the second curve the
coating material with refractive index 1.98 was replaced by the equivalent layer
combination 0.46H 0.07L 0.46H with nL = 1.38 and nH = 2.08
relative to the ,\/4 position, only quarter-wave-thick layers were subjected
to the search. Selection criterion was a reflectance level below 0.4% at six
equally spaced wavelength positions (this would correspond to eleven positions for a non-symmetric design). For each layer a choice of ten discrete
values, increasing on an equal percentage basis from 1.38 to 2.37, was offered.
2.4 Numerical Synthesis
In Fig. 2.1 we tried to show the interplay between process, design and control in the evolution of superior products: Improvements in one area call for
improvements in the other two.
In recent times, enormous improvements have been made in the field of
computers and automation. The impact for control was that more complex
designs can be implemented. The impact for design was that it is no longer
necessary to understand in full the effect of layer parameters on the performance of the whole layer system. For fully automated deposition systems it
seems to suffice that a design is entered into the controlling computer and
then tuned to generate a satisfactory product [2.29]. Tuning, so to speak,
replaces understanding.
This opens the door to numerical designs, where the design is just a
list of thicknesses and materials rather than a formula specifying groupings,
subsystems, periodicities, and matching layers that would allow an understanding of why that particular layer sequence generates the desired spectral
performance. Assurances are even given that mathematically this particular
38
2. Design Strategies for Thin Film Optical Coatings
numerical design is a "global optimum" [2.30]. Two numerical design methods
have been particularly successful: the flip-flop method [2.31] and the needle
method [2.32].
The flip-flop method works as follows:
(1) Select a total physical thickness for the coating. Divide this thickness
into thin layers of equal thickness.
(2) Assign some initial index, either high or low, to each layer.
(3) Evaluate a merit function based on the desired spectral response
(4) Change the state of each layer (from low to high index or from high to
low) one at a time and re-evaluate the merit function. If the performance
is better in the flipped state, retain the change; otherwise restore it.
(5) If, after testing all layers (a single pass), the merit function has improved,
go back to step 4 for another pass; otherwise end.
And the needle method:
(1) Select a total physical thickness for the coating.
(2) Start with a thick single film (preferably of high index) or a starting
design.
(3) Determine locations inside the layers where thin layers (called needles)
of another material might improve the merit function
(4) Use a ("local") optimization to let the new layers, together with old
layers, grow to an optimal thickness.
(5) Go back to step 3 for another pass until no further improvement of the
merit function can be accomplished.
Though the needle method works with both single layers and multilayers
as starting designs, in practice single layers lead to better results. Apparently,
multilayers as starting designs limit the choices too much.
The author recently used an optimization program with the needle
method implemented (TFCalc 2.9 [2.26]) to design an edge filter similar to
Figs. 2.3 and 2.5. The reflectance targets were:
A = 350 to 466 nm:
A = 478 to 750 nm:
T=O%
T = 100%
The targets were spaced 2 nm apart. The starting design was only a single
thick layer of Ti0 2 (31 quarter-wave optical thicknesses at 520 nm). Table 2.3
and Fig. 2.11 show the remarkable result after a two-day effort on a 66 MHz
486iDX2 personal computer.
Note that this design has equal ripple structure in the pass band similar to
the Chebyshev design of Fig. 2.11. But while the Chebyshev design still needs
to be converted to reality (from in-between refractive indices to refractive
indices of existing materials and from equal entrance and exit media with
refractive index 1. 76351 to non-equal entrance and exit media with prescribed
refractive indices) the needles design is reality!
2.4 Numerical Synthesis
39
Table 2.3. Edge filter synthesized with "needles" (TFCalc 2.9 [2.26]) (physical
thicknesses in nm)
Layer no.
Phys. thickn.
Material
Layer no.
Phys. thickn.
Material
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
2.27
107.78
47.47
44.37
37.62
75.49
40.49
73.01
40.51
73.01
40.51
61.60
40.45
71.01
41.73
59.90
41.58
49.57
94.61
0.97
73.01
46.61
45.55
36.26
60.16
21.21
air (1.0)
Ti02
Si0 2
Ti02
Si02
Ti02
Si0 2
Ti0 2
Si02
Ti0 2
Si0 2
Ti02
Si0 2
Ti0 2
Si0 2
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti0 2
Si0 2
Ti02
Si0 2
Ti0 2
Si02
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
28.20
30.54
42.11
42.00
62.64
22.52
82.39
30.81
57.23
37.60
66.20
33.93
60.63
37.50
63.75
37.61
56.77
41.84
59.58
42.38
48.79
53.09
51.02
34.26
57.36
46.34
20.78
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti0 2
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
Si02
Ti02
glass (1.52)
100
11,1\ ,1 1\ I I 1\ /1
I
III II I II
~I/IJ 1/ \1 " \I "
80
I
I
60
' '"
.~
"
r
\ 1\ 1\
\.1 \/ \J
1\
1I
\ 1
40
99.8
100 times magnified
-
400
99.6
99.4
I
I
I
I
I
I
20
100.0
v
I
I
I-
o
\
99.2
500
600
700
99.0
Wavelength Inm
Fig. 2.11. Long-wave pass designed by the needle method. The design is given in
Table 2.3
40
2. Design Strategies for Thin Film Optical Coatings
2.5 Results of Recent Design Contests
2.5.1 Berlin Contest 1991
The subject of this contest [2.33] was an antireflection coating for (wide-angle)
lenses to be used with normal and infrared photographic film. A RMS figure
of merit was defined by taking into account the reflectances at normal and 30°
light incidence. 44 designs were submitted by 28 authors. The contributors
had approximately three months to work on the problem. Figure 2.12 shows
the results.
Several contributors used a general starting design as proposed by Baumeister [2.34]. Dobler [2.35]' for example, used
>'0 = 1100 nm. His design emerged as third best. The winner was W. H.
Southwell, Rockwell Science Center, Thousand Oaks, California 91360. He
used the flip-flop method [2.31].
In the design contest, five coating materials were allowed: MgF2' Si0 2,
Ah03, Ta205, and Ti02. If we look at Southwell's design given in Table 2.4
we note that two materials, Si0 2 and Ah03, were not used at all. The third
material, Ti0 2, was only used in the first six layers. So it appears that
TiOdMgF2 were the flip-flop layers in the first portion of the design and
Ta205/MgF2 in the second portion of the design. The design has ten layers
with thicknesses below 10 nm.
1.0
c
0.8
0.6
u.
0.4
0.2
0
0
0.4
0.8
1.2
1.6
2.0
Total thickness film
Fig. 2.12. Figures of merit accomplished during a recent design contest as a function of total physical thickness. Designs with refractive index span 1.38-2.32 are
marked with crosses, designs with refractive index span 1.38-2.16 with squares.
Dashed line: minimum of squares, straight line: minimum of crosses
2.5 Results of Recent Design Contests
41
Table 2.4. Antireflection coating of Southwell [2.33] (physical thicknesses in nrn)
Layer no.
Phys. thickn.
Material
Layer no.
Phys. thickn.
Material
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
109.1
32.1
13.2
76.7
36.4
18.5
113.9
13.2
28.5
7
21.3
77.5
5.9
43.5
8.3
21.1
41.6
18.3
24.1
7.7
21.8
17.8
17.9
129.9
13.6
13.9
Air (1.0)
MgF2
Ti02
MgF2
Ti0 2
MgF2
Ti02
MgF2
Ta203
MgF2
Ta203
MgF2
Ta203
MgF2
Ta203
MgF2
Ta203
MgF2
Ta203
MgF2
Ta205
MgF2
Ta205
MgF2
Ta205
MgF2
Ta205
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
32.9
12.5
248.4
5.1
60.3
10.1
35.4
9.6
23.4
19.2
14.5
19.9
7.9
107.2
11
29.3
15.7
11.9
31.1
13.3
24.7
6.6
45.4
5.8
24.9
5.6
25.2
MgF 2
Ta205
MgF2
Ta02
MgF2
Ta205
MgF2
Ta203
MgF2
Ta203
MgF2
Ta203
MgF2
Ta205
MgF2
Ta205
MgF2
Ta205
MgF2
Ta205
MgF2
Ta205
MgF2
Ta205
MgF2
Ta203
MgF2
Glass (1.52)
2.5.2 Tucson Contest 1995
The subject of this design contest [2.36] was a wide band hot mirror. A defect
function (that was to be minimized) was defined as
D
= 200 - Tvis - Rir + exp((L - 50)/50) ,
(2.14)
with
719
Tvis
= (320)-1 LT(A)~A
~A
= 1nm,
400
1998
R ir
= (1250)-1 L
R(A)~A
~A
= 2nm.
(2.15)
(2.16)
750
L is the total number of layers, T is the transmittance, and R is the reflectance
(in per cent). Any of the four following (artificial) materials could be used:
42
2. Design Strategies for Thin Film Optical Coatings
n=
n=
1.46 - iO
1.62 - iO
n = 2.05 - iO.0002
n = 2.35 - iO.0005
The incident medium was air and the substrate index was 1.47. The number
of layers was not allowed to exceed 99. Physical thicknesses below 10 nm were
not allowed.
The best design was submitted by Alexander V. Tikhonravov and Michael
K. Trubetskov, Research Computer Center, Moscow State University, Moscow, 119899, Russia. The visual transmittance was optimized to such an
extent that the 75-layer coating could serve as an antireflection coating with
an average reflectance of Ro = 100% - Tvis - Avis = 0.21%! Figure 2.13 gives
the spectral performance.
Table 2.5 gives the visual transmittance T vis , the visual absorptance Avis,
the infrared reflectance Rin the defect function D, the number of layers,
the total physical thickness, and the design method used for the eight best
designs, ranked by defect function D.
As we see from Table 2.5, a numerical design was again the clear winner.
Its defect function D was only 73% of the best classical design (Lemarquis).
In order to bring out the differences between the eight designs of Table 2.5,
in Fig. 2.14 we first plotted the refractive index against the accumulated
physical thickness. The curves start from the substrate and are relative to
the total physical thickness.
The difference between the numerical designs 1-5 and the classical designs
6-8 shows clearly. In designs 6-8 we can identify four sub-stacks, some with
two materials and some with four (for suppressing orders [2.37]). In designs
1-5 very little physical interpretation appears possible.
silica
alumina
tantala
titania
100
IrV 'yYfV lfV~
(\
90
II
80
70
~
60
0:::
50
e..
!-=" 40
30
20
(
10
o
0.4
It.!
0.5 0.6 0.7 0.8
1.5
2
V
3
Wavelength film
Fig. 2.13. Spectral performance of the winning design of the TUcson 1995 hot
mirror contest. T is given in the visible and R in the infrared spectrum
2.5 Results of Recent Design Contests
43
Table 2.5. Characteristics of the eight best designs of the 1995 Tucson contest
Designer
Tvis
Avis
Ric
D
Number
of layers
Tot. phys.
thickness
Design method
Tikhonravov 1
Tikhonravov 2
Southwell 1
Noe
Southwell 2
Lemarquis
97.33
96.96
96.68
96.66
96.05
96.07
2.46
2.76
3.01
3.03
3.26
3.31
97.25
97.82
96.66
96.22
96.70
96.57
7.07
7.19
8.34
8.94
9.30
9.63
75
84
76
80
86
91
8204
9089
8253
7049
8272
11260
Perilloux
96.13
2.88
96.32
9.73
89
8611
Paul
96.56
2.95
95.94
9.82
92
8562
needles
needles
flip-flop
needles
flip-flop
equiv. layers +
optimization
equiv. layers +
optimization
equiv. layers +
optimization
Tikhonravov 1
Tikhonravov 2
Southwell 1
Noe
Southwell 2
Lemarquis
Perilloux
Paul
Fig. 2.14. Refractive index as a function of physical thickness of the eight best
designs of the 1995 Tucson contest. The physical thickness is relative to the total
physical thickness
We then grouped the physical thicknesses in 10 nm intervals:
group
physical thickness interval
Onm
lOnm
20nm
0- 9.99nm
10-19.99 nm
20-29.99nm
and plotted their frequency in Fig. 2.15.
2. Design Strategies for Thin Film Optical Coatings
44
30
30
1)' 20
c
Tikhonravov 1
Ql
:J
Tikhonravov 2
1)' 20
c
Ql
:J
0-
0-
Ql
~ 10
U: 10
50 100 150 200 250 300 350
Physical thickness Inm
50 100 150 200 250 300 350
Physical thickness Inm
30
30
>u
c
20
Southwell 1
>u
c
Noe
20
Ql
:J
Ql
:J
0-
0-
Ql
~
U: 10
LL
10
50 100 150 200 250 300 350
Physical thickness Inm
50 100 150 200 250 300 350
Physical thickness Inm
30
30
1)' 20
Southwell 2
c
Ql
:J
0-
~
LL
0-
~
10
LL
20
Perilloux
Paul
1)' 20
c
0-
0-
~
50 100 150 200 250 300 350
Physical thickness Inm
Ql
:J
Ql
:J
LL
0
30
30
c
10
0
50 100 150 200 250 300 350
Physical thickness Inm
>u
Lemarquis
1)' 20
c
Ql
:J
~
10
LL
50 100 150 200 250 300 350
Physical thickness Inm
10
0
0
50 100 150 200 250 300 350
Physical thickness Inm
Fig. 2.15. Frequency of thickness intervals for the designs of Table 2.5 and Fig. 2.14.
(Physical thickness interval 0-9.99 nm is labelled 0, 10-19.99 is labelled 10, etc.)
2.6 Design Strategies for the Different Deposition Technologies
45
The curves bring out that the classical designs have a compacter thickness
distribution and fewer thin layers than the numerical designs.
Another important criterion for the evaluation of designs is their robustness against thickness variations. In Table 2.6 we show the result of varying
all thicknesses randomly within a range of ±2%. 1000 variations were calculated. The minimum and maximum defect functions D min and Dmax were
determined.
The quantity 100 (D min - Dtheor) / Dtheor in Table 2.6 should be an indication of the optimality of the design (Tikhonravov 1 and 2) and 100(Dmax D min ) / Dtheor an indication of the robustness. This time the winner is a classical design.
2.6 Design Strategies for the Different
Deposition Technologies
For the design methods we use the following groupings:
(1) Equivalent layers: We include in this category all exact methods except
Chebyshev synthesis.
(2) Chebyshev synthesis: Designs are selected and scaled for refractive indices
between 1.46 and 2.35. Note that entrance and exit media are equal and
both have an intermediate refractive index (antireflection coatings have
to be added on both sides if used between air and glass).
(3) Refining/optimization: We include in this category all methods where
the final design is an improvement of the starting design. The number of
layers before and after the designing process are essentially the same.
(4) Numerical design (flip-flop/needles): In this category the starting design
is either standardized or not required. The computer makes most of the
decisions.
Table 2.6. Lowest (Dmin) and highest (Dmax) defect functions of the best eight
designs of the Tucson contest 1995 after 1000 cycles of random thickness variations
within ±2%. D of Table 2.5 is now Dtheor
Design
-DtbOQr
Dmax Dmax-Dmin 100 Dmax-Dwi!l
Dtheor Dmin 100 D min
Dtheor
Dtheor
Paul
9.82
Southwell 2
9.3
8.94
Noe
Perilloux
9.73
Tikhonravov 1 7.07
Tikhonravov 2 7.19
Lemarquis
9.63
Southwell 1
8.34
9.96
9.37
9.20
9.83
7.32
7.50
9.93
8.55
1.43
0.75
2.94
1.08
3.50
4.24
3.16
2.52
10.88
10.24
10.08
10.83
8.07
8.35
11.13
9.82
0.92
0.87
0.88
0.99
0.76
0.85
1.19
1.27
9.34
9.36
9.83
10.22
10.69
11.85
12.38
15.22
46
2. Design Strategies for Thin Film Optical Coatings
For the deposition technologies the groupings are:
(1) Evaporation: Evaporation is accomplished by incandescent sources heated
either by electron beam or by a filament. Coating materials are evaporated either directly (e.g., ZnS, MgF 2 ) or with the aid of a gas (e.g.,
Si0 2 , Ti0 2 ). The thicknesses are monitored optically (measuring reflectance/transmittance during deposition) or mechanically (quartz crystal) [2.38]. Ion bombardment during deposition may be used to improve
the microstructure [2.39].
(2) Sputtering: The coating material is now knocked out of a target by impacting ions. The ions may be generated either by a RF magnetron or
an ion gun. The major improvements on evaporation are
(a) the higher energy of the depositing molecules (around 10 eV versus
0.2 eV) leads to denser and more stable films [2.40], and
(b) the linear relationship between deposition rate and applied voltage
(in evaporation the relationship is exponential) allows better deposition
automation and precision. Reactive dc magnetron sputtering, as used
for large area coating, is not considered here because the optical quality (absorption) is not good enough for coatings with a high number of
layers.
(3) Pulsed plasma CVD: A gas is bled into the vacuum chamber and deposited by a RF or microwave pulse. The coating material deposits and
the remaining gas is flushed out. Reactive gas and flushing gas may be
the same. Thickness control is by the number of pulses [2.41]. Non-pulsed
CVD, though possible [2.42], is less precise than pulsed CVD.
For the evaluation of the strengths and weaknesses of the different design
methods we use the following criteria:
(1) Physical significance: If the effect of each layer on the performance of
the whole layer system is well understood, monitoring schemes can be
devised to automatically compensate the errors [2.43].
(2) Intermediate indices: Some deposition technologies allow the mixing of
different materials either by co-deposition from different sources [2.15] or
by alternating many very thin layers of two materials [2.41].
(3) Layers < 10 nm: Non-continuous deposition processes have great difficulties in depositing thin layers. They are very difficult to monitor
and they are more vulnerable to residual gas attack during the time
when sources are being changed. Because pulsed continuous processes
like plasma impulse chemical vapour deposition (PICVD) are built up
from layers < 1 nm anyway [2.41], layers> 1 nm can easily be accommodated.
(4) Curve fit: Quality of fit between specification and design characteristic.
(5) Total thickness: The sum of all the physical layer thicknesses affects the
mechanical properties (adhesion, water resistance, stress, etc.) of the optical coating. On the other hand, the higher the total thickness the more
2.7 Conclusion
47
nearly perfect is the optical performance of the coating. The maximum
allowable total thickness is much higher for chemically deposited layers
than for physically deposited layers; see [2.44J and [2.45J.
(6) Number of layers: For non-continuous processes like evaporation and
sputtering each new layer means de-activating the previous source and
activating another. Also a change of the monitor strategy might be required. For continuous processes like chemical vapour deposition, changing materials means just opening another valve!
Table 2.7 lists the strengths and weaknesses of the different design methods
and Table 2.8 summarizes their impact on the implementation.
2.7 Conclusion
Comparing Tables 2.7 and 2.8 we conclude that evaporation is limited to
the design methods of equivalent layers and refining/optimization. These design methods allow error compensation, which in turn compensates for the
weaknesses in deposition control. Chebyshev synthesis designs are difficult to
implement because intermediate indices are hard to accommodate. Numerical
(flip-flop/needles) designs are difficult to produce because weaknesses in deposition control make it impossible to handle the associated thin layers and,
due to the lack of understanding the effect of each layer on the performance,
error compensation is not possible either.
Table 2.7. Strengths and weaknesses of different design methods
Physical significance
Intermediate indices
Layers < 10 nm
Curve fit
Equivalent
layers
Chebyshev
synthesis
Refining/
optimization
Numerical
design
high
not required
seldom
adequate
medium
necessary
no
sometimes
excellent
reduced
not required
less seldom
good
poor
not required
often
excellent
Table 2.8. Design-relevant process parameters (visual/near IR)
Error compensation
Intermediate indices
Layers < IOnm
Total thickness
Number of layers
Evaporation
(e-beam/ion assisted)
Sputtering
(ion beam / RF)
Plasma CVD
(pulsed)
required
almost impossible
very difficult
critical
minimized
helpful
possible
difficult
critical
minimized
not required
easy
easy
much less critical
irrelevant
48
2. Design Strategies for Thin Film Optical Coatings
In addition to equivalent layers and refining/optimization, sputtering can
accommodate Chebyshev designs because they allow intermediate refractive
indices and rely less on error compensation.
The relatively new pulsed plasma CVD can accommodate all designs and
especially allows the implementation of the optically superior numerical (flipflop/needles) designs. The timing of the emergence of pulsed plasma CVD
technology and the flip-flop/needles design methods seem to verify the pattern expressed in Fig. 2.1 and Table 2.1.
References
2.1 R.L. Burden, J.D. Faires: Numerical Analysis (Prindle, Weber & Schmidt,
Boston 1985)
2.2 J.F. Tang, Q. Zheng: "Automatic design of optical thin-film systems - merit
function and numerical optimization method", J. Opt. Soc. Am. 72, 15221528 (1982)
2.3 A. Herpin: "Caleul du pouvoir reflecteur d'un systeme stratifie queleonque" ,
Compt. Rend. 225, 182 (1947)
2.4 L.I. Epstein: "The design of optical filters", J. Opt. Soc. Am. 42, 806-810
(1952)
2.5 A. Thelen: "Equivalent layers in multilayer filters", J. Opt. Soc. Am. 56,
1533-1538 (1966)
2.6 W. Geffcken: "Interferenzfilter mit verminderter Bandstruktur", German
Patent 902191, filed Oct. 29, 1949
2.7 A. Thelen: "Design of optical interference coatings 1992", Proc. SPIE 1782,
2-7 (1992)
2.8 F.C. Rock: "Antireflection coating and assembly having synthesized layer of
index of refraction", US Patent 3 432 225, filed May 4, 1964
2.9 A.J. Thelen: "Three layer anti-reflection coating", US Patent 3 185020, filed
Sept. 7, 1961
2.10 W. Geffcken: "Schicht zur Anderung des Reflexionsvermogens aus einer
Mehrzahl abwechselnd iibereinanderliegender Teilschichten aus zwei Stoffen
von verschiedener Brechzahl", German Patent 742463, filed Oct. 14, 1942
2.11 R. Herrmann: "Quaterwave layers: simulation by three thin layers of two
materials", Appl. Opt. 24, 1183-1188 (1985)
2.12 P.I. Richards: A special class of functions with positive real part in a halfplane, Duke Mathematical J. 14, 777-786 (1947)
2.13 H.J. Riblet: "General synthesis of quarter-wave impedance transformers",
IRE Transactions on Microwave Theory Tech. MTT-5, 36-43 (1957)
2.14 A. Thelen: Design of Optical Interference Coatings (McGraw-Hill, New
York 1986)
2.15 C. Misiano, E. Simonetti: "Co-sputtered optical filters", in Vacuum and Thin
Film Technology, ed. by J. Yarwood, P. Anderson (Pergamon, Oxford 1978)
pp.403-406
2.16 S.D. Smith: "Design of multilayer filters by considering two effective interfaces", J. Opt. Soc. Am. 47, 43-50 (1957)
2.17 J. Mouchart: "Thin film optical coatings, 5: Buffer layer theory", Appl.
Opt. 17, 72-75 (1978)
2.18 Z. Knittl: "Control of polarization effects by internal antireflection", Appl.
Opt. 20, 105-110 (1981)
References
49
2.19 A. Macleod: "Design of an antireflection coating for glass over the region
400nm to 900nm", Proc. SPIE 1782, 602-611 (1992)
2.20 P.W. Baumeister, J.M. Stone: "Broad-band multilayer film for Fabry-Perot
interferometers", J. Opt. Soc. Am. 46, 228-229 (1956)
2.21 P. Baumeister: "Design of multilayer filters by successive approximations",
J. Opt. Soc. Am. 48, 955-958 (1958)
2.22 A.L. Bloom: "Refining and optimization in multilayers" , Appl. Opt. 20, 6673 (1981)
2.23 J.A. Dobrowolski, R.A. Kemp: "Refinement of multilayer systems with different optimization procedures", Appl. Opt. 29, 2876-2893 (1990)
2.24 Li Li, J.A. Dobrowolski: "Computation speeds of different optical thin-film
synthesis methods", Appl. Opt. 31, 3790-3799 (1992)
2.25 P. Baumeister: "Computer software for optical coatings", Photonics Spectra,
143-148 (Sept. 1988)
2.26 TFCalc 2.9, Software Spectra, Inc., 14025 N.W. Harvest Lane, Portland, Oregon, 97229 USA
2.27 J.A. Dobrowolski: "Completely automatic synthesis of optical thin film systems", Appl. Opt. 4, 937-946 (1965)
2.28 A. Thelen: "Reflection reducing coating", US Patent 3 854 796, issued
Dec. 17, 1974
2.29 H.A. Macleod: "Thin films evolve from black art to science" , Photonics Spectra, 103-104 (Jan. 1993)
2.30 A.V. Tikhonravov: "Some theoretical aspects of thin-film optics and their
applications", Appl. Opt. 32, 5417-5426 (1993)
2.31 W.H. Southwell: "Coating design using very thin high- and low-index layers" ,
Appl. Opt. 24, 457-460 (1985)
2.32 S.A. Furman, A.V. Tikhonravov: Basics of Optics of Multilayer Systems (Editions Frontiers, Gif-sur-Yvette, France 1992)
2.33 A. Thelen, R. Langfeld: "Coating design contest: Antireflection coating
for lenses to be used with normal and infrared photographic film", Proc.
SPIE 1782, 552-601 (1992)
2.34 P. Baumeister: "Detailed knowledge of optical coating design techniques may
be superfluous to produce usable coatings", OSA 1992 Technical Digest Series 15, 8-10 (Opt. Soc. Am., Washington, DC 1992)
2.35 H.R. Dobler: Private communication, Aug. 6, 1992
2.36 A. Thelen: "Design of a hot mirror - contest results", OSA 1995 Technical
Digest Series 17, 2-7 (Opt. Soc. Am., Washington, DC 1995)
2.37 A. Thelen: "Multilayer filters with wide transmittance bands", J. Opt. Soc.
Am. 53, 1266-1270 (1963)
2.38 H.A. Macleod: "The monitoring of thin films for optical purposes", in Vacuum and Thin Film Technology, ed. by J. Yarwood, P. Anderson (Pergamon,
Oxford 1978) pp. 383-390
2.39 J.M.E. Harper, J.J. Cuomo, R.J. Gambino, H.R. Kaufman: "Modification of
thin film properties by ion bombardment during deposition", in Ion Bombardment Modification of Surfaces, ed. by O. Auciello, R. Kelly (Elsevier,
Amsterdam 1984) pp. 127-162
2.40 P.J. Martin: "Review ion-based methods for optical thin film depositon",
J. Mater. Sci. 21, 260-284 (1986)
2.41 J. Segner, M. Heming, H. Hochhaus, J. Otto, R. Langfeld: "Optical coatings
by plasma impulse CVD" , paper 1782-12, presented at the conference "Thin
Films for Optical Systems", September 14-18, 1992 (Technische Universitat
Berlin, Berlin, FRG)
50
2. Design Strategies for Thin Film Optical Coatings
2.42 D.Z. Rogers: "Manufacture of optical interference coatings by low pressure
chemical vapour deposition", Proc. SPIE 1168, 19-23 (1989)
2.43 H.A. Macleod, D. Richmond: "The effect of errors in the optical monitoring
of narrow-band all-dielectric thin film optical filters", Opt. Acta 21, 429-443
(1974)
2.44 N. Boling: "Optical coatings made by plasma chemical vapour deposition",
J. Opt. Soc. Am. 4, 102 (1987)
2.45 J. Segner: "Plasma impulse chemical vapour deposition - a novel technique
for the production of high power laser mirrors", Mater. Sci. Eng. A 140,
733-740 (1991)
3. Coating Technologies
3.1 Physical Vapour Deposition
Ulrich Jeschkowski, Hansjorg Niederwald
Coating solid surfaces with thin films by condensation from the vapour phase
is called physical vapour deposition (PVD) if only physical effects are involved. If the film forming process involves chemical reactions one has the
special case of reactive evaporation.
The deposition is achieved mainly by the following steps:
• creating a vapour phase by evaporation or sublimation of the coating material in vacuum,
• transporting the emitted particles through the residual gas phase from the
source to the substrate,
• condensation of the film forming species on the substrate and layer building
by nucleation and diffusion.
Detailed overviews ofPVD and related techniques are given by Pulker [3.1]
and Frey and Kienel [3.2].
3.1.1 Non-Reactive Evaporation
The first step in depositing a thin film is heating the material in vacuum
until it evaporates or sublimes at adequate rates. In the optical coating industry, technical vacua are in the range of 10- 6 to 10- 5 mbar. The pumping
is mostly done in a two-stage process: roughing down to about 10- 2 mbar by
mechanical displacement pumps and generation of high vacuum by diffusion
pumps, turbomolecular pumps or cryopumps.
There are many different methods of heating the material for deposition:
direct and indirect resistance heating; heating by thermal conduction, radiation or induction; electron beam heating; laser irradiation or arcing. One
of the oldest methods, put to standard use for a long time, is the indirect
resistance heating of containers made of W, Mo, Ta, Pt, or C. The sources
can have various forms like boats, strips, coils, or baffled crucibles. To avoid
direct contact with the current-heated hot container walls, inserts are sometimes employed. They mostly consist of Ah03 or graphite. For low-melting
52
3. Coating Technologies
materials borosilicate glass has also been taken. The disadvantages of currentheated evaporaters are layer contaminations by container material and the
limited lifetime of the sources.
Evaporation by Electron Beam
A widely used evaporation source today is the electron beam gun. This costly
but versatile and stable source type is best suited for melt-forming materials
such as metals. The water-cooled walls of the gun hearths prevent any reactions with the hot material, thus excluding the danger of film contamination.
The high-energy electrons enable high surface temperatures; thus the evaporation of high-melting metals or dielectrics becomes possible. Commonly
used metals for optical thin films are: AI, Ag, Au, Si, Ge, Ni, Cr, or Inconel.
Melting dielectrics as Si0 2, A1 20 3 , MgO, MgF2' Sb 2S3 are easy to evaporate
and behave well under the electron beam. Materials such as ZnS, Sb 20 3 , and
SiO can cause trouble by spitting or decomposing.
Beam shaping methods permit the energy density in the hearth to be influenced in such a way that for each material the optimum power concentration
is obtained and an area of approximately uniform ablation is established. The
electron beam gun allows one to change the rate of evaporation over a wide
range. Its fast response makes it well suited to control by quartz crystal rate
monitors. Constant rates are established quite easily.
Substrate Temperature
The substrate temperature has a strong impact on the layer structure and
the stability. MgF 2 layers for instance are only hard and stable when put
on heated substrates. Yet the surface temperature for the deposition of ZnS
should not be too high because the condensation coefficient drops with rising
temperature.
Thickness Uniformity
To get a maximum of the usable substrate area coated in uniform thickness, the source-to-substrate geometry must fulfil certain conditions. A simple arrangement consists of a central rotating substrate and an eccentrically
positioned source at distance R from the axis of rotation. A good start for
optimizing thickness homogeneity is to choose the height H of the substrate
plane above the source in such a way that the H / R ratio is near 1.35. Fine adjustment is often necessary according to the actual vapour cloud distribution
of the chosen material [3.3J. More complex arrangements use double rotation of the substrates together with correction shields to get good thickness
uniformity. A limiting factor is always the changing source-to-substrate distance during the coating run because the source filling level decreases unless
constant refilling is done during the process.
3.1 Physical Vapour Deposition
53
3.1.2 Reactive Evaporation
Many metal oxides decompose on heating and thus form non-stoichiometric
coatings that exhibit strong absorption. In order to get a fully oxidized metal
oxide film, the use of O 2 as additional reaction gas is necessary. For Ti0 2 coatings, for instance, a total pressure of about 1 x 10- 4 mbar must be maintained
by refilling the coater with O 2 after the pump-down to below 5 x 10- 6 mbar
[3.4]. Because the melts of metal oxides such as Ti0 2, Ta205, Hf0 2, Zr02, or
Nb 20 5 show thermal dissociation, it is best to start with oxygen-deficient suboxides. To form homogeneous layers with a constant refractive index, stable
conditions with respect to evaporation rate, gas flow, and substrate temperature must be provided. The gas flow is controlled by a flow meter or an inlet
valve. The inlet valve can be operated by a total pressure gauge or a mass
spectrometer. If the total pressure is held constant, the portion of oxygen is
not constant during the run. It is always rising.
The reaction takes place near or at the substrate surface. Here the reactants must be present in the appropriate concentration to form a stoichiometric compound. Even at high substrate temperatures beyond 300°C
the reaction is often not complete. To get absorption-free coatings, a baking
process in air has then to be done after venting the coater.
Another way to obtain fully oxidized films is to feed more oxygen to the
reaction. However, this procedure generates porous and mechanically weak
layers due to the high pressure. Moreover, the refractive indices are lowered. Such layers show large wavelength shifts during venting because of the
high content of moisture that is being incorporated into the porous structure. For the same reasons their optical data are a function of temperature.
Consequently, the application of such filters is limited if they are exposed to
significant temperature changes. It is therefore better to use as little O 2 as
possible to get dense and hard layers with the highest refractive index and
to do the baking after venting, if necessary.
Several attempts were made to modify the starting material in order to
get dense, drift-free Ti0 2 layers. The application ofthis important high-index
material suffers from the variety of different oxygen phases, which are difficult
to control. Therefore it has been suggested that one start with a mixture of
different contents of Ti0 2 and Ti 20 3 [3.5] or dope Ti0 2 with 5% Ce02 [3.6].
3.1.3 Energy-Enhanced Evaporation
Thin film research and development has endeavoured to feed energy into the
growing film in order to eliminate the deficiencies inherent in thin films produced by thermal evaporation in vacuum (such as, for example, low packing
density or high void fraction, insufficient adhesion to the substrate, or substoichiometric composition, which results in undesired optical absorption) and
to produce the desired film properties [3.7,8].
54
3. Coating Technologies
The oldest and most widespread method is the heating of the substrates.
It significantly improves the adhesion, density, and stoichiometry, especially
in connection with reactive evaporation, i.e., by adding a gaseous reaction
partner, which is incorporated into the growing film, for example, O 2 for
many metal oxides. However, the energy input into the film by substrate
heating is limited due to the limited thermal stability of most substrates
(glasses). Process t emperatures vary between 200 and 350°C, which means
l:!..E = kT '" 0.05 eV average particle energy.
A way to increase the energy input into the growing film during evaporation is to expose the substrate simultaneously to a particle beam. The process
using charged particles, known as ion-assisted deposition (lAD) , is the best
investigated; it is shown schematically in Fig. 3.1. We distinguish two cases:
(a) The ions are reactively incorporated into the growing film.
(b) The ions - usually rare gas ions - influence the layer composition mainly
by their kinetic energy.
Substrate holder
I
I
I
I
I
I
/
To pump
+--
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
/
I
I
/
I
/
I
I
/
I
/
I I
/
I
\ / I
/
\ \ I I /
\ \I I /
"' \ \I
"'
Fig. 3.1. Main features of a set-up for ion-assisted deposition
3.1 Physical Vapour Deposition
55
In case (b), the ion impact during coating disturbs the development of the
typical columnar structure and leads to a densification of the layer composition. The effective surface temperature is significantly elevated, resulting
in a higher chemical reactivity and an increased diffusion of the substrate
and the film atoms. Loosely bound gas atoms and molecules, for example
water molecules, are removed, while other contaminants, for example organic
molecules, are cracked and removed by the ion impact. These effects lead to
improvements for a number of film characteristics because mechanical stress,
porosity and moisture receptivity are reduced and step coverage and higher
packing density are achieved. Other properties such as surface texture, film
structure, adhesion, and optical constants (e.g., refractive index) are positively influenced as well [3.9,10]. Generally, films grown under the influence
of energetic ions tend to have more bulk-like properties than layers grown
under thermal evaporation, which means higher hardness, better chemical
and thermal stability as well as better (optical) homogeneity [3.11]. Typical
particle energies for ion-assisted coating techniques range from around 10 eV
to several hundred eV [3.12~14].
An ion source widely used in lAD is the Kaufman ion source. Inside the
source a plasma is contained between a hot filament cathode and a cylindrical anode. A magnetic field, which is generated by permanent magnets,
compresses the plasma. Ions with high kinetic energy are extracted as a beam
through one to three grids. Because of the insulating substrates in many coating applications, the beam has to be space-charge neutralized, which is easily
effected by a hot filament that emits the necessary electrons. The hot filaments lead to problems in the use of this kind of ion source. The significant
erosion of the filaments limits the operation time and results in a measurable contamination of the growing film, which sometimes spoils for instance
the optical properties. Therefore, ion sources working without hot filaments
have been developed. In the radio-frequency ion source [3.15], for example,
the plasma is generated inductively by rf field radiation. A container of fused
silica holds the plasma and the ion beam is extracted by a set of carbon
grids. Electron cyclotron resonance (ECR) ion sources operate similarly and
generate the plasma by exciting the electron cyclotron resonance inside the
container, thus reaching higher plasma and ion current densities.
The reactive ion-assisted deposition method, in which the ions react chemically with the evaporated material, mainly concerns metal oxides, but also for
example nitrides and carbides [3.16, 17]. Investigations in this direction were
stimulated by the fact that films of metal oxides are often optically absorbing in the UV, VIS or near-IR spectral range because they are insufficiently
oxidized. Ebert constructed a simple ion source to improve the stoichiometry
especially of oxide films [3.15]. A fused-silica container includes a cylindrical
hollow cathode. Between the cathode and the substrate holder a discharge
with oxygen ions (O~ and O 2) and electrons is generated through a small
56
3. Coating Technologies
nozzle. Typical discharge data are 40 V and 1 A. This method enables the production of metal oxide coatings with a significantly lower absorption [3.16, 18].
Another energy-enhanced coating process, which is used for some applications of optical coatings on glass, is low-voltage ion plating (LVlP) developed
by Pulker et al. [3.19] from the ion plating process of Mattox [3.20]. Figure 3.2 shows the set-up schematically. The coating material is evaporated
by an electron beam evaporator. The crucible is at a positive potential with
respect to the low-voltage ion source and thus serves as the anode for an
argon plasma discharge at about 80 V and 200 A. The substrate holder is
electrically insulated. The distance between the evaporation source and the
substrate holder is about 70 cm. The evaporation material is melted under
a closed shutter. Then the ion source is switched on and the discharge between the ion source and the anode is established. The plasma, however, is
diffusely spread all over the vacuum chamber. Under these conditions, a bias
of several 10 e V builds up between the insulated substrate holder and the
plasma, with the substrate holder serving as the negative electrode. A significant portion of the evaporated particles are ionized and thus accelerated
towards the substrate. The resulting strong densification of the coatings, for
instance in the case of metal oxides, is one of the major advantages of this
process. A good adhesion, a smooth surface, and quite often an amorphous or
quasi-amorphous structure are features of LVlP films. The stoichiometry of
LVlP films is worse than that of lAD films, and sometimes leads to quite high
optical absorption [3.21]; in some cases an undesired intrinsic stress occurs
in the layers. Because of the high packing density, LVlP coatings incorporate
and absorb almost no water and their optical performance shows very high
environmental and thermal stability. The properties of LVlP films of a variety
Low-voltage
plasma source
Substrate holder
Ar
+
-----. To pump
Gas inlet
Electron beam evaporator
Fig. 3.2. Schematic set-up for low-voltage ion plating
3.1 Physical Vapour Deposition
57
of important optical materials are listed in [3.19]. Metal coatings with high
reflectivity and good adhesion can also be produced by LVIP [3.22].
3.1.4 Sputtering
Sputtering is the ejection of atoms from the surface of a solid target through
energetic particle bombardment. It was discovered as an erosion of the cathode in gas-discharge experiments more than 130 years ago. Today sputtering
is well understood and has a great variety of applications, especially for the
coating of surfaces. Overviews are provided by a number of helpful articles
and books [3.23- 28] .
Sputtering is characterized by the sputter yield S, which is the ratio of
the number of ejected atoms to the number of incoming energetic particles
(sometimes atoms, but predominantly ions) . S increases with the energy of
the incoming ions to a maximum, normally lying between 10 and 20 keV, and
then decreases again. At low energies there is a threshold for S; for Ar as
sputtering gas, the threshold lies roughly between 20 and 30 eV, depending
on the target material. S increases with the ion mass and depends on the angle of incidence with a maximum between about 60°C and 80°C. Moreover,
S depends on a number of target properties such as material (atomic mass),
composition, crystallinity, crystal orientation and the constitution of the surface (e.g., roughness, existence of a passivation layer). Most of the sputtered
particles are neutral atoms and their energy distribution has a high maximum
at several1OeV.
For coating applications a number of different sputtering processes are
available. Figure 3.3 schematically shows the standard configuration for diode
sputtering. At a pressure of about 10- 2 mbar, a gas discharge is generated.
Argon is usually used as sputtering gas. The Ar+ ions are accelerated to
energies of about 500- 3000 eV. The energetic ions then hit the cathode. Due
to momentum transfer and also - to a small extent - thermal evaporation
from the strongly heated surface, target particles are released. They migrate
r -_ _ _ _---L_ _ __ _~/ Target
\ A;e
,
8
~
G
o
:::k
+ -•. . . - - - - '--- - - -
Argon ions
Electrons
Target particles
Substrate
Fig. 3.3. The mechanism
of diode sputtering
58
3. Coating Technologies
through the process chamber and are deposited on the substrate as well as
on other surfaces inside the process chamber. By application of a dc voltage,
conducting targets can be sputtered in this configuration. Insulating targets,
however, would prevent sputtering by collecting a surface charge.
In order to sputter insulators, too, and to enhance the ionization in the
sputtering gas for gaining higher sputter rates, instead of a dc discharge a
hf discharge is applied; a typical frequency is 13.56 MHz. With the help of a
capacitor in the target lead, one can achieve a situation in which the sputtering ions, because of their low mobility as compared to that of the electrons,
hit only the target and not the substrate. However, the electrons still reach
the substrate and heat it up significantly, which is dangerous especially for
glasses and plastics.
To prevent the heating-up and achieve even higher sputter rates, today
hf magnetron sputtering is the most common sputtering process. It is shown
schematically in Fig. 3.4. Permanent magnets behind the water-cooled target
generate a magnetic field that keeps the positive ions close to the target and
the hot electrons away from the substrate.
The significant numbers of charged and energetic particles facilitate the
production of films by reactive sputtering [3.17,29]. Starting from a metallic
target, which usually yields a much higher sputtering rate than a compound
target such as the oxide, the reaction partner is added to the sputtering gas.
This process is widely used for the production of protective and decorative
films (mostly various nitrides, but also carbides and borides), for example on
machine tools and consumer articles, respectively [3.30]. Also most oxides for
optical applications are produced in this way [3.1]. The main advantages of
sputtering are:
• The high kinetic energy of deposited particles [3.31] produces very dense,
and in many cases amorphous, films.
• Compounds can be deposited with good uniformity: fractionation as in
thermal evaporation plays only a minor role.
• Substrates with complex surface shapes can be coated uniformly due to the
random trajectories of the sputtered atoms at the relatively high working
pressures.
Another sputtering process with different properties is the ion beam sputtering. The set-up is shown schematically in Fig. 3.5. In a high-vacuum environment an ion beam of defined energy is directed onto a target at a de-
I N', ----,1 S '1- - - " NY Magnets
L.::J
L-.J
L.::J /' Cathode
Iiiiiii~!!iiiiiiiiiiijijiiji~"~ijijijiiiii;:;- Target
~
Ef- Electrons
p p p
Ions
~ Sputtered particles
_iiii_iiEFiiiiIllliiEFiiii_EFiiiiillllliiiiiiii' - Substrates
Fig. 3.4. The mechanism of magnetron sputtering
3.2 Chemical Vapour Deposition
59
Target
ion source
Fig. 3.5. Basic set-up for
the (dual) ion beam sputtering process
fined angle of incidence. In the early experiments a duoplasmatron ion source
was used [3.32]; later on this was replaced by more effective sources such as
the Kaufman ion source [3.33- 35]. To avoid contamination from the erosion
caused by the plasma inside the ion source, efforts are made to replace the
Kaufman-type sources by others, for example by the ECR (electron cyclotron
resonance) source. In order to sputter insulating targets, too, the Ar+ ion
beam is neutralized by electrons that are emitted from a hot filament and
coupled into the beam via a plasma bridge. Multilayers can be produced by
simply bringing other targets into the position of the first [3.36]. The advantages of ion beam sputtering are basically due to the fact that the substrate
is held under high vacuum and is well shielded from the sputtering process at
moderate pressure. The result is a process with a minimum of substrate heating and a very low film contamination, which still has the advantages of the
film growth by energetic particles. For optical applications this means films of
high density and stability with very low scattering and absorption [3.37- 39J.
3.2 Chemical Vapour Deposition
Wolfgang Mohl
3.2.1 Techniques of Chemical Vapour Deposition
Any process of chemically reacting a gas-phase compound of a material to be
deposited, in combination with added gases, to produce a solid formation on
a surface that grows atomistically on a suitably placed substrate, is referred
to as chemical vapour deposition (CVD).
The growing use of CVD methods is due to the ability to produce a large
variety of films and coatings, for example of metals, semiconductors, oxides,
and of compounds in either crystalline or amorphous form, with high purity
standards and unique properties. The advantages of CVD are the relatively
60
3. Coating Technologies
low costs of equipment and operating expenses, and the capability of creating
films of widely varying stoichiometry.
The three main CVD techniques are thermal CVD, plasma CVD, and
laser CVD. Many variants of CVD processing have been investigated and
developed in recent years, including low-pressure (LPCVD) [3.40]' plasmaassisted (PACVD) and plasma-enhanced (PECVD) [3.41], laser (LCVD)
[3.42], and plasma-impulse CVD (PICVD) [3.43J. Moreover, physical and
chemical vapour deposition have been combined to perform hybrid processing while preserving features of both techniques. The gas-phase reactions are
induced either by thermal or non-thermal activation, according to the CVD
variant chosen.
CVD, plasma CVD, and laser CVD can use the same precursors, but there
are differences resulting from the mechanism of deposition. CVD and LCVD
use high temperatures, and consequently thermodynamic processes govern
the nature of the resulting deposit. In plasma CVD the same precursors react at temperatures several hundred degrees Celsius lower, and the deposition
processes are controlled by kinetics. The lower deposition temperatures prove
to be very advantageous for sensitive substrate materials (thermally instable compounds [3.44]) and may cause the formation of metastable phases of
the deposit. But all CVC techniques need volatile precursors whose chemical
composition is changed during the deposition process. In thermal CVD the
precursor forms a deposit when it comes into contact with a hot surface. In
plasma CVD the vapour of the precursor is decomposed by the contact with
the plasma. In LCVD the precursor is decomposed in a photochemical process or by pyrolysis upon contact with a surface that has been heated by a
laser. With the progress of laser processing of materials, new opportunities,
including localized deposition and tailoring of reaction pathways, i.e., laterally resolved materials structures, are created. Review articles and books
dealing with these aspects of CVD can be found under [3.45-51J.
The numerous variations in design and operating parameters frequently
make it difficult to compare the performance of individual methods or reactors, even when depositing the same material. Regardless of the process type,
the associated equipment must have the following capabilities:
• supply and monitoring of reactant and diluent gas flow into the reactor,
• transfer of activation energy into the reactor to induce chemical reactions
and subsequent deposition,
• removal of by-products and depleted gases.
In the following a few comparative data concerning the above-mentioned
techniques are listed:
(a) thermally activated CVD, restricted by high deposition temperatures
typical temperatures
operating pressure
deposition rates
T = 700-1000°C
p = 50-300 mbar
a = 0.5-2 !"Lm/min.
3.2 Chemical Vapour Deposition
61
Plasma CVD can be subdivided into:
(b) plasma-assisted CVD (PACVD): Electrons promote the dissociation of
the reacting gas. An additional ion bombardment enhances the deposition
rates and/or the properties of the deposit.
(c) plasma-enhanced CVD (PECDV): inducing chemical reactions that do
not occur without plasma.
In cases (b) and (c) the plasma provides the reaction energy that allows lower
deposition temperatures.
typical temperatures
operating pressure
deposition rates
T
p
a
= room temperature
= 0.1-1 mbar
to <"-' 200°C
= 1-4I-lm/min.
Note: PACVD/PECVD allow low temperatures but require absolutely clean
surfaces, which can be obtained by plasma cleaning methods, to promote
adhesion [3.52]. We will concentrate here on the discussion of plasma phase
CVD techniques.
3.2.2 Peculiarities of the Various Techniques
Advantages of Plasma CVD
The term plasma CVD is used as a universal description of film deposition
where initial gas phase reactions are induced by electrical discharges. Such
films are typically carbides, oxides, nitrides and oxynitrides of elements such
as AI, B, Ge, Si and Ti.
Film deposition in a glow discharge system is a dynamic irreversible kinetic process that begins with homogeneous reactions in the plasma bulk
and near the surface and terminates in heterogeneous reactions with, and on,
the surface. The deposition processes, including the foregoing homogeneous
and heterogeneous reaction sequences, are mainly controlled by the plasma
properties and the excited and/or radical states.
The advantages are:
•
•
•
•
•
•
•
lower deposition temperature,
high deposition rate,
improved adhesion,
thermal stability of deposited material,
higher density,
high temperature modifications (e.g., diamond, Si 3 N 4 , SiC),
improved crystallinity.
It is possible to produce thin films at temperatures lower than those at
which the thermochemical reaction occurs by applying a cold plasma to enhance the chemical reactions. Thus previously unfeasible high-temperature
reactions can now be performed on thermolabile substrates.
62
3. Coating Technologies
Simplified Model of Film Growth by Deposition
from the Gas Phase
Three basic steps are necessary for the film formation:
• generation of the film-forming species,
• transport to the substrate,
• film growth on the surface.
Under plasma conditions the influence of the plasma on these three basic
steps has to be considered [3.53].
There are two main interactive plasma effects on the CVD process:
• plasma properties
- electron density,
- electron energy,
- EEDF (electron energy distribution function);
• process conditions
- gas pressure,
- flow rate,
- surface temperature,
- substrate bias.
During the three basic steps, the precursor compound has to undergo several
types of reaction pathways:
• formation of reactive species (excited states, ions, free radicals),
• intermediate species that deposit on the surface,
• mobile formations terminated by reactions that form the stable films.
All the above-described steps are controlled by the process and the plasma
properties.
Properties of Low-Pressure Plasmas (Non-Equilibrium State)
As already mentioned, plasma parameters such as electron density, electron
energy, and EEDF are most important to the CVD process under plasma
conditions. The plasma is sustained by ionization events as a result of electron
bombardment. The electron energy determines the ionization cross-section,
which increases with energy and levels off at about 50-100eV. Under these
conditions high electron densities are obtained, which are in turn responsible
for a high chemical reactivity of the plasma. To produce the desired plasma
chemistry, and hence the proper film formation, the parameters should be
independently controlled.
The absence of a thermodynamic equilibrium in the LP (low-pressure)
plasmas prevents the prediction of the chemical reactions and the scaling-up
of the process. It becomes more difficult because ofthe non-Maxwellian nature
of the EEDF in the LP plasma; the rate constants from the cross-section yield
3.2 Chemical Vapour Deposition
63
incorrect results if a Maxwell-Boltzmann distribution is assumed. The LP
plasma, a cold plasma, in its non-equilibrium state induces high temperature
chemistry at low temperatures. Note: 1 eV ~ 11600 K. Telectron ranges from 1
to about 8 e V [3.54]. The rest of the plasma (ions, neutrals) stays cool because
of collision kinetics [3.55].
The energy input from the plasma phase into the surface promotes the
formation of thermodynamically stable phases (films), because it occurs via
a large number of relatively small energy quanta, such as recombination radiation, quenching of radicals or ionized states, relaxations from exited states
and ion bombardment. The impact of the ion bombardment, for instance, is
more likely to induce the growth of a crystalline film, whereas an amorphous
phase is induced in the absence of ion bombardment.
3.2.3 Variation of Processing Conditions and Properties
Film Properties and Deposition Parameters
The properties of thin films strongly depend upon the deposition technique
and the chosen conditions. In order to obtain the desired film, optimum deposition conditions have to be found by carrying out experiments in a welltailored fashion with varying parameters. But scaling-up problems are often
unavoidable. In industrial applications there is an urgent need for plasma
deposition techniques, because they are superior to conventional thin-film
deposition techniques in many ways.
The final step, surface deposition (i.e., the growth of the film), depends
on the following deposition conditions:
• The generation rate of the intermediate species determines the deposition
rate and the stoichiometry.
• The mean free path length and the collision frequency in conjunction with
the partial pressure of the different compounds (gas composition) affect
the growth and the properties.
• The gas flow rate and the dwell time control the stoichiometry of the films
formed.
• The temperature affects the surface processes, i.e. the mobility and the
rate of chemical reactions on the surface.
• The ion bombardment during the deposition (the ion energy is given approximately by the bias voltage) has a very positive effect on the film properties, especially on the adhesion and the defect formation. Good adhesion
requires a high sticking coefficient, which, however, counteracts the mobility of the film-forming species with the surface. Mobility is mandatory for
undisturbed film formation without defects. With the help of ion bombardment the negative influence of a high sticking coefficient is compensated
without losing the good adhesion. The ion bombardment, moreover, reduces absorbed impurities and trapped gases.
64
3. Coating Technologies
The nature and the complexity as well as the degree of interdependence
among many of the variables - plasma parameters for example influence
process parameters - make it necessary to evaluate the relationships between
the process and plasma parameters.
Some selected examples:
• To extend the lifetime of the valve seats and stems in nuclear power plants,
these parts were protected by a multilayer ceramic coating showing good
wear resistance in high-temperature water environment [3.56].
• Transparent SiC material for use in severe environments has been fabricated by hot-wall CVD [3.57].
• Superconducting films with high Tc have been produced by the microwavegenerated plasma CVD method [3.58].
• The formation mechanisms of TiN films and the investigation of plasma
species by optical emission spectroscopy and quadrupole mass spectrometry have been examined [3.59].
• The protection of the soft surfaces of eye-glasses consisting of organic material (e.g., CR 39) by CVD coatings has been compared with other technologies such as ion-assisted deposition or lacquering [3.60].
Equipment for Plasma CVD
The standard set-up for the formation of solid deposits by plasma CVD
(Fig. 3.6) is a parallel plate reactor with two electrodes of 10-50 cm in diameter and a spacing of a few centimetres, in which an electrical gas discharge
is initiated [3.61].
Apart from the parallel plate reactor, other arrangements are occasionally used that separate the plasma from the substrates. The plasma itself
Gas
Load lock system
Counter electrode
Substrate
L_r::::=::;==~=~==j
Substrate
electrode
Pump
Pump
Fig. 3.6. CVD system for laboratory use [3.72]. Frequency 13.56MHz, equipped
with load-lock, movable counter electrode; typical power ratings: 50-450 W
3.2 Chemical Vapour Deposition
65
can be induced by electromagnetic radiation, for instance rf or microwave
(2.45 GHz) [3.62,63]' and the combination of two frequencies (dual-frequency
mode).
In conventional systems there is usually no spatial separation of the
plasma generation and deposition ZOnes. This has several drawbacks because
the ion density and the ion energy cannot be adjusted independently. Figure 3.7 schematically displays a dual-frequency mode system. The plasma is
sustained by 2.45 GHz, and the energy of ions striking the substrate surface
is adjusted by the so-called bias voltage, which is generated and controlled
by a second frequency that is attached to the substrate.
The rf-induced negative dc self-bias voltage significantly changes the deposition rates and the film composition. These effects provide a powerful
tool for producing "tailored" films, while preserving the important advantages, namely the high deposition rates and low substrate temperatures [3.64].
Pulsed microwave plasmas (PICVD, 2.45 GHz) are most advantageous for the
production of superior-quality optical materials [3.65]. The reaction chamber
for room temperature deposition experiments is shown in Fig. 3.8.
With this technique the almost monolayer-by-monolayer growth control
opens up a completely new degree of freedom in the design of optical coatings.
Important Parameters
The most important parameters controlling the film growth by plasma CVD
are:
• partial pressures of reactants and dwell time,
• rf power,
• substrate bias and surface temperature.
Microwave
ECR magnet
ECR magnet
I
I
Orifice
Substrate
Fig. 3.7. Dual-mode
microwave/radio frequency plasma deposition system [3 .72]
66
3. Coating Technologies
Microwave power
Microwave power
Microwave window
a)
c::
Pump
Plasma
~
Plasma region
Substrate
b)
Substrate
~
Gas
Gas
To pump
Fig. 3.8. Two versions of PICVD coaters. (a) For planar substrates, with diameters
of up to about 20 cm; (b) for dome-shaped substrates, with a diameter at the
opening of up to about 18 cm [3.73]
The above variables also affect parameters such as the deposition rate and
plasma parameters such as the electron density, the electron energy and the
EEDF. For example, the partial pressure of the reactant gas together with the
rf power determines the rate of dissociation of the reactive gas and hence the
deposition rate. The same parameters determine the electron energy and the
electron density. The substrate bias (ion energy) is a function of the electron
temperature and the pressure in combination with the rf power (which in
turn controls the surface bombardment), so the substrate bombardment by
ions and the growing of the film both depend on the same set of process
variables, i.e., pressure and rf power. In some cases, the interplay of process
and plasma parameters may make it difficult to obtain very high deposition
rates by PECVD processes because of the competition between homogeneous
and heterogeneous reactions.
Plasma chemistry controls the types and the concentration of the excited
species, ionized states and energetic neutrals, which in turn influence the reaction pathways or steps involved in the overall reaction for film formation and
the physical location of the reaction sites. Deshpandeyand Bunshah [3.66] discuss in detail the role of "plasma" in PECVD. Excellent reviews on PECVD
are given by Reinberg [3.67], Hallahan and RosIer [3.68], Rand [3.69], Yasuda
[3.70], and Hallahan and Bell [3.71].
3.3 Sol-Gel Coating Processes
Wolfram Beier
3.3.1 Sol-Gel Chemistry
Conventionally, nonmetallic inorganic materials (glasses, ceramics) are produced by high-temperature processes such as melting or firing at tempera-
3.3 Sol-Gel Coating Processes
67
tures up to 1600 DC. But chemical routes to synthesize such materials have
also been developed or are being researched. The various chemical techniques
are often subsumed under the term "sol-gel methods" , because they usually
involve a sol or solution as precursor which is converted to a gel. Subsequently, the gel intermediate is densified by relatively mild heat treatment
(400-1000 0 C). The sol-gel methods have their greatest potential in the field
of thin inorganic films and coatings.
Usually alkoxides are used as starting compounds, that is, organometallic substances of the form M(OR)n (M: metal of valence n, R: alkyl group
C xH2x+d. By two groups of reactions, hydrolysis and condensation, the
alkoxides are converted to three-dimensionally connected networks. An example is the formation of silica glass from TEOS (tetraethoxysilane).
Hydrolysis:
Si(OC 2 H 5)4 + 4 H 2 O ---+ Si(OH)4
Condensation:
---+ Si0 2
Si(OH)4
Overall reaction:
Si(OC 2 H 5)4 + 2 H 2 O ---+ Si0 2
+ 4 C 2 H 5OH.
(3.1)
+ 2 H 2 O.
(3.2)
+ 4 C 2 H 5OH.
(3.3)
Thus, two moles of water are needed as a minimum amount to completely
convert one mole of TEOS. More generally, for an alkoxide of a metal M with
valence n, the following reactions will take place
M(OR)n
M(OH)n
M(OR)n
+ n H2 0
+
---+ M(OH)n
---+ MO n / 2
n/2 H 2 0 ---+ MOn/ 2
+ n ROH
+ n/2 H 2 0
+ n ROH
(hydrolysis).
(condensation).
(overall).
(3.1a)
(3.2a)
(3.3a)
Equations (3.1, 3.2) and (3.1a, 3.2a) are simplified. In real systems, the reactions are much more complicated, especially because the hydrolysis occurs
in successive steps. In the case of silicon-ethoxide this means
Si(OC 2 H 5)4
Si(OH)(OC 2 H5h
Si(OHh(OC 2 H 5h
Si(OHh(OC 2 H5)
+ H2 0
+ H2 0
+ H2 0
+ H2 0
---+
---+
---+
---+
Si(OH)(OC 2 H5h
Si(OHh(OC 2 H 5h
Si(OHh(OC 2 H 5)
Si(OH)4
+ C 2 H 50H.
+ C 2 H 50H.
+ C 2 H 50H.
+ C 2 H50H .
(3.4)
(3.5)
(3.6)
(3.7)
Furthermore, the condensation reactions (3.2, 3.2a) already start before the
hydrolysis is completed. Thus, not only reactions such as
(OHhSiOH
+ HOSi(OHh
---+ (OHhSi-O-Si(OHh
+ H20
(3.2b)
are proceeding, but also reactions such as
(OH)x(OR)ySiOH + ROSi(OH)v(OR)w ---+
(OH)x(OR)ySi-O-Si(OH)v(OR)w
+ ROH
,
(3.2c)
where x + y = v + w = 3. Even the direct condensation of alkoxy groups is
possible under certain circumstances:
68
3. Coating Technologies
(OH)x(OR)yMOR + ROM(OH)v(OR)w -7
(OH)xCOR)yM-O-M(OH)v(OR)w
+ ROR,
(3.2d)
where the valency of M is 4. In (3.2c) alcohol, and in (3.2d) ether is set free
instead of water (3.2, 3.2a, 3.2b). Equations (3.2b) and (3.2c) show that siloxane bonds (Si-O-Si), which constitute the basic structural units of silicate
glasses, are formed by sol-gel condensation.
Thus many different oligomers of partially hydrolyzed and differentially
condensed species rapidly form in aqueous alkoxide solutions. In most cases
only few or no reaction constants are known. For this reason, the phenomena
cannot be theoretically calculated, nor exactly predicted. Though many factors are still unknown, the system Si(OR)4 -7 Si0 2 is the best investigated
one by far. In some other cases (e.g., Ti02) much empirical knowledge has
been gathered and in still other cases even basic experiments are lacking.
The reaction kinetics and the time-dependent size distribution of oligomers
and agglomerates in the solution are important for the following steps of solgel processing. They determine the results of gelation, lyogel-to-xerogel conversion (drying), gel-glass transition (heat treatment), and crystallization.
Therefore it is extremely useful to investigate the phenomena that already
proceed in the solution. Only if these reactions are at least qualitatively understood, can the succeeding steps be controlled and changed as desired. The
reactions in the solutions depend on many parameters, for example:
•
•
•
•
•
•
•
•
compositions and concentrations of alkoxide(s) and solvent(s) used,
amount of added water,
catalyst used (type, concentration),
further additives: chelating substances such as diketones or desiccating
controlling chemical additives (DCCA) such as NH 2CHO,
sequence in which the components are added,
time schedule of mixing, for example ageing (pre-hydrolysis) of components
or intermediates that react slowly,
further conditions of mixing (e.g., mixing efficiency, surface-to-volume ratio, ultrasonic agitation, atmosphere),
temperature.
Considering this, it is understandable that up to now the reaction kinetics
of only a few sol-gel precursors have been be revealed. This holds especially
for multicomponent systems. But in the last decade the number of investigations on this topic has increased (see e.g. [3.74] and [3.75] on ternary alkoxide
precursors for Si0 2-Ti0 2-Zr02 coatings).
If a sol contains several different alkoxides and/or alcoholic solvents, ester
exchange reactions become possible and even more complicated kinetics will
result. Up to now only few studies on this subject exist; for information about
the exchange of different prop oxide groups see [3.76].
Different techniques are suitable to investigate sol-gel mixtures. By viscosity measurements they can be characterized macroscopically. If the viscosity
3.3 Sol-Gel Coating Processes
69
is determined as a function of the shear gradient, microscopic insights can
also be gained (shape of the agglomerates,. see [3.77], variations under shear
stress, see [3.78]). Concerning the time dependence of the viscosity, many
sol-gel solutions exhibit nearly constant low values over quite long periods.
But when the clusters have grown and are stitched together to a certain
extent, the percolation limit is reached. Now the viscosity increases rather
quickly [3.74] and the fluid converts to a gel.
This behaviour is important for sol-gel applications on an industrial scale.
Of course, long working periods are desired and newly developed solutions
should be optimized accordingly. Transferring laboratory results to the production scale is not trivial, mainly due to the different surface-to-volumeratios of the sol batches.
The knowledge of the viscosity-time schedule is important especially for
solutions with non-Newtonian behaviour. The latter can result for example
from an excessive evaporation of solvents within a short time. Such sudden
viscosity changes occur during the following technical processes:
• dip coating (upper part of the meniscus),
• spin coating (rim of the wetted region, critical if the amount of the solution
is the limiting factor),
• drawing fibres directly from the solution.
Of course, solutions cannot be characterized by viscosity studies alone.
During a technical coating process, a solvent may be added discontinuously
to the reservoir to replace the fluid drawn out, or to fix the viscosity at a low
level. Such a solution is not expected to contain the same cluster distribution
as a freshly prepared one. Some kinds of aggregates do not solve again if a
new solvent is added, i.e., reactions as given in (3.2c) need not be reversible.
Therefore analytical techniques become necessary by which the molecules,
clusters and agglomerates can be detected directly, for instance in older dip
coating solutions.
NMR techniques can be used for many sol-gel systems to explain the
reactions going on immediately after the mixing of the components. The
reaction kinetics in Si-Ti-Zr-alkoxide solutions for example was clarified by
high resolution NMR [3.79]. This method is less suited for exploring the subsequent processes, for example, the formation of oligomers and aggregation.
NMR is again a powerful tool to investigate the structures of solid gels and
gel glasses, especially if magic-angle spinning (MAS) techniques are used.
MAS NMR 29Si studies were performed for instance for the gel glass systems
Li 20-Si0 2 [3.80] and Si0 2-Ti0 2-Zr02 [3.81].
As mentioned above, the initial reaction steps in the solutions can already
determine the structure and the properties of the resulting gel-glass products. For example, chainlike structures can be produced by acidic hydrolysis,
whereas the basic conditions often lead to the formation of spherical clusters.
As is shown in Figs. 3.9 and 3.10 for the system Si0 2, this microstructure
70
3. Coating Technologies
Fig. 3.9. SEM picture of HCI catalysed TEOS gel with fibrous structure
[3.82]
Fig. 3.10. SEM picture of NH 4 0H
catalysed TEOS gel with granular
structure [3.82]
(nm scale) of agglomerates in the sol is reflected in a similar macrostructure
(!-lm scale) of the gels, which consist of fibrillar or globular units, respectively.
29Si NMR can also be used for mixed alkoxide solutions (Si- Ti) ; however,
for pure Ti alkoxide sols no direct NMR signals are recordable, because Ti
nuclei have no suitable resonances. But enough other NMR resonances remain
to be recorded, because organic groups are set free continuously. The status
of such solutions can be detected by 1 Hand/or 13 C NMR techniques. The
control of "T solutions" is important because they are used for Ti0 2 coatings
on flat glass substrates (solar shielding glass). Long-term stability of the
solutions is desired to ensure a continuous and undisturbed (i. e. economical)
production.
Particles of down to 5 nm in size can be detected by dynamical light
scattering techniques (using a laser scattering photometer), though uncertainties still exist concerning the quantitative evaluation. Several investigations describe the insights to be gained from light scattering. For example,
the 90 DC scattering intensity was recorded in strongly basically catalysed
TMOS/methanol/water mixtures and it could be deduced that monodisperse
Si0 2 particles form under these conditions [3.83]. Also information about the
fractal geometry (see Sect. 3.3.2) of special agglomerates can be extracted
from scattering results [3.84].
Thus many "S solutions" can be well characterized by this method; T solutions, however, are more difficult to deal with, b ecause they certainly contain particles smaller than 3 nm. But even pure titanium-ethoxide solutions
have been investigated by means of laser light scattering. This alkoxide forms
trimers if solved in cyclohexane or CC1 4 [3.85]. The shape of the particles depends on the solvent used [3.86]. In ethanol and n-butanol mono disperse ,
3.3 Sol-Gel Coating Processes
71
spherical agglomerates are forming, whereas irregularly shaped clusters are
produced in tert-butanol.
If the clusters become too small, X-ray scattering is a good tool. By means
of small-angle X-ray scattering (SAXS) particles that are typically sized between 1 and 100 nm can be detected; therefore this method is increasingly
used in the sol-gel field. Up to now mostly model solutions have been investigated (see e.g. [3.87]). By means of SAXS the gap existing between NMR
(molecular scale) and light scattering (!-lm scale) can be closed.
A typical example demonstrates the possibilities of SAXS. Two Schott
"S30 solutions" (equivalent oxide concentration: 30 giL) were investigated;
they contained mainly TMOS, water and ethanol. One solution was four
months older than the other. Their SAXS results are presented in Figs. 3.11
and 3.12; the scattering intensity I is plotted as a function of the scattering
parameter q = (47f sin 0) I A.
At small angles, the scattering signal of the older sample (Fig. 3.12) exceeds that of the fresh solution by a factor of 2. This result is indicative of
larger or more particles in the aged solution. It is highly probable that solutions with such different sets of clusters also have different properties, for
example concerning reactivity and wetting behaviour.
In terms of SAXS, small q values mean large inhomogeneities. The abovementioned upper limit of 100 nm for detectable particles corresponds to the
smallest angles at which one can operate. The lower limit of the detectable
structures decreases to about 0.016 nm if one switches to wide-angle X-ray
scattering (WAXS). Thus short-range-order phenomena and atomic distances
can be investigated by means of WAXS.
It can be concluded that SAXS is a very powerful tool for examining the
size, structure and fractal dimension of the agglomerates that form in sol-gel
solutions. The results for example help in deciding whether the particles grow
by diffusion-limited or by reaction-limited mechanisms in a specific solution.
20.------------------------------------,
18
16
14
12
0=
0=
.S!
..., 10
291190 PL02A
291190 PM02A
8
6
4
2
o~--~~~~~~~~~~
0.00
0.10
0.20
q
0.30
Fig. 3.11. SAXS investigation of a S30
solution; age: 2 weeks
3. Coating Technologies
72
20
18
16
14
12
!
i
•,
,,
%
\
!;!
...., 10
0=
0=
8
291190 PL01 A
291190 PM01 A
6
4
2
0
0.00
0.20
0.10
q
0.30
Fig. 3.12. 8AX8 results of a 830 solution; age: 18 weeks
Sol-gel solutions, gels and gel glasses can also be characterized by IR
and Raman spectroscopy. By means of these methods the concentrations of
alcohols and water, for instance, are detectable in situ during the alkoxide
reactions. By measuring the absorption at 1650 cm- 1 , the hydrolysis of TEOS
can be registered as a function of the water / alkoxide ratio and the kind and
concentration of the catalyst.
With the help of Raman spectroscopy the conversion of Ti propoxide
can be detected within four hours. The very first hydrolysis steps proceed
fast, to be followed by slower reactions [3.88]. The final Ti0 2 powder still
contains some residual Ti-OR groups (i.e., Ti-O-C bondings) that withstood
hydrolysis.
The sol-gel system Si0 2-Ti0 2-Zr02 has also been analysed by IR spectroscopy (IRS). Especially the influence of stabilizers such as acetic acid,
acetylacetone, ethyl acetoacetate and diethanolamine was explored [3.89]. It
was proved that no traces of these additives remain after heat treatment at
250 DC.
Gas chromatography (GC) can also be used to detect reaction paths in
alkoxide solutions. Alcohols set free by the hydrolysis can be quantitatively
determined (see e.g. [3.90]). Analogously, kinetic information can be deduced
by detecting the water concentration. According to the net reactions (3.3, 3a),
the H 20 amount must decrease monotonically during the formation of oxides
from alkoxides. Indeed the GC analysis of HOH and ROH is limited, because
it is unspecific with respect to the many above-mentioned partial reactions.
Using GC, the evolution ofthe alkoxide concentration can also be detected
(for tetramethoxysilane, see [3.91]). Unfortunately it is difficult to detect all
educts and reaction products with the same separation column, because some
of them are polar (e.g. water, methanol), whereas others are nonpolar (alkoxides). Furthermore, some reactive species can bind to the packing material.
This effect can be avoided by trimethylsilylation, which converts all reactive
3.3 Sol-Gel Coating Processes
73
SiOH and SiOR groups to non-reactive SiOSi(CH 3 )s units. However, in practice, the problem of an overall and simultaneous GC technique has not yet
been solved.
The wide field of sol-gel chemistry cannot be covered here completely; for
a much more detailed treatment, see [3.92].
3.3.2 Sol-Gel Fractals
The formation of structures with fractal geometry during the different steps
of sol-gel processing shall be briefly addressed. A characteristic feature of
fractal objects is their self-similarity over several orders of magnitude: Every
part of such a structure resembles - after a proper magnification - the original
configuration. Not only line fractals are possible, but also surface and mass
fractals. For mass fractals, the mass M within a sphere of radius R is given
by
(3.8)
where D is the dimension of the structure and c is a constant.
For normal (non-fractal) mass objects, D = 3. If a smaller D is evaluated
(e.g., the fractional value 2.5), a structure offractal dimension, i.e. a fractal, is
present. Interestingly, the density d of such material is not constant. The d of a
sphere decreases with increasing radius R. This follows from the combination
of
(3.9)
and (3.8), which leads to
(3.10)
For the above-mentioned example it follows that d = C2R-o.5. It results
further that a porous, fractal object contains pores of every size, up to sample
size! This is an implication of the self-similarity concept.
Beginning with the condensation of molecular clusters, linear chains are
often formed in solutions. After a time, side chains adhere to the main chains
and second-order side chains to the first-order side chains, and so on. The gels
evolving in this way are often mass fractals, whereas their interior surfaces
are surface fractals.
In the field of sol-gel products, silica gels have so far been investigated
the most with respect to fractal properties. Important analytical tools for
this purpose are the above-mentioned light and X-ray scattering techniques.
In TEOS solutions, for example, the growth of linear chains is observed if
acids such as HCI are used as catalysts. Fractal growth, On the other hand,
occurs if hydrolysis cannot be completed, or if the reaction is much faster
74
3. Coating Technologies
than the transport of new educts. HF-catalysed TEOS (one of the fastest
reacting systems) is an example of the latter case.
Generally, filament or cluster structures form in acidic or basic TEOS solutions, respectively. Such different conditions and various molecular clusters
in the solutions can determine structural features of the product up to much
larger dimensions (f..Lm scale, see Figs. 3.9, 3.10) [3.82]. Again the first reaction steps in the solution already determine whether gels or gel glass coatings
will exhibit fractal features or not.
The SAXS results of Figs. 3.11 and 3.12 will now be considered again,
this time with respect to fractal properties. If the intensity and the scattering
vector are both plotted logarithmically (see Figs. 3.13 and 3.14), the fractal
dimensions of the agglomerates can be immediately deduced from the slopes
of the linear portions of the curves.
The linear parts of the curves in Figs. 3.13 and 3.14 have slopes of -1.24
which are consistent with the fractal dimension 2.24. The two S30 solutions
are distinguished by the fact that the fractal concept can be applied up to
different upper limits (smallest angle). The aged sample contains in addition
some larger fractal clusters; this argument can even be stressed quantitatively:
In the aged solution the deviation from linearity occurs at a fourfold smaller
angle; thus the largest aggregates will be four times larger than those in the
younger solution.
The fractal phenomena of TMOS gels have been investigated by meanS of
synchrotron radiation scattering. It was shown that basic samples are volume
fractals built up from smaller volume and area fractals [3.93]. Contrary to
that, all acidic gels were volume fractals consisting of much smaller structural
units (radius < O.4nm).
No fractal dimensions were found in samples that had been dried from
TMOS gels under hypercritical conditions. This finding can be interpreted
10.----------------------------------.
..!!1
...,
0=
0=
291190 PL02A
291190 PM02 A
0.1+-"""----.--,r-,,-rTTTr----.--.~
0.004
0.01
q
0.1
Fig. 3.13. Log-log
plot of the 8AX8 results from Fig. 3.11
(830 solution; age:
2 weeks)
3.3 Sol-Gel Coating Processes
75
10
0=
0=
291190 PL01 A
291190 PM01 A
0.3 -l--~-r-""""----'--"'--"""""r-T""....-r"T"T""--"""':
0.004
0.01
0.1
q
Fig. 3.14. Log-log
plot of the SAXS results from Fig. 3.12
(S30 solution; age:
18 weeks)
if all pores have roughly the same size (possibly as a result of the special
treatment). Besides, a narrow pore-radius distribution can also be obtained
by adding drying control additives (DCCA) such as formamide.
In conclusion, the fractal concept has been successfully applied in the
field of gels and gel glasses, that is, many different structures, produced by
various techniques, were well described. But it still remains to be proved that
the ideas of fractal geometry can be used for exact scientific predictions or
economically relevant improvements of sol-gel processing and sol-gel products. In the following fields, the application of the fractal concept may lead
to a deeper understanding and consequently to better products:
• sol-gel reactions, especially during the step of polycondensation,
• "tailor-made" realization of special porosities or shapes of the interior surface for all kinds of sol-gel products (e.g., nano-membranes with respect
to the volume, catalyst carriers with respect to the surface),
• "tailor-made" realization of the refractive index and other specifications
for thin sol-gel coatings by constructing adequate (fractal or non-fractal)
microstructures.
Computer simulations have been performed for simple cases [3.94,95]. In
principle, such calculations must be checked and continuously optimized by
measurements. The final aim of all these activities is to enhance the understanding of all stages of sol-gel processing and to find correlations between
the microscopic phenomena and the properties ofthe products (mostly oxidic
thin films).
3.3.3 Dip Coating
The formation of thin coatings on flat glass by dipping and subsequent
withdrawing was already experimentally and theoretically investigated by
76
3. Coating Technologies
Schroeder some decades ago [3.96,97]. His research covered many oxidic
systems, but the systems Si0 2 and Ti0 2 were most explicitly dealt with.
Schroeder analysed the optical film thickness nd as a function of the inclination angle, the concentration of the solution (equivalent oxide content), and
the drawing speed. He found the following dependency between the geometric
thickness d and the drawing speed 1/ and also tried to explain it theoretically:
d
=
const. 1/2/3 .
(3.11)
For Si0 2 films, Schroeder also investigated the thickness/speed relation as
a function of the age of the stock solution. Aged alkoxide mixtures lead to
thicker coatings, presumably due to the continuously increasing viscosity that
results from polycondensation reactions.
Thus a good foundation for the sol-gel dip coating process was already
built many years ago. Though many of these early findings were empirical,
the main goal was reached: to enable the fabrication of products such as
optical filters, antireflection coatings, etc., in a reproducible and economical
manner.
After Schroeder, several other authors treated the dip coating process,
especially regarding the exponent given in (3.11). Yoldas and O'Keeffe, for
example, determined a value of 1/2 for it [3.98]. This value was also taken by
Mukherjee, who worked with TEOS solutions [3.99]. Another investigation
proved that even much lower values (down to 1/10) are valid for Si0 2-Ti0 2Zr02 alkoxide solutions, depending on the solvent used [3.100].
The theoretical treatment was considerably improved by Strawbridge and
James [3.101,102], who derived formulae that consider the surface tension
and the different densities of solution, solvent and film. (Normally, only an
overall density, the drawing speed, and the viscosity are considered.) The
authors were able to fit their experimental results on the TEOS-C 2H 5 0HH20-HCl system very well to theory. But until today no ab initio theory exists
for multicomponent alkoxide solutions that would predict the film thickness
correctly when all physical constants and processing parameters are known
and are put in.
Directly above the meniscus, freshly formed dip coating films are mainly
unreacted films of the alkoxide solution used. They still contain large amounts
of solvents. On the other hand, all organic compounds diffuse out and evaporate quickly due to the high surface/volume ratio, whereas the small thickness
of the film allows the water molecules to diffuse into every point of it.
Thus, above the very meniscus, processes develop within seconds that
would take days or even months if centimetre-sized gel monoliths were to be
produced from the same solutions. These processes are: hydrolysis, condensation, growth of primary clusters, aggregation to larger particles, gelation
and out-diffusion of the reaction products.
Brinker and Hurd contributed to the fundamentals of dip coating. In
order to record at least some of the phenomena in situ, they investigated
3.3 Sol-Gel Coating Processes
77
freshly forming, drying films by position sensitive ellipsometry [3.103,104].
They used this method for Ti0 2 films (from Ti-ethoxide solutions) and for
Si0 2 films (from colloidal sols of particles between 15 and 20 nm in size, made
according to the Stober process). They obtained the index of refraction and
from that derived the content of solid phase as a function of position and
time.
As is to be expected, pre-aggregated sols lead to thicker, but more porous
coatings than stabilized sols. The formation of distinct microstructures is
strongly influenced by capillary forces, which are lower in colloid gel films
(here Si0 2 ), because they contain larger structural units (l/r dependency).
In an analysis that considers not only draining but also evaporation, Hurd
and Brinker assume an evaporation singularity that leads to a sharp drying line. At this discontinuity of the (curvature-dependent) evaporation, the
relatively thick fluid film switches to a thin and quite dry gel coating. Experimental data that would allow an adequate consideration of the capillary
pressure are lacking [3.105].
In special cases the fluid meniscus may take even more complicated
shapes [3.106]. In its upper part an additional thickening may be generated
by Marangoni transport of fluid (i.e., induced by surface tension gradients,
which in turn result from temperature or concentration gradients).
Modelling of the processes in the meniscus must consider many parameters. If objects of complicated shapes are to be coated, not only the drawing
speed and the inclination must be included, but also the concentrations of all
constituents of solution and atmosphere. Viscosity and the different surface
energies which determine the contact angle are important, too. All these parameters depend on temperature and/or concentration and on position. (In
an inertial system that moves with the glass plate they are time-dependent.)
The actual meniscus (region of high solvent concentration) spans only
a few mm. The zone above it may be even more important due to the
in-diffusion and out-diffusion processes that strongly influence the final microstructure of the coating. In this region the fluid should stop flowing lest
possible defects of the substrate surface are reproduced in the film. In a region
15-25 cm above the stock solution surface, the sol-gel film becomes visible
(interference effects).
Current topics of research concerning dip coating are:
•
•
•
•
•
•
water content in technical solutions and in the atmosphere,
use of aged solutions,
additives (e.g. to affect the crystallization of Ti0 2 ),
coating with thick films (or with multiple films),
coatings with new compositions, especially multicomponent mixtures,
interactions between coating and substrate.
3.3.4 Spin Coating
Although in the electronic industry the process of spin coating with alkoxide
solutions has been intensively used for decades, the glass scientists took but
78
3. Coating Technologies
little notice of these activities. One reason for this lack of interest was the
nomenclature: in glass science thin "sol-gel" glass films are very common,
whereas in the field of silicon wafer processing these films are termed "SOG"
(spin-on glasses).
In an early, typical SOG study, undoped and doped Si0 2 films were spun
on Si wafers [3.107]. The electronic data of the semiconductors are presented
as functions of the spinning parameters and atmosphere, etc. The use of
SOG for creating blocking layers and dopant diffusion sources may be more
economical than the use of hot gas diffusion techniques.
The spin coating process in general is analysed in [3.108] for simple systems (consisting of a solvent and a component that converts to the solid). If
the atmospheric currents above the spinning disk are laminar, simple functional dependencies exist between the process parameters and the properties
of the resulting films. Under this condition the film thickness is inversely proportional to the root of the angular frequency. It is also demonstrated that
the evaporation of the solvent is more important to the film formation than
the centrifuging of the excess fluid.
In 1957 a basic derivation was already presented, concerning the contour
of a fluid film on a rotating disk [3.109]. It was shown that a smooth surface
stays smooth during spinning and that initial disturbances or waves (e.g. of
sinusoidal shape) are levelled very fast. Only due to this phenomenon can
spinning be applied for the production of films with extremely high flatness.
In Sect. 3.3.3 possible influences of the Marangoni convection were mentioned. Not only in dip coating, but also in spin coating, such surface tension
effects playa role [3.110]. The so-called "Benard cells" are unwanted and
several investigations have sought means to avoid them.
The basic work of Bornside et al. identifies and analyses four phases during
the spinning process: deposition, spin-on, spin-off, and evaporation [3.111].
In a model which describes the complicated processes well, the quantities
concentration and thickness are set to be constant in the radial direction,
whereas normal to the disk surface concentration, viscosity and diffusivity
are taken as variable within the film [3.112]. The evolution of the coating
thickness is successfully treated as a function of the process parameters and
time. Moreover, the paper addresses the phenomenon of "skin formation". A
skin appears when the viscosity exceeds a certain limit. There are two ways
to avoid a skin: (a) saturation of the atmosphere with a solvent, (b) addition
of a second solvent with a different evaporation rate.
The microstructures of sol-gel spin coatings and of the respective solutions were correlated by Brinker et al. [3.113]. If chainlike aggregates are
present in the sol, they are aligned during spinning. In this case, differences
between the central and the outer parts of the final film are detectable by
FTIR spectroscopy. One the other hand, homogeneous coatings (with identical structures near centre and rim) result if the sol contains spherical clusters.
3.3 Sol-Gel Coating Processes
79
It is interesting to compare the properties of sol-gel films fabricated by
dipping and spinning. For Si alkoxide solutions, crack-free wet films with
thicknesses of 1100 nm can be obtained by spinning (cf. 700 nm obtained by
dipping) [3.114]. But spun coatings exhibit more shrinkage, so that the final
thickness (after heat treatment) proved to be similar for both techniques. The
maximum thickness obtained for annealed films was about 600 nm. The residual porosity of the spun films was greater, even after baking at 1100 DC. The
porosity could be reduced by increasing the water content of the solutions.
Obviously the species can form a denser packing during the withdrawing process because it runs much more slowly. Additionally, dip films show slightly
smaller surface roughness values compared to spin coatings. Replacing the
solvent ethanol by propanol leads to greater roughness, too.
During the withdrawal, only small speed gradients (1-100 S-l) occur in
the solution. In contrast, speed gradients of up to 105 S-l occur during spinning, especially shortly after the start. Therefore the shape of the aggregates
and the resulting non-Newtonian effects are of greater relevance to the spinning process. This is emphasized by Macosko et al., who treat the rheology
of Si0 2 solutions [3.115]. Basic catalysed TEOS solutions and colloidal Si0 2
sol (Ludox®) were characterized by viscosimetry and investigated with the
help of a cryo-TEM technique. In both kinds of samples the viscosity rose
from 10- 1 to 103 dPas, but the increase occurred much more abruptly for
TEOS. The above-mentioned TEM technique proved to be useful, because
an approaching gelation can be detected in advance by direct imaging of
particles.
Obviously, the sol-gel spin coating technique is well understood and controlled for simple systems, yet it is still difficult to produce spin-on coatings of multicomponent compositions. For example, the development of Agcontaining and Pd-containing multicomponent solutions for spinning purposes was complicated by stability and wetting problems [3.116]. To overcome
these difficulties, unusual solvents had to be added. Another general problem
is the fabrication of multiple spin coatings. Therefore considerable research
efforts are still necessary, concentrating on the following points:
• spinnability of multicomponent solutions (containing several alkoxides,
salts, solvents, catalysts, etc.),
• multiple coatings (problem: to avoid or at least reduce the heat treatment
between the single spinning steps),
• coating of non-planar substrates.
At the end of this section it should be emphasized that other sol-gel
coating techniques exist apart from dipping and spinning, the most important
being spraying. This method can be especially economical if glass substrates
that are still hot from production are coated on-line and any extra heat
treatment may be omitted. Examples are the coating of flat glass at the end
of the float glass line and the "hot end coating" of glass bottles immediately
after blowing.
80
3. Coating Technologies
3.3.5 Heat Treatment
As all other gel glasses, sol-gel glass films have to be heated at least to temperatures around Tg to ensure that all chemical reactions are nearly completed.
(Ceramic sol-gel coatings must be fired up to similar temperatures.)
The terms "heat treatment" or "firing" mean the complete sequence of
drying and annealing steps, i.e., every thermal processing above RT until the
final glassy or ceramic oxide film has formed.
Concerning practicable T-t programs, Schott has gathered comprehensive
knowledge; thermal processing has been developed empirically over several
years. Nevertheless there still exists a considerable optimizing potential, especially with regard to making the production more economical while maintaining, or even improving, the film quality. It is particularly desirable to
shorten the whole firing procedure. Today, thermal treatment is the bottleneck of industrial sol-gel processing; the greatest potential for economy exists
in shortening those steps.
Moreover, new applications would become economically attractive if firing could be done faster. This holds especially for depositing multicoatings.
Schott has already gathered some experience (e.g. concerning the influence of
humidity) for the necessary annealing steps between the single coatings. Several investigations were performed on a semi-production scale, for example
to increase the Taber abrasion hardness of Ti0 2-Si0 2-Ti0 2 reflective coatings on float glass panes. Among other parameters, the temperature of the
intermediate heat treatments was varied. A maximum value for the Taber
abrasion resistance was reached at 140°C oven temperature for intermediate baking. (Actually the temperature of the substrate is even lower, namely
80°C, at the optimum conditions.)
Many published papers also treat the question of optimized T-t programs
for reaching distinct film properties. Mukherjee, for instance, investigated
the solidifying of anti-reflective coatings in the system Na20-B203-Si02 and
found that a treatment at 500°C is superior to a 300°C anneal [3.99]. The
heat treatment must not last too long. A shortening of the baking step from
5.5 to 1.5 h led to a distinct reduction of reflectivity.
Generally, investigations on multicomponent coatings are still empirical;
theoretical treatment and understanding are lacking. To improve this situation it will be necessary to record the film structure during the densifying
process by means of modern analytical methods.
Hirashima et al., for example, used ellipsometry for monitoring the film
compaction by heat treatment [3.117]. The thin films were dip coatings of the
systems Ta205, Nb 20 5, and hydrogen-containing V 20 5. During drying only
ethanol evaporated and the microstructure did not essentially change before
thermal treatment. Concerning Nb 20 5 coatings, it showed that films with a
granular microstructure can be compacted to a higher degree than those with
a fibrillar nature.
3.3 Sol-Gel Coating Processes
81
A problem of general importance is the cracking of thick films (and the
cracking of stacks of thin films beyond a certain number of layers). The
reason is, of course, the considerable shrinkage of gel layers (by factors of two
or more) during their thermal hardening.
In principle the shrinkage can be drastically reduced by adding oxidic
microparticles of the target composition to the sol. It is questionable, however,
whether such admixtures are concordant with the processing parameters and
quality demands.
During baking, lateral movements may occur within thick coatings, even
if the chemical bondings between film and substrate resist the thermal treatment and impede slip movements directly at the interface. But if the thermal
expansion values differ too much, cracks may also occur directly at the interface. Films may be peeled off the substrate or the single films of a stack may
separate from one another in such a case. Shrinkage normal to the substrate,
on the other hand, is much less critical. This phenomenon only leads to a
homogeneous compaction.
The problem of thermal-expansion mismatch can also be eliminated by
suitable additives, but in this case they must be of different compositions.
Thus it becomes difficult to maintain the physical specifications of any functional coating - quite apart from problems with processing parameters and
quality demands.
Garino determined lO!-lm as the maximum thickness value for crackfree Si0 2 gel coatings. But this finding holds only for baking temperatures
below 200°C and for coating solutions with small water/TEOS ratios (R
values) [3.118]. Relatively thick films can only be obtained if the chemistry
of the solution and the annealing scheme are optimized and harmonized.
Presumably, small R values are favourable because the corresponding solutions have low surface tensions. Keddie et al. studied the compaction of Ti0 2
spin coatings and Si0 2 spin coatings on silicon wafers. Using a Rutherford
back scattering technique they found that the absolute density of Ti0 2 films
increased when the temperature was raised. But the relative density (compared to pure anatase) was only 0.7, even at 750°C [3.119]. Notably, a certain
anatase content was already detected at 450 °C. To the contrary, Si0 2 films,
which had been prepared for comparison, were already completely densified
at 450°C. The same authors showed that Ti0 2 coatings can be densified to
a maximum degree by especially high heating rates [3.120].
So far, the baking of films has been performed mostly by heat or IR radiation. If the light of halogen lamps (which has a higher radiation temperature)
is used for that purpose, the absorption within the coating will increase and
the process will develop more rapidly. The baking step may be shortened still
more by using laser radiation [3.121].
An investigation dealing with the heat transport and the temperature
distribution field during CO 2 laser baking of Si0 2 showed that the 1O.6-!-lm
radiation penetrates lO!-lm at the most (attenuation of the initial intensity
82
3. Coating Technologies
to 0.15%) [3.122]. The resulting temperature distribution is almost independent of the thermal conductivity of the substrate and of the heat transfer
coefficient.
These circumstances, which enable the energy transfer into strictly defined paths, are favourable for the consolidation of thin sol-gel films whose
thicknesses usually lie far below 10 J..!m. The direct energy deposition during
laser curing effects the heating of the layer within milliseconds. The cooling
happens with similar rapidity.
If alkoxides are used, the rapid heating causes carbon residues to be enclosed in the layer [3.123]. This effect is usually unwelcome; yet the resulting
changes of properties (e.g., increased refractive index and increased hardness)
might also be deliberately induced and exploited.
In a more recent investigation [3.124], optical layer waveguides were produced in the sol-gel system Si0 2 - Ti0 2 , whereby the hardening was performed
either by traditional (oven) heat treatment or by 1O.6-J..!m laser irradiation.
This comparative study proved that the transmission losses were markedly
higher in the laser-tempered layers.
It was experimentally proved that sol-gel layers may also be cured by NdYAG laser irradiation [3.125]. After a 40-nm-thick Au-Pd sputter layer/coating had been deposited, paths of 150-600 J..!m width and 20-100 nm depth
were lasered.
Drying and tempering can also be done by microwaves, which, however,
do not directly couple with the gels. To nonetheless sinter gels densely by microwaves, in the case of bulk samples a conductive susceptor was used [3.126].
Heated up by the microwaves, the susceptor transmitted the energy to the
sample through thermal radiation.
In a likewise indirect manner KNb0 3 gels (spun films from ethoxides)
were processed by microwaves [3.127]. In comparison to conventional thermal
treatment the temper duration was shorter and crystallization set in earlier.
If layers are to be consolidated in this way, planar (possibly masklikestructured) susceptors might be used. But for two reasons this seems to be
an impracticable solution: First, the educts cannot be removed, and second,
the local resolution is impaired by the insertion of a thermal transmitter.
The first argument in particular speaks against the deposition of a thin
metal coating on the sol-gel film before laser tempering. Fabes and his group
have been investigating this indirect method as well as direct CO 2 lasering
for several years [3.123]. Indirect lasering works with a Nd-YAG laser and a
thin sputtered Au or Pd coating.
A special problem arises if the (porous) sol-gel films which are to be densified are infiltrated by organic dyes. Practically no organic dye can withstand
the temperatures that would be needed to completely densify the oxide matrix. Therefore lower temperatures and/or shorter annealing times must be
accepted, even though they may have an unfavourable impact on the longterm stability of the product.
3.4 Thermal Coating Processes
83
3.4 Thermal Coating Processes
Joachim Disam, Dirk Gohlke, Katharina Lubbers
Introduction
During the processing of optical or industrial special glass, the moulds are
subjected to extremely high thermal, corrosive and abrasive loads. The materials requirements are therefore resistance to high temperature and temperature shock, high oxidation stability, excellent corrosion resistance to the
aggressive products due to glass vaporization, and sufficient durability [3.128].
If one of the above criteria fails to be met the glass surface quality in the
finished products will be reduced and, consequently, the mould equipment
life will be lowered.
The above requirements can only be met with high-alloyed steels or nickelbased and cobalt-based alloys. Because these materials are very expensive it
is reasonable to repair worn mould equipment at low cost or, if non-expensive
base metal can be used, to apply a high-standard coating. The coatings must
also meet high quality requirements: They must be non-porous, exhibit a
defined grain size, be adjustable by thermal treatment and their hardness
must conform with that of the base metal. The oxide phases and existing
carbide bands have to be moulded into the matrix so as to affect the hardness
of the coating as little as possible; otherwise there is the danger of cracking.
The adhesive strength has to be guaranteed even in cases of prolonged use
and the coating thickness should be variably adjustable from 0.2 to 7 mm.
All the criteria can be met by one thermal coating process. The coating
technique and the appropriate layer material must be chosen according to the
application and must fit the often predetermined basic materials. The techniques, layers and basic materials unfortunately are not freely interchangeable.
3.4.1 Processes and Materials
In glass-producing facilities the predominant application is the coating of
tools such as moulds, dies and rolls. In most cases, coatings with thicknesses
between some hundred microns and some millimetres are needed. The most
appropriate coating technologies are therefore laser cladding, surfacing and
powder metallurgical processes such as hot isostatic pressing (HIP) and thermal spraying. The coating processes can be classified according to their adhesion mechanisms. Laser cladding, surfacing and hot isostatic pressing result in
a metallurgical bonding of the substrate and the coating, whereas with thermal spraying there is only mechanical adhesion. Because detailed descriptions
of coating processes can be gathered from the textbooks [3.129-134]' only the
basic principles will be considered in the following. Table 3.1 gives an overview
of the various coating techniques and applicable layer materials, the adhesion
84
3. Coating Technologies
properties, the density of the layers, and the advantages and disadvantages
of the respective techniques. The table shows that the coating technique cannot be freely chosen but depends on the respective applications and on the
chosen layer material. Ceramics, for instance, can neither be deposited by
surfacing nor by laser powder coating. Likewise, materials containing hard
material formers such as titanium or aluminium are only suited for processes
performed under vacuum or protective gas.
Flame Spraying
In the flame spraying process the coating material is rapidly melted, partially
or completely, in the nozzle system of a spray gun and is accelerated toward
the substrate. Special applications may require an inert gas to atomize and
propel the molten material. The surface material for flame spraying may be
in the form of wire, rod, cord or powder. In the case of wire flame spraying,
the wire-shaped material to be deposited is supplied to the flame through a
suitable wire feeding equipment. Figure 3.15 gives a schematic view of flame
spraying.
The fuel gas in the oxy-fuel gas-flame is mainly acetylene, but propane and
hydrogen are also used. The acetylene-oxygen flame reaches a temperature
of 3150 ec, the propane-oxygen flame 2850 ec, and the hydrogen- oxygen
flame 2660 ec. Nevertheless the substrate keeps relatively cool because the
flame does not reach it. Flame-sprayed coatings have a maximum density of
only 80% and a relatively low adhesion strength. On the other hand, flame
spraying is a very cheap process with a high deposition efficiency.
High-Velocity Oxy-Fuel Gas Spraying
In high-velocity oxy-fuel gas (HVOF) spraying a hot gas jet ejected with
supersonic speed is used for the partial or complete melting of the coating material and for the acceleration of the powdery spray material. With
Compressed air
Fig. 3.15. Principle of flame spraying
steels, bronzes,
ceramic
steels, Ni-based and mechanical
Co-based materials, adhesion
bronzes, ceramic,
WC/Co-materials
steels, Ni-based and mechanical
Co-based materials, adhesion
bronzes, ceramic,
special materials
steels, Ni-based and
Co-based materials,
special materials
(Cr, Mo, cermets)
Ni-based and Cobased materials,
cermets
Ni-based and Cobased materials
Flame spraying
High-velocity oxyfuel gas spraying
Plasma spraying
Vacuum plasma
spraying
Powder metallurgy /
hot isostatic
pressing (HIP)
Laser-spray coating
100
max. 92
max. 95
max. 80
~
%
Density
metallurgical
bonding
metallurgical
bonding
~
~
100
100
mechanical
min. 98
adhesion;
partly
metallurgical
bonding
mechanical
adhesion
metallurgical
bonding
steels, Ni-based
and Co-based
materials, bronzes
Deposit welding
Adhesion
mechanism
Coating materials
Process
Table 3.1. Thermal coating processes and their applications
high costs
high costs
coating preparation
(cutting and grinding)
necessary
coatings of superior
quality
small heated zone
superior quality
high costs
high deposition rate
oxide-free coatings
spraying of highreactive materials
high deposition rate high costs
materials with high
melting temperature
economical
high deposition rate
dies and moulds
for press glass
dies and moulds
for press glass
every kind of
moulds
moulds, rolls
moulds, rolls
tools without
abrasive wear
diffusion barrier
rolls, moulds
wear-resisting
parts
high porosity
low adhesion
not suitable for moulds
not suitable for
high-reactive materials
economical
high deposition rate
tools for the
manufacture
of TV panel
components
large heat-affected zone
preheating the work
piece up to 400 DC homogenizing is necessary
economical coating
process
Applications
(Schott)
Disadvantages
Advantages
01
00
00
ct>
00
00
(")
0
ct>
..,"'0
()q
S·
~
0
Q
~
S
..,ct>
P"
~
~
<:;oj
86
3. Coating Technologies
high-velocity flame spraying, the impact velocities of the particles hitting
the substrate are several times higher than with conventional flame spraying
and, depending on the spray powder applied, still almost twice as high as
with atmospheric plasma spraying. The high impact velocities achieved with
a continuous particle flux enable the production of dense layers with strong
adhesion.
Besides achieving high particle velocities, HVOF spraying has the decisive
advantage that the heating of the particles is easier to control than with other
techniques. Thus the microstructure of the spray material is less changed
and thermally activated processes, which may negatively influence the layer
properties, are avoided.
Plasma Spraying
In plasma spraying a plasma jet is used as heating source. The plasma is
formed inside a plasma gun by a DC arc that is ignited between the tungsten cathode and the nozzle, which serves as anode. A stream of gas, which
passes this arc, becomes dissociated and ionized. The reaction leads to a
high-temperature plasma of approximately 20000 K. The high velocity of the
plasma jet is generated by the thermal expansion of the gas. After the spray
powder is injected into the plasma by a carrier gas it is melted and accelerated
and finally impinges on the substrate (Fig. 3.16).
Due to the high plasma temperature it is possible to spray even materials
with high melting temperatures. The plasma spray process effectively melts
most metal powders, refractory metals and oxides.
Conventional plasma spraying is carried out at atmospheric pressure. The
properties of these coatings are often unsatisfactory because of the porosity
Tungsten
cathode
Copper
Main power
supply
anode
+
Powder
Substrate
Fig. 3.16. Schematic
view of plasma spraying
3.4 Thermal Coating Processes
87
of up to 5%, the degree of partly unmelted particle contents, and the oxide
inclusions caused by air entrainment into the plasma jet. To improve the
coating quality, low-pressure plasma spraying is operated in a large chamber
containing a controlled atmosphere at a defined pressure of 27- 133hPa. The
advantages of low-pressure plasma spraying over conventional plasma spraying are the deposition of dense, oxide-free coatings with a high bond strength
between deposit and substrate and the possibility of spraying high-reactive
materials; disadvantageous are the higher costs.
Laser-Spray Coating
The laser-spray coating process - in contrast to flame spraying, HVOF gas
spraying, and plasma spraying - produces a metallurgical bonding between
the coating and the substrate. The surfacing material can be delivered as
preplaced powder, wire feed or blown powder. The blown powder process
offers the widest operating range. Figure 3.17 shows the principle of a laser
beam powder feed coating process.
The powder blown into the laser beam and forwarded onto the surface
is melted within a fraction of a second and is bonded to the surface (singlestage process). The laser beam induces the formation of a very thin molten
surface coating by simultaneously affecting the powder and the surface of
the substrate. The surface coating guarantees the metallurgical bonding of
the coating material to the substrate. Clad thicknesses of 0.3- 3.0 mm can be
reached in a single pass.
The laser spray coating technique can be applied to a variety of metallurgical combinations, and the process has several advantages over conventional
cladding processes. For example, the components can be coated on a small
\
/tz
pOWder gas
stream
Laser beam
\
I
1..::<.;
\
\
PSi
\
\
\
,
~ ~:~t:~.~.;'
' •• ,~· • • I
,~~·:{2P'
'.' •• " • ' ,£
\,~<::::',;/>d
(?:: ::// /
'{ .... , :: :'1
~----,,....--+...,....,...:.:.~,:......
-~"~~~
Fig. 3.17. Principle of
laser beam powder feed
coating process
88
3. Coating Technologies
area and with high precision reproducibility. The relatively high costs are a
disadvantage.
Powder Metallurgical Coatings
The most important powder metallurgical coating process is diffusion bonding by hot isostatic pressing (HIP). The HIP process requires a leak-tight
encapsulation of the whole material (Fig. 3.18). The process parameters are
temperature, holding time and pressure. In addition, the heating time and
the cooling time play an important role. Usually the HIP temperature ranges
from 50% to 80% of the coating melting temperature. Metallurgical factors
must be considered to avoid eutectics or the formation of undesired phases.
Generally the pressure is set to 100MPa [3.135] . The holding time controls
the diffusion, the grain growth, and the phase transformations in the materials. The heating time mainly depends on the heating conductor and is
of only secondary importance, whereas the cooling time governs the phase
equilibriums.
Composites of identical metallic materials represent the simplest case of
diffusion bonding. For composites of different metallic materials a clear distinction has to be made between systems forming brittle intermetallic phases,
and others. The presence of brittle phases markedly reduces the composite
strength. The appropriate choice of the HIP parameters helps to limit the
formation of such phases. The HIP diffusion bonding process is of particular
interest if conventional techniques fail or if high bond strengths are required.
?
b
~
I
p
Filling of the
capsule with
powder and
closing
Hot isostatic pressing
(HIP)
Fig. 3.18. Principle of the HIP process
Encapsulation
and finish-machining
3.4 Thermal Coating Processes
89
Tungsten Inert Gas Hardfacing
Thngsten inert gas hardfacing (TIG) belongs to the electric arc welding processes; it is a very economical coating process with a wide range of applications. The electric arc burns between a permanently negatively polarized
tungsten electrode and the positively polarized work piece. A surrounding inert gas stream protects the tungsten electrode (Fig. 3.19). Usually argon with
a purity of 99.99% is used as inert gas. For some applications a mixture of argon with 5% hydrogen is used because hydrogen reduces the oxide inclusions
in the coating. The surfacing material can be fed manually or mechanically
as a wire or a rod. During the deposit welding process, the inert gas shields
the tungsten electrode, the electric arc, and the surfacing material. Therefore
the density is nearly lOO% and almost no oxide inclusions are present. The
metallurgical bonding ensures a very good adhesion of the coating. On the
other hand, the substrate is contaminated very deeply by the surfacing material, the heat input is high, and the materials undergo thermally activated
changes so that the formation of brittle phases is possible.
3.4.2 Applications
In the following the various applications of thermal coating processes in special glass production will be explicated by examples from Schott's operational
practice. The examples show why, depending on the application and on the
layer material required, different coating techniques are employed.
For example, a repair-type coating applied to a Nimonic 90® roller (see
Fig. 3.20), which is used in the manufacture of glass ceramic panels, can
demonstrate the convenience of using the low-pressure plasma spray (LPPS)
method. The average glass contact period of this roller is about lO h. Then
Welding direction -
Fig. 3.19. Schematic view of tungsten inert gas hardfacing
90
3. Coating Technologies
Fig. 3.20. Nimonic 90® roller
the surface is freed from glass evaporation products, inspected for scratches,
roughnesses and flatnesses, and prepared for reuse by grinding and polishing.
After about nine months the roller is used up because its diameter is so much
reduced. In the repair case the coating thickness in the finished condition is
projected to be 5 mm.
Before the actual coating took place, the samples were heated up to 920 °C
and cleaned by means of the plasma flame and the transferred electric arc.
Afterwards the rollers received a 6-mm-thick coating. The spray powder consisted of protective-gas-atomized Nimonic 90®. Its chemical composition is
given in Table 3.2.
Figure 3.21a shows that the coating exhibits few carbide inclusions in
the polished condition. Etching the ground surface, one clearly observes the
formed spraying texture with a grain structure (Fig. 3.21b).
If the coating is subsequently subjected to a thermal treatment at 1080°C
over varying periods of time, grain growth in the coating is started. With an
annealing time of up to 48 h more uniform grain sizes are produced (Fig. 3.22).
Prior to the final machining operation, the roller was subjected to a diffusion
treatment at 920°C for 24 h.
In the past, the surface layers best meeting the high demands on porosity
and homogeneity in the glass forming process were powder-metallurgically
Table 3.2. Chemical composition of the Nimonic 90® powder (in wt%)
Cr
20.0
Co
18.2
Ti
2.7
O2 content: 500 ppm
Al
1.4
Fe
1.5
Mn
0.8
Si
0.7
Cu
0.2
C
0.1
Zr
0.1
B
0.02
S
0.01
Ni
bal.
3.4 Thermal Coating Processes
-~
91
~._""Ir.. a)
b)
Fig. 3.21. Ground segments of the coating in (a) polished and (b) etched condition
from base metal/coating interface, from the centre and toward the coating surface
Fig. 3.22. Grain-size distribution of the coating after time-dependent thermal
treatment at 1080 °C
deposited by hot isostatic pressing (HIP). This is exemplified in Fig. 3.23
by the lower part of a mould for reflector production. Compared with the
conventional configuration, the tool life achieved with this coating was three
times higher and at the same time the cutting rate was doubled.
92
3. Coating Technologies
1/"111'"
1
I
I
11111111
Fig. 3.23. Lower part of a mould for reflector production. Coating material: cobased alloy (Stellite 6®); thickness: 1.5 mm; process: hot isostatic pressing (HIP)
References
3.1 H.K . Pulker: "Coatings on glass", in Thin Film Science and Technology, ed.
by G. Siddall, Vol. 6 (Elsevier, Amsterdam 1984)
3.2 H. Frey, G. Kienel (Eds.): Dunnschichttechnologie (VDI Verlag, Dusseldorf 1987)
3.3 G. Deppisch: "SchichtdickengleichmaBigkeit von aufgedampften Schichten in
Theorie und Praxis", Vakuum-Techn. 30, 67- 77 (1981)
3.4 H.K. Pulker, G. Paesold, E. Ritter: "Refractive indices of Ti02 films produced by reactive evaporation of various titanium- oxygen phases", Appl.
Opt. 15, 2986- 2991 (1976)
3.5 Balzers: Auslegeschrift DE 24 19 122 B2 (1974)
3.6 H.W. Lehmann, K. Frick: "Optimizing deposition parameters of electron
beam evaporated Ti02 films", Appl. Opt. 27, 4920- 4924 (1988)
3.7 C.K. Hwangbo, L.J . Lingg, J.P. Lehan, H.A. Macleod, J.L. Makous,
S.Y. Kim: "Ion assisted deposition of thermally evaporated Ag and Al films",
Appl. Opt. 28, 2769- 2817 (1989)
3.8 H. Anders: Dunne Schichten fur die Optik (Wissenschaftliche Verlagsgesellschaft, Stuttgart 1965)
3.9 W.G. Sainty, R.P. Netterfield, P.J. Martin: "Protective dielectric coatings
produced by ion assisted deposition" , Appl. Opt. 23(7), 1116-1119 (1984)
3.10 P.J. Martin, R.P. Netterfield: "Optical films produced by ion based techniques", Progress in Optics XXIII, ed. by E. Wolf (Elsevier, Amsterdam
1986) pp. 115- 178
3.11 H. Pulker: "Characterization of optical thin films", Appl. Opt. 18(12), 19691977 (1979)
References
93
3.12 J.R. McNeil, A.C. Barron, S.R. Wilson, H.C. Herrmann: "Ion assisted deposition of optical thin films: low energy versus high energy bombardment" ,
Appl. Opt. 23(4), 552-559 (1984)
3.13 J.E. Greene: "Low energy ion bombardment during film deposition from
the vapor phase: effects on microstructure and microchemistry", Solid State
Technol. 14, 115-122 (1987)
3.14 F. Varnier: "Ion assisted deposition effects on the surface structure of a Ti02
thin film", Vac. Sci. Technol. A 8(3), 2155-2159 (1990)
3.15 J. Miiller, G. Steininger: "Sputtering of metal oxides with an rf ion source",
in Surtec 1989, ed. by H. Czichos, G.E. Vollrath, Proc. Int. Congress for
Surface Technology, Berlin, Germany, Oct. 11-13, 1989 (Hanser, Miinchen
1989) pp. 391-396
3.16 J. Ebert: "Activated reactive evaporation", Proc. SPIE 325, 29-38 (1982)
3.17 R.F. Bunshah (Ed.): Deposition Technologies for Films and Coatings (Noyes,
New York 1982)
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3. Coating Technologies
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3.65 J. Segner: "Plasma impulse chemical vapour deposition - a novel technique
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3.66 C.V. Deshpandey, R.F. Bunshah: "Plasma assisted deposition techniques and
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3.90
3. Coating Technologies
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W. Beier, A.A. Goktas, G.H. Frischat, C. Wies, K Meise-Gresch, W. MiillerWarmuth: "Kinetics of the formation of Si02-Ti02-Zr02 gels from alkoxide
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M.D. Sacks, R-S. Sheu: "Rheological properties of sol-gel materials", J. NonCryst. Solids 92, 383-396 (1987)
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C. Wies, K Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. Goktas, G.H.
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C. Wies, K Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. Goktas,
G.H. Frischat: "High resolution solid state nuclear magnetic resonance of the
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Phys. Chern. Glasses 31, 138-143 (1990)
W. Beier, M. Meier, G.H. Frischat: "Charakterisierung von Si02-Gelen und
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E.C. Ziemath. W.L. Pretti, M.A. Aegerter, J.E.C. Moreira, T. Lours,
J. Zarzycki: "Light scattering dynamic study of the gelation process", J. NonCryst. Solids 100, 211-214 (1988)
E.C. Ziemath, M.A. Aegerter, J. Moreira, M. Figueiredo, J. Zarzycki: "Light
scattering of Si02 monodisperse microspheres prepared by the sol-gel route" ,
Mat. Res. Soc. Symp. Proc. 121, 311-316 (1988)
W.R Russo, W.H. Nelson: "A structural study of titanium tetraethoxide in
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M. T. Harris, C.H. Byers: "Effect of solvent on the homogeneous precipitation
of titania by titanium ethoxide hydrolysis", J. Non-Cryst. Solids 103, 49-64
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B. Himmel, T. Gerber, H. Biirger: "WAXS- and SAXS-investigations of structure formation in alcoholic Si02 solutions", J. Non-Cryst. Solids 119, 1-13
(1990)
KA. Berglund, D.R Tallant, RG. Dosch: "Time resolved Raman spectroscopy of titanium isopropoxide hydrolysis kinetics", in Proc. 2nd Int.
Conf. Ultrastructure Processing of Ceramics, Glasses, and Composites, ed.
by L.L. Hench, D.R Ulrich (Wiley, New York 1985) pp. 94-99
U. Wellbrock, W. Beier, G.H. Frischat: "Preparation of Si02-Ti02-Zr02 gel
glasses and coatings by means of modified alkoxide solutions", J. Non-Cryst.
Solids 147/148, 350-355 (1992)
J.B. Blum, J.W. Ryan: "Gas chromatography study of the acid catalyzed
hydrolysis of tetraethylorthosilicate [Si(OC2H5)4]", J. Non-Cryst. Solids 81,
221-226 (1986)
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3.93 A.F. Craievich, D.L dos Santos, M.A. Aegerter, T. Lours, J. Zarzycki: "Structural study of fractal silica humid gels", J. Non-Cryst. Solids 100, 424-428
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3.94 E.J.A. Pope, J.D. Mackenzie: "Theoretical modelling of the structural evolution of gels", J. Non-Cryst. Solids 101, 198-212 (1988)
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3.119 J.L. Keddie, E.P. Giannelis: "Microstructural evolution of Ti0 2 sol-gel thin
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4. Properties and Characterization
of Dielectric Thin Films
Introduction
The properties of thin films are strongly influenced by the deposition parameters and conditions, i.e., they are strongly related to the coating technologies
described in Chap. 3. Variations are possible, for example in mechanical characteristics, in colour, in film thickness, etc. To efficiently produce coatings for
a certain application it is necessary to know the parameters that influence a
desired film property during the deposition process. For reactive evaporation,
as described in Sect. 3.1.2, these are: substrate temperature, deposition rate,
oxygen partial pressure and total pressure. For films produced by the sol-gel
technique, as described in Sect. 3.3, the relevant parameters are: composition of solution, drawing speed, interaction with the surrounding atmosphere
during drawing and with the substrate during densification.
Macroscopic and microscopic film properties are necessary for a sufficient characterization of dielectric thin films. These are, for example, macroscopic properties such as refractive index, absorption, film thickness, adhesion, stress, density, scattering of light, and hardness, and microscopic quantities such as the composition and chemical binding state of elements, stoichiometry, topography, roughness of the surface, formation of interfaces, crystalline or amorphous state and also the structure of crystals.
Various analytical methods are available for obtaining such information.
Some relevant techniques, which can be efficiently employed in industrial surface labs, are summarized in Table 4.1. Thin film and surface sensitive methods in general use photons, electrons and ions as probes, but also forces, for
example in atomic force microscopy (AFM) [4.1]. The different probes interact
with a fixed volume of the samples and create emitted photons, electrons or
ions, which are sampled by a detector. The acronyms of some relevant methods are given in Table 4.1. Photons are used as probe in optical spectroscopy
to determine the transmittance, T, the reflectance, R, and the absorptance,
A, in infrared spectroscopy (IR) [4.3], Raman spectroscopy [4.2,3]' angle resolved scattering (ARS), total integrated scattering (TIS) [4.4], photothermal
deflection techniques (PTD) [4.35,36]' laser calorimetry (LC) [4.37]' grazing incidence X-ray reflectivity (GIXR) [4.5,6]' small angle X-ray scattering
(SAXS) [4.8], grazing incidence X-ray fluorescence analysis (GIXF) [4.5,6],
X-ray diffraction (XRD) [4.9]' extended X-ray absorption fine structure
100
4. Properties and Characterization of Dielectric Thin Films
Table 4.1. Problem-oriented analysis
Film properties
• Macroscopic
-
Density
Stress
Adhesion
Optical quantities (n, k)
Scattering
Hardness
Thickness
Thermal conductivity
• Microscopic
- Composition
- State of oxidation
- Structure
- Roughness of surfaces
- Formation of interface
• Electrochromic
Methods
GIXR, RBS
Bending
Micro indenter
T,R,A
TIS, ARS, PTD
Milling, etc.
R, T, A, TEM, GIXR, stylus
PTD
SIMS, IBSCA, SNMS, RBS, NRA,
ESCA, AES, EDX, WDX, GIXF
ESCA, AES, SIMS, XANES
XRD, ED, Raman, IR, SAXS, EXAFS
AFM, TEM, GIXR, Microscopy
(Normarski, Fizeau, Mirau)
TEM, GIXR
CVG, TVG, VMG, QCM
(EXAFS) [4.7], X-ray absorption near-edge fine structure (XANES) [4.7] and
in electron spectroscopy for chemical analysis (ESCA) [4.10,11]. Electrons
are the probe in transmission electron microscopy (TEM), energy dispersive
X-ray analysis (EDX) , wavelength dispersive X-ray analysis (WDX) , electron diffraction (ED) [4.12] and Auger electron spectroscopy (AES) [4.10,11].
Ions are applied as primary particles in secondary ion mass spectroscopy
(SIMS) [4.13]' secondary neutral mass spectroscopy (SNMS) [4.14]' ion beam
spectrochemical analysis (IBSCA) [4.15,18]' nuclear reaction analysis (NRA)
[4.16,18], and Rutherford backscattering (RBS) [4.16,18]. Electrochemical
techniques, which are applied for investigating electrochromism, combine
electrical quantities such as current C or voltage V with changes in film
properties. Most commonly used are current voltage graphs (CVG), transmittance voltage graphs (TVG) [4.19], voltamassograms (VMG) and quartz
crystal microbalance (QCM) techniques [4.20].
The different methods of thin film analysis can support the development
of thin films, the optimization of processes and the control of production and
can help in trouble-shooting. For this type of work a special approach is necessary, because the instruments used in general are fairly expensive and because
the analysis of thin oxide film is highly complex and time-consuming [4.17].
Therefore, the definition of the object of investigation, for example the selection of the appropriate film parameter, and the characterization with the
method of the suitable analytical features, form the basis for problem-oriented
analysis. Only well-defined problems and samples allow an efficient applica-
4.1 Surfaces of Substrate Glasses
101
tion of the different techniques. A selection of samples, classified according
to their significance, is the starting point for a successful investigation. Moreover, a critical assessment of the potentials of the available methods with
regard to the defined working hypothesis and the problems to be investigated is indispensable for a problem-oriented analysis; the selection of the
most powerful instrument is one of the key problems to be solved. This approach, which is typical of industrial thin film labs, is illustrated in Table 4.l.
Some characteristic film properties are connected with methods that give the
desired information in principle [4.17]. Obviously, different methods can be
applied to investigate a certain problem. The film composition, for example,
can be analysed by SIMS, IBSCA, SNMS, RBS, NRA, ESCA and AES, but
other, more sophisticated techniques can be used as well.
Each method has its merits, but also its shortcomings. Both should be
known in detail to select the appropriate method for a special object of investigation. The demands on the methods are fairly high. For a sufficient characterization of thin oxide layers, the techniques should detect all elements with
high lateral resolution, a well-defined and preferably low information depth,
and high element sensitivity. In addition, they should give information about
the state of oxidation and the binding state of elements, and should work
non-destructively. Often a quantitative determination of elements and compounds is desired, i.e., the matrix effect should be negligible. Owing to the
high complexity of oxide layers, a multi-method approach is necessary for an
accurate description of these coatings [4.17].
4.1 Surfaces of Substrate Glasses
Klaus Bange
Homogeneous substrate surfaces are an essential precondition for defect-free
coatings, because polishing defects, for instance, can act as preferred nucleation sizes, and insufficient cleaning may influence the adhesion. Therefore,
checking the polished glass surfaces prior to coating is essential, especially
for high-quality optics and optical coatings. A variety of suitable methods
for surface inspection are available to detect topographical defects induced
by surface roughnesses such as scratches, digs, inclusions and spatters. Very
sensitive instruments are necessary because polished optical surfaces can have
roughnesses of less than 0.1 nmrms. The suitable methods differ from each
other regarding the useful magnification and the time needed for application.
The visual inspection of polished glass surfaces is still common practice
with optics producers. This method is insufficient for high-quality optics because the resolution is very limited. It can be improved by dark field illumination, which makes visible the light scattered by surface inhomogeneities
such as scratches, digs or residues. A certain improvement is obtained if
the inspection is done by a microscope with a 25-50-fold magnification. The
102
4. Properties and Characterization of Dielectric Thin Films
use of polarized light and the interference effect in microscopy also enhance
the visibility of small defects; differential interference contrast (Normarski)
microscopy [4.4] exhibits a particularly high vertical resolution, even at relatively low magnification (100 x), which gives a reasonable field of view.
Contact and non-contact methods can be used for the measurement of
surface roughness. All contact instruments measure the surface profiles with
a diamond stylus probe that touches the surface. Variations in height are
measured by moving either the stylus or the surface. Optical (non-contact)
surface profilers make use of a microscope interferometer, such as Mirau (common path), or Fizeau (fringes of equal chromatic order), and usually require
a test sample with optical constants that do not vary over the area being profiled, because differences in the phase change on reflection will be converted
into height differences [4.4]. Most of the methods mentioned are used by optics producers for the routine inspection of polished surfaces, because their
average time for application is relatively short. A problem of this technique is
the lateral resolution, which lies around 1 J..Lm for optical profilers and around
0.2 J..Lm for contact profiling instruments.
By other methods, which usually are not used for routine surface inspection, average values for the roughness can be obtained, and surface profiles
can be measured with a very high lateral resolution. Grazing incidence X-ray
reflectometry (GIXR) allows one to investigate the surface roughness of large
areas and has the advantage of giving additional information on the modified surface layers, for example on thickness and density [4.21,22]. Scattering
measurements such as total integrated scattering (TIS) and angle-resolved
scattering (ARS) are also well suited for measuring the surface roughness,
because the conversion to rms roughness is straightforward [4.4]. The spot
size of the laser limits the lateral resolution of these techniques.
The methods that are applied to determine the position of local irregularities and the type of the defect should be surface sensitive for the detection
of the uppermost atomic layers and should be able to detect the surface topography of selected areas (windows) with a high lateral resolution. Electron
microscopy techniques can be used for surface inspection, and information
on the nm scale can be obtained on replicas of the surface by TEM [4.23].
Glass surfaces can also be inspected by the fairly new method AFM. A direct
examination of the surface is possible with AFM measurements, which can
be realized in air without special surface preparation. This keeps the necessary time for investigation of a particular glass surface relatively low with the
advantage of very high lateral resolution [4.24].
The capacity of AFM is demonstrated in Fig. 4.1 on two differently
polished Schott BK 7® glasses. The pseudo-three-dimensional representation
shows that the average roughnesses are 0.98 and 1.23 nm. In the surface with
the higher rms roughness, the traces of polishing seem to be deeper, and small
particles, possibly residues of the polishing material, are to be found on top
of the surface [4.24].
4.1 Surfaces of Substrate Glasses
10
103
t
E
o
I
5
+
E
c:
I
o
Fig. 4.1. Low resolution AFM images on differently polished Schott BK 7® glass
surfaces
Optical glasses in particular are prone to form surface layers during polishing and cleaning procedures when they are in contact with aqueous solutions.
In general such layers are unwanted because they usually appear as corrosion
or staining of glass surfaces, as they can degrade its chemical, mechanical and
optical properties considerably. The interaction between the glass surface and
the cleaning solution is fairly complex; it depends on the glass composition,
the pH-value of the solution, the temperature, the time and so on. In principle
104
4. Properties and Characterization of Dielectric Thin Films
six different types of surface layers on glass surfaces are reported [4.25], which
are induced by the interaction with a surrounding medium. Powerful instruments are necessary to control the influence of the cleaning procedure because
the chemical stability of glasses differs widely and thermodynamical calculations for the multicomponent systems give only a rough approximation. The
leaching process of glass layers can be controlled by spectral reflectance measurement if the effect is strong enough. But in general only some few nm in
the surface region are chemically changed; therefore surface sensitive methods are necessary for the investigation of the glass surface. Hydrogen depth
profiles of very thin surface layers can be quantitatively analysed by NRA.
For the other elements the changes in the composition of the glass surface region are obtained by RBS. ESCA and AES also give quantitative information
about the chemical composition of the glass surface and the bulk glass. SIMS
and IBSCA can be used to detect changes in composition of less than 1% because they are highly sensitive to the different chemical elements. Extensive
investigations on leached glass surfaces are reported in [4.26].
4.2 Macroscopic Properties of Thin Films
Klaus Bange, Clemens Ottermann
The functionality and reliability of coated devices are often strongly related
to the macroscopic features of thin oxide films. The optical properties of
the respective materials, for example, are directly used for the production of
interference systems. Mechanical characteristics of thin films such as stress or
adhesion also directly influence the product qualities. Various analytical tools
which are available for an accurate analysis of the macroscopic properties of
thin films will be briefly reviewed in this section. Typical results obtained by
these methods are given in Sect. 4.4.
4.2.1 Density
It is generally accepted that most deposition techniques create film densities that deviate strongly from the bulk density of the respective material [4.18,27]. Gas incorporation or crystalline disorder may produce holes
and pores in the films and an increased number of vacancies in the crystals
reduces the film density. Differences in the film densities can influence various film properties such as the refractive index, film adhesion, crystallization
behaviour, stress and also the chemical stability. Problems can arise in films
with a reduced density especially when they are used at temperatures where
water desorption and adsorption can occur [4.28]. This mechanism can lead
to the destruction of thin film systems [4.18].
The density of thin films can be estimated by a number of different techniques.
4.2 Macroscopic Properties of Thin Films
105
• The refractive index n at visible wavelengths is a reasonable indicator of
the film density of pure, stoichiometric and undoped materials. In Ti0 2
films that are prepared by different techniques and consequently have different densities, for example, the refractive index n can vary by as much
as 0.4 [4.29].
• Indirect and qualitative indicators of the film density are the hydrogen content determined by NRA with the 15N technique [4.30] and the infrared absorption of oxygen-hydrogen groups whose concentration is strongly linked
with n [4.31]. Although it is difficult to quantify these measurements, they
make it possible to detect the penetration of water or oxygen-hydrogen
groups into the pores of less dense films.
• A fairly accurate value for the film density can be calculated from the
mass of the film, monitored by an oscillating quartz during the deposition
process, and the film thickness, measured by a stylus technique or by an
optical technique. The error typically is about 10%.
• The film density can be quantitatively determined by RES [4.32,33]' provided that the precise stoichiometry (also hydrogen content) and the film
thickness are known. This is not guaranteed in general for thin oxidic films
on glass.
• The density of thin films can be determined with high accuracy by means
of GIXR, as has recently been demonstrated [4.22]. When X-rays cross
the interface between two media, they are refracted, according to Snell's
law, just like visible light, but the index of refraction n for hard X-rays
is slightly smaller than 1 and complex [4.34]. For that reason, X-rays are
refracted away from the surface normal when propagating from air into
matter and there exists a critical angle, at and below which total external
reflection occurs. For most materials this angle is below 0.5 0 in the hard
X-ray range. So the determination of the critical angle gives the mass
density of the reflecting medium with the accuracy for the density being
determined by the accuracy of the angle of incidence [4.5,6,22].
Figure 4.2 shows the reflectivity on a logarithmic scale versus the angle
of incidence for Ti0 2 layers deposited by reactive evaporation (RE) and ion
plating (IP) with film thicknesses of 120 nm and 95 nm, respectively; the deposition parameters are given in [4.18,22]. The different critical angles show
that the IP layers are denser than the RE layers. The experimental data
can be simulated in principle by one homogeneous layer. However, the fit is
qualitatively better if an additional layer is assumed between the substrate
and the Ti0 2 film for RE, and on the film surfaces for IP. Typical values of
fit parameters for the Novot-Croce model are given in Table 4.2. The inhomogeneity in density must be considered for films deposited by the different
techniques.
Apparently different reactions take place at the substrate/film interface
and on the film surface, depending on the deposition conditions. The depth
density profiles of Ti0 2 -coated glass, illustrated in Fig. 4.3, are character-
106
4. Properties and Characterization of Dielectric Thin Films
10°
10-1
z..16 2
tl
~163
.:;
•
0::
10-4
165
0.5
0
1.0
Angle of ni cidence
~
1.5
Ideg
Fig. 4.2. Reflectivity of 8000eV photons from Ti02 films deposited by ion plating (upper curve) and reactive evaporation (lower curve) . The fit with the solid
line assumes transition layers with six adjustable parameters, whereas for the fit
characterized by the dashed line no transition layer is assumed
Table 4.2. Fit parameters for the reflectivity (density p in gcm- 3 , roughness a
in nm, thickness 8 in nm) of Ti02 films deposited by ion plating (IP) , reactive
evaporation (RE) , dip coating (DC) on Schott BK 7®
Sample
Ti02/IP /
Schott BK 7®
Ti02/RE/
Schott BK7®
T i02 /DC/
Schott BK7®
Substrate
Intermediate
layer
p
p
a
2.49
0.99
2.49
0.42
3.82
4.4
2.49
1.06
3.69
3.7
8
Main layer
Surface layer
p
8
a
p
8
a
3.93
92.27
0
3.79
3.6
0.87
0
3 5. 7
110.8
2.24
2.8
3.23
71.3
2.22
3.74
3.7
0.68
a
ist ic of IP, RE and dip coating (DC) and demonstrate the influence on the
techniques.
4 .2.2 Optical Properties of Coatings
Thin film materials are optically characterized by the extinction coefficient
k and the refractive index n, whereby the extinction coefficient is given by
the imaginary part of the complex refractive index. Generally, the optical
properties of thin films are somewhat different from those of the respective
bulk materials. The observed values of the refractive indices are usually lower
and the extinction coefficients are higher than the optical constants of the
4.2 Macroscopic Properties of Thin Films
107
5
IP
-......
4
RE
3
BK7
'"E
()
~
a.
?;>
./
DC
J
/
2
Ti02
·iii
c
<1l
0
3-4 nm
o
o
t
Glass substrate
f}-4 n~)
·1
Film thickness /a.u .
t
t
Bulk layer
t
Surface layer
Intermediate layer
Fig. 4.3. Density profile of Ti0 2 layers on Schott BK 7® glass deposited by ion
plating (IP), reactive evaporation (RE) and dip coating (DC)
same bulk material. Additionally, the resulting film properties are strongly
influenced by the deposition method and the selected deposition conditions.
The reproducibility of the optical constants is highly important for reliable
production.
The essential feature of dielectric optical films is their desired low absorption in the relevant region of the spectrum. For that reason the films are
usually characterized by their transparency and by the refractive index. The
transmission-versus-wavelength spectra of optical thin films may be divided
into three parts. The desired and necessary region of high transmittance is
located between the short-wavelength absorption edge, which depends on the
electronic structure of the material, and the long-wavelength limit, which is
determined by lattice vibrations or, in semiconducting films, by the free carrier absorption. The quality of the transparent region depends strongly on
the material itself, especially on its stoichiometry and impurity, which may
cause absorption.
108
4. Properties and Characterization of Dielectric Thin Films
The transparency of thin films is often slightly lower than that of bulk
materials and is strongly related to the deposition conditions. A frequent reason for the increased extinction of the films is the true absorption induced
by small deviations from stoichiometry or by contamination. Another reason
is the light scattering induced by surface and volume imperfections such as
surface roughness, internal grain boundaries, density fluctuations originating
from crystallinity, porous microstructure, pinholes, cracks, etc. The contribution of light scattering and optical losses can be given by
I=R+T+L
with
L = A+S,
(4.1)
where L is the loss, A the absorption, S the scattering, R the reflectance,
and T the transmittance.
In the visible range, the refractive index n of a material is mainly determined by the polarizability of the valence electrons and the density according
to the Lorentz-Lorenz theorem [4.38]
n 2 -1
47ra m NA
n2 + 2 = -3- M P ,
(4.2)
where am is the molecular electric dipole polarizability, NA is Avogadro's
number, M is the molecular weight, and p is the mass density of the material. The refractive behaviour of thin oxide films is also influenced by the
composition, impurities or doping, crystallinity and by the type of bonding.
Several models have been developed to explain the effective refractive index of mixed films, i.e., composition of different materials or a mixture of
solid materials and voids, which differ in their basic assumptions on structure and kind of composition. Besides the Lorentz-Lorenz law, the models of
Maxwell-Garnett, Bragg-Pippard, and the effective-medium approximation
by Bruggeman are often used to describe optical thin films [4.39-43].
The general physical background for the interaction of light with layered structures, i.e., with coatings on substrates, has been extensively described [4.44-49]. Based on these principles, a cornucopia of different methods has been developed to analyse nand k of optical transparent films with
low scattering losses [4.50,51]. Besides the Brewster angle determination and
the minimum deviation technique, the following methods are most common:
• Light propagation in waveguiding structures depends on the effective refractive index neff of the substrate/film/superstratum system. neff is different for the different polarization modes (TE or TM) of the incident light
and can be obtained from the respective coupling angles [4.49]. The intrinsic refractive index and the thickness of the film can be deduced from
the measured neff values by taking into account the well-known indices
of the substrate and the superstratum. The coupling angles are precisely
determined by prism couplers [4.52] or grating coupling techniques [4.51].
These techniques are also capable of determining anisotropies in the optical properties of the films [4.54]. The combined absorption and scattering
4.2 Macroscopic Properties of Thin Films
109
losses are obtained from the attenuation length of the light in the waveguiding film [4.49,194J.
• Spectroscopic ellipsometry is also an important method for analysing the
optical properties of thin films. A circularly polarized incident beam will
be reflected from the considered surface with elliptical polarization according to the Fresnel formula [4.45J. The refractive index n, the extinction
coefficient k and the film thickness d can be deduced from the complex
amplitude ratios of the reflected electrical field to the incident electrical
field parallel or normal to the plane of incidence by taking into account
a model for the layer structure. Spectroscopic investigations are necessary
because nand k both depend on the wavelength and their dispersion must
be known for most optical applications. Spectroscopic ellipsometry with
additional variation of the angle of incidence is a powerful tool [4.55J for
analysing multilayer systems or coatings with inhomogeneous refractive index profiles [4.56J or anisotropies [4.57], very thin films with thicknesses of
approximately 1 nm [4.58J, or opaque films on opaque substrates [4.59J.
• Optical properties of films of unknown thickness can also be deduced from
the transmittance T and reflectance R. To determine the three film properties d, nand k at a given wavelength, the measurement of three independent
quantities is usually necessary. In some special cases (e.g., absorbing film
on non-absorbing substrates) the transmittances and the reflectances, measured from the coated side (film side: Rr) and the substrate side (reverse
side: Rb) of the sample under normal incidence, are sufficient [4.60J. For
oblique incidence of polarized light, these quantities may also be deduced
from reflectance [4.61J and transmittance [4.62J.
• Spectrometry is the fastest method to obtain the optical properties of films
used in interference systems for the visible or near-infrared spectral region.
These films possess in general thicknesses of the order of 100 nm (quarter wavelength), and the transmittance or reflectance spectra exhibit pronounced interference patterns, as shown in Fig. 4.4. The film thickness d
and the extinction coefficient k can be deduced from the spectral positions
of the interference extrema, from the difference between the maximum and
minimum T (or Rf, R b ) values, and from the deviation in respect to the
transmission of the uncoated substrate, whose optical properties should be
well known [4.63J. A variety of different methods has been developed for
deducing the optical film properties [4.50,64, 65J. However, analytical descriptions of the dispersions are needed for the determination of the spectral
dependencies of n(,X) and k('x). Typical dispersion formula for oxidic media
such as Cauchy, Sellmeier or Forouhi-Bloomer expressions and the corresponding polynomial expansions are generally accepted [4.47,50, 66-70J.
Fitting procedures are used to determine the corresponding parameters of
the dispersion functions [4.66,71, 72J. In this case the transmittance and/or
reflectance spectra of the samples were calculated by assuming a model for
their layered structure and taking into account the known properties of the
no
4. Properties and Characterization of Dielectric Thin Films
1.0 - . - - - - - - - - - - - - - - - - - - - - - ,
~7\7'\-;:;:--~:--------(C)
0.8
(a)
Ia>
c 0.6
c.>
ro
::::
·E
(/l
c 0.4
~
I-
0.2
~---(b)
0
300
500
700
Wavelength "A Inm
900
1100
Fig. 4.4. Spectral transmittance T of a 730-nm-thick W03 film on soda-lime glass
sheets in (a) transparent and (b) coloured state; (c) represents the transmittance
of the uncoated substrate
substrate material. The parameters Pi are optimized by fitting to obtain a
minimum deviation between the experimental and the calculated spectra:
[Tca1 ()..,Pl,P2,P3, ... ) - Texp()..)F = minimum,
and
[R cal()..,Pl,P2,P3, ... ) - Rexp()..W = minimum.
(4.3)
Spectrometry is not only well suited for the determination of the optical
properties of single films, but also enables the analysis of multilayer systems [4.66,68]. Work is in progress to extend the field of application to the
analysis of rough and inhomogeneous films [4.62,73].
The following dispersion approximations are sufficient for film materials
used in interference filters or electro chromic applications [4.66,68]:
+ aI/)..2 + a2/)..4 + ... ,
1 ).. + bo + bI/).. + b2 /)..2 +... .
n 2()..) = ao
k()")
=
L
(4.4)
Three-parameter approximations for nand k are adequate for lowabsorption materials with normal dispersion. Additional parameters are
needed for more strongly absorbing films with anomalous dispersive behaviour [4.66]. These polynomial coefficients and the geometrical film thickness d are the only parameters describing the spectral behaviour of Rand T
that can be calculated from the matrix formalism given by Macleod [4.48] or
from the respective expressions deduced for single layers [4.73]. These parameters can be determined by a minimization of the following figure of merit
by using a least-squares-fit routine (i.e., a modified Levenberg-Marquardt
algorithm):
4.2 Macroscopic Properties of Thin Films
x2 =
L [Texp(Ai) - Tcal(Ai, d,Pl,P2,'" )]2
+ L [Rexp(Ai) - Rcal(Ai, d,PbP2,'" )]2
111
Ai
(4.5)
Ai
In Fig. 4.5, X2 is depicted as a function of the thickness d, which is kept
fixed in the evaluation of the transmission spectrum of the transparent W0 3
film in Fig. 4.4. A clearly pronounced minimum is observed for the film thickness of 730 nm. Here the X2 value is more than two orders of magnitude
smaller than for the other minimum structures in the plot. The side minima,
originating from descriptions of the T interference pattern by layer thicknesses, possess a wrong interference order for the light propagating through
the film [4.18]. Uncertainties for the fitted optical film properties can be obtained for the error matrix of the fitting routine. Typical uncertainties are
dd = ±1 nm and dn(A) = ±0.01 in the visible spectral region for 100-nmthick Ti0 2 films.
The stability of the procedure for a clear determination of the optical
properties in the visible region depends on the film thickness and the difference in refractive index between film and substrate material [4.18]. For
highly refractive materials such as Ti0 2 , film thicknesses above 70 nm suffice, whereas for Si0 2 thicknesses above 120 nm are required. The lower limit
for the exact determination of absorption is about k = 10- 3 , depending on
the film thickness, the quality of the absolute transmission measurement and
the correct knowledge of the optical properties of the substrate.
Coatings optimized for high-energy radiation (e.g., laser mirrors) possess absorption properties beyond the detection limit of the spectrometric
100.00...--------------------,
10.00
!:!::
1.00
0.10
0.01+-----...------.------,---'
800
1000
400
600
Thickness d Inm
Fig. 4.5. X2 divided by the number of measured points F as a function of thickness
d, which is kept fixed in the evaluation of the spectral transmittance of the 730-
nm-thick transparent W03 film (see Fig. 4.4)
112
4. Properties and Characterization of Dielectric Thin Films
method. Today further advanced techniques, namely laser calorimetry (LC)
[4.37,75J and photothermal deflection (PTD) [4.35,36,76,77]' are available
to measure even small absorption and to distinguish between surface and
interface losses. Because both methods use high-power lasers for the absorption measurements, the absorption coefficient k can be only determined for
the given laser wavelength. In addition, substrates possessing extremely low
absorption (e.g., fused silica) are required for both methods.
For LC the coated surface area of the samples is irradiated by a laser
beam. The temperature of the sample increases due to the absorption of the
film. The determination of the temperature rise is an absolute measure for
the absorption loss. LC is a stationary technique. Therefore the measurement
of low absorption values is very difficult because the necessary thermal equilibrium requires an effective shielding from the thermal fluctuations in the
surrounding, i.e., the samples must be isolated by high vacuum chambers. In
addition, measurements with LC are time-consuming because it takes a long
time for the equipment to reach thermal equilibrium conditions [4.37,75J.
PTD is a time-modulated technique that requires less effort in shielding,
allows a fast exchange of samples and possesses high spatial resolution. The
probe is irradiated by a pulsed laser beam focused on the film with high intensity. The energy absorbed inside the coating induces a temperature gradient
and thus a refractive index gradient. Both are modulated with the repetition
rate of the laser. The index gradient can be measured by the deflection of a
second probe laser with low intensity. The deflection angle is proportional to
the absorbed optical power [4.76J. Because PTD is only capable of a relative
determination of the absorption losses in films, samples investigated by LC
are needed for an absolute calibration of the deflection angles.
Absorptive losses in films originate from losses in the bulk material and
from contributions of the substrate-film and film-air interfaces. The contributions can be distinguished by the method described in [4.37J, which takes
into account the local variations of the electromagnetic field of the laser light
propagating through the film. It requires the investigation of four films produced under the same deposition conditions, but with a difference of AI4 in
thickness. The different contributions are deduced by comparing the absorption values obtained from these samples [4.37, 78J.
Light scattering is the other loss factor that defines the quality of thin
films for high-quality optical coatings. Volume imperfections and interface
roughnesses are the origins of scattering losses [4.79J. Two methods are widely
used for their determination: total integrated scattering (TIS) and angleresolved scattering (ARS) [4.78,80, 81J. Both methods usually involve a laser
beam which is focused on the sample under normal or oblique incidence. Light
that is backscattered by the sample possesses an angular distribution. ARS
investigations measure the angular dependence of the scattered intensity,
whereas the total backscattered intensity is measured in TIS arrangements
which use integrating optical spheres [4.78, 80J.
4.2 Macroscopic Properties of Thin Films
113
ARS is sensitive to the surface roughness of the samples. Its correlation
length can be deduced from the angular dependence of the scattered light.
The TIS value is directly proportional to the square of the rms roughness
of the surface [4.80]. Both quantities are essential for the rating of polishing
processes and for the characterization of growth behaviour of films deposited
under different conditions. In addition, TIS measurements can be used to
separate volume and surface scattering contributions. In this case, a sequence
of films, which are produced under identical conditions but differ in thickness
by ..\/4, have to be investigated [4.78].
Finally, the spectral position of the fundamental absorption edge is also
of interest for the optical characterization of thin films. It is determined by
the band gap energy E g , which depends on the specific composition and on
the band structure of the coating material. Eg can be deduced from spectral
transmission behaviour by the method described by Tauc [4.82]. Some dispersion models are also capable of determining the band gap energy [4.67,83].
4.2.3 Electrical Conductivity
The oxidic materials used for dielectric interference filters are usually insulators. ilansparent and conductive coatings are required for applications such
as liquid-crystal displays or electro chromic windows and mirrors. ilansitionmetal oxides, deposited under specific conditions, meet both these conditions.
Films consisting of tin oxide, indium oxide or tin-doped indium oxide (ITO)
are widely used because these materials possess high transparency in the visible spectral region combined with a fairly low electrical resistivity. Moreover,
these materials are of interest for heat energy and solar energy conversion because of their high reflectivity for infrared light. The oxides of other transition
metals such as W, Ni or Ti are highly insulating in their highest oxidation
state, but show remarkable conductivity in a substoichiometric composition.
Several models, originally developed for amorphous semiconductors, are
used to describe the conducting mechanism of these materials [4.84]. The most
popular models are tunnelling and hopping mechanisms such as quantummechanical, atomic and polaron tunnelling, atomic hopping or correlatedbarrier hopping, or charge-density waves; reviews of these models are given
in [4.85-90]. These models can be distinguished by the different frequency
and temperature dependencies which they predict for the film conductivity.
Therefore the different explanations can be classified by ac measurements
of the conduction properties as a function of frequency (impedance analysis [4.91]) at different temperatures. The investigations usually require quite
extended frequency and temperature ranges for a clear distinction [4.84].
With respect to application, the dc resistivity of conducting films is of
major interest; it is expressed by the sheet resistance of a layer Ro = p/d,
where d is the film thickness and p is the specific resistivity of the film
material. Resistivity measurements are inherently geometry-dependent and
sensitive to boundary conditions. Two techniques for the determination of
114
4. Properties and Characterization of Dielectric Thin Films
sheet resistivities are commercially available and widely used: Van der Pauw's
method [4.92-94] and the four-point probe [4.95,96]. The basic principle of
both methods (i.e., the application of current and the measurement of voltages with four contacts) is similar, but their geometrical arrangements differ.
Consequently, for the determination of Ro several corrections of the measured results are necessary, depending on the geometrical conditions of the
samples and on the testing equipment [4.94,96]. The Hall mobility in thin
films [4.93] can also be determined with Van der Pauw's method due to the
two-dimensional arrangement of the electrical contacts in this technique.
4.2.4 Mechanical Properties
The mechanical properties of thin films on substrates are different from those
of bulk materials because of their unique microstructure and density, the large
surface-to-volume ratio, the reduced dimensions and the constraints caused
by the substrate. A thorough understanding of the processes that control
these properties is important for a variety of scientific and technological applications. Various aspects of the mechanical properties have been reviewed
so far [4.97,97-99]. In particular, the mechanical properties of optical coatings must be adequate to guarantee mechanical stability and integrity for
their period of use. To fulfil this demand, sufficient knowledge about the
origins and relationships of mechanical film properties is needed in order to
ensure their control during the production process. The following mechanical
properties are of interest for thin films:
•
•
•
•
•
stresses,
hardness and plastic properties,
elastic moduli,
interfacial adhesion strength, and
fracture mechanics.
Mechanical and structural film properties depend strongly on deposition conditions and are additionally influenced by substrate parameters such as chemical composition, surface roughness and crystallinity. To understand the mechanical behaviour of optical thin films, it is therefore necessary to measure
their mechanical parameters directly on their substrates.
Stresses
Nearly all inorganic and metallic compound films are in a state of stress.
The most important stress for oxide film/substrate systems is the interaction
stress (also frequently referred to as "internal" or "intrinsic" stress; see [4.99]
for nomenclature). This kind of stress results directly from the interaction
between the film and the substrate upon which it is deposited. The film
itself would be stress-free after it is removed from the substrate, i.e., the film
4.2 Macroscopic Properties of Thin Films
115
possesses no internal stress components or gradients to curl the detached
layer. The common sign convention is positive for tensile stresses and negative
for compressive stresses.
Interaction stresses will arise in any film/substrate system in which the
equilibrium dimensions of the film and the substrate change relative to each
other after separation. Such virtual dimensional changes can occur during
the deposition (Le., growing conditions) or after the deposition (Le., thermal strain or structural changes). If the strain c of the film attached to the
substrate is c = dl/l (relative change in length), then its stress O"f can be
calculated from
O"f
=
(~)
c = Mfc ,
1- Vf
(4.6)
where Mf = ErI(l - vr) is the biaxial modulus of the film material with
Young's modulus Ef and Poisson's ratio Vf, which in general are not well
known.
The temperatures during the deposition of oxide layers usually differ from
those at which the films are used. In addition, many coated systems are applied in a wide temperature range. If the film and the substrate have different thermal expansion coefficients, a temperature-dependent strain dc th will
cause an additional stress component in the films, the so-called thermal stress
dO"th. If the bending of the substrate is negligibly small, the strain and the
corresponding stress arising from thermal expansion may be calculated from
dO";
h
=
Ef
1- Vf
- - d ct
h
= -Ef1 - Vf
lTD (af(T) - as(T))dT ,
TM
(4.7)
where To is the reference temperature (i.e., the substrate temperature during deposition), and TM is the momentary temperature of the system. af(T)
and as(T) are the temperature-dependent expansion coefficients of the film
and the substrate, respectively. A positive strain dc th corresponds to a tensile stress. If the temperature dependencies of the expansion coefficients are
negligibly small in the temperature range dT = (To - TM), then dO"th can be
calculated using [4.100]
dO"fth = -Ef- ( af - as ) dT .
1- Vf
(4.8)
The total stress of the film/substrate system comprises the interaction
stress obtained during production at deposition temperature and the thermal
stress component. The forces originating from film stress are superimposed
on the whole film/substrate system and bend the substrate elastically. The
resulting change in the substrate curvature strongly depends on its geometrical shape; two types are especially interesting and widely used: the beam
(or cantilever, a long narrow strip) and the disk.
116
4. Properties and Characterization of Dielectric Thin Films
A disk will adopt the shape of a spherical cap if the film stress and the
elastic properties of the round-shaped substrate are isotropic. Usually the
film thickness d is much smaller than the substrate thickness ts. On that
condition, a relationship (Stoney's equation) between the biaxial stress af in
the film and the resulting radius of curvature Rs of the film/substrate system
can be deduced by a simple biaxial bending formalism
(4.9)
where Ms = Es/ (1- vs ) is the biaxial modulus of the substrate material with
Young's modulus Es and Poisson's ratio Vs. Ms generally is well known or easy
to measure and no information about the elastic properties of the film material is required in this relation. If the stress varies in the thickness direction of
the film, then af represents an average film stress. For coatings consisting of
several layers, af is the thickness-weighted average of the stresses in all films,
as long as the total coating thickness is still small compared to ts. For the
bending of a beam-shaped substrate a similar relationship can be deduced
with the same dependencies on the film and substrate properties [4.101]. Additionally, the strain of the film due to the interaction with the substrate
also changes the strength of the inter-atomic bonds in the film materials
and the lattice parameters of crystalline materials. These changes in the film
and/or substrate properties can be used for stress determination. Various
methods for stress measurement have been developed. The most popular are
the bending-substrate methods (beam or disk), but Raman spectroscopy and
X-ray diffraction (XRD) are also used.
Several techniques are used for the detection of the substrate bending and
the evaluation of its radius of curvature. They consist either in mechanical
pick-up systems or in optical and electrical pick-up systems, which have the
advantage of combining non-contact measurements with high sensitivity and
enable in situ investigations of film stresses [4.101, 102]. Therefore, these techniques are widely used for detecting capacitive changes or deviations in the
direction of reflected laser beams, or for evaluating surface topographies by interference measurements. Film stress can be evaluated only uni-directionally
with the bending-beam technique. Biaxial stress investigations are possible
with the bending-disk method, which enables the detection of anisotropic
stress distributions [4.102] due to preferential orientations in film structures
or in physical properties.
Changes induced in the lattice potential energy of crystalline films by pressure or temperature are manifested by variations in vibrational frequencies
resulting from lattice expansion or contraction. The temperature-dependent
variations in the vibrational frequencies of thin films often differ from those
observed in the respective crystalline bulk material. Alternations in the microstructure of the film will also influence the intrinsic vibrational frequencies
under an applied stress [4.103]. Raman investigations possess the sensitivity
4.2 Macroscopic Properties of Thin Films
117
and dynamics to measure these influences on vibrational frequencies. But
extensive temperature-dependent and pressure-dependent measurements on
bulk materials are needed to calibrate the observed frequency shifts [4.103].
The strain in thin crystalline films can also be detected by X-ray diffraction. A deviation of the lattice parameter a from the respective bulk value
ao establishes the strain. The stress is then calculated from the elastic constants of the film and the geometry of the experiment. For example, the usual
diffractometer geometry is widely employed to measure the spacing of planes
parallel to the substrate. The stress can be calculated from
E£ ao-a
1 - 1/£ ao
a£=----- ,
(4.10)
where E£ and 1/£ are the Young's modulus and the Poisson's number ofthe film
material and ao and a are the unstrained and strained lattice parameters. The
X-ray technique is also capable of measuring anisotropic and triaxial stress
distributions in crystalline thin films [4.104].
Oxidic coatings for optical purposes are often amorphous and have film
thicknesses between 40 and 100 nm. In this case, stress determination by
XRD or Raman spectroscopy is quite difficult. The investigation of thick layers is easier. But it has to be taken into account that stress depends on film
thickness for oxide layers [4.106]. Therefore bending-substrate techniques, especially bending-disk methods combined with interference-optical measurements of the substrate curvature, are superior to other techniques and are
widely used for stress determination of optical films [4.105].
For the bending-disk technique, surface topographies of round-shaped
substrates are measured and analysed before and after coating by a ZYGO
Mark IV interferometer system [4.105]. The interference fringe patterns are
analysed by an image processing algorithm, and the corresponding information about the surface curvature is transferred to a PC system. The difference
between these data sets shows the influence of the additional film stress on
the substrate curvature. Figure 4.6 gives typical results for Ti0 2 films under
tensile and compressive stress. These curvature profiles can be described by
a two-dimensional polynomial with the following parametrization:
where z(x, y) is the height of the profile at the location (x, y), ao contains an
arbitrary off-set, and b1 and b2 are parameterizations of a plane corresponding
to adjustment uncertainties of the samples. The parameters Ci and di contain
information about the shape of the resulting substrate bending, i.e., the uniformity and anisotropy of the film stress. The curvature possesses rotational
symmetry under the ideal condition of an isotropic stress distribution on a
round-shaped substrate
(4.12)
118
4. Properties and Characterization of Dielectric Thin Films
Compressive
o
10
20
30
40
Tensile
50
Coordinate x Imm
Fig. 4.6. Typical results for substrate b ending (fused silica, 1mm thick, 48 mm in
diameter) due to (a) compressive stress and (b) tensile stress in lOO-nm-thick Ti0 2
films
The radius of curvature can be approximated to b e R = 1/(2c) for slight
substrate bending. The film stress is determined according to (4.9) , taking
into account the film thickness determined by optical means described in
Sect. 4.2.2. The uncertainties in stress determination, which are deduced
from reproducibility and stability studies, are smaller than 10 MPa for a 100nm-thick film on fused silica substrates with a thickness of 1 mm.
The stress of a layer may not stay constant in time. Reorientations and
diffusion processes in the film material will alter the microstructure of the
film and cause a relaxation of its strength. Moreover, the stress in oxide layers
is known to be influenced by post-deposition temperature treatment and by
water absorption from the humidity of the ambience [4.105-lO7].
Hardness and Plastic Properties
The most common t echniques for the measurement of hardness and plastic/elastic properties of sub micron thin films and multilayers on substrates
are microindentation and nanoindentation investigations. These methods define hardness as the applied load, divided by the area of the remaining indentation. The experimental techniques, which are highly sophisticated, up
to now are the only means to test elastic, plastic, and fracture responses on a
fine scale with high spatial resolution. Nanoindentation techniques are attractive for the evaluation of optical films because the test depth can be smaller
than the typical coating thickness and the coated system can be investigated
in situ.
A nanoindentation set-up is described in detail in [4.99]. The displacement
of the indenter h as a function of the applied load P is the result of an
indentation experiment. In Fig. 4.7 the relation between hand P is shown
schematically. Load and displacement can be measured with high accuracy,
but a correct interpretation of the resulting data is only possible if the actual
4.2 Macroscopic Properties of Thin Films
119
geometrical interaction of the indenter tip with the sample is known, because
the contact area must be determined correctly. A variety of techniques have
been used to obtain this information.
Because the load is incremented on the indenter in contact with the sample
material, the resulting displacement consists of both plastic and elastic deformation of the sample. A plastically deformed zone extends below the contact.
Several models attempt to explain the expansion of this zone [4.108,109] and
the resulting interaction and mixing of the mechanical properties of film and
substrate, which depend on film thickness and indentation depth. But a clear
determination of film hardness strongly depends on the mechanical composition of the film/substrate system, as is shown by simulating the indentation
by finite element calculations [4.110,111]. The quality of the hardness determination differs remarkably according to whether hard coatings are combined
with soft substrates or hard substrates with soft coatings. Further limitations
on hardness determination by indenter techniques are described in [4.99,112].
Finally, the displacement, hp, determined from the unloading process as
defined in Fig. 4.7, is a measure for the plastic deformation properties of the
film/substrate system.
:::>
~
c..
"0
Cll
o
....I
Unloading
hfinal
hp
Displacement h /a.u.
Fig. 4.7. Scheme of a typical load-displacement curve for nanoindentation showing
the difference between the plastic depth hp and the final depths hfinal and the
definition of the slope dP / dh
120
4. Properties and Characterization of Dielectric Thin Films
Elastic Moduli
An accurate knowledge of the elastic properties of the coating is desirable for
choosing the correct coating material and film thicknesses to prevent plastic
deformation, cracking of brittle materials, or wear under mechanical load.
The exact measurement of the elastic moduli of thin films proves to be difficult. Nevertheless several methods have been developed for determining the
Young's modulus and the Poisson's ratio, or at least a combination of these
quantities such as the biaxial modulus [4.98,99, 113]. First results indicate
that the elastic properties of the film material and the bulk differ by up to
25% [4.114]. The elastic properties depend strongly on the production conditions [4.115] and on other film properties, for example on density [4.116].
Load-sensing and displacement-sensing indentation techniques are becoming increasingly popular for measuring the elastic moduli of thin films. They
rely on the assumption that displacements in materials mostly recover elastically during the unloading of the indenter. In this case, the elastic punch
theory can be used to determine the modulus E from a simple analysis of
indentation [4.138]. Figure 4.7 schematically shows the dependence of the
indentation load P as a function of the displacement h. For a flat cylindrical
punch, the elastic modulus is determined by the slope dP/ dh of the unloading
curve according to
dP
dh =
2
-..fi VAEr
(4.13)
,
where A is the contact area of the punch. The reduced modulus Er contains information about the elastic behaviour of the punch (Ei, Vi) and the
respective properties of the film (Ef , vr).
1
(1 - v1)
Er
Ef
-=
+
(1 - vl)
Ei
.
(4.14)
For punches with non-cylindrical shape, additional corrections are necessary. The indentation behaviour of coatings is strongly influenced by substrate
properties in the case of thin films such as oxide layers for optical purposes.
Therefore this method of modulus determination is subject to similar restrictions and uncertainties as the hardness investigation mentioned above.
Another method determines the elastic film properties by the dependence
of the thermal stress component on the biaxial modulus of the film material;
see (4.8). Here the modulus and the thermal expansion coefficient of the film
material are unknown in principle. Both quantities can be simultaneously
determined from the measurement of thermal stress by depositing the films
on two different substrates [4.100] with widely different thermal expansion
properties. In some special cases, the thermal expansion coefficient of the
film can also be separately determined by Raman investigations [4.117] and
additional thermal stress measurement.
4.2 Macroscopic Properties of Thin Films
121
Other techniques for the evaluation of elastic film properties use optical
methods such as Brillouin spectroscopy [4.118], or the mechanical treatment
of the coated substrate (e.g., the mechanical reSOnance method [4.119]), substrate bending [4.114]' or beam-deflection experiments On micrometer-scale
cantilever beams etched in silicon wafers [4.99]. The measurements on freestanding films, for example the bulge test, have also been developed to a high
degree of sophistication [4.98]. However, the values for the elastic properties
may differ from those gained for films attached to substrates because of the
different stress and strain conditions.
The elastic properties of thin films are also determined with surface acoustic wave (SAW) techniques [4.120]. They measure the different dispersions of
the wave propagation in the coated substrate and the uncoated surface. The
Young's modulus and the Poisson's ratio of the film material can be deduced
from this difference in propagation behaviour if the density and the thickness
of the layers are known.
Interfacial Adhesion Strength
For all practical applications of thin films, a certain degree of adhesion is
required to guarantee the fUnctionality of the coated part. The quality of adhesion mainly depends On the interface layer between the coating and the substrate. Adhesion is defined as the work that is necessary to separate atoms or
molecules at the interface. Several mechanisms of interfacial bonding can be
distinguished, for example mechanical linking, electrostatic forces and chemical bonding. The adhesion of coatings that are used in technical applications
usually originates from a combination of all these effects.
The adhesion of films must be measured because it decisively influences
the fUnctionality of coatings. More than 350 methods for the determination
of adhesion are known [4.121]' ranging from basic to highly sophisticated, but
nOne is capable of measuring the interfacial adhesion directly. In most cases
there exists nO precisely defined "interface" between the substrate and the
film. The transition region is elongated and possesses chemical and mechanical properties that differ from those of the film and the substrate materials.
Therefore it behaves more like an "interphase" [4.121]. In addition, forces
originating either from film or substrate properties (e.g., stresses), or from
the ambience of the coated part are typically superimposed On the interfacial
forces.
The result of the established adhesion-measurement techniques is therefore the so-called "practical adhesion", which is defined as the work or the
force required to remove the coating from its substrate. The locus of an adhesional failure in a film/substrate system will be the weakest point in the
compound. In practice a complete interfacial failure is quite rare and the
separation is mostly of interphasic nature or even cohesive within the film or
substrate.
122
4. Properties and Characterization of Dielectric Thin Films
Oxide films on inorganic substrates usually possess quite good adhesion
properties. Therefore, most of the techniques reported in [4.121] either cannot
be applied to these systems, or are not good enough for the semi-quantitative
determination of film adhesion. In addition, quick and easily interpretable
tests are required for standard use in production. Therefore practical investigations such as pressure-cooker or Taber scratch-shear tests are widely
applied. More sophisticated methods, which enable a better evaluation of adhesion properties, are pull-off or topple tests and the extensively used scratch
tests (or dynamical indentation method) [4.122], which are also applicable for
testing quite thin layers for optical purposes [4.127,128].
All scratch test methods use a single-point contact, usually a spherical Rockwell C120° diamond cone. This scratching stylus is drawn across
the coated sample with a dynamical increase of the load of the indenter and thereby produces an increasing elastoplastic deformation of the
film/substrate system along its path, analogous to the indentation testing
for hardness investigations. The deformation causes damages in the coating,
which are adhesive or cohesive in nature. The minimum load at which an
adhesive failure occurs is called the critical load Le; it is representative of
the coating adhesion [4.122]. The critical load is affected by several factors
besides adhesion properties [4.131], the most important being coating thickness, elastic substrate and film properties (Young's moduli), friction between
the sliding stylus and the coating surface, surface contamination and stresses
in the layers [4.123,132].
Several models have been developed to describe the dynamical interaction
of the stylus with the sample [4.108] or the distribution of induced stress
fields in combination with fracture mechanics [4.124]. But to explain the
scratch test results on films for technical applications, these models are still
not conclusive enough, and greater efforts are needed with respect to finite
element calculations.
With modern equipment, scratch testing is reproducible and the adhesion
of a number of quite similar coatings can be ranked. However, this implies
a clear identification of the failure modes. Much effort is put into real-time
failure identification during scratch testing (e.g., the detection of acoustical
emissions or changes in the surface friction properties due to the detachment
of the films). But up to now the scratch track still must be carefully observed
by optical microscopy or scanning electron microscopy investigations. A lot
of different failure modes are analysed and their direct relevance to adhesion determination has been reported for a variety of different film/substrate
systems [4.129,130,133]. In some cases the reproducibility may be insufficient for an exact Le determination because of lateral inhomogeneities of the
coating. This situation requires the performance of many scratches and an
additional statistical evaluation of the data. It is convenient to determine the
cumulative failure probability as the result of adhesion testing by a Weibull
statistical analysis [4.133].
4.2 Macroscopic Properties of Thin Films
123
Scratch testers developed by CSEM are widely used [4.122,129]. A
schematic representation of the set-up is given in Fig. 4.8. The scratching
stylus is drawn linearly in one direction across the coated surface. An acoustical emission detector is used for the real-time detection. A steep increase
in the acoustic emission signal indicates coating detachments and the corresponding load is interpreted as the critical load [4.122]. The equipment is
optimized to work also with small loads [4.129]' so that the investigation of
quite thin layers, for example optical coatings, is possible as well. Additional
refinements are in progress. More information about this detection principle
is given in [4.122].
A different scratch technique has been developed by Baba et al. [4.125,126].
The testing principle is shown schematically in Fig. 4.9. The tester measures
the friction properties of surfaces microscopically. The indenter mount swings
at 30 Hz with an amplitude Xo of 100 !-lm due to an external force perpendicular to the scratch direction. The sample stage moves relative to the stylus
with velocities v of up to 40 !-lms- I . The sample holder is designed as a ramp
with a variable tilt angle 0:, which enables the application of a steadily increasing load by an increasing pressure of the sample towards the elastically
mounted stylus along the scratch path. The load increase can be calculated
by taking into account the tilt angle of the stage, the scratch length, and the
elastic properties of the indenter mount.
The coefficient of friction is determined from the measured phase difference between the external force and the corresponding motion of the diamond
stylus in contact with the coating, which is influenced by the frictional properties of the surface [4.125,126]. This response signal is picked up by a cartridge,
.--dx/dt
Fig. 4.8. Scheme of a scratch test set-up detecting the acoustic emission signal and
the tangential force FT of the stylus as a function of the applied normal force FN
124
4. Properties and Characterization of Dielectric Thin Films
Cartridge
v
Stage
Fig. 4.9. Scheme of the scratch test method of Baba. The sample, fixed on a ramp
with tilt angle a and speed v, is pressed with progressively increasing load F against
the stylus of a phonographic cartridge swinging with amplitude Xo
which is commercially used in phonographic systems. Frictional properties are
obtained with respect to a dynamically increased load of the microindenter.
The critical load is ideally correlated with a steep rise in the friction properties
of the surface due to the brittle cracking of the films [4.125]. The principle
and technique of the measurements are described in detail in [4.125,126].
Testing equipment based on Baba's principle is commercially available from
Shimadzu and Rhesca (Japan). The sensitivity of this technique is adequate
for the evaluation of optical coatings [4.127,128].
Fracture Mechanics
Because oxide films are frequently brittle, they tend to crack under mechanical load. Often the cracks are caused by stresses in the film material [4.134, 135]. Shear stress and peeling stress occur near the free ends of the
coated components and in-plane normal stress occurs in the interior region.
The shear stress and peeling stress may delaminate the coating from the
free ends. The in-plane normal stress, which may be tensile , causes cracks
in the film, which in turn induce a release of strain energy in the system.
The compressive stress is responsible for the spallating or buckling of the
layers [4.134, 137].
Cracks in the interior region of the coatings will form new free ends in
the remaining film . These free ends give rise to new shear stress and peeling
stress components within the film, which may cause a delamination of the
coating. In addition, strong stresses in coatings on brittle substrates like
glass tend to drive the crack, which originally began in the film, into the
substrate and thus cause the coated system to fail. Evaluations of the stressand-strain release rate due to cracking reveal that the crack initiation and
the crack spacing of the coating is a function of the film thickness and its
elastic modulus [4.135,138] . Though cracking seriously impairs the quality of
optical coatings, simple and accurate techniques for measuring the strain at
fracture of thin materials are still lacking [4.136]. Extending and refining the
4.3 Microscopic Properties
125
existing techniques is indispensable for future investigations of oxidic coatings
on glass.
4.3 Microscopic Properties
Klaus Bange
In addition to the macroscopic quantities described in Sect. 4.2, an adequate
description of thin films also necessitates knowledge about the microscopic
film properties (e.g., composition and chemical bonding state of elements,
stoichiometry, topography, roughness of surfaces, formation of crystalline
or amorphous interfaces and crystal structure). Various analytical methods,
which have been developed in the past decades, are available to obtain such
microscopic information.
4.3.1 Composition
The chemical composition determines the physical and chemical properties
of thin films to a similar degree as the deposition technique and the selected
deposition parameters and conditions. Besides the expected components such
as oxygen and metal, technical oxide films usually contain various other elements, some of which are intentionally added by doping materials to create
special film properties, whereas others are unwanted. The film composition
can be determined by various methods. Many techniques have been developed for the investigation of surfaces. If an ion source is used for sputtering,
the third, depth dimension can be probed perpendicularly to the surface.
The chosen method should have a good depth resolution to obtain elementversus-depth profiles. Destructive techniques, which remove the film during
the depth analysis, are SIMS, IBSCA, AES, ESCA, and SNMS. Two widely
used non-destructive techniques are RBS and NRA. Electron microscopy
combined with EDX or WDX also enables the chemical analysis of thin films
under certain experimental conditions. Grazing incidence X-ray fluorescence
analysis (GIXF) is the established routine technique for measuring thin films
and surface concentrations down to approximately 1010 impurity atoms per
cm 2 [4.139]. Other sophisticated techniques can be used as well. Each method
has its merits and its shortcomings. Both should be known when selecting
the appropriate method for a special problem to be investigated.
The film composition is not only affected by the impurities of the starting
materials or the deposition technique, but can also be changed during testing
procedures or applications. These variations and changes in concentration
are usually very small and can be appropriately studied by SIMS, which is
very sensitive and allows a quick qualitative detection of low concentrations
of elements and also of small changes [4.10]. For many elements the detection limit is in the ppm to ppb region. SIMS includes the collection and
126
4. Properties and Characterization of Dielectric Thin Films
analysis of positive and negative secondary ions that are ejected from an ionbeam-bombarded surface. The technique yields excellent sensitive-qualitative
information, but in general is destructive to chemical compounds, and quantitative results are difficult to obtain. A fairly similar technique, SNMS , uses
electron impact post ionization of sputtered neutral particles by the electron
component of a special low-pressure hf plasma. SNMS in principle allows
quantitative surface analysis and thin film analysis. Until recently, SNMS
has been applied mainly for the investigation of conducting samples and thin
film systems; now the application of SNMS has been extended to insulating
samples and layer systems [4.14].
During the argon bombardment of the sample, detectable light is also
emitted. These optical signals (IBSCA) are characteristic of different elements
in the sample [4.15]. IBSCA is not as sensitive as SIMS, but the characteristic
lines are more realistic indicators of the concentration of elements than the
mass spectrometer signals because the matrix effects of the optical signals
of neutral particles are less pronounced than those of ions. The feasibility
of IBSCA by the use of an optical multichannel analyser is demonstrated
in Fig. 4.10. Some spectra are shown for a multilayer system on glass as
a function of sputter time (i.e. , of depth) for the wavelength region of 250800 nm. The spectral resolution is 0.6 nm for the data shown, but a resolution
of < 0.1 nm is possible. From such data, depth profiles can be deduced [4.18].
Like ESCA, Auger electron spectroscopy (AES) is a secondary electron
detection method which uses high-energy electrons (in the keY range) for
excitation [4.11]. The advantages of the method are the high lateral resolution, which mainly depends on the focus of the excitation beam, and the
rJ)
70
60
50
5
40
.~
20
M
o
C
~ 30
Z.
.sl 10
E
250
300
350
400
450
500
550
Wavelength A Inm
Fig. 4.10. IBSCA spectra as a function of time on a multilayer system
4.3 Microscopic Properties
127
almost matrix-independent element-sensitivity ratios, which facilitate absolute quantification. The detection of all elements except H and He is possible.
The escape probability of the energetic secondary electrons is the determining
parameter for the information depth (depth resolution). Their mean range
is between two and ten monolayers (about OA-2nm) in most materials. The
detection limit of AES is approximately 1000 ppm; the detection sensitivity is principally limited by an appreciable background due to random loss
of energy by the secondary particles, as in other energy-analysing methods
(ESCA). Chemical information can be gained by AES for some special cases.
Electron spectroscopy for chemical analysis (ESCA) uses the emission of
electrons from a solid surface by excitation with X-rays [4.10]. The main
field of application of ESCA is the detection of the chemical bonding state
of elements, which influences the binding energy. Different valence states in
compounds are recognized by specific energy shifts. The detection capability
of elements is similar to that of AES (no H) and the detection limit is in the
range of 1000 ppm. Because X-rays cause less damage than electrons, ESCA
is especially useful in the analysis of sensitive materials, for example organic
films. The quantification in ESCA is also analogous to AES but it is more
reliable, because the photo-ionization cross-sections are well known, the peak
intensities are not influenced by backscattered electrons as in AES, and the
background is lower and easier to subtract.
Only a few non-destructive methods allow an exact quantitative analysis
of the element composition in thin films. Usually a combination of different
methods is necessary to obtain a full quantitative data set. The quantitative
determination of the hydrogen content in thin films is particularly difficult.
But this quantity strongly influences certain film properties, especially those
of optically active layers, and for other materials is a good indicator for several
film characteristics. A quantitative measurement of the hydrogen concentration is possible with nuclear reaction analysis techniques (NRA) [4.30] with
some experimental expenditure. The 15N method additionally enables hydrogen depth profiling because the first resonance reaction IH 5 N, el,) 12C,
occurring at 6AMeV, has a FWHM of only 1.8keV.
The depth resolution at the surface of the investigated sample depends on
the chemical composition and is for example 10 nm for Ti0 2. With increasing film depth the resolution decreases. This is demonstrated on a hydrogen
depth profile of an interference filter shown in Fig. 4.11. The alternating Si0 2
and Ti02 films are well separated in hydrogen content and depth. For the
high-refractive material the H/Ti ratio is about 0.18 and for the low-refractive
film H/Si ~ 0.25-0.3 [4.140]. All layers in the coating systems have the same
film thickness, as can be deduced from the deposition data and from optical
measurements. With increasing beam energy, i.e., increasing depth, a reduction in the resolution is recognized in the experimental data. The systematic
increase results from an increased energy straggling of the 4He ions [4.140].
e
128
4. Properties and Characterization of Dielectric Thin Films
0.3
0.3
en
0.2
0.2 F
I
I
0.1
0.0
0.1
7
8
9
Energy IMeV
10
11
0.0
Fig. 4.11. Hydrogen concentration profile on a Ti02jSi0 2 multilayer system deposited by reactive evaporation
For an exact quantitative analysis of the other elements, especially for
the heavier ones, Rutherford backscattering spectroscopy (RBS) is an excellent tool. This non-destructive technique allows an exact determination of
the oxygen/metal ratio and enables the analysis of impurity elements. The
technique is based on elastic scattering of the incident primary ions (normally 4He). The relative energy loss depends on the scattering angle and on
the difference between the primary ion mass and the target atom mass. For
thin film analysis usually primary ions with an energy of 2-2.8 MeV are used,
and the backscattered particles are detected at an angle of approximately
160 0 • The elements in the films are identified and quantified by simulating
the experimental data with a standard program (RUMP). RBS is insensitive to light elements (H is not detectable). The sensitivity increases with
increasing mass number and is proportional to Z2, but the mass resolution of
heavy elements decreases. For an exact analysis of the film composition the
hydrogen content should be known because this light element also produces
a certain loss of energy in the primary ions. The analytical potential of RBS
is demonstrated in Fig. 4.12 on a T~05/Si02 multilayer system. The measured RBS spectrum and the RUMP simulation are shown. The nine strong
maxima arise from 4He scattering on tantalum in the tantalum oxide layers.
The minima originate from the 4He energy loss in traversing the silicon oxide
layers (1.3-2.6 MeV). The modulation in the oxygen content is also recognizable in the energy range < 1.1 MeV. The observed decrease of resolution at
increasing depth can be explained by straggling effects [4.140].
4.3.2 Oxidation State
Most of the oxide layers are produced by reactive processes, which are described in detail in Chap. 3. The desired oxidation of the metal is generated by
the use of oxygen-containing atmosphere in combination with an additional
supply of energy to increase the rate of the reactive reactions. By the very
4.3 Microscopic Properties
129
Energy IMeV
0.5
1.0
2.0
1.5
2.5
80.-~------~-.----~----~------~~
60
"C
Qj
'>,
"C
.~ 40
Cii
E
o
z
20
\
..~~.~ .... .fiIi
~~'<:!!J1'~~
O~----.-----~-------.-------r~~
200
400
Channel
600
800
Fig. 4.12. RBS spectrum and RUMP simulation of a Ta20s/Si02 multilayer system produced by ion plating
different process conditions of the various deposition technologies, different
chemical bonding states between the oxygen and the metal can be produced,
which in turn can alter macroscopic film properties, for example, optical
quantities or electrical conductivity. Therefore, knowing the oxidation state
of the metal in oxide layers is important for the optimization of deposition
process parameters. Different analytical methods allow conclusions about the
chemical binding states of elements in oxide layers [4.10,11].
The detection of chemical bonding is the classical field of ESCA because
the binding energy of the electrons is influenced by the chemical bonding
state. The detection of the chemical bonding is enhanced by the use of
monochromatic X-rays, which are prerequisite for obtaining energy resolution of the order of O.4eV. Different valence states in compounds are recognizable by a specific energy shift. The so-called "chemical shift" is a shift in
the peak position of the ESCA lines. It can be observed for the same element
in different chemical surroundings and is caused by the rearrangement of the
average charge distribution of the valence electrons. Such rearrangements
primarily result from changes in the oxidation states of compounds, but Can
also be caused by changes in structure. The concept of electronegativity is
of great practical value for interpretation of spectra and for the identification of chemical species. Oxidation processes correspond to a partial decrease
of the valence electron density of the oxidized atom; hence the shielding of
the remaining atomic electrons - including the core electrons - decreases,
yielding an increased binding energy related to a higher oxidation state. An
example of the chemical shift due to oxidation states of the transition metal
is shown in Fig. 4.39. Because chemical bonding alters the state of the outer
130
4. Properties and Characterization of Dielectric Thin Films
atomic level, the chemical effects in AES are in general stronger than those in
ESCA. However, the interpretation is often difficult and relies on the comparison with fingerprint spectra of the respective compounds. Detrimental effects
of the electron beam and difficulties in data acquisition and interpretation
often impede an exact chemical effect detection. Nowadays the situation is
rapidly changing for the better because data acquisition has been improved.
Another spectroscopic tool that is useful for probing the oxidation state
of films is XANES (X-ray absorption near-edge fine structure), which detects
the oscillatory structure of the X-ray absorption coefficient in the immediate
vicinity of the absorption edge, i.e., for hv < 50 eV above the edge [4.7].
XANES directly probes unoccupied electron states of the photo-ionized ion.
These states are sensitive to chemical bonding and hence to crystallographic
order and ion electronegativity. XANES has the advantage that no vacuum is
required for the measurements and that materials with very low concentration
of absorber can often be studied with ease.
In SIMS, cluster ions and molecule fragments can in principle be used
to work back to the chemical environments of the target atoms. Particularly, a special low-energy ion technique has proved an outstanding tool to
identify molecules. Large organic molecules that are immersed in a surface
can be directly identified because often only one proton is stripped. For general purposes, the chemical information is less direct and usually blurred
by high-energy primary ions and high-current density, which are typical of
SIMS. Information about the chemical compounds in thin films is obtainable by methods that determine the physical structure of the sample, for
instance TEM, EELS, or the specially designed XRD, whereas information
about molecular bonds can be gained by IBSCA.
4.3.3 Structure of Oxide Films
The structure of thin films is strongly related to the deposition technique
and the respective deposition conditions. The local arrangement of atoms in
a film may be either regular (crystalline) or irregular (glassy). A regular threedimensional arrangement of atoms or ions in space constitutes a crystalline
structure that is imposed by directional bonding and close packing with the
aim of minimizing the energy of the solids. However, many films have noncrystalline structures. Their subunits are packed together randomly and they
lack the long-range order of crystals because they have only limited mobility
or are sterically hindered at the equilibrium solidification temperature.
The production of films proceeds through nucleation and growth stages
[4.141]. Such stages involve adsorption, surface diffusion, chemical binding
and other atomic processes at the surface. It is generally accepted that there
are three possible modes of growth on surfaces, which depend on thermodynamic conditions. If the atoms or molecules of the deposits are more strongly
bound to each other than to the substrate, small clusters are nucleated directly on the substrate surface, which then grow into islands in the condensed
4.3 Microscopic Properties
131
phase (island or Volmer-Weber mode). The opposite characteristics are obtained if the atoms are more strongly bound to the substrate than to each
other. Then atoms condense first and form a complete monolayer on the
surface, which is covered with a somewhat less tightly bound second layer
(layer-by-Iayer or Frank-van-der-Merwe mode). An intermediate case is obtained if, after the formation of the first monolayer, or of a few monolayers,
the subsequent layer growth is unfavourable and islands are formed on top
of this intermediate layer (layer-pIus-island or Stranski-Krastanov mode).
There are many possible reasons for this mode to occur, and almost any factor that disturbs the monotonic decrease in binding energy, which is characteristic of layer growth, may be the cause. This growth mode is very common
in technical systems. The growth mode which is present in an actual system
depends on the surface, the deposit, and on thermodynamics and kinetics.
Various models describe the influence of the deposition parameters on the
structure of thin films. For evaporated films a three-zone model is reported
by Movchan and Demichishin [4.142]. Characteristic film structures are given
as a function of the ratio Ts/TM of the substrate temperature Ts to the
melting temperature TM of the deposit material. A more generalized model
is described by Thornton for sputtered films [4.143]. The structure in this
model also depends on the ratio Ts/TM and additionally on the argon pressure
in the sputter system, which determines the kinetic energy of the particles
in the gas phase. An additional transition zone is placed between zone one
and zone two at low temperatures. A similar structure model by Messier
demonstrates the influence of the negative substrate bias, which is applied
during magnetron sputtering, on the transition between zone one and the
additional zone [4.144]. Films deposited by reactive ion plating can also be
described by the Movchan-Demichishin model, but an additional fourth zone
is necessary for the vitreous phase. This zone four is placed behind the hightemperature zone three [4.145]. The vitreous modification is assumed to result
from the superthermal film formation condition (thermal spiking).
Oxidic thin films, which are deposited in general in a non-equilibrium
state, possess an atomic arrangement between the two limiting cases crystalline and vitreous. The film structure includes crystallographic arrangements of atoms in a lattice with lattice defects, columnar crystals, vitreous
columns, clusters, i.e., structures in a micrometer to nanometer scale and of
local order. Very different analytical techniques have to be used to determine
the various film structures. The crystallographic structure of thin films can
be determined by X-ray or electron diffraction techniques. In the solids, the
interaction of the probes with a group of atoms creates either a constructive
or a destructive interference in a certain direction and thus causes the wellknown diffraction phenomena that can be described with a classical Bragg
equation. Diffraction data can be used to obtain information about [4.9]:
• identification of crystalline phases including qualitative and quantitative
analysis of mixtures of phases,
132
4. Properties and Characterization of Dielectric Thin Films
• distinction between the amorphous and crystalline states,
• precession measurement of lattice parameter and thermal expansion,
• determination of the degree of preferred orientation and crystalline texture,
and
• measurement of certain physical characteristics, such as small crystallite
sizes, strain, perfection, lattice disorder and damage.
A knowledge of the crystal structure is essential for the identification of
the materials, for the understanding of the film properties and their behaviour
under various external conditions and for characterizing the material at all
stages of preparation.
The deposition of oxide films on glass at moderate substrate temperature usually creates vitreous structures (i.e., amorphous according to ED
and XRD). An exception is Ti0 2 . Raman and infrared spectra give information about the structures of such materials [4.3]. Infrared spectroscopy
(IR) is mainly used for the characterization of chemical structures, but also
for the identification of unknown substances and for tracing chemical reactions either in the bulk or on the surface of materials. The IR spectrum of
a material is often its "fingerprint" and an unknown material may be positively identified by comparing its spectrum to a set of an authentic standard.
The spectra are usually presented as a plot of absorption versus wavelength
or wave number. From a mechanical perspective, molecules in any state of
matter can be viewed as consisting of masses (atoms) connected by springs
(chemical bonds). If energy is absorbed, certain frequencies are excited. The
exact frequency depends on the atomic masses, the force constant of the
chemical bonds connecting them and to a lesser extent on the interaction
between non-bonded atoms. The vibrational frequency of atoms in molecules
corresponds to the frequency found in the IR radiation of the electromagnetic
spectrum. An interaction only occurs if the vibration involves an oscillation in
electric dipole moment because then the IR radiation of the same frequency
is absorbed and vibration is excited. Thus, strongly polar substances such as
water are very strong absorbers of IR radiation, whereas non-polar molecules
show rather weak absorption. Observed absorption bands in the IR of oxide
films correspond to vibrational modes in the molecular bonding. Stretching
and bending modes give general information about the deposited material;
the vibration modes in addition allow one to distinguish reactions with water
or hydrogen.
Comparable information about the molecular structure can be obtained
by Raman spectroscopy, which is based on the observation of scattered light
spectra [4.2]. Electromagnetic radiation can interact with matter in a number of different ways, some of which involve the transfer of energy between
photons and molecules or phonons. The interaction with a particular object
at a given wavelength depends on the molecular structure of the object. Light
may be transmitted, reflected, absorbed or scattered by the molecules. Most
light scattering occurs without gain or loss of energy and is referred to as
4.3 Microscopic Properties
133
Rayleigh scattering. However, a small fraction of the light that is scattered
by matter exhibits a constant frequency shift with respect to the incident
light, indicating that energy is being transferred to or from the molecule.
The frequency shift, which represents the difference in energy between an
incident and a scattered photon, is equivalent to the difference between two
vibrational energy states of the molecule. This energy difference is measured
in Raman spectroscopy. Raman shifts can be positive or negative, depending
on whether energy is lost or gained during the interaction with matter. Lines
in the Raman spectrum that are shifted to longer wavelengths, representing
a loss of energy, are called Stokes lines, and lines that are shifted to shorter
wavelengths, indicating an increase of energy, are called anti-Stokes lines.
Because they result from the more frequently occurring molecular transition
from ground to excited state, Stokes lines are of greater intensity, and are
more often studied than the anti-Stokes lines.
Raman spectra can provide qualitative, quantitative and structural information about a variety of materials. Most significantly, the Raman spectrum
yields information about the molecular rather than the elemental structure
of a sample. Because the presence of particular atomic groups and chemical
bonds is indicated by characteristic frequencies in the Raman spectrum, the
chemical functional groups in a compound can be identified by these group
frequencies. The further application of chemical intuition often leads to the
identity of the compound itself. The intensities of the bands can be used
to determine the concentration of a specific compound even in the presence
of other compounds. Concentrations of a few wt% are easily measured. A
wealth of structural information can be used to distinguish the various geometric forms of a compound, to define the conformation and symmetry, or to
determine the degree of crystallinity of a substance. The geometric forms can
be distinguished by comparing the observed Raman spectrum with the theoretical values. Raman spectroscopy can also be used to trace phase changes
as a function of temperature.
Raman spectroscopy and infrared absorption spectroscopy are complementary analytical techniques. Both provide vibrational information about
the molecule, but difference data are conveyed in scattering and absorption
spectra. The complementary nature of the two techniques is exemplified by
their selection rules. A vibration is Raman-active if there is a change in the
induced dipole or polarizability of matter; it is infrared-active if there is a
change in the permanent dipole, i.e., ionic species have no Raman spectrum
and homonuclear diatomic molecules have no infrared spectrum. A particular molecular vibration may be observed in both spectra, but the intensity differs. The lower the symmetry in a thin film, the greater the overlap of its Raman-active and infrared-active vibrations. In general, non-polar
compounds and symmetric vibrations are more readily observed in Raman
spectroscopy, whereas infrared spectroscopy favours polar compounds and
asymmetric vibrations. Specific problems with thin oxide layers sometimes
134
4. Properties and Characterization of Dielectric Thin Films
require special substrates or an optimized experimental set-up for Raman
investigations [4.18].
In virtually all oxide thin films the fluctuation in electron density induces X-ray scattering at small angles. Structures that can be studied by
SAXS, detected at scattering angles 2() between 20" and 2 0 , have dimensions in the range 1-1000 nm [4.8]. The features responsible for the X-ray
scattering may be compositional fluctuations or density fluctuations associated with crystallites, voids, or additives. In practice, the structure of crystalline solids can be absolutely determined from good diffraction data of the
crystalline material over a reasonable region of reciprocal space. With vitreous or amorphous materials this is impossible because they are normally
isotropic on a macroscopic scale. Therefore the most that can be obtained
by experiment is a one-dimensional correlation function from which the underlying three-dimensional structure can never be deduced uniquely. Thus,
X-ray studies of amorphous film structures fall naturally into two stages: the
actual experiment, which culminates in the derivation of a real-space correlation function, and the interpretation of the correlation function in terms of
a three-dimensional structure. That means, from the experimental SAXS results - the intensity variation versus scattering angle or scattering reciprocal
length (reciprocal space) - the radial distribution function is extracted by a
Fourier transformation. To analyse such a radial distribution function, various structural models have to be tested. An example is given in Sect. 4.4.5.
Interatomic distances, i.e., the distances between a central absorber atom
and the shell atoms, and the number of atoms in a shell (coordination number) can be determined by EXAFS. In the energy region of EXAFS, photoejected electrons are excited to final states far above the Fermi level [4.7].
At these energies the total final density of states varies smoothly with the
energy, and the oscillating behaviour in the absorption coefficient results only
from variations in the matrix element. In this case, a single scattering model
can be used. Waves can be viewed as an outgoing wave superimposed on a
wave backscattered from the surrounding atoms. Depending on the energy,
this backscattered wave interferes either destructively or constructively with
the outgoing wave and thus produces oscillation in the absorption coefficient.
From this data the coordination number or the interatomic distances can be
calculated. Amorphous materials have been studied with these techniques. By
the use of synchrotron radiation with high intensities and resolution, highquality spectra can be acquired very quickly. Extremely high intensity and
appropriate detection techniques also enable the study of the dilute concentration, in the ppm range, of an absorber dispersed in a host matrix.
4.4 Examples of the Characterization of Thin Film Materials
135
4.4 Examples of the Characterization
of Thin Film Materials
Klaus Bange
The problem-oriented use of the tools described above is exemplified here
for various oxide coatings. We discuss two high-refractive materials (Ti0 2,
Ta205) and one low-refractive film (Si0 2), which are often applied for the
coating of optical components and architectural glasses in visible and nearinfrared wavelength range. Additionally, two optically active materials (W0 3 ,
NiO x ) are described; these enable the production of devices with changeable
optical properties. The influence of the deposition techniques is extensively
reported for Ti0 2 films (Sect. 4.4.1). Results for Ti0 2 films produced by physical vapour deposition (PVD), for instance for ion plating (IP), reactive evaporation (RE), ion assisted deposition (lAD) and ion beam sputtering (IBS)
are reported and compared with data obtained for layers deposited by a CVD
process, for instance plasma impulse chemical vapour deposition (PICVD) or
by sol-gel techniques, for instance spin coating (Se) and dip coating (DC).
The influence of the deposition techniques is also described for Si0 2 films
(Sect. 4.4.2) and Ta205 films (Sect. 4.4.3), but with the focus on other film
characteristics. Results of studies on photochromism, thermochromism, and
particularly electrochromism of W0 3 films are given in Sect. 4.4.5, in which
ten different colouration procedures are described. A unified model for the
various transport mechanisms of the different colouration procedures is discussed for films deposited by evaporation. In Sect. 4.4.4, a model for the
electrochromism of hydrous nickel oxide films is outlined, which explains the
colouration mechanism of evaporated materials. In summary, this section
demonstrates, by selected topics, the use of the analytical methods on some
relevant film materials and gives useful information about various macroscopic and microscopic film properties which represent the basis for different
functions in products.
4.4.1 Titanium Oxide
Olaf Anderson, Klaus Bange, Clemens Ottermann
Applications involving low loss (i.e., low absorption and low scatter) optical
coatings have stimulated a considerable amount of activity in the preparation
and investigation of transparent dielectric films with high refractive index.
Ti0 2 is one of the most important materials, whose particular interest results from commercially available products, for example interference filters,
antireflecting coatings for shop windows, and selectively reflecting coatings
for architectural glass (described in Chap. 5). Various deposition methods
can be used to produce Ti0 2 coatings with the desired optical, mechanical
and chemical properties. In practice, the employed deposition techniques are
136
4. Properties and Characterization of Dielectric Thin Films
often selected empirically in relation to a special demand on the film. Optical transmission and reflection data are traditionally used to determine the
optical parameters of thin film material. For example, it is well known that
the refractive index of Ti0 2 prepared by different deposition techniques can
vary by up to dn = 0.4. The various microscopic properties in such Ti0 2
layers therefore can be assumed to differ widely. Microscopic studies of Ti0 2
coatings that, for instance, quantitatively correlate optical absorption with
crystal structure, composition, density, stress, adhesion, roughness, diffusion
processes and thermal conductivity of the film are lacking. A summary of various data on Ti0 2 films deposited by very different techniques with a focus
on these topics is given in the following.
Structure and Crystallization Behaviour
Three phases are known for crystallized Ti0 2 , namely brookite, anatase and
rutile. The orthorhombic brookite modification is formed in thin films only
under special hydrothermal conditions and/or in the presence of a defined
amount of sodium [4.146, 147J. On soda-lime glasses this structure can be
produced by the sol-gel method. The most stable thermodynamic form of
Ti0 2 is the rutile modification. Anatase changes its modification irreversibly
by paramorphosis into rutile. Both modifications are tetrahedral, with titanium atoms being surrounded octahedrally by six oxygen atoms. Differences
between the two modifications are due to the distinctions in the lattice constants.
In thin layers (d < 350nm), Ti0 2 exhibits a complex and somewhat
mysterious crystallization behaviour, which depends upon many parameters. Besides the different coating methods [4.27], a lot of parameters influence the morphology and structure of the films. For the PVD methods
these are the substrate temperature during the coating process [4.148,149],
the partial pressure of oxygen in the coating chamber [4.151, 152J, and the
growing velocity of the films [4.153J. The chemistry of the coating solution [4.154J and the environmental conditions during the hydrolysis of the
layers (especially moisture) [4.155J are the important parameters for the solgel methods [4.152, 251J. In all deposition procedures the behaviour of the
films is determined by the mixing of Ti0 2 layers with other materials [4.156],
the thickness of the layers [4.149]' and the thermal annealing after deposition [4.150, 158-160J. It must be also considered that Ti0 2 layers behave
differently in multilayers as compared to single layers [4.161-163J. Optical
properties such as refractive index, absorption and loss can be controlled over
a wide range [4.151, 164J and other quantities are changeable (e.g., roughness of surface and interfaces, physical density, chemical resistance, electrical
properties and stress). These measurable macroscopic properties are strongly
correlated with the microscopic properties within the layers, for example
morphology and crystal structure.
4.4 Examples of the Characterization of Thin Film Materials
137
All PVD and CVD methods produce amorphous layers below 200°C. The
highest substrate temperature (Ts), up to which the film growth is amorphous, depends upon the coating method and the physical density of the
film. This temperature decreases with increasing film density. With ion plating (IP), which of all methods yields the highest density, it is possible to
produce crystalline layers at Ts cv 220°C. Layers produced with reactive
evaporation (RE) possess a lower density and grow below 250°C in amorphous form, above 300 °C in a crystalline modification. Between these two
temperatures there exists an additional dependence on the deposition rate:
with an increasing rate the films become more crystalline. Thin Ti0 2 layers
produced by an lAD method possess densities that are comparable with those
of IP layers and reveal a comparable behaviour [4.165].
It is not always useful to select a substrate temperature at which the
Ti02 film grows in crystalline form. For CVD methods normally lower temperatures are used so that the layers grow amorphously. But above a specific
temperature, crystallization takes place during post-deposition annealing of
amorphous Ti0 2 films (or by heating-up during application). This temperature depends on the density of the film, and therefore on the growing procedure, and also on the thickness, the composition (doping or impurities), and
the duration of the annealing procedure. These temperatures lie between
cv 250 ° C for very dense IP layers and 450 ° C for sol-gel films containing a
certain amount of Si0 2.
TEM images, depicted in Fig. 4.13, show typical morphologies of crystalline Ti0 2 layers. The films are produced by IP, RE, PICVD and sol-gel
on soda-lime glasses. All films are annealed after deposition at 500°C for one
hour. In DE films (Fig. 4.13a) and RE films (Fig. 4.13b) the anatase crystals
formed at this temperature are small, whereas in IP films they have a size
of about 1 !lm (Fig. 4.13c). In PICVD films large anatase crystals are found,
too, but their structure differs due to the incorporation of a certain amount
of chlorine (Fig. 4.13d). For sol-gel-derived films, modification of parameters during the densification process leads to sodium titanates (Fig. 4.13e)
[4.146,147] which are partially amorphous or possess a brookite modification
(Fig. 4.13f).
The crystallographic properties of Ti0 2 films that are deposited from organic solutions depend on the type of glass substrate [4.147]. An anatase
structure is always formed on alkali-free substrates. However, various types
of "disturbed" Ti02 structures appear and the type depends on the rate of
heating and to some degree on the initial compound if the glass substrates
contain easily migrating sodium ions (which are present in window glass or
plate glass). For example, layers deposited from a TiCb(OC2H 5 h solution
begin to form the brookite modification when heated rapidly (> 25°C min- 1 )
to 450°C (Fig. 4.13f). When, however, these films are heated slowly « 10°C
min -1), the diffusion of N a + ions out of the glass surfaces leads to the formation of crystal phases whose diffraction patterns resemble those of Nax Ti0 2
138
4. Properties and Characterization of Dielectric Thin Films
Fig. 4.13. TEM micrographs of Ti02 layers produced by different methods on
soda-lime glass substrates after a temperature treatment at 500 DC for one hour.
(a) DC, (b) RE, (c) IP, (d) PICVD, (e) modified DC, (f) SC
4.4 Examples of the Characterization of Thin Film Materials
139
or Na20·xTi02 [4.166]. Many comparable phases with non-stoichiometric
compositions within a wide range may be formed, too [4.167-169]. The crystallization of the Ti0 2 film can be partially inhibited by the migration of
sodium ions out of the glass surface. The films with different structures that
are shown in Fig. 4.13 also differ markedly in their optical and mechanical
properties.
In general the following results are obtained:
• The crystallization temperature decreases with increasing density of an
amorphous layer [4.107].
• The crystallization temperature decreases with increasing film thickness
[4.149,161,162]. Below a thickness of about 50nm the crystallization temperature increases strongly [4.161,162]. This is attributed to the increasing
influence of the surface energy. Up to 600°C no crystallization takes place
for layers with thicknesses below 12 nm.
• Impurities or doping strongly increase the crystallization temperature
[4.170].
• Amorphous layers with high densities crystallize very rapidly, that means,
over a small temperature range, whereas layers with lower densities crystallize slowly over a wider temperature range [4.105,107]. This behaviour is
due to the fact that in layers with higher densities only the atomic positions
are rearranged during crystallization, whereas in layers with lower densities
a transport of material by diffusion takes place. In thin layers the formation of the rutile modification generally occurs in the temperature range
of above 800°C to 900 DC. The exact temperature for this metamorphosis depends on the parameters during deposition and other pretreatments.
Impurities or doping can hinder or promote the transformation. Under special conditions the formation of the rutile modification is observed at about
350°C.
Composition
Oxide films deposited at industrial scale usually contain various other elements besides the expected components such as oxygen and metal. In part,
these are intentionally introduced by doping to obtain special film properties,
but the films also contain undesired components in very different concentrations. Special impurities in the film indicate that the film composition is not
only influenced by the deposition technique, but can also be changed during
testing procedures or applications. Table 4.3 summarizes typical compositions
of Ti0 2 films deposited by different techniques [4.17].
The hydrogen content of the films is determined by NRA. It is an indicator of the density of films, which has a strong influence on certain film
properties and is a good indicator of several film characteristics. Figure 4.14
shows some hydrogen depth profiles for Ti0 2 films, deposited by three different PVD techniques. The high amounts of hydrogen at the film surfaces
4. Properties and Characterization of Dielectric Thin Films
140
Table 4.3. Composition of Ti0 2 films deposited by different techniques
Sol-gel
PVD
CVD
DC
SC
RE
lAD
SP
IP
PICVD
TiOl.9Clo.osNao.5CaO.01Ho.04
Ti01.75 Clo.24 N aO.l Sio.o5Ho.6
Ti02.0SCO.25Sio.03Ho.l
Ti02.04CO.1Ho.25
Ti02.o4Aro.o5Tao.oo3Ho.o5
Ti01.99Moo.002Ho.Ol
TiOl.9SClo.3Ho.05
(at 6.4 MeV) indicate different coverages of adsorbed water at the interface
air/film. At an energy of approximately 7.3 MeV on the energy scale (which
is also a depth scale), the film/glass interface is located. The high hydrogen
concentration at this position indicates the presence of the hydrated glass
surface underneath the film. The low value at the interface of the IP process
indicates different predeposition conditions. The hydrogen content is quantitatively obtained and varies between H/Ti '"" 0.1 for RE and 0.01 for IP.
The small amount of hydrogen in the IP layers suggests a high film density.
As shown in Table 4.3, the sol-gel technique creates films with a fairly high
hydrogen content [4.214,215].
RES is used for an exact quantitative analysis of the other elements appearing in Table 4.3 [4.253]. The various layers obviously contain different
impurities that can be related to the deposition technique. The undesired
elements in the sol-gel process are due to solution or diffusion products from
the glass, whereas in the PVD and CVD techniques special deposition conditions may introduce impurities such as argon, which is a gas in the sputtering
0.3
... Ti02 (RE)
• Ti02 (SP)
+Ti02 (IP)
i=
0.2
I
0.1
6.5
7.0
7.5
Energy IMeV
Fig. 4.14. Hydrogen depth profiles of Ti02 films deposited on glass by reactive
evaporation (RE), sputtering (SP), and ion plating (IP)
4.4 Examples of the Characterization of Thin Film Materials
141
process, or tantalum and molybdenum, which are crucible materials, or chlorine, which is part of the starting material. The specific RE film listed in
Table 4.3, which was prepared on carbon, contains carbon and silicon in addition to titanium and oxygen. The carbon in the layer is probably induced
by residual gas components, and silicon is a vacuum chamber impurity from
previously deposited silica films [4.17, 253].
Electronic Structure
The fabrication method and stoichiometry also influence the electronic structure of Ti0 2, which can be analysed by ESCA. Pronounced features are measured at binding energies of 458.6eV and 464.4eV, induced by the Ti2p3/2
and Ti2Pl/2 states [4.171]. The ESCA spectra are fairly similar and no additional oxidation state is obtained. The measured binding energies of the Ti 2p
peaks and the splitting of the doublet (5.8 e V) indicate an oxidation state of
4+ for Ti. Therefore, the differences in the spectra can only be expressed
by changes in the full width at half-maximum peak height (FWHM) of the
Ti 2P3/2' The results are summarized in Table 4.4.
The values of the FWHM in principle fall into two extremes: The FWHM
for RE films is approximately 1.2eV, whereas IP films exhibit a fairly large
FWHM of approximately 1.4 eV. Despite the distinct film characteristics, a
straightforward explanation of the differences in the FWHM of the 2p of Ti
is impossible. The decrease in the FWHM of IP and RE films by heat treatment (IPT, RET), for example, can be interpreted either as a result of a
transition from amorphous to crystalline state, or as a result of subsequent
oxidation. Moreover, differences in density or hydrogen content may influence
the FWHM. It is a general phenomenon that oxidic thin films produced by IP
exhibit a larger FWHM than films produced by other techniques [4.171]. The
non-equilibrium ion-assisted deposition conditions create films with very high
density and low hydrogen content. The non-crystalline amorphous or vitreous structures of IP films also suggest a fairly unusual local arrangement
and binding structure between the metal and oxygen atoms, which will certainly influence the spatial distribution of the valence charge. This concept
of structure-induced charge transfer may be significant for the explanation of
the properties of amorphous oxidic thin films produced by ion plating.
Table 4.4. Full width at half-maximum peak height (FWHM) of Ti 2p state of
Ti0 2 thin films deposited by different techniques
Sample
FWHM/eV
IP
RE
DC
IPT
RET
1.37
1.22
1.20
1.32
1.19
142
4. Properties and Characterization of Dielectric Thin Films
Density
The density of films is one of the most important parameters of Ti0 2 layers and influences many other properties, for example the crystallization behaviour, the stress of the layers [4.107] and the refractive index. The density of
thin films can be measured absolutely with high precision by GIXR (grazing
incidence X-ray reflectometry), as described in Sect. 4.2.1. Table 4.5 summarizes the results for different Ti0 2 layers. In the following the densities
are expressed on a relative scale which is related to the density of anatase
(Panatase = 3.84gcm3 ). In IP layers, densities higher than 1 are detected,
whereas DC layers typically only have relative densities of about 0.75-0.85.
The densities obtained for other deposition techniques lie between these two
extremes [4.22].
The widely varying density of the films is due to their different porosity, which is indicated by incorporated water that has been absorbed from
atmospheric moisture and also changed the indices of refraction. In most processes the hydrogen content is fairly low after deposition but increases within
hours or days during storage in ambient conditions [4.29]. In Fig. 4.15 the
H/Ti ratio is shown as a function of density for Ti0 2 layers produced by
several methods. In RE layers typical Ti/H values of about 0.2 are detected.
Dense IP layers have a Ti/H value of below 0.01, whereas DC layers have a
Ti/H value of about 0.15-0.25, depending on annealing temperature. In general, post-deposition temperature treatment decreases the hydrogen content
of films. If the RE or DC layers are condensed during this treatment in some
specific cases, the reduction of the hydrogen content is permanent.
The different hydrogen contents of the layers are typical of the deposition
methods and correlate with the refractive index and the density of the layers.
With decreasing hydrogen content (H/Ti: IP < lAD < PICVD < SP <
RE < DC), the density and the refractive index increase (see Fig. 4.15).
This phenomenon is due to the decreasing porosity of the layers, into which
water, hydrogen, or oxygen-hydrogen groups are incorporated and absorbed.
The incorporation of hydrogen is not influenced by the crystallization process
itself because dense and amorphous IP layers contain the same quantity of
hydrogen as crystalline ones. Oxygen-hydrogen groups may be incorporated
into the films in a similar way as into Si0 2 (discussed in Sect. 4.4.2).
Table 4.5. Relative density (Pr = Pfilm/Panatase; Panatase = 3.84gcm- 3 ) of Ti02
thin films prepared by different deposition techniques
Deposition technique
Relative density pr
IP
RE
PICVD
DC
SP
0.95-1.02
0.75--0.90
0.8 -1.0
0.75-0.85
0.90-1.0
4.4 Examples of the Characterization of Thin Film Materials
0.3
0.2
•
a)
• •••
..
•
•
•
0.1
II)
II)
c:
,.
Q)
>
•
2.5
><
Q)
"0
.1: 2.4
tsIII
••
•
0.0
0
2.3
.t=
Q)
a::: 2.2
300
~ 200
~
t:l 100
•
••
t.
••
••
••
.
•
•
•
1----.. ------./.. -• +-',.
..........
.'\
.•
t
•
c)
-----------------------------------~--\
\
••
\
-100
400
a.
b)
\
\
\
\
o
III
143
350
••
•
..
RE
IP
PICVD
SC
~ 300
!5
(Vf
250
200
2.8
3.0
3.2
"
"
,
-- --
...:
a.
E
8
d)
/---+~
\
\
\
\
\\
\
\
\
\
I
I
T
3.4
3.6
3.8
4.0
Density /g cni 3
Fig. 4.15. (a) H/Ti ratio, (b) refractive index n at the wavelength of 550nm,
(c) stress 0-, and (d) change in stress do- as a function of density p for Ti02 films
produced by RE, IP, PICVD, and SC. The solid line in (b) represents the prediction
of the Lorentz-Lorenz theorem; the dashed lines in (c,d) are drawn to guide the
eye
144
4. Properties and Characterization of Dielectric Thin Films
The energy and mobility of the particles during the deposition process is
the most relevant parameter determining the density of the layers. For IP,
the energies of the incident particles reach 20-40 eV. Other lAD methods
work with equal or higher energies. In the RE process, in which the kinetic
energy ranges from 0.1 to 0.5eV, the mobility of the particles on the layer
surface is defined by the temperature of the substrate. DC layers have to be
converted into an oxide form by temperature treatment (at about 450 0 C).
In this procedure only the thermal energy of the particles inside the layer
during diffusion is available, which causes the relatively low density of these
layers.
With GIXR, described in Sect. 4.2.2, very thin intermediate layers with
different densities are detected at the interfaces (air/layer or layer/substrate)
[4.22]. This vertical inhomogeneity in the structure of the deposited films becomes visible if data that are typical of the deposition process are measured
with great precision. It turns out that the surface layer of films deposited
by ion plating is reduced in density, whereas the intermediate layer between
the film and the substrate of films produced by reactive evaporation is increased in density. Apparently, a reaction takes place at the substrate surface.
Sol-gel layers on glass have an intermediate layer at the interface between
film and substrate which is caused by diffusion processes during temperature
treatment.
Optical Properties
Titanium dioxide is widely used as high-refractive index material for optical
applications in the visible spectral region. As a further advantage, the losses of
Ti0 2 layers are low (low absorption and low scatter). The optical properties
of the layers strongly depend on the production technique and its conditions,
as is shown in Fig. 4.16. Here the spectral dependencies of the refractive
indices n(A) are depicted for Ti0 2 films produced by several methods. The
refractive index of the IP film is the highest, exceeding that of the layer
deposited by dip coating (DC) by dn '" 0.4. The DC and RE Ti0 2 films
possess a polycrystalline anatase phase, the other layers are amorphous.
All films depicted in Fig. 4.16 show a strong dispersion in the region
of shorter wavelengths because of the fundamental absorption of monocrystalline Ti0 2 at about 3.5eV (350nm) [4.172]. Several models for the transition type in Ti0 2 layers have been discussed; they are summarized in [4.173].
Usually Tauc's method [4.82] is applied to determine the band gap energy Eg of Ti0 2 films and the corresponding transition type, but the results published so far are contradictory. Evidence is found for indirect allowed transition by Tang et al. [4.174]' whereas a direct allowed transition
is mentioned in [4.173,176]. Meng et al. [4.176] claim Ti0 2 to be a semiconductor with a direct band gap, but dipole-forbidden transition is mentioned in [4.161,162,175]. In addition, the results obtained for Eg depend
strongly on the chosen deposition parameters [4.175,176]. Band gap energies
4.4 Examples of the Characterization of Thin Film Materials
145
- - 18S
2.8
- ---- IP
._._- PICVD
. - - - SP
c
.... ..... RE
.. - .. - SC
------ -
-.-.: .---:-:.: -:: .---: :.---:
----- - - -
.....................
2.2
400
... .. - .
600
800
Wavelength A Inm
1000
1200
Fig. 4.16. Refractive index n{oX) as a function of wavelength oX for Ti02 films
deposited by several methods
between 3.0 and 3.8eV are quoted, depending on the O/Ti ratio of the film
material [4.173], the film morphology (amorphous, anatase or rutile crystal
phase) [4.174,177], and on post-deposition annealing [4.173]. Consequently it
must be assumed that the dispersion of n( A) is also strongly dependent on
film properties and deposition conditions.
However, the results given in Fig. 4.16 suggest that the dispersion of the
different layers is fairly similar, irrespective of the differences in the absolute
values and the crystalline state. This finding seems to be also in agreement
with dispersions obtained for Ti0 2 layers [4.178]. They are mainly distinguished only by a certain constant factor no, which can be chosen as the
refractive index at a given wavelength Ao (e.g. 550nm). An approximation
which describes the general behaviour ofn(A) for all films in Fig. 4.16 is given
by
(4.15)
A is given in J.l.m. n550 is the refractive index at wavelength Ao = 550 nm. In
the visible region (400- 1000 nm) the deviations from the individual spectral
n(A) curves are less than ±1%. This approximation is also valid in the spectral
range up to 2200 nm for stoichiometric RE and IP Ti0 2 films .
The achieved absolute value of refractive index mainly depends on the
chosen deposition method, but also on the specific production parameters.
In Table 4.6 typical results are listed for n at 550 nm wavelength for Ti0 2
layers produced by different techniques. Most of the films quoted in Table 4.6
are amorphous, some consist of polycrystalline anatase, but none of rutile.
In addition, refractive indices higher than 2.7 are found at 633 nm wavelength for Ti0 2 films with a polycrystalline structure of rutile, deposited by
a filtered arc evaporation method [4.182] or by reactive ionized cluster beam
146
4. Properties and Characterization of Dielectric Thin Films
Table 4.6. Typical ranges of refractive indices at 550 nm wavelength for Ti0 2 films
deposited by the quoted methods
Deposition method
Refractive index
at 550 nm
Reference
Sol-gel deposition
(spin coating or dip coating)
2.28
2.23-2.30
2.19-2.21
2.18-2.20
2.23-2.31
2.45
2.21-2.36
2.49-2.53
2.42
2.46-2.47
2.46-2.52
2.42-2.54
2.45
2.34-2.51
2.3-2.52
2.44-2.48
-2.51 (tempered)
2.43-2.49
2.49-2.52
2.56
[4.18]
[4.18]
[4.178]
[4.178]
[4.18]
[4.78]
[4.178]
[4.178]
[4.181]
[4.18]
[4.78]
[4.178]
[4.253]
[4.178]
[4.180]
[4.18]
[4.18]
[4.179]
[4.18]
[4.78]
Activated reactive evaporation
Reactive evaporation
Rf diode sputtering
Reactive magnetron sputtering
Ion beam sputtering
Ion-assisted deposition
Plasma impulse CVD
Reactive ion plating
deposition [4.183J. In Table 4.7 the refractive indices at Ao = 550 nm and the
corresponding densities of mono crystalline bulk Ti0 2 are listed for comparison [4.184, 185J. The crystal phases are uniaxial or biaxial, respectively, and
the refractive indices are different for the different axes.
In PVD and CVD methods the refractive index depends strongly on substrate temperature, total gas pressure, deposition rate, kind of target material or precursor/reacting-gas composition and on the specific energy of the
deposited particles [4.173,176,179,191, 233J. In the sol-gel techniques the
chemical compositions of the solutions and the temperature of the densification process ofthe films determine their refractive behaviour [4.186,187, 192J.
Table 4.7. Densities and refractive properties of several Ti0 2 bulk materials
Crystalline phase
Density pig cm- 3
Refractive index at 550 nm-
anatase
brookite
rutile
3.84-3.90
4.13-4.17
4.22-4.26
2.51,2.59
2.612, 2.613, 2.73
2.63,2.94
- values at 550nm are interpolated (see (4.15)) from the results measured at
546.1 nm and 589.3 nm given in the references
4.4 Examples of the Characterization of Thin Film Materials
147
In general, the deposition conditions that increase the density of films are also
responsible for an increase of n(>').
The refractive indices of Ti0 2 films are significantly lower than the bulk
values for most of the deposition conditions in Table 4.6. Especially porous
films as SC or RE layers with a large capacity for taking up water (see
Fig. 4.16a) show the lowest n values. An effective refractive index is determined by the composition of film material and voids (Sect. 4.2.2). Therefore
the porosity, i.e., the film density p, is an important factor which is responsible for the absolute value of n. An approximately linear relation between n
and p is expected according to the Lorentz-Lorenz theorem.
In Fig. 4.15b the refractive index of SC, RE, PICVD and IP Ti0 2 films is
plotted as a function of the film density. The corresponding optical properties
of anatase are also shown. The refractive indices of the different films increase
linearly with density and almost independently of the production method
used and the morphology of the films. The results given in Fig. 4.15b are
in good accordance with published data for PECVD [4.150] and SC Ti0 2
films [4.188], but differ remarkably from the results given in [4.189]. Moreover,
the correlation between nand p does not agree with the prediction of the
Lorentz-Lorenz theorem (see Sect. 4.2.2, and solid line in Fig. 4.15b).
The absorptive losses of Ti0 2 films strongly depend on the production conditions [4.148,166,178]' the stoichiometry [4.173]' the impurity content [4.78] and the morphology [4.182,192] of the layers. The spectral extinction coefficient k(>') of pure amorphous Ti0 2 coatings is usually smaller
than 0.001 in the visible spectral range [4.178,190,192]. These low absorption
properties of Ti0 2 single layers must be determined by PTD, ellipsometry, or
wave-guiding investigations because spectral transmission measurements are
not sensitive enough. Ti0 2 films for high-power laser or wave-guide applications are optimized for very low absorption losses. The extinction coefficients
typically take values of 1 x 10- 4 for RE layers and 2 x 10-4 for IP layers [4.193]'
but for IP films values as small 5 x 10- 6 are reported [4.179]. The absorptive
losses of PICVD films are also small; values of k < 1.5 X 10- 5 [4.194]' measured by the waveguiding technique, are in agreement with k = 1 X 10- 5 found
by PTD investigations [4.193]. Ti0 2 films produced with ion-beam sputtering
(IES) show k values down to 1.1 x 10- 5 [4.78]. Interfacial losses discovered
by PTD technique (Sect. 4.2.2) are negligible for IES and PICVD layers. RE
Ti0 2 layers have losses at the interface air/film, whereas IP Ti0 2 layers possess absorption at both interfaces, Le., at air/film and film/substrate [4.193].
The scattering losses of Ti0 2 films measured by TIS investigations depend also on the deposition conditions. The microstructure (Le., the grain
size and the surface roughnesses of crystalline films) is the major source of
scattering [4.78,195]. A dramatic increase in elastic scattering of two orders
of magnitude is induced in initially amorphous Ti0 2 films by crystallization
during thermal annealing. Especially Ti02 films with a crystal structure of
rutile are expected to show strong scattering because of the pronounced biaxial refractive properties of the grains (Table 4.7). Therefore films of this type
148
4. Properties and Characterization of Dielectric Thin Films
are not desired for optical applications even though they possess the highest
refractive indices for Ti0 2 coatings.
Reactively evaporated Ti0 2 layers exhibit a columnar structure typical
of this process, even in amorphous films. The voids within the films and their
pronounced surface roughness are responsible for the scattering losses [4.78].
The respective losses of denser IP and IBS Ti0 2 films are smaller because
of their reduced microstructure and their smoother surfaces. Ti0 2 films produced by these methods possess large compressive stress. If the film thickness
exceeds a certain threshold value (typically of the order of 100 nm, depending on production conditions) rutile grains are spontaneously formed. These
crystal grains significantly increase the scattering [4.78]. Moreover, the highreflecting Ti0 2 coatings increase the scattering effect of the substrate surfaces. Therefore the polishing quality must be improved to reduce the scattering losses of highly sophisticated coatings.
Stress
Ti0 2 films deposited on a substrate usually exhibit an interaction stress
which is inherent in the film/substrate system (Sect. 4.2.4). Ti0 2 films possess remarkably different stresses, depending on the coating technique and
the respective deposition parameters. Tensile stresses of up to +3 CPa have
been reported for ion-assisted deposition [4.197]' whereas for films produced
by rf sputtering compressive stresses of up to -5 CPa are obtained [4.198].
Stress is found to depend on film properties such as thickness; in RE Ti0 2
films [4.199]' for example, tensile stresses of 300 MPa are measured. The annealing of amorphous Ti0 2 layers also causes variations in the film stress
due to changes in the morphology [4.103]. In addition, impurities in films or
admixtures influence the stress properties, too [4.78,200].
Stress is also strongly related to film density. This is demonstrated in
Fig. 4.15c, where stress values are depicted as a function of film density, obtained at about 100-nm-thick Ti0 2 layers deposited on fused silica under different deposition conditions. The substrate temperatures during production
range from 140°C to 300 °C, depending on the deposition method. Thermal
stress contributions are estimated to be small compared with the measured
total stress values, so that their influence can be neglected. Therefore the
stress data in Fig. 4.15c represent mainly the interaction stress of the film.
In their strong dependence on density these pure Ti0 2 films show a similar
characteristic behaviour as films produced by sputtering [4.212,213]. Films
with densities far lower than those of Ti0 2 in the crystalline state of anatase
possess tensile stress. The stress increases with increasing density up to a certain maximum of about 400 MPa. A further increase in density leads to stress
reduction, and films with densities of the order of anatase show compressive
behaviour.
Differences in coating properties that alter the growth conditions of the
films, especially the mean distances between the atoms, are believed to be
4.4 Examples of the Characterization of Thin Film Materials
149
the origin of this phenomenon. The observed density dependence of stress can
be explained by a potential model of the interatomic interactions. Forces are
assumed to act on particles whose separations are different from those of the
equilibrium structure. Films with densities equal to or higher than that of
their crystal phase possess repulsive forces causing compressive stress. A tensile force is obtained in films of lower density, showing attractive interactions
between the atoms. Tensile stress will increase with decreasing density. But
a stronger decrease in film density leads to a more porous film structure with
reduced strength. Therefore, tensile stress decreases with decreasing density,
as shown in Fig. 4.15c. The contamination of the film material with nonbridging species such as oxygen-hydrogen groups, which will not support the
bonding network of the film material, also leads to a reduction in film stress,
as shown in Fig. 4.15c.
The annealing of amorphous Ti02 films in general causes a transition to
the anatase phase which increases the film stress [4.105,107]. The temperature for that phase transition and the speed of the crystallization depend
on film thickness, density and contamination. Figure 4.17 shows the stress
dependence on annealing temperature for an about 100-nm-thick Ti0 2 film
produced by different methods and under different conditions. For this representation only films without significant amounts of impurities are chosen. The
samples are annealed at temperatures between 25°C and 600°C for 20 min
per pass. After each annealing step the stress is measured at room temperature. In all cases the amount of tensile stress increases due to phase transition [4.105]. The corresponding starting temperature and range are mainly
determined by density.
700
ca
D..
:2
600
_____ RE
____ IP
500
_____ PICVD
-.A-SC
400
b
300
~
200
U)
U)
(J)
100
0
-100
0
• ••••••
100
r····~
200
300
400
500
Annealing temperature T rc
•
600
Fig. 4.17. Stress (j as a function of annealing temperature for Ti0 2 films deposited
by PICVD, RE, SC and IP
150
4. Properties and Characterization of Dielectric Thin Films
Figure 4.18 shows TEM images of different films, which are obtained for
temperatures corresponding to the beginning, middle and end of the rise in
stress. In the RE film with the lowest density (2.98gcm- 3 ), the formation of
crystal grains begins at a temperature of 270°C and is completed at about
300 °C under the conditions mentioned above. With increasing film density
(PICVD: 3.78gcm- 3 , IP: 3.87 gcm- 3 ) the temperature range and the starting temperature for crystallization are continuously reduced. In the IP film
the phase transition is completed within a temperature range of only 20 K,
starting at a temperature of 240°C. In addition, the size of the formed crystal
grains also depends on density. In RE films the grains have sizes of 0.2 Il-m;
in IP and PICVD films with densities close to that of anatase they have sizes
of about 11l-m. For comparison, Fig. 4.17 also shows the annealing behaviour
of a sol-gel film which is already crystalline after the production process. In
this case no major increase in stress is observed because there is no phase
transition.
Figure 4.15d depicts the changes in stress da as a function of film density
for Ti0 2 films deposited by several methods. In films with densities below
bulk value, da increases with increasing density. The increase in stress is much
smaller in layers with higher densities. The stress of Ti0 2 layers relaxes as
a function of time after deposition. The amount and speed of the relaxation
process depend on the production conditions, but the relative humidity of
the surroundings has no influence [4.105].
Adhesion and Other Mechanical Properties
Knowledge of the stress properties of Ti0 2 films is also important in respect to the adhesion properties of the coating. Adhesion strongly depends
on pretreatment and production conditions. The adhesion strength increases
with increasing substrate temperature [4.201-203] and with increasing energy of the condensing particles [4.201,204,205]. Good adhesion properties
are achieved by one or more of the following three mechanisms: intermediate
phase formation, new bonding configurations at the interface and lowering of
the interfacial energy between the material.
Adhesion properties are obtained by scratch-test investigations using
Baba's method (described in Sect. 4.2.4); the adhesion properties of Ti0 2
layers deposited on fused silica substrates are obtainable by several different
methods [4.127,128]. Figure 4.19 shows the resulting critical loads Lc and
their respective stress properties, obtained for 100-nm-thick Ti0 2 films. Lc
is normalized to the critical load obtained for cracking of an uncoated fused
silica substrate surface (cohesive failure). As-deposited films, with the exception of SC layers, show compressive or tensile stress and good adhesion
properties. IP and PICVD films deposited by plasma assistance have the best
adhesion properties, which also seem to be influenced by film stress. IP Ti0 2
films show compressive stress and the highest critical load. Tensile stress reduces the film adhesion. The tensile stresses of RE and PICVD layers is quite
4.4 Examples of the Characterization of Thin Film Materials
w
0:::
151
c
o>
Il.
Fig. 4.18. TEM images of Ti02 films produced by RE, IP and PICVD obtained
after annealing for 20 min at given temperatures
152
4. Properties and Characterization of Dielectric Thin Films
c:::J Before
_
After annealing
Ti0 2 I fused silica
R
= 15 ~m
E
§,
(.)
...J
l5.
IP
RE
PICVD
SC
E
o
u
Fig. 4.19. Normalized critical load Lc and stress (J" of 100-nm-thick Ti0 2 films
deposited on fused silica by IP, RE, PICVD, and SC before annealing (light grey
bars) and after annealing (dark grey bars) at 600 0 C for 30 min
similar and the adhesion also shows no major difference beyond the quoted
uncertainties. The tensile stress of the SC layers is lower, but the adhesion is
heavily reduced. This phen6menon may be caused by the different, namely
polycrystalline, morphological structure of the SC layers. In addition, the
thermal energies of the sol-gel process are lower than the kinetic energies of
the particles of other deposition methods; this may also reduce the bonding
strength in the interface region.
The Ti0 2 films are annealed at 600°C for 30 min to investigate the influence of the temperature treatment on the adhesion properties. All amorphous
films show a significant decrease in adhesion properties due to the annealing
procedure, whereas the adhesion of the SC layers increases due to the higher
densification temperatures during the treatment. The annealing of amorphous
Ti02 layers at temperatures above 400°C causes a phase transition to polycrystalline anatase state correlated with an increase in tensile stress, as shown
in Fig. 4.19. The reduction of the adhesion strength seems to be correlated
with the increase in stress. But this conclusion may be equivocal because
4.4 Examples of the Characterization of Thin Film Materials
153
the morphology of the films was changed by the phase transition. There is
evidence that the hardness and the Young's modulus of Ti0 2 films also differ
from the respective bulk quantities [4.182,200,201]. Moreover, these mechanical properties are expected to be density dependent.
Roughness
The roughness of Ti0 2 layer surfaces and interfaces is an important parameter for the fabrication of optical components with low optical losses [4.4]. The
roughness of surfaces and interfaces can be expressed by the rms (root mean
square) value. Additionally, the analytical method that was used to determine
this parameter must be described, because different methods yield different
rms values due to their different correlation lengths which, in general, are not
comparable with each other.
The surface roughnesses of Ti0 2 layers are predominantly determined by
the deposition method and, to a lesser degree, by post-deposition temperature
treatment [4.24]. The topography of the surface can be directly visualized by
AFM. Figure 4.20 presents two AFM images of Ti0 2 layers on highly polished
Schott BK 7® glass. The surface of the RE layer is distinctly rougher and more
coarsely structured than the surface of the IP layer. The rms values of IP
layers (4.58 ± 1.65 nm) and RE layers (7.72 ± 0.52 nm) support these findings.
After crystallization (due to post-deposition temperature treatment), the rms
roughness values of crystalline film surfaces are two to four times higher than
those of amorphous film surfaces.
The surface roughness can also be measured with high precision by
GIXR [4.22]. Typical rms values for Ti0 2 are 0.87 nm for IP, 2.24 nm for
RE, and 0.68 nm for DC layers. The remarkably low value for DC layers is
due to the special fabrication method by the sol-gel route. Additionally, the
interface roughness between film and substrate, which is also detectable by
GIXR, is significantly lower in IP films than in RE films [4.22]. ARS and TIS
also give information about the roughness of surfaces and interfaces through
the associated optical losses (Sect. 4.2.2).
Diffusion Process in Dip Coating Layers
Sol-gel films are transformed into oxides during a post-deposition thermal
treatment [4.147]. For Ti0 2 this "baking" happens at between 400°C and
500°C (Chap. 3). On alkali-rich substrates (soda-lime glass), considerable
amounts of sodium and potassium diffuse across the glass surface into the
deposited layer when the temperature exceeds 200°C. Above 500 °C diffusion
of calcium takes place, too. As described above, the diffusion of sodium is
crucial for the formation and crystallization of the Ti0 2 layer and it depends
on the temperature/time treatment in a complicated manner. Diffusion and
crystallization are additionally influenced by the preceding drying of the solgel films at temperatures between 100°C and 250 °C.
154
4. Properties and Characterization of Dielectric Thin Films
a)
100
1
o
I
IJm
b)
100
1
o
I
Fig. 4.20. Atomic force microscopy images of Ti02 film surfaces on Schott BK 7®
glass by (a) reactive evaporation and (b) ion plating
To demonstrate diffusion, three depth profiles of sodium for a Ti0 2/8i0 2/
Ti02 coating on soda- lime glass are shown in Fig. 4.21. The samples are
heated up to 400 e e (densification, solid line), annealed at 500 e e for one
hour (dashed line), and tempered at 680 e e (dotted line). After fabrication
(densification at 400 e C), a depletion layer is present on the surface of the
glass substrate, and the lacking sodium is located within the three layers.
A subsequent thermal treatment at 500 ee for one hour causes further diffusion processes that change the sodium profiles remarkably. The depletion
layer on the glass surface nearly vanishes and the sodium content within the
three layers increases. A thermal treatment at 680 ee, which happens during
the tempering of the samples, changes the diffusion profiles, too. The depletion layer vanishes completely and the sodium content within the layers
decreases. This effect is caused by the complete crystallization of the Ti02
layers, whereby sodium is squeezed out of the layers. The 80 2 content in the
ambient atmosphere during tempering causes the formation of Na280 4 on
4.4 Examples of the Characterization of Thin Film Materials
155
Soda-lime glass
200
,,--"'---
Na depth profiles
-------
,/~/.".-.
,
/
,/
::J
~
Z.
'iii
c:
.l!l
c:
f'
150
100
I
\'\1,
',
,
o
500
1000
1500
/
.,.,
\\._ ......
50
2000
II
-------. 680·C
_._._.- 500·C
- - 400·C
2500
3000
3500
4000
4500
Time/s
Fig. 4.21. SIMS depth profiles of sodium for a three-layer system
(Ti02jSi02jTi02) on soda-lime glass after temperature treatment at 400 °C (densification), 500 0 C (additional thermal treatment), and 680 0 C (tempering)
the surface of the layers. The decrease of the sodium content in Ti0 2 layers
during crystallization is a typical phenomenon that is also connected with the
densification in this process stage. At the interface between the Si0 2 layer
and the underlying Ti0 2 layer a significant sodium enrichment is obtained,
caused by the superior blocking of the Si0 2 layer of the diffusion of sodium.
During the tempering process, sodium and calcium show opposite diffusion behaviour, which is demonstrated in Fig. 4.22 for a Ti0 2 layer on
soda-lime glass. Whereas sodium fills up the depletion layer on the glass Surface by diffusion from the bulk into the surface region, a depletion layer for
calcium is formed during tempering. The two diffusion processes are coupled
and opposite.
As described above, depending on the thermal treatment, anatase, brookite, or a mixture of both modifications are obtainable on soda-lime substrates
in thin Ti0 2 layers which can contain a certain amount of Nax TiO y phases.
In some cases the crystallization process is inhibited by the sodium diffusion
so that the films remain amorphous. Often the substrates are coated with
a thin layer of Si0 2 (30 nm) before the Ti0 2 film is deposited because the
described diffusion processes depend on the kind and the quality of the sodalime glass (Le., different manufacturers or temporal deviations). This ensures
a widely uniform and constant surface quality of the substrate and, due to
the defined blocking of alkali diffusion, helps regulate the formation of the
Ti0 2 layer.
156
4. Properties and Characterization of Dielectric Thin Films
Soda-lime glass
................................................................
250
Na depth profiles
200
::i
.!::
Z-
150
'iii
100
E
50
c
$
//......
.'.
:~
- - Densified
.............. Tempered
0
................
200
:)
.!!!
Z-
'iii
c
Q)
'E
.........
Ca depth profiles
,
150
... "
100
- - Densified
.............. Tempered
50
".
0
o
500
1000
1500
2000
2500
3000
3500
Time Is
Fig. 4.22. SIMS depth profiles of sodium and calcium for a Ti0 2 (DC) layer on
soda-lime g la ss after densification (solid line) and after tempering (dotted line)
Laser Annealing
Besides by temperature treatment inside an oven, sol-gel thin films can also
be densified and transformed into an oxidic form by CO 2 laser irradiation
(,\ = 10.6 ~m) [4.208- 210J. Because it is impossible to densify large areas of
sol-gel films with a single laser shot , a scanning laser beam system must be
used. This method has several advantages:
• The temperature region between 300 °C and 700 °C can be passed quickly.
At a heating rate of > 30°C S-l the formation of anatase is suppressed
and above 700°C the crystallization in the rutile modification takes place.
Ti0 2 layers with a refractive index of up to 2.6 can be reached, which in
general cannot be used as optical coatings.
• Only a small amount of the power from the incident laser beam « 1%) will
be absorbed in the sol- gel film itself. The primary heating occurs within
a small region at the substrate surface. The depth of this region can be
restricted by selecting suitable parameters to less than 10% of the diameter
of the laser beam. Because the temperature of the bulk glass remains below
80°C, a treatment of tempered glass substrates is possible without inducing
stress and relaxation.
• Due to the short local heating period, sodium enrichment at the substrate
surface is observed, but no diffusion of sodium into the thin films .
4.4 Examples of the Characterization of Thin Film Materials
157
• Ti0 2 crystallizes in the anatase modification. The average grain sizes are
a factor of four to ten smaller than those obtained in the oven process.
• Structured films for decoration purposes or sensors are easily obtainable
by using masking techniques and by washing off the nondensified film.
The most relevant parameters determining the quality of the laserdensified films are the power density of the laser beam, the scanning beam
speed, the distance between the laser tracks, the diameter of the beam, the
pulse frequency and the pulse duration. The optimum choice of these parameters depends on the kind of substrate and on the chemistry of the coating
solutions. A significant parameter concerning the uniformity of the films is
the stability of the whole laser system. The stability has to be < ±2% for the
integral output power, and < ±1 cm S-l for the scanning speed to obtain
a uniform refractive index to within ±0.005 over the whole sample. Further
observations [4.210]:
• The leached surface layer thickness is decreased in connection with the
dehydration of the substrate surface. An increase of the sodium concentration at the surface for uncoated substrates is obtained, beyond a threshold
value, which depends on the type of substrate.
• Surface melting occurs with the formation of a wall-type relief structure
by increasing the deposition energy above this level. For coated substrates
this leads to cracking and delamination of the films, which is a drawback
of this deposition method.
• Extinction coefficients are correlated with the refractive index for these
sol-gel Ti0 2 films. They do not differ significantly from those obtained by
conventional densification from the same sol-gel solution, in spite of the fast
densification process. This is shown in Fig. 4.23. On the left, the dispersion
relation of densified Ti0 2 thin films is compared with those deposited by
other methods. The data are partly taken from [4.18]. Substrates with
densified films of n = 2.4 exhibit a large number of cracks. On the right of
Fig. 4.23, the extinction coefficient versus refractive index of a variety of
laser-densified and oven-densified Ti0 2 thin films is shown. Even for the
samples with the highest refractive indices there Seems to exist a correlation
between these optical film properties. But the extinction coefficient is more
than one order of magnitude higher than for PVD layers. This phenomenon
is probably caused by a nonstoichiometric formation such as Ti0 2 - x and by
the contamination with carbon, which increases with increasing refractive
index.
• The hydrogen content closely correlates with the density and the refractive
index.
Thermal Conductivity
The damage thresholds of Ti0 2 layers under laser irradiation depend, among
other things, on the optical and thermal properties of the films and on the
158
4. Properties and Characterization of Dielectric Thin Films
4.0 . - - - - - - - - - - - - - - - - ,
•
2.8
c:
X
Q)
"0
.!:
3.0
N
I
0
~
2.6
•
•
•
•
~
.Ii o·
.:.!.
c:
Q)
>
~
0
U
c:
-.-
'" 2.4
~
cr:
~
w
2.2
200
2.0
1.0
•
400
600
800
1000
Wavelength A Inm
1200
•
....
0.0 +-----,~-____r--__,_---l
1.8
2 .0
2 .2
2 .4
2 .6
Refractive index n at 550 nm
Fig. 4.23. Left: dispersion relation of densified Ti02 thin films compared with
those deposited by other methods; from top to bottom: ion plating, laser-densified
(cracked), reactively evaporated, oven-densified, laser-densified. Right: extinction
coefficient vs. refractive index for a variety of laser-densified and oven-densified
Ti02 thin films ; open/closed circle: oven/laser-densified on Tempax®; open/closed
squares: oven/laser-densified on float glass [4.210]
film thickness. In comparison with traditional electron evaporation, films produced by ion-assisted processes are denser, harder and more stable with low
loss and good adhesion. The films show less or no columnar microstructures
and have refractive indices close to their corresponding bulk material. For
these reasons various attempts have been made to employ these techniques
in the production of components for high-power laser applications.
Figure 4.24 summarizes the results on RE and IP films [4.207]. The data
demonstrate the dependence of damage threshold and thermal conductivity
on the optical thickness nd. IP Ti0 2 films show a lower damage threshold
than RE samples. For both kinds of samples the strongest correlation is found
between absorption and damage threshold, indicating that absorption is the
relevant factor in the damage process [4.207] . Against expectation, the thermal conductivity does not correlate with the damage threshold data [4.211];
presumably the averaged thermal conductivity of thin films, although important in heat dissipation, does not always play the major role in the short-pulse
laser damage process of thin film coatings.
A mechanism based on stress perhaps explains the differing damage
thresholds of the two Ti0 2 films. Under laser irradiation the local temperature rise, induced by optical absorption and thermal inhomogeneities, leads
to a thermal expansion of the films and creates a relaxation of the tensile
stress in the RE samples, but an enhancement of the compressive stress in
the IP films. The results suggest that the damage threshold of an IP Ti0 2
film can be improved with optimized deposition parameters (e.g., an appro-
4.4 Examples of the Characterization of Thin Film Materials
'I 0.8
:.::
'I
E
~ 0.6
15
• RE
o
N
I
IP
~
g 0.4
I
"0
c
0
E
Q;
.c:
I-
0.2
0.0
I
0.0
0.5
I
I
1.0
Optical thickness n·d (I-a
....,<.>
I
::;;10
(5
I
.c:
(/l
~
£;
Q)
OJ
I
co
E
co
• RE
I
E
'>
<.>
(ij
159
o
I
I
5
IP
I
I
0
1.5
2.0
=514 nm)
0
0.0
0.5
1.0
Optical thickness n·d (j"o
1.5
2.0
=514 nm)
Fig. 4.24. Left: thickness-dependent thermal conductivity for RE (closed circles)
and IP (open circles) Ti02 thin films measured by the mirage method. Right:
thickness-dependent laser damage threshold (0.532Il-m, 10 ns Nd:YAG) for RE
(closed circles) and IP (open circles) Ti02 thin films [4.207]
priate arc current) which lead to a low compressive or tensile stress in the
films [4.107].
4.4.2 Silicon Oxides
Olaf A nderson, Clemens Ottermann
Silicon oxide (Si0 2 ) is a versatile material, which serves for example as lowrefractive index material in optical coatings, as diffusion barrier or protective
coating on glasses, as electrical insulating layer in integrated circuit devices
and as solid-state electrolyte with good ionic conductivity in optical switching
devices. Various deposition techniques are used for the preparation of Si0 2
films to create very different film properties. The microscopic and macroscopic
quantities influenced by the deposition conditions will be discussed in this
section.
Si0 2 layers are usually amorphous and therefore lack the long-range order
present in crystalline materials. Elementary Si0 2 cells have a tetrahedral configuration, each oxygen atom being bound to two silicon atoms by a sharing
of valence electrons. Neighbouring silicon atoms share an oxygen atom (oxygen bridging) with a mean value of the bond angle of 144 0 , although a spread
between 110 0 and 180 0 is present [4.216]. Si0 2 bonds have mixed covalent
and ionic character. However, the covalent energy of the silicon-oxygen bond
is almost four times higher than the polar energy; therefore Si0 2 should be
considered to be covalent with partly ionic character rather than the other
way round [4.217].
160
4. Properties and Characterization of Dielectric Thin Films
Density
The density of Si0 2 films is characteristically determined by the deposition
conditions, i.e., the deposition process and the parameters used. A high density is important to prevent the adsorption of water vapour from the ambient
atmosphere, which leads to a shift in the spectral characteristics of the layers (e.g., aging) and to the appearance of water absorption bands in the IR
spectral region. Additionally, the efficiency as diffusion barrier against alkali
increases with higher density. The achieved density is 1.7 g cm -3 for layers
produced by sol-gel techniques. Layers deposited by lAD or IP have a density
of 2.3gcm- 3 , which is higher than that of fused silica (p = 2.2gcm- 3 ).
In general, Si0 2 layers grow with an amorphous structure independent
of the deposition process used. Annealing up to 1000 ce causes no phase
transition to crystal structure. The density of a film therefore is mainly determined by its porosity, which can be decreased by increasing the substrate
temperature, by changing the deposition rate or the gas pressure, by choosing different precursors for eVD processes, or by an additional bombardment
with energetic ions (50-100eV) from an auxiliary ion source during deposition [4.218].
The density of Si0 2 films can also be increased by thermal treatment after
deposition. The content of incorporated hydrogen decreases and permanently
remains at this lower level. The refractive index increases simultaneously. Because no crystallization takes place, the changes are attributed to a decrease
in the porosity of the layer [4.219]. In contrast, a decrease of the refractive
index after annealing does not necessarily indicate a decrease in density because the oxidation of existing suboxides in the layer (SiO x , x < 2) will
also reduce its refractive index [4.219]. Flash annealing of silica films immediately after deposition relaxes the bond angle distribution and increases
their stability against attacks of atmospheric water [4.224]. Two main mechanisms contribute to the chemical inertness: (a) the relaxation of the Si-O-Si
bond angle distribution to larger bond angles, and hence less reactive configurations, and (b) the film densification, which limits the incorporation of
atmospheric H 20 and also of etchants such as buffered HF. The densification of sol-gel Si02 films which have a porous structure with residual Si-OH
groups can be explained by the siloxane bond formation derived from the
condensation between silanol groups during heat treatment [4.21]. However,
they remain porous even after annealing at 500 ce.
UV light irradiation during deposition improves the quality of Si02 films
produced by reactive ion-beam sputtering. Good densification and stoichiometric properties are obtained due to chemically active atoms that are produced by irradiation and enhance the chemical reactions for the Si0 2 growth.
Additionally, the deposition rate increases by a factor of five in comparison
with the process without UV treatment [4.225]. The creation efficiency of
oxygen-vacancy centre defects and non-bridging oxygen-hole centre defects
4.4 Examples of the Characterization of Thin Film Materials
161
in amorphous Si0 2 by ionizing radiation depends significantly on the OH
content of the network [4.226].
The density of thin Si0 2 films can be determined directly and with high
precision by GIXR (grazing incidence X-ray reflectivity). With this method
even small differences in density of substrate and layer of about 1-2% can be
measured. In Fig. 4.25 the reflectivity of Cu K", radiation is depicted versus
the angle of incidence for a 120-nm-thick Si0 2 layer deposited on fused silica by the ion plating process. The densities of film and substrate differ by
2.5%; this causes the weak oscillation of the measured curve. Typically, the
uncertainty in density determination by GIXR is less than ±1 % for flat and
well-polished samples.
Composition and Electronic Structure
Besides the expected components Si and 0, other elements may be contained
in silica films as dopants or impurities. Layers produced by CVD processes
contain various amounts of hydrogen and sometimes, as additional impurities, chlorine or carbon, which influence the density and the porosity. Silica
films produced by sol-gel techniques contain a relatively high amount of hydrogen or oxygen-hydrogen groups due to their manufacturing method. The
impurities contained in Si0 2 layers deposited by PVD techniques are typical of the respective method: Ar, for example, is typical of sputtering, and
crucible oxides are typical of thermal evaporation. Substoichiometric SiO x
films possessing an excess of Si can also be produced by specific deposition
conditions.
Hydrogen or hydrogen-containing components are important contaminants because they strongly influence the material properties of the silica
1 00 ~~--------------------------------~
10- 1
120 nm Si0 2 (I P) / silica
silica : p = 2.20 g cm- 3
Si0 2 : p = 2.15 9 cm- 3
10-2
-'='
:~
10-3
~ 104
c;:::
Q)
a:: 10"5
10-6
10"7
~~~~_!1IMtJ
0.0
0 .5
1,0
1.5
2 .0
2 .5
3 .0
3 .5
4 ,0
Angle of incidence 1') /deg
Fig. 4.25. Reflectivity of a Si02 (IP) layer on fused silica for Cu K", radiation
(8.04 keY) versus angle of incidence
162
4. Properties and Characterization of Dielectric Thin Films
films. In most PVD processes the layers are nearly free of hydrogen after
deposition. The porosity of the films enables an incorporation of water from
atmospheric moisture, which also causes changes in the refractive properties [4.204,205]. The hydrogen content increases significantly for hours or
days and only reaches its final value after 3~4 weeks [4.29,215]. The H/Si
ratio detected in IP films is 0.05, and in sol~gel layers it is 0.4.
Hydroxyl incorporation into oxides has two origins: (a) intrinsic pathways, which are associated with the homogeneous chemical reaction responsible for film growth, and (b) extrinsic pathways, which refer to incorporation after film deposition. IR investigations show that most of the detected
OH derives from post-deposition or extrinsic sources, i.e., from the deposition chamber ambient during cooling and from atmospheric moisture. OH
incorporation from atmospheric moisture occurs in spatially correlated nearneighbour Si~OH groups, whereas OH groups incorporated in the deposition
chamber are randomly distributed in the Si0 2 host material [4.220]. Atmospheric H 2 0 preferentially reacts with Si~O~Si groups with smaller bond angles. These bonds are more reactive than relaxed Si~O~Si bonds and perhaps
more vulnerable to attack because of the localized increase in the bond-free
volume that is evident in the increased etch rate of these films in buffered
HF [4.22,221,223].
For all Si0 2 films the Si 2p states are observed at 103.5 eV in the
ESCA spectra, suggesting a Si4+ valence state independent of the deposition method. No additional features can be recognized in the data, and the
deconvolution of the spectra only results in the decomposition of the doublet
structure. But analogous to other oxides, the FWHM of the Si 2p peaks does
depend on the deposition method. The data are summarized in Table 4.8.
All FWHM results are approximately the same except for IP. The film produced by IP exhibits the broadest FWHM, analogous to Ti 2p described in
Sect. 4.4.1. The decrease of the FWHM of the Si 2p peak after heat treatment
cannot be explained by a transition from an amorphous to a crystalline state
(as in the case of Ti0 2 films) because the films remain amorphous.
It is a general phenomenon that thin oxide films produced by IP exhibit a
larger FWHM than films produced by other techniques. The non-equilibrium
ion-assisted deposition conditions create films with a very high density and
Table 4.8. Full width at half-maximum peak height (FWHM) of Si 2p state of
Si0 2 thin films prepared by different techniques
Sample
FWHM/eV
IP
1.94
1.84
1.70
1.74
1.76
RE
DC
IPT
RET
4.4 Examples of the Characterization of Thin Film Materials
163
a low hydrogen content. The density of Si0 2 films produced by IP lies in
same range as the bulk density of quartz glass, in some case it may even be
higher [4.227]. Also, the non-crystalline, amorphous and vitreous structures
of IP films suggest a fairly unusual local arrangement and binding structure
between the metal atoms and the oxygen atoms which will certainly influence
the spatial distribution of the valence charge. This will lead to changes in
the core level binding energies, which are smaller than those produced by a
change in the oxidation state. Studies of the network of quartz glass such
as amorphous Si0 2 show that small changes in the Si-O-Si bond and the
bridging bond angle are correlated with variations in the valence charge that
influence the chemical binding state of the Si 2p level [4.228]. This concept of
structure-induced charge transfer may be significant for the explanation of
amorphous oxidic thin films produced by ion plating.
Because sol-gel films are more porous than evaporated films, some intermixing of Ti0 2 and Si0 2 is expected at the interfaces of multilayer systems.
In spite of this, there is no evidence for interdiffusion in a sol-gel Ti0 2 /Si0 2
bilayer after heating at 450°C [4.250]. Neutron reflectometry measurements
showed that the interface width is 0.8 nm. This finding is in accordance with
the roughness of a sol-gel silicon dioxide surface after the same heat treatment, suggesting no interdiffusion or mixing at the bilayer interface.
Optical Properties
Silicon dioxide thin films possess some relevance for optical coatings due to
their low refractive indices, low absorption, and low scatter losses. The optical
properties of Si0 2 layers strongly depend on production conditions such as
precursor concentration, process temperature, gas pressure and deposition
rate [4.225,229-231]' and on film properties such as stoichiometry [4.153,232].
The refractive index n(A) of stoichiometric silica films possesses only a
small dispersion in the visible spectral region. It varies by a mere 1.2% between 400 and 850 nm [4.233]. The gap energy of stoichiometric Si0 2 films
is higher than 5.3eV (220nm) and may be reduced by increasing impurity
content or structural changes [4.70,234]. Therefore pure silicon dioxide layers
are also well suited for UV applications.
The following equation gives a sufficient approximation of the spectral
behaviour of Si0 2 films in the visible range:
(4.16)
A is given in f.lm. n550 represents the refractive index of the layer at a wavelength of 550 nm. The absolute values of n(A) depend on the chosen deposition
conditions. Representative results obtained for several deposition methods are
summarized in Table 4.9.
The total variation in refractive index with deposition conditions is remarkably small for stoichiometric silica, which is amorphous in all the cases
164
4. Properties and Characterization of Dielectric Thin Films
Table 4.9. Typical ranges of refractive indices at 550nm wavelength for Si02 films
deposited by various methods
Deposition method
Refractive index
at 550 nm
Reference
Sol-gel techniques (SG)
(DC, SC or spraying; oven or laser burn-in)
1.42-1.45
1.42-1.46
1.44-1.46
1.47-1.48
1.47
1.455
1.43-1.49
1.44-1.46
1.48-1.50
:::;1.50
1.49
1.50
1.46-1.53
1.46
1.45-1.46
1.45-1.47
1.46-1.50
1.45-1.47
[4.18]
[4.206,229]
[4.231]
[4.18]
[4.235]
[4.181]
[4.153]
[4.225]
[4.78]
[4.227]
[4.387]
[4.18]
[4.233]
[4.232]
[4.236]
[4.388]
[4.230]
[4.18]
Reactive evaporation (RE)
Reactive sputtering (SP)
dc, rf, magnetron SP
Ion beam sputtering (IBS)
Reactive ion plating (IP)
lon-assisted deposition (lAD)
Chemical vapour deposition (CVD)
Plasma-enhanced CVD (PECVD)
(photo, ECR, glow discharge)
Plasma impulse CVD (PICVD)
listed in Table 4.9. It is interesting to note that the n550 values, which are
obtainable by several deposition methods, are larger than the respective refractive indices of amorphous bulk silica material [4.185] given in Table 4.10.
Refractive indices of films deposited by glow discharge PECVD, lAD, or IP
are even close to the n values of quartz.
The density of Si0 2 films seems to be the decisive factor in determining
the refractive behaviour. Refractive indices as low as 1.35 are reported for
films with high water incorporation due to their porous structure [4.217],
whereas densities of the order of 2.3 g cm -3 are obtained for layers with n550
values of 1.47 [4.235]. The packing densities of RE layers lie between 0.88 and
0.99, depending on substrate temperature; the packing density of films deposited at room temperature by low-energy lAD is 0.93 [4.233]. The density
Table 4.10. Densities and refractive properties of several silicon dioxide bulk materials
Silica phase
Density p/gcm- 3
Refractive indices at 589.3 nm
Quartz
Cristobalite
Tridymite
Nat. lechatelierite
Fused silica
2.64-2.66
2.32
2.26
2.19
2.20
1.544, 1.553
1.487, 1.484
1.469, 1.470, 1.471
1.4588 (amorphous)
1.4585 (amorphous)
4.4 Examples of the Characterization of Thin Film Materials
165
and refractive index of CVD silica layers are 2.25 g cm -3 and 1.45, respectively [4.115]. Refractive indices of up to n633 = 1.95 are obtained for substoichiometric SiO x films with an excess of silicon [4.131,232,236]. However,
their brownish colour and high absorption make these films unfit for most
optical applications.
The absorption of stoichiometric silicon dioxide films in general is extremely small and quite difficult to determine. In the visible range, extinction coefficients k smaller than 1 x 10- 4 are obtained for reactive magnetron
sputtered films [4.181]. Waveguide investigations and PTD measurements
(Sect. 4.2.2) indicate k < 1 X 10- 5 for ion-beam-sputtered films [2.18] and
Si0 2 coatings produced by lAD or IP [4.233,238]. The scattering losses of
Si0 2 layers are negligibly small compared with those of Ti0 2 films [4.164].
TIS and ARS investigations on ion-beam-sputtered Si0 2 films show no significant increase in scattering due to the coating in comparison with the
uncoated fused silica substrate.
Stress in Si0 2 Layers
For Si0 2 films compressive stress is reported for most of the production
methods. Absolute stress values also depend on the production conditions
and range from -330MPa to -120MPa for PECVD layers [4.230]. Silica
films deposited by PECVD on silicon wafers have compressive stresses of up
to -600MPa [4.240]. The stress relaxation induced by annealing at 500°C
reduces u to about -100 MPa. Compressive stress values in the range from
-240MPa to -150 MPa are reported for PECVD Si0 2 films [4.106]. Stress
is found to depend on film thickness and decreases with increasing thickness.
A corresponding result is obtained for SiO x produced by ion-assisted deposition [4.241] with compressive stress values between -150 MPa and -50 MPa.
The compressive stress of films produced by rf sputtering is -77 MPa [4.200]
and that of silica layers obtained by thermal oxidation is -350 MPa [4.239].
In contrast, tensile stresses with values between 120 MPa and 900 MPa are
reported for silica layers produced by CVD [4.239].
The interaction stress of Si0 2 layers deposited by spin coating, plasma
impulse chemical vapour deposition, reactive evaporation, and ion plating is
depicted in Fig. 4.26. The ranges of stress are shown for 100-nm-thick silica
layers produced under different conditions typical for the production of optical coatings. The stress in the Si0 2 films correlates with density, analogous
to the Ti0 2 films described in Sect. 4.4.1. Films with low density, for example sol-gel films, exhibit tensile stress, whereas films with higher density,
for example silica layers deposited by RE and IP processes, show compressive stress. Additional impurities in the film material, which occur in some
PICVD coatings, reduce the amount of compressive stress. This finding is in
agreement with the results on Ti0 2 films (Sect. 4.4.1) and with respective
investigations on silica films reported in literature [4.239]. Tensile silica films
deposited by CVD show a remarkable dependence of stress on the relative
166
4. Properties and Characterization of Dielectric Thin Films
400
SC
200
til
a.
~
\:)
'"'"~
0
PICVD
RE
-200
U5 -400
IP
-600
-800
Si0 2
Fig. 4.26. Stress of lOO-nm-thick Si0 2 films deposited by spin coating (SC), plasma
impulse chemical vapour deposition (PICVD), reactive evaporation (RE) and ion
plating (IP). Ranges of variation of stress with respect to different production conditions are depicted by shaded rectangles. The open rectangles represent typical
changes in stress for particular samples within one month of production; the directions of the changes are indicated by arrows
humidity of the surrounding atmosphere. The stress decreases at 60% relative humidity by about an order of magnitude within one hour, but increases
again in dry air. This indicates that in a less dense, porous film structure,
stress relaxation occurs by water incorporation. The tensile nature of stress
is also in agreement with reduced film density (Sect. 4.4.1).
Si0 2 films deposited by sol-gel methods show a similar dependence. In
Fig. 4.27 the stress of spin-coated silica films is depicted as a function of storage time after production in different media. In an ambient with a relative
humidity of 75% the stress decreases approximately twice as fast as in films
stored in dry air. However, the relaxation of silica layers produced by RE and
IP is not influenced by humidity. Typical results for stress relaxation during
storage after production are also shown in Fig. 4.26. The open rectangles represent typical changes in stress for distinct samples in ambient atmosphere
within one month after production. The directions of the relaxation are indicated by arrows. In general, stress is reduced by relaxation.
All films shown in Fig. 4.26 are amorphous. In contrast to Ti0 2 films, annealing these films up to a temperature of 600 °e causes no abrupt change in
stress because no phase transition occurs within this temperature range. The
stress in IP layers is not dependent on temperature, whereas the other films
show a smooth change towards tensile stress. This is indicated in Fig. 4.28,
where stress values, obtained directly after production and annealing at
600 °e for 30 min, are depicted for RE, se, and PleVD films deposited with
different substrate temperatures.
4.4 Examples of the Characterization of Thin Film Materials
167
160
140
t1l
c...
~
\----.
120
b
If)
If)
~
Ci5
---.----------.
• 0% r.h .
• 75% r.h.
------....-----.
100
80
o
5
10
15
20
25
30
35
40
Storage time t Id
Fig. 4.27. Variation of stress as a function of storage time after production for
lOO-nm-thick Si02 layers deposited by spin coating in dependence on the relative
humidity (r.h.) of the environment
Adhesion and Other Mechanical Properties
The adhesion of silica films on glass substrates is influenced by several deposition conditions. The adhesion of sol-gel films increases markedly with increasing densification temperature [4.219,242]. For RE films a similar behaviour
is found with respect to substrate temperature [4.202,203], whereas RE films
annealed after production show the opposite tendency [4.219]. Moreover, adhesion is influenced by film thickness and stress [4.202,204] and by the chemical composition of the layer material. Inorganic films possess better adhesion
on glass substrates than oxide coatings with organic compounds [4.202,203].
Figure 4.28 depicts the adhesion properties of 100-nm-thick Si0 2 films
deposited on fused silica substrates by several methods. Critical loads L c ,
normalized to the load for cohesive fracture of uncoated substrates, are
shown for RE, SC and PICVD layers obtained with a swinging scratch-testing
microtribolometer after Baba, described in Sect. 4.2.4. With the exception of
PICVD layers deposited on fused silica at 70 DC substrate temperature, the
adhesion of as-deposited films is as good as the mechanical stability of the
substrates. This result from IP Si0 2 films is not given in Fig. 4.28. Annealing
the films for 30 min at 600 °C does not change their adhesion properties irrespective of changes in stress properties shown in the Fig. 4.28. The adhesion
properties of Si0 2 films cannot be inferred from the stress influence because
of the high adhesion quality of these films.
PICVD Si0 2 layers deposited at low substrate temperature contain carbon with a carbon-silicon ratio of the order of 1. These films are actually
plasma-polymerized layers with residual organic contamination. However, annealing at 600 DC decreases the carbon content of the layer, while its critical
168
4. Properties and Characterization of Dielectric Thin Films
c::::J Before
After
"nr,,,,, llinn
E
~
o
....J
~
E
o
Si02' silica
u
-400 ~---------------------------------....J
RE
PICVD
140°C
PICVD
70°C
SC
Fig. 4.28. Normalized critical load L c and stress (J of 100-nm-thick Si0 2 films
before (light grey bars) and after (dark grey bars) annealing at 600 ° C for 30 min.
The layers are deposited on fused silica by RE, PICVD (substrate t emperatures
70°C and 140 °C) and SC
load increases markedly. After the heat treatment the adhesion properties
are as good as those of the other films, in agreement with the results given
in [4.202, 203].
Young's moduli are obtained on CVD Si0 2 films by microindentation,
as described in Sect. 4.2.4. An increase from 47 GPa to 65 GPa is found for
increasing substrate temperatures between 375°C and 475 °C [4.115] ; these
values are consistent with the 74 GPa reported for steam-oxidized films. A
biaxial elastic modulus E/(l-v) of (100± 10) GPa and a thermal expansion
coefficient of (0.6 ± 0.2) x 10- 6 K- 1 are found for PECVD Si0 2 films [4.240].
These Young's moduli differ significantly from the respective values for fusedsilica bulks of 73- 74 GPa [4.185] and seem also to depend on film density.
Furthermore, ultramicroindentation hardness is measured at silica films deposited by sputtering, CVD, and thermal oxidation [4.243]. The hardness
4.4 Examples of the Characterization of Thin Film Materials
169
of SP and CVD films is about 21 GPa. Thermally oxidized layers possess a
higher hardness of 25 GPa.
Blocking of Alkali
Besides being applied as interference-active layers in optical systems, thin
silica films are widely used as blocking layers to prevent diffusion losses from
the glass components, or as protective layers against attacks from the environment. Above all, thin Si0 2 layers have to prevent the corrosion of glass
surfaces due to ambient moisture and the diffusion of alkali (Na, K) from
glass substrates into the adjacent surrounding or into alkaline-sensitive coatings such as Ti0 2 or tin-doped indium oxide (ITO). The influence of the diffusion of sodium on the crystallization behaviour of Ti0 2 films is described
in Sect. 4.4.l.
The transport of ions within the glass matrix has been well known for
a long time [4.244]. Alkali contained in commercial soda-lime glass diffuses
from the interior to the surface when the glass is immersed in water or exposed to a high-humidity atmosphere with temperatures around the dew
point. At the surface, alkali dissolves in water or forms chemical compounds
with materials in the surrounding. The primary driving force for this alkali
transport is the concentration gradient. The speed of the diffusion process
strongly depends on the glass composition and on temperature. The amount
of alkali transported to the surface can be substantial, as indicated by experiments. Quite high sodium losses are induced in soda-lime glass surfaces
by Na2S04 microcrystals, which form when S02 gas is blown onto the hot
surface during draw up from the melt. In spray-deposited Sn02 films on hot
sheet-glass surfaces, NaCI microcrystals are created by the interaction with
the SnCl4 solution.
A protective coating should therefore have barrier properties in both directions. It is well known that Si0 2 layers fulfil this demand, but up to now
the optimum deposition methods and parameter sets have not been defined
for obtaining the best protection or diffusion barrier properties with Si0 2
coatings. This deficiency is reflected in the literature, which is not free from
contradictions. Doping with phosphor is reported to increase the blocking
effect of Si0 2 sol-gel layers in one investigation [4.233], while just the opposite is stated in another [4.246]. However, agreement seems to exist that
doping the Si0 2 layers with Ti0 2, Ta205, or Zr02 decreases their blocking
properties. A possible explanation for this finding is that these materials can
form crystalline phases within the amorphous Si0 2 matrix which reduce the
density of the films [4.146]. But, as a matter of fact, some patents claim the
exact opposite [4.245]. The contradictions may originate from small variations in the deposition conditions that cause big differences in the protective
properties of the respective layers; this is particularly true for sol-gel films.
The above-mentioned increase of blocking properties of Si0 2 films by
phosphor doping is explained by the binding properties of phosphor with re-
170
4. Properties and Characterization of Dielectric Thin Films
spect to sodium ions. Similar results are reported for doping with boron. Si0 2
layers deposited by CVD methods have a good blocking efficiency [4.248]. A
strong signal is obtained for the Si-H bonding at 880 cm- 1 by IR investigations of these layers. The polarization of this bonding is likely to be responsible for the observed effective protection against alkali diffusion [4.249].
Si0 2 layers with low densities, e.g. films produced by sol-gel techniques
or reactive evaporation, also have reduced blocking properties. The blocking
capability of these layers can be drastically increased up to the barrier quality
of PICVD or IP films by a temperature treatment at 500°C for one hour.
This is demonstrated in Fig. 4.29 for an 80-nm-thick Si0 2 layer deposited
by RE on soda-lime glass. SIMS depth profiles of Ca, Mg, Na and Si are
shown in the upper part of the figure; they were measured after the sample
had been treated in water at 95°C for 24 h. A strong leaching of sodium is
obtained in the glass surface. Because the Si0 2 layer itself remains unchanged
without showing any sign of corrosion, the leaching of the glass must happen
directly through the layer. The other sample, which after deposition was
annealed at 500°C, shows no leaching of sodium in the glass surface. Owing to
the temperature treatment, this layer is densified and possesses an excellent
('......
200
::::J
~
~
·iii
c
150
100
Q)
E
50
0
---/
,
,,
,,,
,
,,
,,,
,
,,
,,,
,,
200
~
~
·iii
100
c
.l!l
c
50
0
l I
---
- - - Ca
--._- Mg
--Na
------- Si
iI
il
il
'I
/f"
i _- -----------------
150
/----.7"
"
/1"
As-deposited
,///--.i>/-~:·:------- ,
Ca
!/
_._-- Mg
:
i
- - Na
Si
i
il
0
,' .........
::::J
Glass
Si02
250
I
i
i '
:
i,
:
:,
:
o
I
500
-------
"
~
1000
1500
2000
2500
3000
Time /s
Fig. 4.29. SIMS depth profiles of Ca, Mg, N a and Si for a Si02 (RE) layer on sodalime glass after the leaching experiment. Above: as-deposited; below: annealed at
500 0 C for one hour after fabrication
4.4 Examples of the Characterization of Thin Film Materials
171
blocking capability. A comparable blocking behaviour is obtained for Si0 2
sol-gel layers by post-deposition temperature treatment.
4.4.3 Tantalum Oxide Layers
Klaus Bange
The manifold physical properties of tantalum oxide layers possess a broad
field of application. They are used as highly refractive material in optical
multilayer interference filters, or as substances with low optical losses, for
example in waveguides [4.254-256]. Ta205 layers are applied as dielectric
material in condensers because of their large dielectric constant. Layers with
crystalline structures exhibit piezoelectric properties. Fairly recently Ta205
layers have been used as ion-sensitive membranes in solid-state H+ ion sensors
and in electro-optical and electrochromic devices [4.257,258].
Various methods are available to produce Ta205 layers. Traditional techniques such as thermal oxidation or anodic oxidation have been extensively reported [4.260-264]. Few data are given for films produced by CVD [4.270,271]
and ion-assisted processes [4.272,273]. Some results available from films deposited by rf and dc sputtering [4.265,269] try to correlate film properties
such as refractive index, composition or laser damage with deposition parameters such as gas composition, gas pressure, rf power or substrate temperature.
This section describes results mainly from investigations on films deposited
by reactive electron beam evaporation and reactive ion plating. Ta205 layers
prepared by ion plating (IP) are deposited in a Balzers coater (BAP 800). The
films are produced with a deposition rate of 0.4 nm S-l and an oxygen partial
pressure of 0.1 Pa. The film thickness and the rate are monitored by optical
measurements and by an oscillating quartz to control the film mass. Films
deposited by reactive electron beam evaporation (RE or RET) are produced
in a Balzers BAK 760 with two different parameter sets. RET films grow at
a substrate temperature Ts of 380°C, a rate of 0.2 nm s-1, and an oxygen
partial pressure of 1 x 10- 2 Pa. For the RE sample the background pressure
was 4 x 10- 2 Pa and the Ts < 200°C [4.18]. The interest is based on two very
different types of application. Materials having a high refractive index, low
optical losses and a high temperature stability are suitable for optical filters,
interference filters and edge filters. In electro chromic all-solid state devices
the Ta205 layer functions as a dielectric layer that is electrically insulating
but ionically (H+, OH-) conducting [4.18].
The optical properties of the layers exhibit remarkable differences depending on the preparation method. Figure 4.30 depicts the spectral refractive
index n and the extinction coefficient k. All Ta205 layers show a monotonic
decrease in the refractive index. Whereas the refractive indices of evaporated
films (RET, RE) differ only slightly, IP films possess higher values (dn ~ 0.2).
The extinction coefficient of RET and IP films is fairly low. Bulk losses of
172
4. Properties and C haracterization of Dielectric Thin Films
2.4 ,---....-----,-----,-----,---r---,----,-----,
2.3
.'
c:
x
Q)
"0
.!:
......
....
2.2
Q)
-
>
U
~
Q>
2.1
.......... .
~.:::::.:=.:.; . -.- .-
a::
2.0
RE
IP
RET
0.06
.>It.
C
0.0 5
Q)
·u
~
~ 0.04
8
c:
0
0.03
x
0.02
~
.!:
w
'"
"'" ~I'-..
0.01
0.00
400
600
..........
I-- t--...
800
.........
1000
Wavelength J../nm
Fig. 4.30. Spectral refractive index n and extinction coefficient k of IP, RET and
RE films
37 em -1 for IP and 7 cm - 1 for RET are determined from PDS investigation [4.274]. No interfacial loss can be measured for these materials. Only
in RE films is a high adsorption with a maximum at approximately 400 nm
observed. With increasing wavelength k decreases.
The similar general behaviour of the spectral refractive index with different absolute values indicates that the films possess a different film density
p. This is supported by the CIXR results depicted in Fig. 4.31, where the
reflectivity is given in logarithmic and linear scale. The reflectivity data of
IP and RE films on Schott BK 7® glass, taken with 9000eV photons (below
the Ta L3 edge), are shown as full circles. The critical angle at approximately
0.3 0 shows that IP layers are denser than RE layers [4.22]. The experimental data can be simulated with a homogeneous bulk layer with densities of
p = 6.1 gcm- 3 for RE, p = 7.07 gcm- 3 for RET, and p = 8.26gcm- 3 for IP.
However, the fit is qualitatively better if additional layers with thicknesses of
approximately 3 nm with intermediate densities are assumed. The additional
4.4 Examples of the Characterization of Thin Film Materials
173
10°
10-1
Z.
.s;
10-2
~ 10-3
q::
Q)
0::
10-4
10-5
10-6
0
0.5
Angle
1.0
of incidence ~ /deg
1.5
Fig. 4.31. Reflectivity of 9000eV photons from Ta 2 05 films deposited by ion
plating (IP, upper curve) and by reactive evaporation at Ts < 200 C (RE, lower
curve). The fit with the broken line assumes no intermediate layer, whereas that
with the full line assumes transition layers with six parameters
0
layers are surface layers for IP samples, and intermediate layers between the
substrate and the bulk Ta205 layers for RE and RET samples. Further information is obtained from the fits. The surface roughness a of IP films is
0.76 nm, that of RE and RET films is 1.6 nm [4.22].
The surfaces roughness of different Ta205 films is also examined with
AFM and TEM investigations on PtjC replica. The structures on the surfaces of the Ta205 films and on the polished Schott BK 7® glass surface
resemble each other [4.24]. Polishing traces on the glass surface seem to be
more strongly developed through the Ta205 films. Especially the surfaces
produced by IP show this effect. The rms values obtained for IP and RE are
(0.99 ± 0.06) nm and (1.18 ± 0.28) nm, respectively. All results indicate that
the roughness of the Ta205 films is mainly determined by the roughness of the
glass surface. The different findings suggest that the growth of Ta205 films
proceeds by a two-dimensional rather than by a three-dimensional mechanism because the defects of the substrate are still recognizable on the film
surface [4.24J.
The bulk structure of the Ta20s films is investigated by various methods. No pronounced features are obtained in Raman spectroscopy and XRD,
i.e., the layers are "Raman and XRD amorphous" [4.18]. Electron diffraction
experiments likewise exhibit the non-crystalline state of the materials, but
the nanostructure of the IP film is more homogeneous in TEM micrographs
than the nanostructure of the RET films, in which islands with a diameter of
approximately 100 nm exist [4.259]. Differences in IP and RET are obtained
by cross-section investigation. A weak columnar structure is observed in RET
films, whereas IP films are fairly homogeneous. Additionally, RET films ex-
174
4. Properties and Characterization of Dielectric Thin Films
hibit a changed layer growth in the first layers « 10 nm) at the substrate
interface. This is in agreement with the increased density for the intermediate
layer analysed by GIXR [4.22J.
The film composition is analysed by the combination of NRA and RBS.
The results obtained can be summarized as follows. Layers deposited by
IP can be described by Ta205.o6Ho.o6' In high-density material, oxygenhydrogen groups are probably adsorbed at the few internal surfaces and therefore the material is best characterized by Ta205.o6-8 x 6(OH) with 6 ~ 0.06.
The stoichiometry of RET films is given by Ta205.28Ho.o6. In these materials, which have a lower density, H 20 is probably adsorbed at the internal
surfaces of pores and holes. The description can therefore be modified to
Ta205.28-8 x 6(H 20) with 6 ~ 0.3. It seems that films with a low extinction
coefficient (Le., RET and IP) are in a nearly stoichiometric state. The tantalum oxide layers deposited at low substrate temperature have the composition
Ta205.5H1.2 and exhibit the lowest density and the highest extinction coefficient. The formula can be rewritten as Ta205.5-8 x 6(OH) with 6 ~ 1.2, or in
a more probable way as Ta205.5-8 x 6(H20) with 6 ~ 0.6. Low temperature
deposition seems to create films in a substoichiometric state which contain a
high amount of water [4.18J.
Information about the chemical binding state and the electron structure
can be obtained by ESCA. Figure 4.32 summarizes some ESCA data on IP,
RE and RET films. The experimental data are fitted with a 80% Gauss and
20% Lorentz contribution. A FWHM of 1.3eV is used, which is obtained from
investigations on anodic oxide Ta205 [4.247J. Besides the Ta4f doublet at
22.9 eV and 24.9 eV, which characterizes the highest oxidation state (Ta5+),
an additional binding state is present. The second doublet is shifted by 1 eV
to lower binding energies. This is usually interpreted as the TaH state which
is related to Ta20 (Ta204)' It should be noted that the additional state is
most pronounced in IP and RE films, whereas in RET films it is only a minor
species. In the case of RE samples, which have a high extinction and are substoichiometric, this interpretation is consistent with other results, but for IP
films it is contradictory. Presumably the high-density modification produced
by the IP process creates materials whose bond lengths differ from those
growing in thermodynamic equilibrium. The unusual electronic structure of
IP films, which is also indicated by the high optical bulk loss, may create the
second doublet.
The results obtained on tantalum oxide films prepared by different deposition techniques can be summarized as follows. Films deposited by reactive
evaporation with low substrate temperature (RE) possess a low refractive
index and a high extinction coefficient. They are substoichiometric and have
the lowest density and a high water content which is adsorbed at internal surfaces. Ta205 films evaporated on substrates with high temperature (RET)
exhibit the lowest optical loss, an increased density and an increased refractive index (relative to RE), are stoichiometric and contain a moderate amount
4.4 Examples of the Characterization of Thin Film Materials
Ta 4f
175
RET
28
Binding energy leV
Fig. 4.32. ESCA data of the Ta 4f doublet of Ta205 films prepared by ion plating
(IP) and reactive evaporation at low (RE) and high (RET) substrate temperatures
of water. Films prepared by IP possess the highest refractive index and the
highest film density. The surface roughness of the IP film is at least as low
as the substrate roughness. The stoichiometric material contains a very low
concentration of hydroxyl groups.
4.4.4 Nickel Oxide and Hydrous Nickel Oxide
Klaus Bange
Compounds consisting of nickel oxide and hydroxyl are well known as electrode materials in batteries and have been used and studied for a cen-
176
4. Properties and Characterization of Dielectric Thin Films
tury [4.275-278]. Over the last decade increasing interest has focused on the
electro chromic properties of these materials. Hydrous nickel oxide films, the
material of present interest, can be represented as NiOxH y , but is usually
simply referred to as nickel oxide. The valence of Ni changes by the injection
of charge from NiH to NiH or NiH and is used for charge storage, but is also
connected with variations in optical properties, especially in the absorption
in the visible spectral range. The anodic electrochromism of this thin film
material can be used in very diverse applications (see Sect. 5.3).
Various techniques have been reported for the preparation of these inexpensive film materials. Hydrous nickel oxide films are deposited by reactive
sputtering [4.279]' by electrochemical and chemical methods, for example by
anodic oxidation of nickel electrodes [4.280], colloidal precipitation [4.281]'
cathodic electrodeposition [4.282] and chemical vapour deposition [4.283]. In
this section attention is focused on nickel oxide systems produced by reactive
electron beam evaporation. The reported data are obtained from films prepared in Balzers coaters (BAK 550, BAK 760). The deposition parameters are
varied over a broad range to obtain an optimized electro chromic behaviour.
The oxygen partial pressure P0 2 is varied between (0.6-4) x 10- 2 Pa, the
substrate temperature Ts between room temperature and 200°C, and the
deposition rate r between O.lnms-l and 0.5nms- l . For some experiments
the water background pressure is increased. Detailed information about the
deposition techniques and parameters is given in [4.18].
Most of the process parameters used during reactive evaporation directly influence the microscopic and macroscopic properties. For the electrochromism, especially for the switching rate, the film density is an important characteristic that can be varied by changing the gas pressure. Figure 4.33 illustrates the dependence of the relative density of nickel oxide
films on the oxygen gas pressure Po 2 • At high vacuum a density of :::; 0.8
is obtained, which decreases for P0 2 > 10- 2 Pa, and is as low as :::; 0.5 at
P0 2 = 5 X 10- 2 Pa.
The optical properties of as-deposited films also depend on the evaporation conditions. Figure 4.34 summarizes some results of the refractive index
and the extinction coefficient at >.. = 550 nm as a function of oxygen gas pressure during the reactive evaporation. The refractive index is :::; 2.25 at low
P0 2 and low rates; it decreases to 2.1 at P0 2 = 4 X 10- 2 Pa and a low rate
of 0.2 nm S-I. This effect is caused by a diminished film density, as apparent
from Fig. 4.33. Higher substrate temperatures increase the refractive index.
The data obtained indicate that the refractive index of films deposited in
BAK 550 is systematically lower than that of films deposited in BAK 760.
The optical absorption in the as-deposited material is also influenced by
the deposition parameter. Figure 4.34 gives data on films deposited in a
BAK 760 coater which show that the absorption is high at low P0 2 and decreases with increasing pressure. The extinction coefficient changes in the visible range (>.. = 550 nm) between 0.015 and 0.05. Higher rates create higher
4.4 Examples of the Characterization of Thin Film Materials
177
1.0
--.---Q.
~
'wc
Ql
"C
Ql
0.5
>
~
~
Qi
c:::
~
~
• Bange et al.
o Agrawal et al.
0.0
10-2
Gas pressure P02 /Pa
Fig. 4.33. Relative density of nickel oxide films deposited by evaporation at different oxygen gas pressure
absorption, whereas increasing substrate temperatures reduce the absorption [4.18]. The influence of the substrate temperature and the deposition
rate on the extinction is very weak. It can be assumed that at a substrate
temperature Ts of 90°C the extinction is slightly increased.
The stoichiometry of the as-deposited material is analysed by ESCA, RBS,
and NRA. The data obtained are summarized in Fig. 4.35. The oxygen-nickel
ratio (OjNi) and the hydrogen-nickel ratio (HjNi) are given as functions of
the oxygen partial pressure. The 0 jNi ratio varies between 1.04 and 1.4. Most
values are in the range from 1.1 to 1.25. One extreme value of 1.4 is obtained
at high pressure (4 x 10- 2 Pa) and low rate. Only a very weak dependence of
the 0 jNi ratio on P0 2 is found. But a high substrate temperature decreases
the oxygen content of the film, and 0 jNi rv 1 at Ts = 170 ° C is obtained. For
P02 > 1 X 10- 2 Pa the oxygen concentration increases. Higher rates increase
the 0 jNi ratio.
The HjNi ratio is also given in Fig. 4.35. HjNi varies between 0.3 and
0.5 at RT. The hydrogen content does not seem to depend systematically on
the oxygen partial pressure, and a variation of the rate does not significantly
alter the hydrogen concentration. With increasing substrate temperature the
hydrogen concentration decreases, and with increasing water partial pressure
the HjNi ratio increases. The film composition of the NiOxHy films prepared
under "standard conditions" (r = 0.2 nm S-l, P0 2 = 6.02 Pa, RT) is represented by x = 1.25 ± 0.1 and y = 0.4 ± 0.15 [4.18,285]. The electrochromism
for this standard material, which exhibits an optimized electrochromism in
all-solid-state devices, is described in detail later (Sect. 5.3).
Different chemical binding states of nickel are indicated by ESCA. In
the energy range of the Ni 2P3/2 state, three peaks appear at binding ener-
178
4. Properties and Characterization of Dielectric Thin Films
2.3
c
c:
2.2
---
r- rC/~AK760
"--< D~
><
CD
"C
.!:
CD
>
ts
2.1
(
~
CD
a:
2.0
1'---...
rL~
'~D
1.9
t--.°
.><:
0.06
t;".
r: 0.1
r: 0.2
r: 0.2
... r: 0.5
Ts RT
D r: 0.2
Ts RT
Ts 90°C • r: 0.2
Ts: 90°C
Ts: 170°C Ts: RT + H2O
'E 0.05
CD
'0
IE
CD
0
0
0.04
0
0.03
c:
nc:
t
I
••
~ 0.02
w
t,
~
(
0.01
0.00
0.6
1
2
Pressure 110-2 pa
4
Fig. 4.34. Refractive index n and extinction coefficient k versus oxygen gas pressure. The influence of the deposition system (BAK 550, BAK 760) is shown for n;
for k various rates r and substrate temperatures Ts are given (see text)
gies of 853.4,854.6, and 855.8eV; at the binding energy of 860.5eV a very
broad feature shows, which is well-known as a shake-up satellite observed for
NiO. The three maxima have different intensities which are influenced by the
deposition conditions. The most pronounced peak at 853.4eV is caused by
NiO (NiH). An interpretation of the peak at higher binding energy is more
complicated. The most plausible explanation is that at 854.6 eV the peak is
produced by Ni( ORh, and at 855.8 eV by Ni 2 0 3 [4.20,296]. The presence of
small amounts of Ni2 0 3 is confirmed by Raman investigation [4.284].
The ESCA data for the 0 Is region also indicate OR groups. Peaks appear at 529.5eV and ~ 531eV for the as-deposited material [4.18,296]. The
maximum at lower binding energies is induced by the oxygen in a Ni-O
bond, whereas the maximum at higher binding energy is induced by hydroxyl
4.4 Examples of the Characterization of Thin Film Materials
_°
r: 0.1
r: 0.2
t. r: 0.2
•
Ts: RT
Ts:RT
Ts: 90°C
• r: 0.5
o r: 0.2
• r: 0.2
179
Ts 90°C
Ts 170°C
Ts RT+ H2O
1.4
0
""~
1.3
Z
0
~
1.2
•
•
1.1
•
~
~
0.8
0
""~
Z
I
0.6
0.4
,
~
~
4
0.6
1
~
(
4
0.2
0.0
2
4
Pressure 110-2 pa
Fig. 4.35. Oxygen-nickel (O/Ni) ratio and hydrogen-nickel (H/Ni) ratio versus
oxygen gas pressure for different rates r and different substrate temperatures Ts
groups. OH/NiO ratios between 0.2 and 0.35 are obtained for the bulk. Especiallyat high substrate temperature a low hydroxyl content is found [4.18].
The nanometre-scale structure of the evaporated material is also rather
complicated. Grains of about 4 nm in size are found in films deposited at
P0 2 ~ 5 X 10- 2 Pa [4.286]. These grains form clusters with a size of 60100 nm. The grain size increases at lower pressure, and a columnar structure
is observed [4.286]. XRD indicates a NiO crystal phase in as-deposited materials [4.18] but is unable to discriminate between a cubic and a hexagonal
structure. Variations are observed in the intensity of the signals of the (111),
(002) and (022) orientations which depend on the preparation conditions. Yet
the available data admit of no straightforward interpretation. The observed
electron diffractograms are consistent with a cubic NiO structure [4.286].
The ensuing model for the as-deposited evaporated material is also consistent with overstoichiometric clusters of about 4-5 nm in diameter with
180
4. Properties and Characterization of Dielectric Thin Films
hydroxyl groups at their internal surfaces and possibly with some hydration.
The films may be represented roughly as NiO (OH)z with z = 0.25 in the
bulk with additional minor species and z = 0.5 at the surface [4.287].
The electrochromic processes (i.e., the ion intercalation/deintercalation
processes) are traditionally studied by electrochemical techniques. Cyclic
voltammetry in particular has been applied for the investigation of nickel oxide films. Figure 4.36 summarizes the results of current-voltage (CVG) and
reflectance-voltage (RVG) measurements, and the change in frequency of the
quartz crystal microbalance (VMG). The data depicted are gathered after ten
cycles in 1 M NaOD for a NiO(OH)x prepared under standard conditions. The
experiments are carried out with a sample consisting of glass/Ni/NiO(OH)x
with an active size of 0.125 cm2. The film thickness is systematically varied
from 155 nm to 620 nm. Similar experiments are conducted in 1 M N aOH.
The reflectance is measured with a helium-neon laser (>. = 632.8nm) [4.287].
The voltammograms evolve upon continued cycling, and the current densities increase during ion intercalation/deintercalation. This process, which
is heavily dependent on the as-deposited film material, stops after approximately ten slow cycles. Because the as-evaporated films can be represented
approximately as NiO(OH)o.25, it can be to assumed that extended cycling in
electrolyte-containing hydroxyl groups leads to an increased hydroxylation.
The changes in the initial cycling are described in detail in [4.18].
A fairly typical current-voltage diagram for films deposited by evaporation after the initial cycling procedure is depicted in Fig. 4.36. During the anodic cycling a maximum is obtained at +400 m V and additionally the current,
which is induced by the oxygen evolution reaction, increases for potentials
E > 500 m V. In the cathodic cycle a minimum is observed at 200 m V, which is
usually correlated with the bleaching process. These two main features in the
CVG are chiefly influenced by the initial cycling process. Comparable electrochemical studies on Ni(OHh layers indicate that both features stem from the
reduction/oxidation of nickel hydroxide to nickel oxyhydroxide [4.288,289].
Changes in the reflectance are obtained over a broad range of potentials
(Fig. 4.36), i.e., the optical response to the applied voltage seems to behave
differently from what is indicated by the CVGs. The reflectance decreases
continuously for E > -250 m V and the absorption in the layer seems to
saturate at about 550 m V. The small hysteresis suggests that the sweep rate
is in the same range as a reaction rate of the electrochromic colouration and
bleaching process.
Analogous optical behaviour over a broad range of applied external potential is observed in reflectogram investigations with nickel electrodes [4.289].
The change obtained in the reflectance in the cathodic potential region has
been interpreted as a formation of Ni(OHh, followed by an oxidation process
for larger anodic potentials. For E > +350 mV, possible oxidation mechanisms are discussed; these consider the so-called "nickel oxyhydroxides" as
hydroxides or hydrated oxides.
4.4 Examples of the Characterization of Thin Film Materials
181
8
'f'
E
u
<I:
..§
c
~
4
CVG
C
~
0
4
:::l
()
8
RVG
C
0.5
~
c
~(])
'$
c:::
~
"
0.4
B
0.3
0.2
200
N
~
'<i
(])
Cl
~ 100
VMG
.s:::
u
:>.
u
c
(])
:::l
0-
J::
0
-0.4
-0.2
o
0.2
Potential E IV
0.4
0.6
Fig. 4.36. Current (CVG), reflectance (RVG), and change in frequency of the
quartz crystal microbalance (VMG) as a function of the applied potential. The
data were taken with a sweep rate of 10 m V S-l after ten cycles in 1 M NaOD for
a NiO(OH)x film with a thickness of 620 nm
Comparable electrochemical experiments have been carried out on samples consisting of glass/ITO /NiO(OH)x with an active size of 4 x4 cm 2 [4.290j.
The transmittance and the current are recorded versus the potential in electrolytes in which the pH is systematically varied over a broad range by the
use of different electrolytes such as 2M KOH and O.lM HCI [4.18, 290j.
The potentials of the CVGs and the optical data shift linearly with pH by
58 m V /pH-unit. An increase in the H+ concentration in the electrolyte is
correlated with an increase in potential. This behaviour can be explained by
182
4. Properties and Characterization of Dielectric Thin Films
the Nernst equation in combination with the Fermi level concept in liquid
electrolyte [4.291,292]. The colouration rate is nearly independent of the pH.
An increasing OH- concentration increases the colouration [4.18].
The spectral transmittance and reflectance change over a broad wavelength region as a function of the amount of injected charge [4.18,295]. While
the refractive index stays fairly constant, the spectral extinction coefficient increases linearly with the amount of injected charge. The spectral colouration
efficiency exhibits a maximum at approximately 400 nm, and with increasing
wavelength the CE decreases monotonically. A CE of 45 cm C- 1 is derived in
the visible range (>. = 550nm) [4.18].
The measured frequency shifts as a function of potential, shown in
Fig. 4.36, are strongly correlated with the observed electrical data. The determined frequency shift is transformed to an equivalent change in mass [4.20].
During anodic colouration, a small decrease in mass (m1) is recognized in
the potential range from -500 m V to +320 m V. For potentials E > E1
(E1 = +320 m V) a drastic change in frequency is seen which indicates a
significant change in mass (m2)' Cycling in cathodic direction decreases the
frequency reversibly, as shown in Fig. 4.36. From the depicted CVG and VMG
data it may be inferred that during the colouration process a species ejection
mechanism starts at potential E < E 1 , to be followed by a more important
species incooperation for E > E 1 .
The typical data set given in Fig. 4.36 is reproduced for different types of
samples with various film thicknesses and also in two different electrolytes.
All curves are carefully analysed after more than ten cycles. The results
for the VMGs and CVGs are summarized in Table 4.11. The first column
shows the electrolyte used, the second the film thickness. The number of
nickel atoms NNi and the mass of the films mf are calculated from the film
thickness, the active area and the density of the film. Mass changes m1 and
m2 are determined from the change in frequency for E < E1 and E > E 2 ,
respectively. Q represents the charge that is analysed by graphical integration
of the CVG during the colouration process. The value for the gas evolution
is subtracted.
The following typical trends can be recognized from the data summarized
in Table 4.11. With increasing film thickness, i.e., with increasing mass mf
and increasing number of nickel atoms NNi, the measured charge Q as well
as the analysed mass m2 increase, independent of the used electrolyte. The
proportionality between NNi, Q, and m2 indicates that the values obtained
are due to mass changes inside the bulk and not by surface effects. The
determined mass m1 is always smaller in NaOH than in NaOD. The mass
m2 behaves differently. It is obvious that m2 in NaOH is generally larger than
in NaOD [4.287].
Figure 4.37 exhibits the spectral absorptance of a 300-nm-thick NiO(OH)x
film on a glass-nickel substrate in the range 400-4000 cm -1. The coloured
state (solid line) and the bleached state (dashed line) are depicted. All ob-
4.4 Examples of the Characterization of Thin Film Materials
183
Table 4.11. Film parameters, data from VMG, CVG, Eqs. (4.22), (4.23), and
(4.24)
Electrolyte
Film parameter
VMG
Thick- mf NNi
ml
ness
!-lg xlQ17 ng
mm
1
1
1
1
1
1
1
1
M
M
M
M
M
M
M
M
NaOH
NaOH
NaOH
NaOH
NaOD
NaOD
NaOD
NaOD
620
460
275
155
620
460
275
155
35
26
15
9
35
26
15
9
2.5
1.9
1.1
0.6
2.5
1.9
1.1
0.6
m2
ng
o 633
-8 528
o 312
-17 240
-53 567
-20 449
-9 284
-24 164
Eq.
Eqs. (4.22)
(4.24) and (4.23)
CVG
m21
Q Qle QI m2 exp mNaOH,OD
(a = 3,4)
NNiu mC xlQ17 eNNi a.u.
a.u.
a.u.
1.5
1.7
1.7
2.3
1.4
1.5
1.5
1.5
13.2
10.9
6.1
4.1
13
11.3
7.2
4.1
0.79
0.66
0.37
0.25
0.78
0.68
0.43
0.25
0.32
0.35
0.34
0.39
0.31
0.36
0.39
0.39
4.7
4.9
5
6.2
4.5
3.9
4.1
4.1
-+ 5; 3.5
-+ 4.67; 3
a.u. = atomic units
served vibrational bands are fairly broad and suggest an amorphous state
and/or a multicompound system [4.297J. The most pronounced feature with
a maximum at approximately 3400 cm- 1 is characteristic of bound OR
groups and water [4.298J. The symmetric band (3600cm- 1 ) and the antisymmetric band (3645cm- 1 ) of Ni(ORh are also situated in that frequency
range [4.299J. Both bands possess a very sharp resonance. The strong and
broad signal is probably due to lattice water incorporated in the porous
evaporated film. Compared with the bleached state, the absorptance in this
wavelength range is strongly enhanced in the coloured film, indicating an increase in the number of OR-containing species. This agrees with the results
0.5
0.4
Q)
u
c:
El
0..
0
C/)
.c
«
0.3
0.2
l~'
/
~
0.1
0.0
--
(\
\
\
\
\
\
\
-----
B
\
\"\
',,~
............ - .......
4000
C
3000
~-
2000
1000
-/1\
Wavenumber /cm- 1
Fig. 4.37. Absorptance of bleached (B) and coloured (C) NiO(OH)x film with a
thickness of 300 nm
184
4. Properties and Characterization of Dielectric Thin Films
on the incorporation of hydrated cations in coloured films that have recently
been achieved for Ni(OH)2 films by QCM experiments [4.300].
The scissors mode of H 20 produces a weaker absorption at approximately 1660cm- 1 [4.299]; no changes can be observed during colouration.
Small changes in the spectra are obtained in the region of 500 cm -1. The
observed mode is due to nickel-oxygen vibrational modes and is induced by
a superposition of the antisymmetric nickel-oxygen stretching and bending
at 580cm- 1 and 470cm- 1 , respectively [4.299]. The nickel-oxygen vibration
feature of the coloured film seems to be shifted to smaller wave numbers. The
rv 100 cm -1 increase in the stretching and bending modes reflects a higher
nickel-oxygen order in the bleached film, which can be created by a reduced
oxygen-hydrogen content.
In summary, these assignments of vibrational modes in the IR spectra
support a colouration mechanism for evaporated NiO(OH)x films that is
connected with an incorporation of oxygen-hydrogen groups or water and
a lower nickel-oxygen bond order. The data on films produced by electrodeposition [4.301] and chemical precipitation [4.300,303] are consistent with
those in Fig. 4.37. Somewhat different results are reported by Lynam [4.302].
The as-deposited evaporated films were hydroxylated by cycling in KOH, and
the resulting bleached material showed a strong peak at :::::: 3640 cm -1, which
was clearly associated with OH in Ni(OHhOnly small differences in the Ni 2P3j2 and 0 Is state have been found
by a comparative ESCA investigation of the bleached and coloured state of
evaporated materials [4.18]. The peak at 835.4eV, caused by NiH, is quite
similar in the coloured and the bleached films. An unambiguous correlation
with NiH and NiH cannot be obtained from the data. In the 0 Is energy
region the ESCA data yield more information. Two peaks characterize the
spectrum in the bleached and coloured state. The first one is located at
rv 531 eV and undoubtedly originates from oxygen-hydrogen groups; the second peak is located at 529.5 eV and is induced by oxide. The oxygen-hydrogen
content of coloured material is reduced in the surface region; the hydroxyl
content in the film bulk is reduced by approximately 30% [4.18]. These results are confirmed by NRA investigations on all-solid-state devices in which
the coloured electrochromic material shows a reduced hydrogen concentration [4.304]. The 0 Is data strongly support the general conviction that the
bleached film contains Ni(OHh and the coloured film NiOOH. Based on the
different experimental findings, which seem contradictory in parts, a model
will now be discussed that satisfactorily explains all available experimental
data for NiO(OH)x films produced by e--beam evaporation.
Films produced by e--beam evaporation consist of a mixture of different
oxidized nickel sites. The exact composition is strongly related to the deposition conditions used [4.18,296]. NRA and ESCA results show that the
content of OH groups in as-received materials is approximately 25% [4.296].
According to the conventional model, these groups are probably responsible
4.4 Examples of the Characterization of Thin Film Materials
185
for the active electrochromic behaviour of the film. The oxygen sites of the
films can be bound either to trivalent or to divalent nickel.
It is assumed in the literature that the cycling of a Ni electrode in liquid electrolyte creates Ni(OHh films with electrochromic behaviour [4.305].
Cycling of evaporated NiO(OH)x films in NaOH indicates a nearly reversible
behaviour in optical and electrical data between the first and the tenth cycle.
These changes, which are too small to be analysed, indicate stable electrochromic properties and prove that electrochromic cycling does not induce
rapid or large changes in the chemical formula of the bulk of the oxide films
used.
The mass variations during the colouration and bleaching of these films
were also recorded after numerous cycles and compared with those ofNi(OHh
films [4.300]. The results for NiO(OHh are very different and confirm the
mass stability in the NiO(OH)x films. If SOme hydrated cations had been
incorporated in the evaporated films during the colouration process, they
must have been ejected in the subsequent bleaching process. But taking into
account the film structure, which is rather dense in comparison with the "colloidal" NiO(OHh films, it is more probable that the cation incorporation is a
minor phenomenon and can therefore be neglected in the following, whereas
the role of water will be considered.
The results of the microbalance technique suggest a small non-reversible
increase in mass during the first ten cycles in NaOD electrolyte. This small
enhancement must be explained by the interaction between electrolyte and
film, which leads to an exchange between H 2 0 and D 2 0 at the defect sites of
the structure, i.e., adsorption and desorption are taking place at the internal
surface or voids.
The column (Qje)jNNi in Table 4.11 indicates that between 30% and 40%
of the Ni atoms should change their valency from 2+ to 3+ during colouration. Thinner films obviously contain a larger amount of electrochromically
active material than thicker ones. The difference may be due to an increased
substrate temperature caused by radiant heat from the crucible during the
deposition of thicker films. The differences in the oxygen-hydrogen content
in the as-received nickel oxide and the number of electro chromic ally active
species after ten cycles (Table 4.11) can be explained by an increase of incorporated oxygen-hydrogen species during the first ten cycles. This would also
explain the small increase in mass.
The reported results of IR spectroscopy favour a model in which the OH
content in the film is increased in the coloured state. On the other hand,
ESCA results on the 0 Is state indicate a weak decrease in the oxygenhydrogen groups for the coloured state compared to the bleached state [4.18],
which agrees well with the decrease of hydrogen content in coloured films as
analysed by NRA in all-solid-state devices containing NiO(OH)x films [4.304J
and in films obtained with other deposition techniques such as magnetron
sputtering [4.306J or anodic oxidation [4.300]. In order to explain the virtual
186
4. Properties and Characterization of Dielectric Thin Films
contradiction to the oxygen-hydrogen increase of the coloured films seen with
FTIR, a model is presented which is mainly based on VMG results but also
draws from the background knowledge of other findings. The potential range
between +0.5 V and El (Fig. 4.36) and the remaining region between El and
+0.6 V will be separately dealt with in the following.
To simplify matters, the electrochromically active compounds of the investigated NiO(OH)x films are assumed to possess the ability to change the
oxidation state of Ni only from 2+ to 3+ during colouration, and the OH
species are assumed to move. Based on these assumptions, the variation in
mass ml is consistent with a proton ejection in the film during colouration. If
a dissolution reaction at the metal/oxide interface is assumed for the nickel
substrate, the decrease in mass will be the same in NaOH and NaOD solutions. The higher decrease in mass detected in the NaOD solution compared
with that in the NaOH solution for E < El disproves the dissolution hypothesis.
The simplest version of such a proton ejection reaction is described by
bleached
E < E1
:
Ni(OH)2 - H+
coloured
f-t
NiOOH + e- .
(4.17)
This process qualitatively satisfies the change in charge and the mass balance.
But from a thermodynamical point of view it is difficult to explain why a H+
ejection process is observed over such a broad potential range for an energetically well-defined Ni(OHh molecule. The lack of the sharp Ni(OHh line
in the bleached state of the FTIR spectra also proves that this species is not
a major component of the film. Raman spectroscopy shows a reinforcement
at the NiH -OH stretching mode wave number in the bleached state, superimposed on the broad oxide band, which remains the same in both bleached
and coloured materials. The broad oxide band demonstrates that colouration
is a local, short-range-order effect, initiated by the hydroxyl present in the
bulk of the film [4.300]. The spectral absorption in the visible range likewise
supports this model. The absorption spectrum of Ni(OHh exhibits two pronounced maxima at approximately 350 and 500 nm, whereas the evaporated
films show a change in absorption over a broad wavelength region [4.66]. This
broad band is more characteristic of an oxide structure.
The colouration/bleaching reactions are evidently due to the change of
divalent to trivalent nickel at a lattice site surrounded by hydroxyls. In the
vicinity of these active sites, and despite their complex surroundings, we can
therefore approximate the phenomenon by the reaction given in (4.17) for
the sake of simplicity.
The mean colouration mechanism which starts at E = El is characterized
by a significant increase in mass that can only be explained by the incorporation of OH-. The electro chromic colouration mechanism for evaporated
NiO(OH)x films can therefore be described as
4.4 Examples of the Characterization of Thin Film Materials
> E 1 : O'Ni(OHh - O'H+
187
B
(4.18)
+ H+
O'NiOOH + O'e-
B
H 20,
(4.19)
O'Ni(OHh + OH- - (O'-l)H+
B
O'NiOOH-H 20 + O'e-
(4.20)
B
NiOOH-(1/0')H 20
(4.21)
E
OH-
Ni(OHh
+ (l/O')OH -
[(O'-l)/O'lH+
+ e-.
The a-value of (4.21) is an arbitrary number greater than zero that describes
the bonding of 0'-1 hydroxyl ions per Ni(OHh during the colouration of the
film. Equation (4.21) furthermore suggests the formation of lattice water due
to the incorporation of OH-. This is supported by the increased absorption in the region of the bound oxygen-hydrogen group shown in Fig. 4.37.
Equation (4.21) describes the ejection of (0' - 1)0' hydrogen ions during the
colouration process which is superimposed on the injection of 0'-1 hydroxyl
groups. A quantitative analysis of the mass balance for (4.21) leads to a mass
increase of (in atomic units)
mNaOH(O') = (l/O')OH - [(0' - l)/O'lH
mNaOD(O')
=
17/O'a.u. - (0' -l)/O'a.u. ,
=
(l/O')OD - [(0' - l)/O'lD
=
18/O'a.u. - 2(0' -l)/O'a.u. ,
(4.22)
(4.23)
per Ni(OHh molecule in NaOH and NaOD electrolyte, respectively. The
smaller mass increase m2 in 1 M NaOD in Table 4.11 compared with 1 M
NaOH leads to the restriction 0' > 1. A further restriction, caused by the
injection of (l/O')OH- which is associated with the ejection of [(0' -l)/O'lH+
during the colouration process in (4.21), is 0' > 2, imposed by (1/0') - (0'1) /0' < 0, because NRA measurements show a decrease of the hydrogen content in the coloured films. The quotient m2/NNiu calculates the so-called
"molar" weight of the particle (in atomic units), using the measured mass
increase m2 absorbed per nickel atom during the colouration process. By dividing that value by the number of charge transfer per nickel atom (Q / e NNi),
the change in mass m2,exp per Ni(OHh molecule is obtained.
(4.24)
Equation (4.24) and the values given in Table 4.11 yield m2,exp' These values
are also shown in Table 4.11 and can be compared with the theoretical data
of the last column. The theoretically expected values from (4.22) and (4.23)
are given for 0' = 3 and 0' = 4. A good agreement is obtained for 0' = 3 between the model calculation given by (4.22) and (4.23) and the mass increase
determined from the experimental results; see (4.24). Because the theoretical
values for 0' = 3 are consistently higher than the experimental data it can be
inferred that the a-value is somewhat higher than 3.
The electro chromic colouration mechanism of NiO(OH)x films deposited
by e- -beam evaporation can therefore be summarized by
188
4. Properties and Characterization of Dielectric Thin Films
bleached
coloured
E<El:
Ni(OH)2 - H+
f-t
NiOOH + e-
E>El:
Ni(OHh + 1/30H- - 2/3H+
f-t
NiOOH·l/3H 20+ e-. (4.26)
(4.25)
For potentials E > El only hydrogen ejection occurs and produces the electrochromic change in optical properties. This process is superimposed on
OH- injection for potentials E > E 1 . The proposed model likewise resolves
the virtual contradiction between NRA and FTIR findings. The increased
FTIR absorption in the OH vibrational mode range of the coloured film is
due to the formation of 1/3H20 lattice water per Ni(OHh molecule. The
decrease in hydrogen content, deduced by NRA in all-solid-state devices, is
consistent with the ejection of 2/3H+ and results in a total decrease of 1/3H+
in the coloured film.
The unique structure of the evaporated films helps in understanding the
first phenomenon (hydrogen ejection) which is not observed in Ni(OHh hydroxide films or porous NiO anodic oxide prepared by other deposition techniques. There the VMGs are perfectly flat up to the potential El [4.300]. In
contrast to the other specimens, as-deposited evaporated films contain a large
amount of trivalent nickel, which implies the presence of a large number of
voids on the cationic sites. In the other materials, where these cationic sites
are occupied, the transport to the oxygen or hydroxyl sites can only be done
by anions. In the evaporated films, protons quickly occupy the cationic voids
and induce the first low-potential reaction.
Experimental data (in particular Raman) describing the stoichiometry of
evaporated nickel oxide films support the hypothesis that NiO is the major
species ofthe Ni(OHh/Nb03/Ni film composition [4.18,284,296,307]. Based
on the observed water content in the coloured and bleached films, and also
taking into account a non-probable minor incorporation of hydrated cations
in the solution, we assume the following composition for the bleached and
coloured films:
bleached:
coloured:
a(NiO) + Ni(OH)2 + J-lH 2 0 + TNa, H 20.
a(NiO) + NiOOH + (J-l + 1/3)H20 + TNa, H 20,
(4.27)
(4.28)
where a varies between 5 and 15, depending on the deposition parameters [4.18]. T is 0 and J-l is 1 for the evaporated material investigated here.
ESCA detail spectra of the 0 Is state show a decrease in the ratio of O-H
bonds to Ni=O bonds for coloured films. The compositions proposed in (4.27)
and (4.28) yield 3/a and (7/3)/(a + 1) for the O-H/Ni=O ratio of the 0 Is
state for bleached and coloured films, respectively, which means that a maximum change of 30% in that ratio can be expected during the electro chromic
modulation. This corresponds well with the ESCA findings, which are only
sensitive to surfaces with a naturally enhanced content of OH groups. On
the whole this is in agreement with the value of incorporated OH in the as-
4.4 Examples of the Characterization of Thin Film Materials
189
deposited materials and with the value of OR which is responsible for the
whole subsequent electrochromic behaviour of the films.
In summary, the presented model explains the experimental data of microbalance, current voltage graphs, FTIR and NRA, and the results of ESCA
studies, and is also consistent with Raman results [4.300]. Dense NiO (e.g.,
in single crystals or thermal films) either cannot be coloured at all or the
colouration stems from lattice defects, or from macroscopic pores as in the
case of anodic oxides, or from microscopic defects as in the present case. The
main promoting forces for colouration are therefore water molecules or OR
anions trapped in these special lattice defects formed during the material
processing.
4.4.5 Tungsten Oxide
Klaus Bange
Tungsten oxide films are among those optically active materials that can alter
their optical properties as a function of changes in external conditions. In
these substances, which recently have been named "chromogenics" [4.308],
very different mechanisms are responsible for the variation in the optical
properties of films. The processes basically fall into three different groups:
• Thermochromic films change their optical properties as a function of substance temperature.
• Photochromism may be defined as a reversible absorption change which
is triggered when a film is exposed to different types of irradiation and
regains the original properties without irradiation.
• Electrochromism is a unique property of thin films or thin film systems in
which they change colour due to an applied potential and return to their
original state upon potential reversal.
All of these reversible effects are observed in tungsten oxide and some
nonreversible processes can be obtained in addition. Materials with controllable light absorptance, transmittance and reflectance are of great technical
relevance because of their numerous potential applications. Single tungsten
layers or layer systems can be used for different types of devices whose optical
characteristics can be modulated as a function of applied external potentials,
gases, but also as a function of temperature or of the intensity of irradiation.
In the near future, optically active thin films may be employed for regulating
the throughput of radiation energy of windows in buildings and cars, or as
optically active filters to maintain comfortable lighting and temperature in
sunglasses, and they may be used in systems with variable reflectance such as
automotive rear-view mirrors, displays, sensors or detectors, or as road signs,
and so forth.
Today, various film deposition techniques are available to prepare tungsten
oxide layers [4.278]. Thermal evaporation, applied by Deb in his pioneering
190
4. Properties and Characterization of Dielectric Thin Films
work [4.309,310] is still a convenient and widely used method for the deposition of tungsten oxide films. Various deposition conditions and parameters
are reported, for example for the deposition rate, the substrate temperature,
the oxygen partial pressure, the total pressure, and so on. Despite the differences in the deposition conditions, a comparable colouration is obtained
for tungsten oxide films prepared by this technique. Sputtering in Ar + O 2
plasma [4.311-313] and chemical sputtering in O 2 + CF 4 plasma [4.314,315]
have been reported as well. The characterizations of these films indicate a
strong structural and compositional kinship between evaporated and sputterdeposited tungsten oxide films. Chemical vapour deposition (CVD) involving
pyrolysis ofW(CO)6 on hot substrates has been used to prepare tungsten oxide films [4.316-319]. In spray pyrolysis, which is a variety of this technique,
a solution is sprayed on the hot substrate surface under conditions that cause
the droplets to evaporate before hitting the surface. The microstructure of
these film materials differs significantly from that of PVD (physical vapour
deposition) films.
Plasma-enhanced chemical vapour deposition (PECVD) is a modern
technique capable of yielding high deposition rates onto substrates with
low temperatures. A maximum deposition rate of about 40 nm s-1 is reported, which by far exceeds the rates achieved by other deposition techniques [4.320-322]. Anodic oxidation, i.e., anodization under potentiostatic,
potentiodynamic, and galvanostatic conditions has been used to produce
tungsten oxide films [4.323-329]. Amorphous and crystalline films are obtained, depending on the anodization voltage. Sol-gel-derived films are made
from colloidal solutions by dip coating, spin coating, or spraying [4.330-336].
Several other techniques can be applied to produce tungsten oxide films, for
example decomposition of oxalatotungstate compounds, electrodeposition,
thermal oxidation, and hydrothermal treatment [4.278].
Evaporated Film in As-Deposited State
These considerable differences in deposition conditions create differences in
the structure and in the optical and electrical behaviour of as-deposited tungsten oxide films. The colouration behaviour of the respective thin film also
depends strongly on the originally deposited form. In order to explain the
various colouration processes of tungsten oxide films, the subsequent description is restricted to films produced by thermal evaporation, which are best
characterized so far. The results on "standard" layers, produced in a commercial system (Balzers BAK 760) by thermal evaporation on substrates at
temperatures below 100 DC, are described. These films exhibit an optimized
colouration behaviour. The total pressure was 2 x 10- 2 Pa. A deposition rate
of 1 nms- 1 was used and the thicknesses of the films varied between 200 and
800nm. Further details are given in [4.18].
W0 3 powder evaporates by sublimation in vacuum at a temperature
> 1200K. W 3 0 9 , W0 3 , W 2 06 and W0 2 predominate in the gas phase,
4.4 Examples of the Characterization of Thin Film Materials
191
but other polymers of W0 3 and fragments are also found. A variation in
water partial pressure drastically changes the composition of the evaporated
species [4.337,338]. At higher water partial pressure the polymers increase
in number and size. From the reported data it can be inferred that under
technical conditions W0 3 monomers and polymers adsorb preferentially at
the substrate. Minor species are substoichiometric fragments. In addition,
water adsorbs in a concurrent process. Because the substrate temperature
is generally low and the background pressure high, the adsorbed particles
are not very mobile in the surface and create porous films with low density.
The as-prepared films are highly disordered and electron diffraction, XRD,
and Raman spectroscopy suggest an amorphous state. But columnar structures are observed. Columns of approximately 20 nm in diameter are found
in 500-nm-thick films. They appear to be very homogeneous, i.e., no grain
boundaries can be recognized [4.18].
Investigations of the film growth of 40-nm-thick tungsten oxide films exhibit clusters with sizes between 2 and 10 nm. The cluster size depends on the
substrate temperature [4.18]. For as-deposited "standard" films, Raman spectroscopy shows a broad peak centred at a wave number of about 750 cm -1,
and a narrower peak at about 950 cm- 1 . The broad W-O stretching vibration at 750 cm -1, which is generally interpreted as amorphous state, can
be deconvoluted into two Gaussian components peaking at approximately
710 and 805 cm- 1 , where two pronounced features in the crystalline film are
also located [4.339,340]. The small narrow peak at about 950 cm- 1 is found
neither in crystalline thin films nor in single crystals. A corresponding vibration mode is found only in water-containing material [4.341-343]. Because
this peak vanishes with increasing annealing temperature, the feature can
be unambiguously assigned to stretching vibration modes of terminal W =0
bonds [4.344].
The presence of a local order is also supported by SAXS investigations.
To analyse the obtained radial distribution functions (RDF), various structural models have been proposed [4.346,347]. Nanba and Yasui employ eight
different crystallographic structures as starting point and compute the corresponding RDFs. The position of the first and the second peaks in the observed
RDFs are consistent with W-O nearest neighbours at about 0.2 nm, with
W-W nearest neighbours at 0.37-0.4nm, and to a smaller extent with W-O
nearest neighbours. All these features stem from a basic octahedral W0 6
building block. A structural model, shown in Fig. 4.38, has been developed.
It is based on hexagonal W0 3 , and W0 3 ·1/3H 2 0, in which three-membered
and six-membered rings are displayed in the x-y plane and four-membered
rings are parallel to the vertical z direction. The cluster size of the model is
1.6 nm in diameter and is consistent with high resolution electron microscopy
evidence [4.18,347]. The three-membered rings can stem from the W 3 0 9
molecules in the vapour.
192
4. Properties and Characterization of Dielectric Thin Films
T5 = 300 · C
Ts = 150 · C
Ts= RT
::J
.!!!
lJ...
o
a:::
o
0.5
Distance Inm
1 0
0.5
Distance Inm
1
0
Distance Inm
Fig. 4.38. Radial distribution functions (lower parts) determined from SAXS (solid
curves) and computed from the structural models shown (upper parts) are based
on connected W06 octahedra [4.346]
The results on the composition of tungsten oxide films can be summarized
as follows: Thin tungsten oxide films deposited by evaporation do not consist of W0 3 , i.e., they are substoichiometric and can be described as W0 3 - z
with z > O. This is indicated by AES, ESCA and RBS. But the layer contains additional hydrogen and water or water fragments , as found by NRA,
ESCA and IR investigations and by a combined TGA and FTIR investigation [4.18, 346]. The complex composition of the as-deposited films is best
described as W0 3 - z - q(OH)q x pH 2 0 [4.384]' whereby the exact values of the
variables are sensitive to the deposition conditions and to post-deposition
treatment. The results on the composition are supported by data on the
film density. The packing density, which is defined as a relative density of
the ideally packed bulk W0 3 (Pbulk = 7.16gcm- 3 ), can vary by evaporation
onto unheated substrates between a relative density of ~ 0.8 and 0.4 as a
function of the total pressure used in the vacuum. With increasing substrate
or annealing temperature, the density can be changed between 0.5 and 0.9
[4.348- 351].
The optical absorptions in the UV are dominated by the band gap of the
semiconductor. The band gap is wide enough to render the material transparent. A band gap of Eg ~ 3.25 eV is found, which is typical for highly disordered tungsten oxide films [4.278]. The optical absorption can be explained
4.4 Examples of the Characterization of Thin Film Materials
193
by assuming an indirect allowed transition. High vacuum evaporation leads to
a distinct absorption in the NIR, centred at approximately 1.2 eV. The spectral refractive index exhibits the typical dispersion for oxides. A refractive
index of about 2 is analysed at a wavelength of 550 nm.
Colouration Procedures
A wide variety of processes and mechanisms can be used for the colouration of a tungsten oxide film with an amorphous state, i.e., on nanocomposited material described above. Coloured films can be obtained by preparing
the tungsten oxide films in high vacuum [4.351,352] or at high evaporation
temperature (> 1600 K) [4.353]. In both cases the films exhibit a light blue
colour. For thermo chromic colouration, films are simply annealed in vacuum
at temperature Ta > 80°C, which exceeds the substrate temperature used
during the preparation. The results of mass spectrometry suggest that oxygen desorbs during that process [4.353]. The bleaching of films is obtained by
annealing in air.
A non-reversible colouration can be produced by the bombardment of
transparent tungsten oxide films with electrons or different types of ions. The
colouration obtained by interactions with electrons is induced by electronstimulated desorption processes [4.354]. The colouration by irradiation with
ions is a more complex procedure. The effect of non-reversible colouration
by ion beam irradiation on amorphous tungsten oxide films is studied for
various types of ions (1 H , 2He , 15N , 16 0 , 23Na , 40 Ar , 70Ga) with energies in
the keY and MeV range [4.355-359]. Different effects (e.g., an increase in the
oxygen-deficient substoichiometric films, a modification in the nanostructure
of the material, or ion implantation) are supposed to be responsible for the
colouration [4.355-359].
Switching from the bleached state to the coloured state can be induced
by UV illumination. The obtained fast and short time changes in the optical
absorptions are small for tungsten oxide films with thicknesses > 300 nm.
Differences in the transmittance spectrum can usually only be observed after
exposure to light for several hours or days. Fairly recently, very thin films with
a higher dynamics have been investigated [4.360-362]. The photochromic behaviour of tungsten oxide is strongly influenced by the film density, the film
thickness, the wavelength of the irradiated light and also by the surrounding
atmosphere. The presence of vapour-containing hydrogen compounds considerably increases the photo chromic response [4.363]. Oxygen in the atmosphere is necessary for a complete bleaching and decreases the absorption.
Most experimental data indicate that the UV-induced colouration process is
probably combined with oxygen desorption [4.353].
Electrochromic colouration can be achieved by various procedures. Most
of the fundamental research on the electrochromism of tungsten oxide films
is realized in liquid electrolyte experiments on the basis of electrochemical
measurement techniques. These established methods enable the control of
194
4. Properties and Characterization of Dielectric Thin Films
the experimental conditions, for example a potential or a current, and the
measuring of time-dependent quantities. Changes in transmittance or absorptance and variations in the spectral refractive index and the extinction
coefficient are generally analysed for different colouration states, and a spectral colouration efficiency can be found. Diffusion constants and electromotive
forces as a function of the injected charge are generally reported [4.364, 365J.
Several other techniques are available to obtain electrochromic colouration
in electrochemical environment without applying an external voltage. The
film colours when the surface is covered with an electrolyte (20% H 2 S0 4 ) and
a tip in the electrolyte touches the film surface. Colouration occurs around the
tip and keeps growing until the electrolyte boundary is reached [4.366, 367J.
Colouration also results when tungsten oxide films are in contact with a
mixture of HCI and Zn [4.368J. Similar effects are achieved by using sulphuric
acid (0.1 N) and a tin foil. Procedures of this type can be very useful in
experiments in which non-conductive substrates are necessary.
Electrocolouration, i.e., colour at ion by application of a high electrical
field, is another procedure to obtain absorption. This colouration process
is also reversible and depends on the relative humidity in the surrounding.
The electrochromic systems described in Sect. 5.3 in general work with different types of electrolytes. In such devices the liquid electrolytes are often
replaced by solid-state electrolytes. The behaviour of tungsten oxide films
in all-solid-state devices and in liquid electrolyte experiments is essentially
identical.
Hydrogen-containing gases colour in the presence of catalysing material,
and oxygen-containing atmosphere bleaches tungsten oxide layers [4.18,378,
379J. Variations in hydrogen content determine the changes in optical density
and the colouration rate. The relative humidity also has a strong impact on
the colouration behaviour of tungsten oxide/catalyser systems. The colouration rate is highest for relative humidities of approximately 30% [4.18J.
Evaporated Films in Coloured State
The coloured state of tungsten oxide films in principle represents a new film
material whose microscopic and macroscopic properties differ from that of
as-deposited or bleached films. The obviously changed visible characteristic
feature is the colour, i.e., the absorption. The changes in the optical density
induced by colouration are best described theoretically by the small polaron
concept or by the intervalance charge transfer model [4.368-371J. The local
structural distortion of the lattice and the formation of a polaron are more
pronounced in films with a high oxygen deficit and an enhanced nanocrystallinity. The experimental results obtained from those films suggest that
the tungsten-oxygen distance is increased by the localization of one extra
electron. Evaporated films with high oxygen content and/or amorphous, less
dense structure exhibit no measurable changes in structure, probably because the oscillator strength of small polaron absorption is decreased, i.e.,
4.4 Examples of the Characterization of Thin Film Materials
195
the obtained results depend remarkably on the film preparation conditions.
For similar reasons the intensity of the terminal W =0 vibration is decreased
by coloured amorphous films, whereas an increase is observed in films with a
higher degree of crystallinity.
The various colouration procedures doubtless create the same changes
in the electronic structure of tungsten oxide films. Changes in the optical
density are directly proportional to the amount of W5+. Such information
can be obtained from ESCA data displayed in Fig. 4.39. Four typical ESCA
spectra for the W 4f state are shown. Films annealed in air at 150°C for
..' . .....
W4f
,_ ...,....'. '
..
.
•J'
.
~
."
'.
..
:::i
.!!!
(J)
c:
e
"0
Q)
...' ."
Qi
o
(5
.--
....
.: t..
~
a..
........
'0
Q)
.0
E
:J
Z
,
x =0
; "
....
--,....~-,/,;
~-,.....
.
.
. .
:....
......
......
.._----"-""=
TA
=150 · C
-------..,-j.
42
40
38
36
34
32
Binding energy l eV
Fig. 4.39. ESCA spectra of a W 4f doublet for an annealed "standard" tungsten oxide (Ts = 150°C), as-deposited film (x = 0) and in two colouration states
(x = 0.09 and x = 0.42). The charge equivalent x describes the number of injected
electrons per tungsten trioxide molecule, which have to be charge-compensated by
the injection of positive ions or by the extraction of negative ions; see the explanations in text
196
4. Properties and Characterization of Dielectric Thin Films
20 h are used as reference. The data indicate that the tungsten oxide layer is
completely oxidized after annealing. The W 4h /2 peak appears at a binding
energy of 35.8eV and the W 415/2 peak at 38.0eV. Relative to the metal
state (31.0eV), the measured W 41 doublet is shifted to a binding energy
approximately 5 e V higher and represents the oxidation state W6+. Transparent as-deposited tungsten oxide films are labelled with x = O. The slight
broadening of the spectrum and the shoulder at a smaller binding energy
suggest that the as-deposited film is not fully oxidized, i.e., the surface contains small amounts of W 5+. The two additional spectra present coloured
films with x = 0.1 and x = 0.4, respectively. The injected charge creates a
new additional oxidation state with a different binding energy that leads to
more complex spectra by superimposing a W 5+ doublet with a W6+. Quantitative information about the relative number of tungsten oxide atoms in
different valance states is obtainable by the deconvolution of ESCA spectra [4.372-374]. In Fig. 4.40 the resulting correlation of W 5+ jW tot to the
introduced charge equivalent x is depicted. An almost linear dependence is
observed for the amount of W 5+ for x < 0.25. The error bars demonstrate
the range of the different fitting solution. The fitting uncertainties increase
with the more complex structure of higher x values. A small offset can be recognized between the theoretical lines and the experimental data points. The
theoretical lines are based on the assumption that all injected charge will create additional W5+. ESR data likewise exhibit this linear increase of the W 5+
feature with colouration [4.375]. Independent of the colouration procedure,
in UPS a band appears near the Fermi level that increases with colouration.
The inserted electrons increase the electron density, i.e., the Fermi level seems
0.3
0
:;:::;
~
:8
S
0.2
+
'"S
0.1
Charge equivalent x
Fig. 4.40. W5+ /W tot ratio, determined from ESCA measurements, as a function
of the injected charge. The circles are the results of analysing the ESCA spectra;
the dashed line represents the expected theoretical dependence
4.4 Examples of the Characterization of Thin Film Materials
197
to move upwards and the excess electrons either enter the otherwise empty
lower part of the conduction band (5dt2g) or a defect band is localized in the
band gap [4.375].
Injection and Ejection Processes
The generation of colour in tungsten oxide films is connected with different
transport mechanisms, which in part are necessary for the charge balancing of
the injected electrons. Whereas the injection of electrons is not disputed, the
charge balancing effect is still under discussion. Reversible electrochromism
is usually explained by the double charge injection model [4.376], in which a
hydrogen intercalation occurs parallel to the injection of electrons, i.e., it is
postulated that the hydrogen content increases in the coloured state. Various
investigations on coloured films have been carried out to verify the double
charge injection model [4.18]. Up to now no experimental evidence has been
found for an increased hydrogen content in the coloured state.
The colouration of tungsten oxide films is produced by an oxygen deficit.
This is reported for films prepared under special selected deposition conditions and also for non-reversible chromogenic processes. Desorption of oxygen is described in thermo chromic and photo chromic processes in which the
coloured state is more substoichiometric than W0 3 - z ' Films coloured with
an electro chromic reaction also exhibit a decreased O/W ratio. This can be
concluded from RBS data and from a careful analysis of the 0 Is signal of
ESCA results [4.18].
To develop a reasonable model for the colouration of tungsten oxide films,
the influence of the reservoir composition on the colour at ion process has to
be taken into account, too. Thermochromic colouration is only observed in
oxygen-free atmosphere and oxygen is necessary for bleaching [4.353,377].
The intensity of the photo chromic colouration decreases in the presence of
oxygen [4.360-362]. The bleaching process of UV coloured films is especially
influential. In gas phase experiments oxygen-containing gases are used for
bleaching [4.378,379]' and in the electro chromic anodic reaction the bleaching
efficiency is increased in aerated solutions [4.380]. From the different results
obtained by the various colouration processes it can be concluded that oxygen
in the surrounding atmosphere tends to increase the bleaching of coloured
films.
Hydrogen-containing environments have the opposite influence on the
chromogenic behaviour. High relative humidity increases the probability of
electrocolouration [4.345]. The photo chromic colour at ion response is accelerated and enhanced by water vapour and alcohols [4.363,381-383]. In gas
phase experiments the colouration is done with hydrogen and with water
vapour, which is called co-catalyser [4.378,379]. The results suggest that
hydrogen-containing species favour the colouration process.
198
4. Properties and Characterization of Dielectric Thin Films
The reported data on the changes in the film composition during the
chromogenic colouration process in the presence of oxygen-containing and
hydrogen-containing reservoirs can be summarized as follows:
• The oxygen content in the film is reduced by the colouration, and the
presence of oxygen in the surrounding atmosphere decreases the colouration
effect.
• No changes are observed in the hydrogen content of films between the
bleached and the coloured state, i.e., the total amount of hydrogencontaining species remains constant.
• The water content and the number of hydroxyl groups remain constant in
tungsten oxide films; but the presence of hydrogen-containing species in
the reservoir is necessary for colouration or enhances the effect.
The experimental results suggest that the chemistry that underlies the
changes in optical properties should be basically the same, i.e., a unified model
with similar reactions may prevail in the different colouration techniques and
may explain the various findings on film composition and the influence of the
composition of the reservoir. The simplest explanation of the changes in the
film composition during the colouration is
reservoir
(H, 0, H 2 0)
e
(W0 3 -
..............
z-
film
q(OH)q' pH 2 0)
(4.29)
0-
The reservoir can contain hydrogen, oxygen, water and dissociation products
of water in different concentrations. The tungsten oxide film consists of a
complex composite of W0 3 - z - q(OH)q . pH 2 0 [4.384], where the variables z,
q and p are determined by the preparation conditions and the film treatment.
During the colouration process an electron e- is injected into the film and
an oxygen ion diffuses across the film interface in the reservoir. In this model
the oxygen concentration in the bleached state is lower than in the coloured
state, as suggested by RBS and ESCA. The hydrogen content and all other
composition components of the film remain constant. Charge neutrality is
achieved by inserting one electron into the film and extracting one oxygen
ion, which is probably removed from W0 3 - z - q species.
Some results of not completely reversible colouration processes may be
explained by that simple mechanism. The models allow one to interpret the
data obtained by thermochromism [4.353] and by ion beam and electron
bombardment [4.354-359]. But this one-channel model for oxygen can explain
neither the measured hydrogen diffusion nor the injection of alkali species
into the film by the use of an alkali-containing electrolyte which is obtained
under various experimental conditions. Moreover, the influence of hydrogencontaining vapour cannot be described.
4.4 Examples of the Characterization of Thin Film Materials
199
To give better coherency between theory and experimental results, the hydrogen transport across the film interface must be taken into account. A more
realistic description of the chromogenic colour at ion process may be given by
reservoir
(H, 0, H 2 0)
film
(W0 3 _ z _ q(OH)q . pH 2 0)
(4.30)
...............
...............
0H
This reaction also explains the measured changes in composition. Hydrogen
diffusion takes place, but the hydrogen content in the film is not changed and
the oxygen concentration is decreased in the coloured state by the amount
indicated by our RBS studies on electrochemically coloured material. Results
on films coloured in alkali electrolyte suggest that, for example, Li+ replaces
hydrogen which had been adsorbed at internal surfaces [4.385]. A similar
mechanism may occur by the use of hydrogen-containing electrolytes. The
injected H+ ion replaces hydrogen adsorbed at internal surfaces, the film behaves like a saturated "hydrogen alloy" , and hydrogen diffuses out of the film
across the film interface into the neighbouring reservoir. The second channel,
the transport of H+ into the film and H out of the film, may explain the unchanged hydrogen concentration determined by NRA. Charge compensation
is realized in this model.
The influence of the composition of the reservoir can be demonstrated by
the reactions given in (4.29) and (4.30). Thermochromism, for example, is
observed only by heating in vacuum; i.e., the oxygen content in the reservoir
is zero. If there is any oxygen in the reservoir, the oxygen desorption process
is superimposed by oxygen adsorption and thus the colouration reaction is
hampered or even suppressed. These influences of the oxygen in the reservoir
are likewise found in photochromism and electrochromism with liquid and
solid electrolyte and especially in gas phase experiments where oxygen is
used for bleaching.
The presence of hydrogen-containing species in the reservoir can be explained by the reactions given in (4.30). While the hydrogen concentration
in the film is unchanged, the hydrogen-containing species in the reservoir are
necessary for charge compensation in the colouration process. The importance of the hydrogen-containing species in the reservoir for the colouration is
demonstrated for photochromism and for different types of electrochromism,
i.e., in aqueous and solid electrolyte, in gas phase experiments and by the use
of high electric fields.
The model discussed first explains the changes in composition during the
chromogenic colour at ion processes. The second model in addition describes
the influence of oxygen and hydrogen in the surrounding atmosphere. But
structural changes, possibly resulting from changes in composition, are also
200
4. Properties and Characterization of Dielectric Thin Films
observed. Moreover, the colouration is affected by water. The published results on the changes in the O-H vibration and in the W =0 mode are somewhat contradictory. Whereas with hydrogen-containing electrolytes the O-H
vibration mode seems to be constant, in experiments with alkali electrolytes
this feature systematically increases with the inserted charge [4.385]. This
phenomenon can be explained by assuming that the different rate of water
incorporation is not correlated with the colouration process. But if the increase in the O-H vibration is correlated with the colouration process, this
can be readily explained by two mechanisms, namely by internal changes induced by the alkali uptake and by a more complex species transport across
the interface. Such processes may be given by
reservoir
(H, 0, H 2 0)
film
(W0 3 -
z-
q(OH)q . pH 2 0)
(4.31)
...............
...............
203H
In this reaction, (H 2 0+H)+ suggests the formation of a solvated proton on
the surface, followed by a proton transport into the adsorbed phase of the
"co-catalyst". In these processes the hydrogen content remains constant, the
oxygen content is reduced in the way found by RBS, and charge neutrality is
given. The reduction of the W =0 sites and the increase in the O-H vibration
can be explained by the reaction given in (4.31). The insertion of a solvated
proton (or alkali) may give a hint as to why water or alcohol is necessary in
the surrounding atmosphere for the colour at ion of the films.
In the non-reversible processes, oxygen desorption is found to be the predominant process. This reaction may be described by the one-channel model
(4.29) for the chromogenic colouration of evaporated tungsten oxide films.
Additionally, it seems that in such processes the basic octahedral W0 6 building blocks are probably damaged by temperature treatment or by the interaction with electrons or ions. This can be concluded from RDFs, where the
first coordination is separated into two, which is interpreted as a strong axial
deformation of the octahedra [4.386].
More complex reactions occur in the reversible processes, where a defined charge transfer across the interface is necessary. In these processes
the main structural and compositional features remain largely unchanged.
Two injected electrons, described in the reaction given in (4.30), are combined by small changes in composition; i.e., the high oxygen concentration
in the tungsten oxide film is only slightly reduced and the hydrogen content remains constant. The phenomenological model given in (4.30) satisfies
the current-doubling effect that is observed in the presence of water and
alcohols [4.381-383] and explains the two channels for the colouration and
bleaching process found in electrochromism and photochromism [4.360-363].
4.5 Properties of Multilayer Systems
201
In these processes the potential at the interface has a dominating impact on
the chromogenic behaviour. This is especially true for electrochemical processes, where external voltages are applied, and for spill-over experiments,
but also for photo chromic processes. The role of the interface potentials in
aqueous electrolyte experiments and in gas phase reservoirs can be described
for different media by a generalized Fermi level concept [4.291-294].
4.5 Properties of Multilayer Systems
Clemens Ottermann
Materials for multilayer coatings are primarily selected on the basis of their
optical, electronic, magnetic or chemical properties. Typically only a few layers are necessary for most applications or decorative purposes, whereas for
sophisticated optical coatings the layered structures have to be more complex. Dielectric mirrors or filters usually require more than 10 layers to meet
the higher demands of tougher specifications. It is desirable to optimize the
properties of the multilayer coating by taking into account the impact of
deposition conditions on the respective properties of the individual films.
This kind of procedure is well established for optical design requirements.
There exist a large number of recipes that allow one to predict the optical
behaviour of multilayer systems from the optical properties of single films;
see for example [4.48,389] and references cited therein. Several computer
codes are commercially available for optimizing coating designs (i.e., the total
number and individual thicknesses of the layers) by taking into account the
spectral refractive and absorptive behaviours of each film.
However, mechanical film properties, especially stresses, are frequently the
limiting factors for producing highly sophisticated optical coatings. Therefore
it is also important to predict their mechanical behaviour. Some effort has
already been invested in discovering the total stress behaviour of multilayered
coatings [4.390]. However, the results indicate that the task is more complex
than for optical properties. The prediction of total stresses requires not only
information about the stresses of the individual films but also necessitates
comprehensive knowledge about the elastic properties (Young's modulus and
Poisson's ratio) of each layer. Up to now, information about these properties
is very scarce for oxidic films. The dependencies of Young's moduli on production conditions are only known for a limited number of materials [4.391].
Further research efforts are required to obtain a more extended database for
a comprehensive optimization of optical multilayer designs with respect to
stresses.
Detailed knowledge about the stress properties of the single films at least
allows one to estimate the mechanical behaviour of multilayer coatings. This
is illustrated by the following example. A dielectric edge filter for sophisticated applications consists of alternating Ti0 2 and Si0 2 single films de-
202
4. Properties and Characterization of Dielectric Thin Films
posited by plasma impulse chemical vapour deposition (PICVD) on fused
silica substrates. In addition, the coated part has to withstand high temperatures (T > 600°C). In a first attempt the design has been optimized only
with respect to optical properties obtained by deposition conditions which
produce Ti0 2 films with a refractive index (n550) of 2.35 at a wavelength
of 550 nm. More than 30 alternating layers are needed to fulfil the optical
requirements; the total thickness of the coating is ~ 2 I-im.
The coating was flaking after the temperature resistance had been tested
by annealing at 800°C for 1 h. An investigation of the failure mode by means
of optical microscopy and SEM shows cracking as the main source of the
damage; see Fig. 4.41. Cracks originating in the multilayer system propagate
into the surface region of the substrate and cause the formation of chips in
the glass. The edges of these chips are bent towards the coated side, which
is a strong indication of tensile stresses in the coating. These large tensile
stresses are responsible for the cracking of the films and have to be reduced.
Si0 2 films produced by PICVD show only moderate compressive stresses;
see Sect. 4.4.2. Amorphous Ti0 2 films, however, with a refractive index n550
of the order of 2.35 possess strong tensile stresses which increase further by
annealing above 300°C; see Figs. 4.15 and 4.17. The findings on Ti0 2 single
layers also suggest two possibilities for stress reduction. A decrease in film
density will induce a decrease in stress and stress change by annealing. The
reduction in density also causes a decrease of the refractive index of the
Ti0 2 films , which forces an increase in the number of alternating layers in
Fig. 4.41. Surface structure of the coated part after annealing (1 h, 800°C) obtained by optical microscopy
References
203
the design to fulfil the optical requirements. The other possibility for stress
reduction is increasing the densities of the Ti0 2 films and, in addition, their
refractive indices due to the correlation between these quantities.
Figure 4.42 shows the result of a screening experiment performed to verify
these predictions. The degree of film damage after extended annealing (120
hat 800 DC) is depicted as a function of the refractive index n550 of the Ti0 2
films, acting as a measure proportional to the film density; see Fig. 4.15a.
The production conditions of the Si02 layers are kept constant for these
investigations. The degree of damage of the coatings is classified by categories,
where "none" indicates no damage and "strong" stands for complete flaking
as shown in Fig. 4.41. The trend of the data obtained for the multilayer
coatings shows the same dependencies as expected from the behaviour of the
single films in Fig. 4.15. This finding indicates the possibility of optimizing
multilayer coating designs with respect to optical and mechanical demands
simultaneously in future.
Strong
Medium
Minor
,,-
'"
None
2.2
/
/
---
\.
\
'" '"
2.25
\
2.3
2.35
2.4
2.45
Refractive index n550
Fig. 4.42. Classification of damage of the coated parts after annealing (120 h,
800°C) as a function of the refractive index of the Ti02 films at the wavelength of
550 nm (closed circles). The dashed curve serves to guide the eye
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of hydrous nickel oxide", Solar Energy Mat. 16, 333-346 (1987)
4.283 T. Maruyama, S. Arai: "The electrochromic properties of nickel oxide thin
films prepared by chemical vapor deposition", Solar Energy Mat. Solar
Cells 30, 257-262 (1993)
4.284 P. Delichere, S. Joiret, A. Hugot-Le Goff, K. Bange, B. Metz: "Electrochromism in nickel oxide films", Proc. SPIE 1016, 165-169 (1988)
4.285 W. Wagner, F. Rauch, K. Bange: "Stoichiometry of NiO",Hy films determined by ion-beam analysis", Nucl. Instrum. Meth. Phys. Res. B 89, 104108 (1994)
4.286 A. Agrawal, H.R Habibi, RK. Agrawal, J.P. Cronin, D.M. Roberts,
RS. Caron-Popowich, C.M. Lampert: "Effect of deposition pressure on the
microstructure and electro chromic properties of electron-beam-evaporated
nickel oxide films", Thin Solid Films 221, 239-253 (1992)
4.287 A. Nemetz, A. Temmink, K. Bange, S. Cordoba de Torresi, C. Gabrielli,
R Torresi, A. Hugot-Le Goff: "Investigations and modelling of e- -beam evaporated NiO(OH)", films", Solar Energy Mat. Solar Cells 25, 93-103 (1992)
218
4. Properties and Characterization of Dielectric Thin Films
4.288 C.M. Lampert, T.R. Omstead, P.C. Yu: "Chemical and optical properties of
electrochromic nickel oxide films", Solar Energy Mat. 14, 161-174 (1986)
4.289 F. Hahn, B. Beden, M.J. Croissant, C. Lamy: "In situ UV visible reflectance
spectroscopic investigations of the nickel electrode-alkaline solution interface", Electrochim. Acta 31, 335-342 (1986)
4.290 K. Bange, U. Martens, A. Nemetz, A. Temmink: "The electrochromism of
single tungsten oxide and nickel oxide films in liquid electrolyte experiments
and all-solid-state devices (ASSDs)", Proc. Electrochem. Soc. 90-2,334-348
(1990)
4.291 H. Gerischer, W. Ekardt: "Fermi levels in electrolytes and the absolute scale
of redox potentials", Appl. Phys. Lett. 43, 393-395 (1983)
4.292 H. Gerischer: "Neglected problems in the pH dependence of the flat band
potential of semiconducting oxides and semiconductors covered with oxide
layers", Electrochim. Acta 34, 1005-1009 (1989)
4.293 J. Janata: "Chemical modulation of the electron work function", Anal.
Chern. 63, 2546-2550 (1991)
4.294 D. Blackwood, M. Josowicz: "Work function and spectroscopic studies of
interactions between conducting polymers and organic vapors", J. Phys.
Chern. 95, 493-502 (1991)
4.295 K. Bange, C. Ottermann: "Electro chromic coatings on glasses", in Prom
Galileo's "Occhialino" to Optoelectronics, ed. by P. Mazzoldi (World Scientific, Singapore 1993) pp. 14-33
4.296 K. Bange, F.G.K. Baucke, B. Metz: "Properties of electro chromic nickel oxide coatings produced by reactive evaporation", Proc. SPIE 1016, 170-174
(1988)
4.297 P.C. Yu, C.M. Lampert: "In-situ spectroscopic studies of electrochromic hydrated nickel oxide films", Solar Energy Mat. 19, 1-16 (1989)
4.298 F.P. Kober: "Infrared spectroscopic investigation of charged nickel hydroxide
electrodes", J. Electrochem. Soc. 114, 215-218 (1967)
4.299 J.F. Jackovitz: "The vibrational spectra of nickel hydroxide and higher nickel
oxide", Proc. Electrochem. Soc. 82-3,48-68 (1982)
4.300 S. Cordoba de Torresi, C. Gabrielli, A. Hugot-Le Goff, R. Torresi: "Electrochromic behavior of nickel oxide electrodes", J. Electrochem. Soc. 138,
1548-1553 (1991)
4.301 M.G. Hutchins, G. McMeeking, Z.C. Orel: "Infrared analysis of electrochromic nickel oxide coatings", Proc. SPIE 1728, 66-72 (1992)
4.302 N.R. Lynam, H.R. Habibi: "Characterization of nickel oxide electro chromic
films", Proc. SPIE 1016, 63-75 (1988)
4.303 R.M. Torresi, M.V. Vazquez, A. Gorenstein, S.1. Cordoba de Torresi: "Infrared characterization of electro chromic nickel hydroxide prepared by homogeneous chemical precipitation" , Thin Solid Films 229, 180-186 (1993)
4.304 W. Wagner, F. Rauch, C. Ottermann, K. Bange: "Hydrogen dynamics in
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4.305 M.K. Carpenter, R.S. Conell, D.H. Corrigan: "The electrochromic properties
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4.306 J.S. Svensson, C.G. Granqvist: "Electro chromic hydrated nickel oxide coatings for energy efficient windows: optical properties and coloration mechanism", Appl. Phys. Lett. 49, 1566-1568 (1986)
4.307 P. Delichere, S. Joiret, A. Hugot-Le Goff, K. Bange, B. Metz: "Electrochromism in nickel oxide thin films studied by OMA and Raman spectroscopy", J. Electrochem. Soc. 135, 1856-1857 (1988)
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220
4. Properties and Characterization of Dielectric Thin Films
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4.330 H. Unuma, K. Tonooka, Y. Suzuki, T. Furusaki, K. Kodaira, T. Matsushita:
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dip-coating method", J. Mat. Sci. Lett. 5, 1248-1250 (1986)
4.331 K. Yamanaka, H. Oakamoto, H. Kidou, T. Kudo: "Peroxotungstic acid coated
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4.332 M.A. Habib, S.P. Maheswari, M.K. Carpenter: "A tungsten-trioxide/ Prussian blue complementary electro chromic cell with a polymer electrolyte",
J. Appl. Electrochem. 21, 203-207 (1991)
4.333 M.1. Yanovskaya, I.E. Obvintseva, V.G. Kessler, B.Sh. Galyamov,
S.1. Kucheiko, R.R. Shifrina, N.Ya. Turova: "Hydrolysis of molybdenum and
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4.334 J. G6ttsche, A. Hinsch, V. Wittwer: "Electro chromic mixed W03-Ti02 thin
films produced by sputtering and the sol-gel technique: a comparison" , Solar
Energy Mat. Solar Cells 31, 415-428 (1993)
4.335 A. Agrawal, J.P. Cronin, R. Zhang: "Review of solid state electrochromic
coatings produced using sol-gel techniques", Solar Energy Mat. Solar
Cells 31, 9-21 (1993)
4.336 J.P. Cronin, D.J. Tarico, J.C.L. Tonazzi, A. Agrawal, S.R. Kennedy: "Microstructure and properties of sol-gel deposited W0 3 coatings for large area
electro chromic windows", Solar Energy Mat. Solar Cells 29, 371-386 (1993)
4.337 A. Azens, M. Kitenbergs, U. Kanders: "Evaporation of tungsten oxides:
a mass-spectrometric study of the vapour contents", Vacuum 46, 745-747
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4.338 E.U. Berndt: Uber die Struktur aufgedampfter Schichten, PhD Thesis (Munster 1986)
4.339 Y. Shigesato, Y. Hayashi, A. Masui, T. Haranou: "The structural changes
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4.340 Y. Shigesato, A. Murayama, T. Kamimori, K. Matsuhiro: "Characterization
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127-139 (1988)
4.342 S. Mohan, A. Mukunthan: "Laser Raman spectrum of tungstic oxide", Current Science 54, 858-859 (1985)
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4.350 H. Morita, H. Washida: "Electrochromism of atmospheric evaporated tungsten oxide films (AETOF)", Jpn. J. Appl. Phys. 23, 754-759 (1984)
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4.353 M.R. Goulding, C.B. Thomas, R.J. Hurditch: "A comparison of thermo- and
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4.354 H. Morita, T. Miura, H. Washida: "Coloration of tungsten oxide films induced
by ion or electron irradiation", Jpn. J. Appl. Phys. 20, 323-325 (1981)
4.355 W. Wagner, F. Rauch, R. Feile, C. Ottermann, K. Bange: "Compaction of
tungsten oxide films by ion-beam irradiation", Thin Solid Films 235, 228-233
(1993)
4.356 N. Koshida, O. Tomita: "Ion-beam modification of amorphous W0 3 film and
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92-94 (1985)
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4.358 H. Morita, T. Miura, H. Washida: "Coloration of tungsten oxide films induced
by ion or electron irradiation", Jpn. J. Appl. Phys. 20, 323-325 (1981)
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4.360 C. Bechinger, S. Herminghaus, P. Leiderer: "Photoinduced doping of thin
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4.364 A. Vertes, R. Schiller: "Concentration-dependent diffusitivity: hydrogen percolation in W0 3", J. Appl. Phys. 54, 199-203 (1983)
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4. Properties and Characterization of Dielectric Thin Films
4.365 G. Bajars, Ya.A. Pitkevich, J. Straumens, A. Lusis: "Electrochemical impedance of a tungsten (VI) oxide injection electrode in protic electrolytes", SOy.
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4.367 R.S. Crandall, B.W. Faughnan: "Measurement of the diffusion coefficient of
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4.373 A. Temmink, O. Anderson, K. Bange, H. Hantsche, X. Yu: "4f level shifts of
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4.375 J.V. Gabrusenoks, P.D. Cikmach, A.R. Lusis, J.J. Kleperis, G.M. Ramans:
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5. Developments at Schott: Selected Topics
5.1 The Pioneering Contributions of W. Geffcken
to the Field of Optical Coatings from 1935 to 1945
Alfred Thelen
Dr. phil. habil. Walter Heinrich Geffcken was the first senior researcher to get
involved in full time optical coating research at the Jenaer Glaswerk Schott &
Gen. Due to circumstances beyond his control he was forced to work in secrecy
and without scientific peer exchange. Several of his results were rediscovered
many years later. When asked in 1992 to contribute to this book an article
on his early work in optical coatings, he gladly accepted. He wrote several
letters presenting his views and suggesting topics. He asked for copies of his
meticulous internal reports which had survived the dramatic transfer from
Jena to the new location in Mainz. But fate did not allow him to finish this
task. Walter H. Geffcken died, 91 years old, on April 4, 1995. Now it is up to
us to finish the task.
A part of Geffcken's work is known from his many patents. Most of these
patents had been declared secret at the time but were published later. Yet,
as Knittl writes in the introduction to his book Optics of Thin Films [5.1]:
"Important design work of deep foresight, unfortunately confined to patent
literature, is due to Walter Geffcken (who also pioneered wet and gaseousreaction deposition processes)." The word "unfortunately" obviously refers
to the fact that the information given in patents is not the same as in scientific publications. Published for a different purpose, it is less comprehensive,
and less rigorous. The full extent of Geffcken's design work, though well documented in internal laboratory reports, never reached the public.
For Geffcken, theoretical work was the link between experiment and invention. He wrote in one of his letters: "I contemplated how I could show
how experiment, theory, and process improvements always interact with each
other and inspire one another to ever higher levels almost as a natural consequence" [5.31].
In the following we will emphasize the theoretical aspects in order to give a
fuller account of his spectacular contributions to the field of optical coatings.
226
5. Developments at Schott: Selected Topics
5.1.1 How Thin Films Came to Schott
Before 1935, Berger [5.2J at Schott had conducted extensive investigations to
determine how the speed of stain formation in acid soluble glasses depends
on thermal treatments during manufacturing. He used the set-up shown in
Fig. 5.1 (from [5.2]).
The acid inside the rectangular tube attacks the test glass. After a while
one observes interference colours through the observation tube generated by
reflections off the acid/leached glass interface and the leached glass/unharmed
glass interface. A table was used to relate interference colours to the thickness
of the leached film.
When Geffcken was given the responsibility of carrying on this work he felt
that the evaluation by a "descriptive" table was unsatisfactory. There were
also some discrepancies: Brownish and blackish colours at the beginning of
the acid attack were not in the table.
Some people tried to explain the brownish and blackish colours by inhomogeneities in the leached film. This in particular bothered Geffcken because
there was no theory that could establish a relationship between brownish and
blackish colours and inhomogeneities in the leached film.
He decided to replace the qualitative method by a quantitative one. He
used monochromatic light to measure the difference in reflectance dR of
etched and freshly polished test glasses as a function of etching time.
He found that dR as a function of time had equal maxima and minima.
There were no anomalies. Geffcken was able to establish perfect agreement
between his measurements and the calculations based on a published formula
for the reflectance as a function of thickness of a homogeneous film with a
refractive index lower than the refractive index of glass.
Now the brownish and blackish colours became explainable: When the
refractive index of the film is lower than the refractive index of the glass
substrate, the reflectance of the growing film decreases (= brownish and
blackish colour) until it reaches a minimum as soon as the thickness is one
quarter of the measuring wavelength. The table used before was for the case
Cover plate
Rectangular
tube
Acid
Etched film
Test glass
Fig. 5.1. Set-up to measure the speed of stain formation in acid soluble glasses
5.1 The Pioneering Contributions of W. Geffcken
227
when the refractive index of the film is higher than the refractive index of the
glass substrate. Then the reflectance of the film increases (= bluish colour)
until it reaches a maximum as soon as the thickness is one quarter of the
measuring wavelength. The table that had been used was for the wrong case!
Having struggled so hard to find an explanation for the brownish and
blackish colours (Le., zones of low reflectance), Geffcken realized that he
might have discovered a thin film phenomena of great technical significance:
a method to reduce the surface reflection of glass surfaces!
It turned out, however, that the reduction of the reflection of glass through
an acid treatment had been known for a long time [5.3] and that the Zeiss
company had recently introduced a process to accomplish the same phenomena by depositing fluoride films through high vacuum evaporation [5.4]. The
disappointment of having been too late did not last long. The researchers at
Schott soon realized that there was plenty of room for improvements in the
antireflection treatments offered so far: The evaporated films were environmentally not very stable and for many applications the reflectance reduction
by a single film was not sufficient. Also, the tremendous potential of thin film
technology for other applications was recognized. The result was the start of
a continuous, still existing commitment to thin film technology.
5.1.2 Multilayer Antireflection Coatings
Continuing with chemical processes, Geffcken showed that treatments with
molten silver nitrate could generate a high-index layer on/in glass. Subsequent acid etching would produce a low-index layer. A two-layer antireflection coating with lower reflectance than a single layer [5.5] was the result.
Yet, secrecy imposed by the German military (coated optics meant an advantage in spotting the enemy at dawn and twilight!) allowed others to publish
with different configurations first: Blodgett [5.6] mentions two-layer films with
evaporated zinc sulphide and dipped skeleton cadmium arachidate films and
Cartwright and Turner [5.7] investigated films of sapphire and quartz.
Geffcken very soon realized that even a two-layer antireflection coating
cannot provide low reflectance for all visible wavelengths. A systematic theoretical study based on vector addition followed [5.8].
Assuming equal phase thickness of all the layers and negligible zigzag
reflections, he could show that the reflectance of a multilayer of non-absorbing
films can be expressed in the form of a Fourier series
La
m
R
=
v
cos 1I~
,
(5.1)
v=l
where R is the reflectance, m the number of interfaces, and ~ twice the (equal)
phase thickness of the layers (~ = 2¢ = (47f/>.)n vdv cos(3v, where nv is the
refractive index, dv the physical thickness and (3v the angle of the light within
the 11th layer). The a v are given by
228
5. Developments at Schott: Selected Topics
ao
al
a2
a3
= G5
+G~
+G~
= 2(GOGl +GlG2 +G2G3
= 2(GOG2 +GlG3 +G2 G4
= 2(GOG3 + GlG4 +G2 G5
+
+
+
+
............ + G;' ,
......... + Gm-lG m) ,
...... + Gm-2Gm) ,
... + Gm-3Gm) ,
(5.2)
and the G'S are related to the refractive indices by
1 nl - no
Go = ,
Cnl +no
C = const. < 1 .
... ,
Equations (5.2) are m + 1 quadratic equations for the m + 1 unknowns
Go, Gl, ... , Gm and solvable. But the solutions of (5.2) are in general complex and cannot be physically realized because refractive indices have to be
positive real numbers.
(The problem of finding positive real solutions to equations of this type is
not only a problem of thin film optics but also of microwave theory [5.9]. It
was solved in general many years later by Richards [5.10] and Riblet [5.11]).
When Geffcken realized that he could not solve (5.2) for the general case
he looked for ways to narrow the problem down. By setting R = 0 for E = 7f
he could establish two additional equations
(5.3a)
(5.3b)
and drop two of the most complex equations of (5.2), namely the first two.
His target Fourier series was the following approximation of a rectangular
curve
Y=
5
1
"6 cosE -"6 cos3E +
1
30 cos5E ,
(5.4)
or, after normalization
Rtarget =
0.5 + 0.595cosE - 0.119 cos 3E + 0.0238 cos 5E .
(5.5)
For four layers (m = 5) Geffcken was able to come up with equations which he
could solve graphically. His first solution was the very disappointing refractive
index sequence
1 11.100 11.365 11.595 11.475 11.506 .
5.1 The Pioneering Contributions of W. Geffcken
229
Other solutions followed. But they all had in common the extremely low
refractive index of around 1.1 for the layer next to air. Because Geffcken
knew that this low refractive index could never be produced he decided to
switch tactics. Instead of forcing a Fourier target on the system he conducted
an exhaustive search [5.12] for four (later also five and six) layer combinations
with refractive indices between 2.25 (Ti0 2 ) and 1.45 (Si0 2 ) and R = 0 at
~
= 7r.
He defined a quantity c by the following equation
(5.6)
and using this with PI
he calculated
P4
= ciao -
1 - P2
= aI/ao, P2 = a2/ao, P3 = a3/aO, and P4 = a4/a O,
and
P3
= ciao -
PI .
(5.7)
He now plotted the reflectance R as a function of P2 with ~, PI and c as
parameters and looked for designs with low R over as wide a ~-range as
possible (c turned out to be a function of the refractive index of the substrate
only).
The results of the search were collected in a table containing 32 designs
(Table 2b of [5.12]). We have selected the following three (Fig. 5.2):
1. 1 1 1.45 2.24 2.24 1. 776 11.499. The second and third layer have the same
refractive index and consequently can be combined to a single ),,/2 layer.
This is the "mother" of all wide band antireflection coatings, the famous
)..j4 )..j2 )..j4 coating [5.13].
2. 1 1 1.45 2.24 2.24 1.823 1.632 1 1.687. Geffcken extended his search also
to five-layer coatings and found that while they did not bring much improvement for low-index substrates they brought a 25% improvement for
high-index substrates [5.13]. These coatings were "rediscovered" over 30
years later [5.14,15]. In Fig. 5.2 we call this coating the ),,/4 )..j2 )..j2
coating because it is closely related to it [5.14].
3. 1 1 1.45 2.24 2.006 1.528 1.5 1.565 1 1.509. This six-layer coating has three
minima and caused Geffcken to speculate that it might be modified into
an antireflection with neutral reflection by relaxing the condition R = 0
for ~ = 7r. This a common design technique for antireflection coatings
today [5.16].
Let us add a fourth design, which was not part of the exhaustive study:
4. IlL 2.115H 0.43L 0.231H 1 1.515 with nL = 1.453 and nH = 2.472. It
was conceived later [5.17] and is discussed in Sect. 2.3.2 of this book.
Today this coating is the most widely used antireflection coating. It was
"reinvented" in 1970 [5.18].
For the implementation of the three-layer broad-band antireflection coating two approaches were chosen:
230
5. Developments at Schott: Selected Topics
4
4
a)
3
b)
3
on lowindex glass
)J4 )J2 )J4
on highindex glass
)J4 )J2 )J2
e::2
~2
e:::
e:::
/
0
400
500
600
Wavelength Inm
0
700
4
400
700
Four layers,
two materials
e::2
e:::
500
600
Wavelength Inm
d)
3
Three minima
e::2
//
--_/
4
c)
3
~"~
/~
e:::
oL---~~~~===---~
400
500
600
Wavelength Inm
700
oL---~~~~--~------~
400
500
600
Wavelength Inm
700
Fig. 5.2. Reflectance of the antireflection coatings: (a) 1 1 1.45 2.24 2.24 1.776 1
1.499; (b) 111.452.242.241.8231.63211.687; (c) 1 11.452.242.0061.5281.51.565
11.509; (d) IlL 2.115H 0.43L 0.231H 11.515 with nL = 1.453 and nH = 2.472. All
layers AO /4 thick at AO = 520 nm
1.
a wet chemical approach spinning on colloidal solutions [5.19] (also described later in detail in [5.20]) and
2. a spraying process depositing Ti0 2 , Si0 2 , and mixtures thereof, by reaction of volatile metal compounds with hot water vapour [5.19,20].
One might wonder why Schott did not follow the general trend of applying
thin films deposited by high vacuum evaporation in those days. The straightforward reason is this: For Schott, glass protection was just as important as
antireflection, if not even more so. There was a strong feeling that a deposition
method without a fusing or other bonding mechanism (like evaporation at
that time) would not guarantee the hardness and chemical stability required.
In fact, the preferred solution would have been a coating-thin-glass-sheet
composite to be fused on later! [5.5].
5.1.3 Theory of Periodic Multilayers
It was Geffcken's express wish to include in this paper his unpublished work
on the extension of the periodic multilayer theory by Rayleigh [5.21]. Knittl
writes about Rayleigh's paper [5.1]: "Not until 1917 can one discover a paper
5.1 The Pioneering Contributions of W. Geffcken
231
concerning a genuine stack of 'plates' of alternately low and high refractivity.
The object of the study was not a technical product, but a work of nature.
This paper, by no less a person than Lord Rayleigh, gave an explanation of
the spectral colours observed in the reflection of light on the covers of some
coleopterous beetles, known to have a laminar structure. We have essentially
here a stop band theory of dielectric multilayers, but, since no artificial stratification of this kind could be produced at that time, the paper fell into
oblivion."
Rayleigh's paper dealt with periodic multilayers consisting of layers with
equal optical thickness. Geffcken extended the theory to periodic multilayers
without the restriction to equal optical thickness. Also, more than two layers
were allowed in the period [5.22]. The only restriction was that all layers had
to be non-absorbing.
He showed that
Rm =
sin( m(3)
sin(a + m(3)
and
Tm
sin(a)
= ---:----'---'-----:sin(a + m(3) ,
(5.8)
where Rm is the complex reflectance and T m the complex transmittance of
the m-period structure, a and (3 are given by
cos a
=
1 + r2
-
t2
cos (3 = --2-t--
(5.9)
where r is the complex reflectance and t the complex transmittance of the
period. Equations (5.8) and (5.9) relate the performance of the periodic multilayer to the performance of the period.
Geffcken now studied periodic structures of the type
(5.10)
where A, B stand for layers with optical thickness (nd) one quarter wavelength thick at a common design wavelength Ao with refractive indices nA, nB
and optical thickness factors a, b. The refractive indices of the massive incident medium and the substrate are separated from the thin film sequence by
vertical lines.
The notation of (5.10) is of course not the equation Geffcken used. It is the
standard notation today [5.15] and most probably goes back to Turner [5.23].
Yet, because the incident medium, the substrate medium and the first
and last layer all have the same index nA, the sequence (5.10) is identical to
the following sequence
nA
I bBaAbB ... a AbB I nA,
(5.11)
So why the "eighth waves" that later [5.24] became such an important element in the introduction of the concept of equivalent layers? Geffcken writes:
232
5. Developments at Schott: Selected Topics
"By using the very practical reference points for the calculation of the reflectance and the transmittance which Rayleigh used, one avoids carrying
around bothersome phase factors!" [5.22].
Neglecting the zigzag reflections again, Geffcken derived for the complex
reflectance vector Rm of structures of the type (5.10) from (5.8) and (5.9)
the equation
Rm
· sinqx .
= - 21"1-.-Slnmx,
SIll X
(5.12)
with "1 = (nA-nB)/(nA+nB), q = nAdA/(nAdA+nBd B), x = (27r/.\)(nAdA+
nBd B), and m the number of periods (aA/2)bB(a A/2). For (5.12) the reference point is no longer the beginning of the thin film structure but the
central plane of symmetry.
(Equation (5.12) also holds if the zigzag reflections are not neglected, when
the quantities x are replaced by a p , mx by a, and 2"1 sin qx by sine [5.20]).
In the pass band, (5.12) describes a sine wave modulated by the "envelope"
2"1sinqx/sinx = /Y.
For nA = 1.45 (Si0 2 ), nB = 2.4 (Ti0 2 ), and nm = ns = 1.45, Fig. 5.3
gives the reflectance as a function of x for various optical thickness ratios q.
The curves of Fig. 5.3 were recomputed because the original pencilled
curves [5.22] were not suitable for reproduction. Figure 5.4 reproduces part
of the original curves.
Geffcken clearly recognized not only that a stop band generally occurs
whenever the total optical thickness of the period is x = 1800 (.\0/2), or
multiples thereof, but also that stop bands can be suppressed. He noticed
that the 2nd order band was suppressed when q = 0.5 (quarter-wave stack)
and that the 3rd order stop band was suppressed when q = 0.33 and q = 0.66
(2:1 stack). Also variations in the structure ofthe secondary reflectance peaks
in the pass band (ripple) were commented on.
Geffcken's theory would have allowed the prediction of the performance of
three-material periodic multilayers and in particular the suppression of two
adjacent stop bands (important for wide band heat mirrors). Yet, as Geffcken
wrote in one of his letters in preparation for this paper: "I want to show how
certain ideas emerge in different heads when their time has come" [5.31]. For
the design of wide band heat mirrors the time had not come yet.
Most of the periodic multilayers of Fig. 5.3 have many very undesirable
reflectance peaks in the pass band (ripple), especially the quarter-wave stack
(q = 0.5). For their elimination, Geffcken [5.25] studied a configuration where
a symmetric periodic multilayer is placed between two symmetrically arranged outer systems at a spacing 8 (Fig. 5.5).
For the configuration of Fig. 5.5 he derived, initially neglecting zigzag
reflections again, for the reflectance of the total system (reference plane is
the symmetry plane)
5.1 The Pioneering Contributions of W. Geffcken
233
e:: 501----+++----+-++---++-H
I-
q = 0.2
o~----~~--~~----~~
o
Period thickness
r
VVVVV
q
=0.80
\.
Period thickness r
100
q
o
Period thickness
r
100
VV WV
VVVVV
o
o
o~----~~--~~----~~
=0.75
360
Period thickness
VVVVv
IV
'V
r
~VV
q
Period thickness
VVVVV
~VVvv
r
540
\ IV
J
o
o
O~----~~--~~----~~
o
I~
\..J
\.
180
e::
501----+-+-+-----1-++---+_-;
I-
100
V
\.
\.
o~--~~--~~--~~
o
IV
vv
540
IVVVVV
=0.70 \.lJ
180
360
Period thickness r
540
e:: 50 1----+-++----+-+-+---+---1
I-
o
q
o
=0.50
180
360
Period thickness
r
.J
540
Period thickness
r
Fig. 5.3. Transmittance of two-material periodic multilayers with different thickness ratios q. nH = 2.4 and nL = 1.45. The total number of layers was 13
234
5. Developments at Schott: Selected Topics
... . . .
• () I~
.
.. .
1\- ....
.,
".0. 1'41
(
~Y\
\
\
-~.
,
~
I
I
,
1./
..
I
,
'
I
Fig. 5.4. Some of the curves of Fig. 5.3 taken from the original pencil drawings
5.1 The Pioneering Contributions of W. Geffcken
235
Inner symmetrical multilayer
Plane of symmetry for
inner and outer system
Outer
layer
Outer
layer
Fig. 5.5. Refractive index as a function of optical thickness of a symmetrical multilayer sandwiched between two outer layers. (j is the phase thickness between the
inner symmetrical multilayer and the outer layers
Tinner + Touter
.(
sin qinnerXinner .
sin qouterXouter .
)
= -1 27]inner.
sIn mx - 27]outer.
SIn 2Xouter
SIn Xinner
SIn Xouter
R
=
,
or, zigzag reflections not neglected,
R _
-
T
-
. 1 + 2 tan 2 G outer ( r;; .
2 tan Gouter
Vysm<Tinner 2
COS Ginner
COS G outer (1 + 2 tan Gouter)
-1
x sin( <Tinner
+ <Touter + 25 -
X
90°)) .
(5.13)
where Rand T are the reflectance and the transmittance vectors of the total
system, y is the envelope of the reflectance of the inner symmetrical multilayer, sin Gouter is the reflectance amplitude of the outer system, sin Ginner
is the reflectance amplitude of the inner system, <Tinner is the phase of transmittance of the inner system, <Touter the phase of transmittance of an outer
layer, and 5 the phase spacing between the outer layer and the inner system.
R = 0 for
vy sin <Tinner =
2 tan Gouter
. (
G
(
2G
) sm <T inner
cos - outer 1 + 2 tan - outer
+ <Touter + 25 -
90
0)
(5.14)
Geffcken proposed an experimental/graphical method for solving (5.14):
1. produce an inner system,
2. measure the reflectance (or calculate vY and sin <Tinner for the inner system),
3. plot tanGouter against 2 tan Gouter/[cos Gouter(1 + 2tan 2 Gouter)] = vY
and read tan Gouter off the graph,
236
5. Developments at Schott: Selected Topics
e outer> and
4. find an outer system with a reflectance amplitude = sin
5. match phases: sin (jinner = sin( (jinner + (jouter + 26 - 90°).
While the equations for ripple reduction already were derived before
1945 [5.25], the implementation in actual designs had to wait until 1948 [5.26].
From the examples given in [5.27] we selected the short wavelength pass
1.45 I 1.22H L (H L)4 H L 1.22H I 1.45, AO = 600 nm. For simplicity's sake
we used nH = 2.475, nL = 1.45. The designs would have shown a slightly
better performance if dispersion for the refractive indices had been included
(see Fig. 5.6).
5.1.4 Other Contributions
In Sects. 5.1.2 and 5.1.3 we have given an account of the major theoretical
contributions and new designs of Geffcken. There were of course other thin
film products that did not need the extensive theoretical preparation. Of
particular importance were the narrow band interference filters, both metaldielectric [5.28] and all-dielectric [5.29], the thin film polarizers [5.30], the
neutral filters, and the metal-dielectric high reflectors [5.20]. Geffcken also
made major contributions to Christiansen filters, optical measurements and
glass technology.
5.1.5 Conclusions
Optical interference coatings are complex structures with many parameters.
Today, elegant theories like equivalent layers, characteristic matrices, Chebyshev prototypes, etc., and readily available automatic design software for
100r-------------~~------------~~_,
80
60
:$?
o
Quarter-wave
Reduced ripple
stack
-.
stack
40
20
400
500
600
700
Wavelength Inm
Fig. 5.6. Reflectance of the short wavelength pass 1.45) 1.22H L (H L)4 H L 1.22H
I 1.45 compared to the quarter-wave stack 1.45 I (HL) H I 1.45, with nH = 2.475,
nL = 1.45, and '>"0 = 600 nm
5.2 Interference Filters
237
personal computers allow the design of impressive coatings. Fifty years ago,
Geffcken designed filters without the benefit of elegant theories, without the
benefit of computers, just relying on a hand calculator, a slide rule and mathematical tables. He left hundreds of pages covered with complex vector equations, long tables and curves. In the best tradition of German science he was
able to funnel this huge amount of basic information into recipes for revolutionary new products. He truly was an ingenious pioneer many years ahead
of his time.
It was very important to Geffcken that proper credit should be given to
his assistant, Mrs. Marga Faulstich. She performed most of the numerical
calculations and the graphical evaluation. She was, like Geffcken, one of the
41 "glass makers" who had the chance to build a new Schott company in
West Germany after World War II. She left the field of optical coatings and
later became an internationally recognized expert in glass making technology.
5.2 Interference Filters
Ulrich Jeschkowski
5.2.1 Coating Technology
Evaporation Technique: Coatings at Ambient Temperatures
An early Schott development is a bell jar coater with four separate compartments. The principle of construction dates back to the years between 1940
and 1950. Five coaters of the largest type with a 1.2 m diameter were built.
The last machine was taken into production around 1969/70.
The bell jar of this coater is divided into four chambers; one contains the
glow discharge arrangement and the remaining three hold the evaporation
sources. The four segments are shielded from each other. Originally only resistance heaters were used. At each place three evaporators may be moved
into evaporation position. In the original configuration three materials were
applied, one for each evaporation compartment (e.g., one metal and two different dielectrics).
The substrates are fixed in a planetary system that carries four plates of
400 mm in diameter. The thickness distribution is optimized by correction
shields for each material individually. Starting in 1970, the resistance heaters
were partly replaced by four-pocket-type electron beam guns with 30 cm3
hearth volume. Now the standard equipment consists of two electron guns
and one three-pocket resistance heater for each coater.
Rate Control and Coevaporation
Together with the installation of electron beam evaporators, the method of
rate control by quartz crystals was established for all sources. Thus Schott
238
5. Developments at Schott: Selected Topics
had the possibility of coevaporating several materials with different rates and
of mixing them in a stable and well-defined way.
This feature was extensively used after 1979, when the use of ThF 4 had
been abandoned for safety reasons. Until then Schott had an in-house fabrication of ThF 4 to ensure the supply and the quality of this important material.
By coevaporation of two different materials an adequate alternative to ThF 4
with respect to refractive index and long-term stability was obtained.
Optical Thickness Monitoring
Originally thickness monitoring was done on different test slides in the plane
of the substrates. The test slides moved in single rotation only, whereas the
substrates had (and have) planetary dual rotation. For metals the transmission was controlled photometrically after stopping the rotation and checking
the correctness of the value.
Dielectrics were monitored during evaporation by visual inspection of dark
lines in a spectroscope. The principle employed made use of frustrated total
reflection. The method was modified by Geffcken [5.32], who used special
prisms with double reflection and thus gained better contrasts and higher
precision. The arrangement had to be calibrated for each material by preparing the prisms with an adequate subcoating in a separate run. The method
was sensitive to the distance relation of source, substrate, and test slides and
suffered from different condensation conditions arising between test slides
and substrates.
Since 1975 optical thickness monitoring had been done directly on the
substrate by measuring the actual transmission of the filter under construction. In the case of dual rotation the signal was discontinuous and had to
be made quasi-continuous by an adequate sample and hold technique. The
signal was put on a strip recorder and the operator had to decide when to
terminate the layers. The higher the number of layers and the tighter the tolerances of the filters the more problems occurred for the operators in terms
of feasibility and reproducibility. Therefore in 1982 Schott started to do the
thickness control and the layer termination by computer.
Coatings at Elevated Temperatures: Reactive Evaporation
The above description refers to processes at ambient temperatures without
heating and without use of any reaction gases. In 1980 Schott started the
production of hard oxide coatings. This was done by the evaporation of oxide
material under controlled oxygen inlet onto heated substrates to achieve stoichiometric, absorption-free layers. These coatings are normally more or less
porous. The filter performance shifts to longer wavelengths during venting
because of humidity incorporation.
5.2 Interference Filters
239
Ion Plating
To get a more dense and bulk-like layer structure, the application of energyenhanced processes becomes necessary. One example of this method is the
ion plating process introduced by Balzers [5.33]' which Schott started to use
in 1988. With this process dense layers, which are free of pores and show
no shift on venting, are obtained. In addition to the conventional reactive
evaporation, the method uses a low-voltage high-current plasma source. By
the ionization of the coating material and the reactive gases a higher energy
input is gained during the layer formation.
Coating by Anodic Vacuum Arc
Another ion-assisted process is deposition by an anodic vacuum arc. Schott
has been employing this method under license [5.34] for the evaporation of
metals since 1988. Partial ionization and high rates effect good adhesion to
the substrate and dense layer structures [5.35].
5.2.2 Computer-Aided Design and Manufacturing
Design of Interference Filters
Since 1982 multilayer interference filters have been designed by an analysing
and refining program written in FORTRAN and running on a HP 1000/ A 700
computer. This in-house software replaced former versions running on IBM
systems in batch mode or on early desktop computers.
The so-called ASM program (analysis and synthesis of multilayers) has,
among others, the following special features:
• Several stacks of the same sequence are combined without editing each
system separately. Asymmetrical groups on both sides of a substrate are
mirrored.
• Different designs (e.g., metal blockers and dielectric bandpass filters) are
put together by binding the n + I-filter to the existing group of n-filters
automatically.
• The signal-to-noise ratio of bandpass filters can be computed using the
actual spectrum of the light source and the detector.
• Coating simulation is done at the monitor wavelength by exact calculation
or by error influence for each layer by the Monte Carlo method.
• The best termination condition for all layers of a filter (absolute or relative
extrema) is analysed.
• Refining is done by the least squares method. One can choose combined
groups that are modified by a factor or by a constant value.
240
5. Developments at Schott: Selected Topics
Manufacturing
Having established the design program, Schott started work on process control by using the HP1000 system. In preliminary tests with small BASIC
PCs we made sure that the optical transmission signal was sufficient for automatic thickness monitoring and layer termination of a filter in progress.
The reliability of PC systems in a noisy environment (electron guns with
10 k V high voltage) not being good enough at that time, we decided to use a
second HP1000jA700 machine for process control. This unit has been working since 1983. It controls four coaters in time sharing mode by interrupts in
real time mode.
The process computer is accessed via terminals at each coater. The production parameters for all filter designs are separately filed for each coater.
A certain design is activated by keying in the defined code name. The layer
system is then calculated at the monitor wavelength to find the theoretical
transmission values at the termination point of each layer. There are options
for different termination methods, for instance extremum, absolute transmission, thickness of quartz monitor, or time. Mostly, relative extrema are used
for the benefit of the self-correcting effect of this so-called "turning value"
method [5.36].
The incoming transmission signal is analysed, using appropriate fitting
algorithms to find the exact time for layer termination. Thus there is no
search for maximum or minimum values but a calculation of the actual time
at which the extreme values will be reached. This method has proved to be
effective and precise.
5.2.3 Products
Metal-Dielectric Filters
Metal-dielectric interference filters (MDI) were invented by GeJJcken [5.37].
In 1952 they went into large-scale production in Mainz. The most common
type in the VIS and the near-IR is the 10 nm bandpass filter. The system has
two cavities of second order. Due to the absorption of the metal layers, the
maximum transmission is limited to around 50%. There are other types with
larger bandwidths of 20, 25, and 50 nm in production. The 50-nm-bandwidth
system is of first order, whereas the 25-nm type is a second-order three-cavity
design.
A special version of the MDI layer system is the linear variable filter that
works in the range 400-700 nm. In this type all layers are deposited in wedge
form.
MDI blocking filters are systems with a relatively large bandwidth and
a good sideband suppression. They have a three-cavity design with a different order of spacers. Normally the four mirror layers are made of metal. A
special solution of this type is the induced transmission filter mentioned by
5.2 Interference Filters
241
Berning [5.38] and Baumeister [5.39]. Here some of the metal mirrors are
replaced by absorption-free quarter-wave stacks. Thus the peak transmission
becomes higher.
MDI System for the UV
In the range 200-300 nm the mirror layers of the MDI filters are made of
aluminium. In order to gain a good optical quality, the evaporation rate of
the aluminium must be very high. The coater is equipped with a cryopump to
avoid any oil contamination and to maintain a sufficiently low water content.
All layers are deposited by electron beam evaporation.
The filters are made up of two-cavity or three-cavity designs, the achievable bandwidths being 20 nm or 12 nm. For smaller bandwidths down to 6 nm
an ADI system is combined with a MDI three-cavity or four-cavity filter,
which provides the sideband blocking.
All-Dielectric Interference Filters
Layer stacks without metal layers are called all-dielectric interference filters
(ADI). The main types are bandpass and edge filters. In the VIS and near-IR
the standard bandpass with 10 nm bandwidth has a three-cavity design. In
the UV from 200 to 300 nm two-cavity designs are normally used. Both types
need blocking filters for sideband suppression. In the UV this can only be done
by using the above-mentioned three-cavity MDI systems. In the wavelength
range above 300 nm absorbing colour glass is available for blocking purposes.
In general the blocking is done by a suitable combination of colour glass,
MDI, and edge filters.
Special classes of ADI systems are multi-cavity bandpass filters with a
relatively large bandwidth for fluorescence applications. The excitation filter
for instance has to shape the illumination radiation in such a way that no
unwanted light disturbs the fluorescence emission. The emission filter must
cut out the wanted fluorescence radiation and suppress the excitation light.
Both cases require filters with bandwidths around 50 nm that have very steep
slopes and a good blocking power. A typical solution for a FITC (fluoresceinisothiocyanate) excitation filter is an eight-cavity design with spacers of high
refractive index.
Filters of this type often suffer from ripples in the pass band. A good
means of smoothing out this interference effect is admittance matching using
the Herpin index of symmetrical quarter-wave stacks [5.40]. The effective
index of the mirror group LHLHL for example is 0.40 for ZnS/MgF 2 at the
centre wavelength of 465 nm. This value can be matched to the surrounding
medium of glass (n = 1.52) by a three-layer antireflection system LHL that
has an admittance of 0.41.
242
5. Developments at Schott: Selected Topics
Edge Filters
Edge filters mainly consist of quarter-wave stacks and are used for broad band
separation of radiation. A typical application is a short pass edge filter for
polymer curing in dental medicine. The filter has to give high transmission of
the excitation radiation around 450 nm and must cut out the remaining longer
wavelengths of the illumination. The high amount of radiation in the VIS
troubles the patient and the dentist and the IR portion can cause a harmful
temperature rise in the teeth. The filters also have to resist temperature
shocks given by the focused energy delivery when the lamp is powered up.
A different type of edge filter is used for IR radiation blocking. It has shortpass characteristics and serves for high throughput of light between 400 and
675 nm. This application often demands a stable filter performance in relation
to high temperature loads. Filters made of reactively deposited Ti0 2/Si0 2
multilayers are normally unstable under temperature cycling. They have a
temperature coefficient of -0.15 to -0.20nm;oC. The changing moisture
content in the porous layers alters the layer index, thus causing the filter
characteristics to drift with temperature. To overcome this problem, Schott
uses the above-mentioned ion plating process [5.41]. This plasma-enhanced
deposition method forms dense, non-porous layers. There is no reaction with
moisture from the outside and therefore no shift to a shorter wavelength with
rising temperature. Ion plating filters made of Ta20s/Si02 multilayers have
a temperature coefficient of +0.001 nm;oC due to the linear expansion of the
material.
Ion plating layers display high compressive stress. They form strong adhesion to the substrate and to each other and therefore are a good choice
if protection of weak surfaces is necessary. In the case of the Schott colour
glass UG 11 the protection effect is combined with a better blocking in the
IR. An ion plating edge filter is applied, which cuts down the leakage around
740 nm, leaves the high transmission in the UV unchanged, and makes the
relatively unstable glass surface more durable.
Temperature stability is also important for some laser applications. There
are special AR and HR systems that must be absolutely stable over time, even
under laser irradiation. Ion plating coatings can fulfil this demand. On the one
hand, the currently used plating process seems to be somewhat unfavourable
for high-power short-pulse laser components because of its inherent tendency
to form small absorption in the layers. On the other hand, the high compressive stress is advantageous if temperature shocks are applied.
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
243
5.3 Plasma Impulse Chemical Vapour Deposition
(PICVD)
Dieter Krause
Chemical vapour deposition is a suitable technique for superior-quality highrate coating with oxidic materials (see Sect. 3.2). Films produced by traditional deposition techniques are usually highly sensitive to environmental
influences such as temperature and humidity of the air and consequently have
unstable physical properties. The reason is found in the crystalline (columnar) structure accompanied by high diffusivity along the grain boundaries or
by porosity in sol-gel-made films.
Advanced coating techniques can improve some of these restrictions by
applying energetic particle bombardment to the growing film [5.42-44]. One
type of ion source is a plasma which can be operated in a continuous or pulsed
mode. A technique using a pulsed microwave plasma has been developed
at Schott for the deposition of the core material in silica tubes to produce
optical fibre waveguides. Because this process will be described in detail in
the volume Fibre Optics and Glass Integrated Optics which is to be published
in this series (see list at front of this book), only a brief overview is given
here. We then concentrate on those modifications of the process which were
used for specific products.
5.3.1 Fundamentals of the PICVD Process
The principle of the PICVD process is most easily explained for a linear, one
dimensional arrangement, as was originally developed for the deposition on
the inner surface of tubes for fibre optic preforms [5.45,46]. The equipment
is shown schematically in Fig. 5.7. Liquid precursors are evaporated in the
gas cabinet at room temperature or elevated temperature. A carrier gas composed of oxygen, nitrogen and, for example, helium transports the reactive
chemicals into the deposition unit. Mass-flow controllers in the control unit
are used to feed the deposition unit with the appropriate gas mixture and
concentration of reaction partners. With the vacuum pump a low pressure
of several millibars is maintained in the receptacle, and the exhaust gas is
pumped into a scrubber. A computer program controls the pulsed microwave
power (2.45 GHz magnetron, peak power in the kW range) and the gas flow.
In Fig. 5.8 we see the principle of the linear tube reactor whose surface is to
be deposited. The gas flows from the left end into the tube. After complete
filling, the microwave pulse at the right end of the tube ignites a plasma
discharge which propagates along the tube. The outer metallic conductor
and the conducting plasma column guide the microwave to the left end like a
coaxial cable at a speed of about 1000 m/s. Within about 2 ms the chemicals
in a length element of the tube react completely and are deposited on the wall
244
5. Developments at Schott: Selected Topics
Exhaust
Control unit
unit
Gas cabinet
Vacuum pump
••gir--l
~
MIcrowave
generator
Fig. 5.7. Scheme of the set-up for microwave PICVD depositions
of that element with almost 100% efficiency. After the microwave is switched
off, the tube is refilled with fresh gas and the exhaust gas is replaced within
about 12 ms in our arrangement. Then the next pulse starts the next period
of the deposition process, which in every period deposits a layer thickness of
one or several molecular monolayers. Thus the deposition rate is controlled
by the mass flow alone if the microwave power is above a certain threshold
and minimum duration.
With the above-mentioned times we get a repetition rate of about 100 Hz
and a deposition rate of up to 5 Jl.m/min, depending on the film material.
We have investigated the kinetics of the reactions with time-resolved optical emission spectroscopy [5.47,48] along the tube. Figure 5.9a shows the
spectrum detected at the beginning of the plasma pulse, Fig. 5.9b shows the
:II
r
1
n
PI.,m. rol"m"
Silica tube
Outer conductor
Furnace (1 000 · C) -
I Gas supply I
=
J
D
Inner conductor -
I
Microwave
generator
II
j
Pump
I
Fig. 5.8. Scheme of the coaxial PICVD reactor as used for depositing material on
the inner surface of a tube
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
245
a)
SiC
425
2000
.l!l
0::::
CI
: SiC
::J
0
.!2
Z.
'iii
.s
..!:
0::::
1000
pI
2~1.6
Si
.Jr\J ~ ~ ~~
1
:'
Si ~I
250
300
350
400
~,
450
500
550
Wavelength Inm
b)
750
, CI
.l!l
0::::
::J
0
.!2
500
+
CI, CI 2
~
CJl
0::::
.s
..!:
'~
250
J
,
CI+
11l
~
250
300
~~ ~
350
400 450
Wavelength Inm
M
500
550
Fig. 5.9. Emission spectra from the plasma reaction SiCl4 + 1/20 2 --+ SiO + 2Cb
at a location close to the plasma ignition point. (a) Spectrum at the very beginning
of the reaction: The SiO line dominates over the minor Si lines and CI, Cb lines.
(b) Spectrum at the end of the plasma discharge: Only CI and Cb lines remain.
Notice the different intensity scale which indicates unchanged CI emission compared
with (a)
spectrum detected after complete reaction at about 2 ms. At the beginning
one observes the dominant SiO line at 425 nm wavelength and several Si and
Cl emissions. No indication of Si0 2 can be found. At the end of the plasma
pulse only the Cl lines remain, which indicates the complete removal of Si
compounds from the plasma volume, that is, complete deposition on the wall
of the reactor.
Figure 5.10 shows time-resolved spectra over the plasma pulse duration at
two different locations: (a) at z = 0 cm of the plasma ignition, including the
spectra of Fig. 5.9, and (b) at z = 24cm upstream of the gas flow and down
246
5. Developments at Schott: Selected Topics
a)
200
300
400
500
Wavelength Inm
b)
1.9
::l
~
Z.
'00
c
2c
z
= 24 em
1fj,1=0.05
200
300
400
500
Wavelenglh Inm
Fig. 5.10. Time-resolved emission spectra from the plasma reaction of Fig. 5.9
at different locations. (a) Close to the plasma ignition point: z = 0 cm. Time 0
corresponds to Fig. 5.9a, time 1.9 ms corresponds to Fig. 5.9b. The deposition is
complete well before the end of the pulse. (b) At distance z = 24 cm from the
ignition point. The start of the reaction is delayed; the deposition is completed
only at the end of the pulse
the plasma propagation. The delay of the reaction onset gives the plasma
propagation velocity in the coaxial guide. In both cases the pulse duration
allows a complete deposition of all Si-containing reaction products. A clear
dependence of the reaction kinetics on time, location, and corresponding microwave amplitude can be derived.
For the Si0 2 deposition from a SiC14 precursor the reaction with O 2
can be described as follows: In a first phase the oxygen is dissociated by the
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
247
microwave power and forms SiO and 2Ch in a homogeneous volume reaction.
In a second phase the SiO molecules diffuse to the wall, and in a third phase
a heterogeneous surface oxidation reaction SiO + 0 forms a solid amorphous
Si0 2 deposit, thus completing the overall reaction SiCl4 + O 2 ---+ Si0 2 + 2C1 2 .
Modelling these kinetic steps yields equations that quantitatively describe the
observed time-dependent emission and deposition.
In a similar investigation we have also modelled the deposition of Ti0 2 .
Different precursors are used for different applications, depending on the allowed substrate temperature, the allowed impurity concentration, the thickness of the thin film stack, and other parameters.
The PICVD process proved to be very powerful not only in the tubular substrate geometry but also for the deposition on planar or threedimensionally formed substrates. In these cases the reaction chamber has
a different arrangement of substrate, gas flow, and coupling elements to the
microwave. Some examples are given in Sect. 3.2 and in Fig. 3.8.
The stability of the process was determined in a set-up for planar deposition. Figure 5.11 shows that in no case does the deposition rate have a
stronger than linear dependence on process parameters such as gas pressure,
precursor concentration in O 2 , microwave power, and pulse repetition frequency. Thus stable working points exist when the microwave power is above
a certain threshold value (about 10 to several 100 W fcm 3 ) and of sufficient
duration to allow complete reaction, diffusion to the wall, and deposition.
Moreover, the pulse repetition frequency must be below the inverse time of
the gas exchange, which depends on the dimensions of the reaction volume.
With this know-how about Si0 2 and Ti0 2 deposition a stable and optimized process can be realized. Some manufacturing processes for existing
products are described in Chap. 6, and various more general aspects are dealt
with in the following sections. A review is given by Segner [5.49].
5.3.2 Impact of the Environment
on the Optical Performance of Thin Films
The extent to which the performance of a product is influenced by the various
possible environmental factors depends on the special application. There are
physical effects such as mechanical scratching and bending or thermal and
optical loads (e.g., high-power radiation), and there are chemical influences
such as humidity, reactive atmosphere (e.g., oxidation), etc., and combinations thereof. This list does not claim to be exhaustive.
One of the most important effects is the pick-up of moisture by films after
physical evaporation or annealing of sol-gel-coated substrates. In edge filters
the spectral position of a steep transmission edge with wavelength gives a
sensitive indicator. Figure 5.12 shows the instability and stability for a reactively evaporated and for a PICVD multilayer interference filter, respectively.
Here reversible temperature cycling between 25°C and 200°C gives an edge
248
5. Developments at Schott: Selected Topics
1.0
1.0
(])
:0
.sa
·in
0
a.
E
(/)
til
(])
(])
0
:;:::
til
·in
0
a.
.~
~
c
::J
0
:;:::
"0
"0
.sa
~c
~
c
(])
0::
"0
"0
0.5
(])
.~
(ij
(ij
z
z
0.5
E
0
E
(;
o
o
0.5
Normalized pressure
~c
o
:;:::
·in
-
:0
:;:::
C
::J
~
19
(/)
o
a.
(])
~ i
.~ 0.5
0.5
(ij
0::
E
(;
1.0
~
~!
(ij
0.5
~c
o
·in
o
a.
(])
"0
"0
o
Normalized precursor concentration in 02
1.0
I
I
I
I
I
I
1.0
0
1.0
E
o
z
z
o
o
0.5
1.0
Normalized microwave power
o ~----------~--------~
o
50
100
Pulse repetition frequency /s-1
Fig. 5.11. The stable deposition rate as a function of several major process parameters
shift for reactively evaporated films of several nm in wavelength with maximum changes near 100°C, indicating water evaporation or incorporation
as the main effect, which may be interpreted as a consequence of porosity.
For the dense and compact PICVD films the edge shift is in the range of
0.004-0.009 nm/K and is thus comparable with that of the best ion plated
films.
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
1.0
I
f\
0.8
~/~.
c:
0
'iii
C/)
'E
'.'
·tv' /\
.\
~~\
0.6
~
0.4
r-
-
c:
0
'iii
C/)
'E
400
\
420
0.6
iplcvJ
'.
b)
\ 1\
~
C/)
c:
~
440
460
0.4
25°C
0.2 - ---100°C
····200°C
II
0.0
500 520 540
25°C \\
---100°C '., . \
.... fOO oCI .\....
0.0
380
\
0.8
...
:\"
f-
0.2
Reactive
evaporation -
i'
.':'
1.0
a)
~)'v ~
C/)
c:
I
249
480
\
\
560
580
600
Wavelength Inm
Wavelength Inm
Fig. 5.12. Temperature cycling of multilayer interference stacks as-deposited at
room temperature 25°C (solid line), at 100°C (dashed line), and at 200°C (dotted
line). (a) Edge shifts of several nm for films from reactive evaporation, (b) stable
edge position for PICVD films
In the lighting industry, today's cold light mirrors for halogen lamps are
exposed to temperatures of up to 350 0 C and oxidizing atmosphere. In the
classical Si0 2 /ZnS coatings the ZnS oxidizes into ZnS04, which reduces the
reflectivity and brilliance of the mirror. The so-called "hard" coating made
out of the Si0 2 /Ti0 2 combination is stable under these operating conditions
and best suited for being produced by PICVD technique. Figure 5.13 shows
the reflectivity of a 23-layer hard-coated cold light mirror in comparison with
aluminium, as-deposited and after an extended heat treatment of 78 h at
E
::J
'c 10
'E .
::J
co
.8
Q)
>
~
2!
c:
o
U
Q)
0::
f!i
0.5
I,,',
I,
,
,
,
I
I
I
O~-r---.----.---'----'-~
400
500
600
Wavelength Inm
700
800
Fig. 5.13. Temperature stability of a cold light mirror consisting of 23 layers (Si0 2
and Ti0 2 ) before heat treatment (solid line) and after heat treatment (dashed line)
at 517°C for 78 h. The reflectivity is unchanged; a small shift of the edge at long
wavelength of +7nm (i.e., 0.9%) is observed
250
5. Developments at Schott: Selected Topics
517°C. The reflectivity remains unchanged in amplitude and edge position.
This type of cold light reflector is mass-fabricated in a PICVD production
line; see Sect. 6.4.
The high purity of the deposit as demonstrated by the extremely low
losses of the core material of optical fibres made by the deposition on the
inner surface of a tube was the motivation to determine the damage threshold
under high-power laser radiation. In cooperation with the laser laboratory of
Lawrence Livermore National Laboratory, laser mirrors for>. = 1.0641l-m
have been developed and tested [5.50,51].
The results for commercial films prepared by electron beam evaporation or
ion plating as well as for doped PICVD films and for the surface of superpolished CVD Si0 2 glass are shown in Fig. 5.14. The damage threshold is in the
range 3-100 GW cm -2, depending on the pulse duration. This corresponds to
a threshold of > 60 J cm- 2 for a 16-ns pulse duration and a system made of
more than 1000 periods of undoped and fluorine-doped Si0 2. For the same
pulse length the thresholds are about 35 J cm- 2 for an Ab03/Si02 sol-gel-
I A. = 1.064 jJm I
Bulk CVD Si02 glass
100
Bulk BK 7
"t
10
E
o
cw .---Bulk
CVD Si02
glass
0.1+--.--T""'T'''T''T'T''TT.,....-~~~~"'T'TT-~~''T'''T''T''T'"...,...j
0.1
10
100
Pulse duration Ins
Fig. 5.14. Laser damage thresholds at wavelength'>' = 1.0641l-m. Multilayer
PICVD Si0 2 films alternately doped with fluorine or germanium compared with
the surface of bulk CVD Si0 2 and with laser mirrors produced by other preparation techniques. (a) Synthetic Si02: superpolished surface; (b) PICVD: Si02: F
and Si02: Ge film stacks; (c) e-beam evaporation: commercial laser mirror (LLNL"Nova"-fusion experiment); (d) ion plating: best experimental value
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
251
made stack and about 18 J cm- 2 for a Hf0 2 /Si0 2 electron-beam-evaporated
commercial mirror. The thresholds of PICVD Si0 2 films are superior to those
of films produced by any other preparation technique; they asymptotically
approach the bulk values of the superpolished silica substrate material for
long and very short pulse durations. This clearly demonstrates the purity of
the deposits and their suitability for all kinds of laser mirrors.
5.3.3 Flip-Flop Layers and the "Design-to-Go" Concept
The excellent stability of the PICVD process and the reproducibility of refractive index and film thickness are due to the complete chemical reaction
and deposition and by the simple control through mass flow of the gas mixture. This, on the one hand, allows one to eliminate all in situ control measurements once the equipment has been calibrated for the applied materials,
which in Schott products very often are only Si0 2 and Ti0 2 .
On the other hand, fast gas switching between these components allows
one to mix the materials to generate refractive indices within the two limits
"low" for Si0 2 with nd = 1.46 and "high" for Ti0 2 with nd = 2.44 (see
Fig. 4.16) on a continuous scale. Unlike all other techniques, PICVD realizes
complicated optical film designs without needing many different materials
with strongly differing properties which, as a consequence, often make the
materials incompatible.
We have made twofold use of this advantage. In the first case we divided
a quarter-wave film (of about 100nm in thickness) into many ultrathin sublayers of either Si0 2 or Ti0 2 , i.e., n· d « A for each sublayer. We obtained
a stack that behaves like a quarter-wave film with arbitrary effective refractivity. The effective index is quasi-digitally controlled by pulse counting only.
Each sublayer is the deposit of 10-1000 plasma pulses and is realized by fast
gas switching between the two components, resulting in the so-called flip-flop
index behaviour. The second use will be described in Sect. 5.3.4.
Figure 5.15 shows a simple three-layer antireflection coating produced on
a float glass by depositing a flip-flop medium-index (M layer), a high-index
(H layer), and a low-index (L layer) film. Figure 5.15a gives the spectral
reflectance and Fig. 5.15b shows a TEM picture of the three films which was
prepared by microtomic cutting. The Ti0 2 layers appear dark compared to
the Si0 2 layers. It is clearly seen that the M layer is built up of nine periods,
each containing a H layer and a L layer, which again are the deposits of
several plasma pulses. The comparison of measured and calculated spectra
proves the excellent index and thickness control of PICVD technology.
The flip-flop concept allows the design of arbitrary index profiles within
the above-mentioned limits by simple pulse counting. We therefore linked
the design computer directly to the control computer of the deposition setup. The filter design defining the index and thickness of individual films
is automatically translated into a sequence of microwave pulses and trigger
points for the gas switching. Shortly after the design work is finished the
252
5. Developments at Schott: Selected Topics
0.16
b)
a)
0.14
0.12
Q)
<::
0.10
u
0.08
u
.!!l
Q)
c;::
Q)
c::: 0.06
0.04
0.02
0.00
/
/
\
/
V i--- V
400
500
700
600
TiOZ
Wavelength Inm
Fig. 5.15. Three-layer antireflection coating on float glass. (a) Spectral reflectance
of the MHL design; (b) TEM picture of a microtomic cross-section of the individual
layers. Ti02 is dark grey, Si0 2 is light grey, showing that MHL = (HL/9)9 HL. The
corrugation of the layers is a preparation artefact
deposition can be started and after a few minutes a sample is ready for
testing. The specification is usually hit already in the first run. This "designto-go" method allows fast improvement cycles in complex optical systems
and an excellent customer service for new applications.
Figure 5.16 gives proof of the stability in the production of a more complex
25-layer bandpass filter made of alternating Si0 2 and Ti0 2 films. The position
of the IR edge is detected and found to be in a 3a range of 6 nm for 17
subsequent production runs without any correction in the settings of the
process. Thus a stability of 3a well below 1% is demonstrated.
1.0
0.8
l,r- 1'1
E
~
U>
Ol
"0
Q)
0.6
'E
U>
<::
~
I-
700 ~-------+--------+-~
Q)
-~
<::
0
'(i;
I
I
10 = 2.0 nm ~ 0.3%
,-~
,
0.4
,,
\
'0
~
,,
~
~
400
500
600
700
Wave length In m
....
800
.
- 30
" 3
,
0.0
+30
760 ~·
~------~--~·~--~~
• .a..
•
•...... -- ••- ItMean
____
___
~
H
0.2
2
I
'oJ
o
4
8
Time Ih
Fig. 5.16. Stability test of the PICVD production of a 25-layer bandpass filter in
17 subsequent runs without any process correction; the stability is 3a < 1%
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
253
The flip-flop concept provides the opportunity to realize any arbitrarily
complex design of advanced optical filters, which often make use of gradient
index structures. An example of practical usefulness is the bandpass filter
with reduced ripples on both sides of the ideally steep edges. One theoretical solution to this problem is a series of Chebyshev functions [5.52]. These
polynomials are often approximated by a stack of films with varying index
and thickness, a made-to-measure problem for the PICVD technique.
In a multilayer design for a bandpass filter the Chebyshev polynomial solution was approximated with 22 layers; see Fig. 5.17. Indices between 1.45
and 2.35 were realized by using only Si0 2 and Ti0 2 as coating materials and
by changing the number of sublayers (each approximately lOnm in thickness) of both materials. The about 4-llm-thick interference stack is built
up of about 400 individual sublayers in total. As shown in Fig. 5.17b, the
agreement between calculated and measured spectral reflection is excellent,
proving again the extraordinary process control of PICVD technology. The
maximum reflectance of > 96% indicates that absorption effects are negligible in the deposited material and at the interfaces generated by the switching
of the gas flow; thus stoichiometry of the films is easily attained.
Many other, and even more complicated, index profiles are possible, including apodization of any kind to suppress side lobes.
5.3.4 Multilayer Stacks and Rugate Filter
The preparation of multiple film stacks without internal stresses, which could
destroy the films by cracking, is easily attained with the PICVD technique
if only small compositional variations are used. Then stacks of thousands of
2.40
1.00
a)
2.20
.S 2.00
Q)
>
-
1.80
n
~
oc
1.60
c:
-
.2 0.60
'0
-
-
c;::
I
-,
Q)
I-
Q)
oc 0.40
0.20
'-
1.40
0
Experiment
-- _. Theory
0.80
x
Q)
'0
"
800 1600 2400 3200 4000
Layer thickness Inm
0.00
b)
,,
,,
,
,
,,
N
~~
400
iAA-
I
P<...
500
~l.\
600
700
Wavelength I nm
Fig. 5.17. Design and realization of a bandpass (in transmission = stop band) filter.
(a) Approximation of the Chebyshev polynomials with 22 layers of varying index
and thickness; (b) calculated (dashed line) and measured (solid line) reflection of
the PICVD-made filter
254
5. Developments at Schott: Selected Topics
quarter-wave layers can be realized, resulting in a total thickness of several
100 Il-m or even reaching the mm range. This opens new design opportunities
for narrow band reflectors or filters with very steep edges.
Negligible absorption is another necessary condition for optical components which are realized with thick coatings. This was demonstrated with
the core material deposition process for optical communication fibres. Analogous doping procedures to change the index in the percent range can be
applied for multilayer coatings. The films are made out of silica with ForGe
doping of 1-10% to reduce or to increase the index, respectively.
In thin film literature the spectral transmittance ranges are usually defined as those limits where the absorption constant is below a defined limit
0: < 10 3 cm-I, i.e., 200 nm :s; A :s; 91l-m for Si0 2 and 400 nm :s; A :s; 31l-m for
Ti0 2 [5.59]. But for high-power applications only significantly lower values
can be tolerated. In fibre optic CDV processes, absorption constants many
orders of magnitude smaller with values of 10- 3 cm- 1 2: 0: 2: 10- 6 cm- 1 are
reached; they are strongly dependent on wavelength and limited by the scattering losses. The definition of a transmittance range thus depends on the
application.
By switching the dopant on and off or by continuously varying the concentration, step index or graded index films of any profile can be prepared in
just the same way as in the manufacturing of fibre optic core profiles.
Periodic multilayer stacks usually have narrow reflection bands, depending
on the index difference 5n, and wide transmission windows. The reflection
spectrum, being determined by the Fourier transform of the index profile,
shows higher order bands and side lobes in addition to the main peak at the
design wavelength. These effects can be avoided by choosing a sinusoidalshaped profile. Mirrors with this ideal profile, so-called "rugate filters" , have
been extensively studied [5.53-59] using a variety of calculation methods.
The preparation technique must be capable of depositing the material
in an absorption-free way at a very high rate and nevertheless with precise
index and thickness control on a length scale of fractions of a wavelength.
The PICVD technique satisfies all these demands.
Figure 5.18 shows some design results, following the calculations of Southwell [5.55]. This coupled-wave theory has the advantage of giving closed-form
expressions for the case of a sinusoidal profile. Reflectivity and bandwidth
for R > 99.9% are given as a function of the modulation index difference
5n and the number of periods N. The bandwidth 5A/ AO (Ao denoting the
design wavelength) is directly proportional to the index difference 5n. The
reflectance can exceed any given value if the number of periods exceeds a
5n-dependent threshold. In Fig. 5.18c we find the necessary values of 5n and
N for a given bandwidth within which the reflectivity is above 99.9%.
By choosing 5n = 0.02, N = 500, and Ao = 1000 nm, one obtains a
reflection spectrum as shown in Fig. 5.19.
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
255
a)
0.03
"0
~
~0.02
.s::
-0
.~
"0
C
0.01
~
O.OO¥-- -- , - - - - - - . - - -----r---.-------j
0.00
0.02
0.04
0.06
0.08
0.10
Refractive index modulation
1 . O'--------::::=-::::~-____::==-___, b)
0.2
O . O!-==-----.-----~--------l
o
100
200
40
~
.s::
300
Number of periods
l1n=0.10
c)
30
-0
.~
-g
ro
!Xl
20
0.05
~
0,
cri 10
a>
---
0.01
200 400 600 800 1000 1200 1400 1600 18002000
Number of periods
Fig. 5.18. Design results of sinusoidal rugate filters showing properties as a function
of index difference 6n and number of periods N. (a) Bandwidth as defined in [5.55]
versus the peak-to-peak index difference 6n; (b) reflectivity at the design wavelength
as dependent on 6n and the number of index periods N (following Eq. (67) in [5.55]) ;
(c) bandwidth of reflectivity exceeding 99.9% as a function of 6n and N (following
Eq. (66) in [5.55])
256
5. Developments at Schott: Selected Topics
100r---------------------------~----~
80
~
!e...
c:
o
60
N =500x
~li,n =0.02
~
~ 40
20
.... l
O~==~==~==~==~~~~
200
400
600
800
Wavelength Inm
1000
1200
Fig. 5.19. Reflection spectrum of a rugate filter with N
of 8n = 0.02; design wavelength AO = 1000 nm
= 500 sinusoidal periods
Real preparations of rugate filters have been designed for .Ao = 1.064 nm,
i.e., Nd:YAG laser radiation. The deposition was made close to fibre optic
process parameters, i.e., planar or tubular silica substrate, deposition temperature between 850°C and 900 °C for F doping and about 1000°C for Ge
doping. F doping was preferred because the thermal expansion mismatch between undoped and doped material is smaller than with Ge doping. Some
results are presented in Figs. 5.20 and 5.21. Figure 5.20 shows SEM pictures
of a broken rugate filter with 500 periods of F -doped silica. Remember that
a period designed for .Ao = 1.064 nm has a thickness of 365 nm, resulting in a
total stack thickness of 182 i-Lm which was deposited in about 2 h (demanding the above-mentioned high stability of the process over long times). The
spectral reflectivity of a 500-period Ge-doped filter shown in Fig. 5.21 has
a maximum value of R = 99.31%, which means that the design reflectivity
of 99.9% and the bandwidth have not been attained. The asymmetrical deformation of the reflection peak also indicates some faults in the ideal index
structure.
Because the rugate filters have only narrow stop bands, most of the transmission range of the materials is transparent [5.59]. By depositing several
rugate filters upon each other with different design wavelengths, a series of
narrow and well-separated reflection peaks can be realized. Figure 5.22 gives
an example where five stacks of 500 periods each have been superimposed to
produce a filter made from F-doped Si0 2 with a total deposition thickness
of about 300 i-Llli and an index difference 8n of 0.01. Another design concept
for the same optical spectrum is a superposition of the five index periods and
amplitudes in the index profile by which the total thickness would be reduced
but the index structure would become much more complicated and process
control more difficult.
5.3 Plasma Impulse Chemical Vapour Deposition (PICVD)
257
a)
Fig. 5.20. SEM pictures of a cross-section of a 500-period F-doped rugate filter.
(a) The overall thickness is about 180 J.l.m. No cracks can be detected. (b) The
periodic structure of the refractive index has been made visible by HF etching
o
100 .-----------------~
80
__------------_.
~
!!..
c 60
o
U
Q)
~ 40
0::
20
I-------~-
1064 nm
O+-----------,-------L---.---------~
900
1000
1100
Wavelength Inm
1200
Fig. 5.21. Measured spectral reflectivity of a rugate filter made out of 500 periods
of Ge-doped silica with t5n = 0.012. The maximum reflectivity of 99.31 % is at the
design position but somewhat below the expected value of 99.9%. The deformation
of the line also indicates some faults in the index structure
258
5. Developments at Schott: Selected Topics
100.---------~----------------------_.
80
;:,e
~ 60
o
~
CD
~40~
20
'v '--'
~
v~~
______________~
o~------------~------------~----~
200
400
600
Wavelength Inm
Fig. 5.22. Measured spectral reflectivity of a rugate filter consisting of five superimposed 500-period stacks of F-doped Si02 with 8n = 0.01. The five different
design wavelengths yield narrow and clearly separated peaks
5.3.5 Summary
PICVD is a
• chemical vapour deposition process, operating with high-purity precursors
and yielding high-quality Si02 and Ti0 2 layers on flat or two-dimensionally
and three-dimensionally shaped substrates with very low absorption coefficients;
• "few millibar" technology with short pumping and processing times and
reduced equipment costs due to excellent process control without in situ
measurements of the growing film and through modular scale-up from laboratory to production equipment;
• plasma process, resulting in dense and amorphous films with excellent stability with respect to environmental influences;
• microwave process with complete reaction and high deposition rate of up
to several jilll/min;
• pulsed process resulting in low thermal load on the substrate and enabling
"digital" control of film thickness and refractive index by pulse counting
via composition or flip-flop technique in a wide continuous range of 1.46 :S
n:S 2.44 for the above-mentioned materials.
Typical deposition parameters for the planar high-temperature coater are
listed in Table 5.1 [5.51].
5.4 Electrochromic Devices
259
Table 5.1. Typical deposition parameters for the planar hightemperature coater for Si0 2 films
Chemical:
O2 flow
SiCl4 flow
Ar flow
Freon flow
200-300 mL/min
50-100mL/min
100mL/min
1-25mL/min
Physical:
Pressure
Furnace temperature
2-4mbar
800-1000°C
Electrical:
Microwave frequency
Average power
Pulse power
Pulse duration
Pulse pause
2.45GHz
=?
Deposition rate
< lkW
4-6kW
1-2ms
4-12ms
1-2 !JlIl/min
5.4 Electrochromic Devices
Klaus Bange, Friedrich G.K. Baucke
Electrochromic devices are active thin layer systems, whose optical properties
are changed by applying an external voltage that causes an internal electrolysis involving a change of the redox state of the "electro chromic layer" (see
Sects. 4.4.4 and 4.4.5) [5.60-66]. Electrochromic devices are thus rechargeable thin layer batteries whose energy content and state of charging are optically indicated [5.67]. The large number of applicable materials and feasible
combinations makes it necessary to summarize the reported electro chromic
devices in a general schematic presentation. As a guideline for the discussion,
Fig. 5.23 gives a systematic overview over the characteristic groups of electrochromic systems and shows their construction from the various required
thin layer types and their interdependence [5.68]. The most relevant and
critical films serve as the organizing principle. For instance, transparent electrochromic systems are grouped according to the type of storage layer (reservoir), which doubtlessly limits their number most severely. Reflecting devices
are characterized by the position of their reflecting layer, which determines
the additional electrochemical function of the metal in the electrochromic
system and, in part, relaxes the requirement of optical transparency for several films. Electrochromic displays are distinguished by the diffusely reflecting
electrolyte but are not further subdivided.
As seen in Fig. 5.23, all layers of transparent electro chromic systems, including electrodes and reservoirs, must be optically transparent when the
device is in the bleached state, which is quite a severe limitation. Reflecting
devices are subject to the same condition if the reflector is added as a separate layer to a transparent system [5.69]. However, the reflecting layer can
also be integrated into the electrochromic system as a reflecting front or rear
~
I
I I
<t>
::;.
::r
M-
o
....,
<t>
(")
~t:l
<t>
'1:1
it
~
<t>
1
d --S
Separate
I
II
II
II
Display
t..
....
Rear electrode
Electrolysis
I
I
Transparent
Integration:'>
Complementary
r
Optically passive
I
<:
Diffusely reflecting
Addition
I
I
1[
2.3 Reflecting;
characterized
by reflector:
2.2 Transparent;
characterized
by reservoir:
2.1 Displays:
2.0 EC devices:
1.0 Layers:
::r
M-
(Jq
Er
~
CJl
CJl
<t>
(=i'
<:
<t>
(=i'
0..
8
M-
...o
...g.
o
CD
(")
<t>
o
....,
~.
~
o .
t:l 0
d1
~. ~
~
(")
...
~.
.'"
(") "rj
~.
t:laq
o
I
'1
j
Front electrode
c:r.l-~
II
I
I
I
II
Nontransparent
II
II
Floating layer
I
Reflecting
I
I
CJl
~
'1:1
(=i'
0..
CD
~
,....
r:.n
<t>
M-
~
g.
r:.n
~
CJl
t:l
,....
.g
g
d1
CD
t:I
SJ'
o
l'-:>
0>
5.4 Electrochromic Devices
261
electrode or as a reflecting layer without an electrochemical function [5.69].
Such integrated reflectors can hide certain other layers, which may thus become non-transparent or have variable transmittance, as several reservoirs
do. Similarly, displays can consist of transparent and non-transparent layers
in addition to the diffusely reflecting electrolyte.
A large number of materials is available for electro chromic layers, electrolytes, storage films, electrodes, and reflectors. Some typical materials used
in the construction of electro chromic devices are summarized in Table 5.2.
For selecting the layer combination, it must be remembered that the electrochromic reaction is always connected with a corresponding counter process. Because all layers have thicknesses in the range of several 100 nm, their
deposition and subsequent treatment are very sensitive processes and necessitate careful handling, in most cases in clean rooms of class 2.
5.4.1 The Layer Components of Electrochromic Devices
The different types of electro chromic thin films are described in detail in
Sects. 4.4.4 and 4.4.5. Besides liquid and polymer electrolytes, which are
insufficient for several reasons [5.67] although they are still applied for experimental work, solid-state electrolytes have been developed [5.70-72] for
the construction of all-solid state electrochromic cells that are enclosed between the substrate glass and a cover plate [5.67]. Evaporation and sputter
techniques are used to prepare the thin-film solid-state electrolytes given in
Table 5.2 [5.73]. The ionic conductivity of these layers is due to water adsorbed at the crystal boundaries of the microcrystalline salts or in the pores
of the amorphous oxides and thus depends on the microstructure of the deposited films. Because of the high optical quality required of the devices, the
crystallites and pores must be sufficiently small to exclude stray light [5.74].
Electrodes and Reflectors
The following is restricted to metal electrodes and reflectors; the properties
of transparent electrodes are reviewed in detail in [5.75]. The requirements
for the films are high. Thus, reflecting layers must have a high optical quality
and hidden metal electrodes must be extremely smooth to avoid device damage by locally high electric field strengths. Unlike silver, which is anodically
dissolved, and aluminium, which may form highly resistant oxide layers on
anodization, the electrodes must not be subject to anodic oxidation [5.69].
In addition, some reflecting layers must be transparent to the diffusing or
migrating charge-balancing ions, but must not dissolve hydrogen because of
possible structural changes of the metal caused by formation of hydrides.
Palladium is thus not suited although its high reflectivity would otherwise
suggest application as a reflecting layer [5.69]. These conditions are only met
by few platinum-type metals and their alloys. The kinetics of proton transport has been discussed in [5.76].
262
5. Developments at Schott: Selected Topics
Table 5.2. Typical materials used for constructing electro chromic devices
• Electrochromic materials:
Cathodically coloured
- W03 (transparent/dark blue),
- Mo0 3 (transparent/dark blue),
- Nb20 5 (transparent/dark bronze),
- Ti02 (transparent/pale blue
- V205 (yellow/greenish blue)
Anodically coloured
- Ir02 (transparent/black),
- IRTOF (Ir, Sn oxide film) (transparent/black),
- NiO (transparent/dark bronze),
- CoOx (red/blue),
- Rh203 (yellow/green),
- Fe4[Fe(CN)6]a (Prussian blue) (transparent/dark blue)
• Electrolytes:
Liquid
- H 2S0 4, H 2S04 + glycerol, LiCI0 4 in PC (propylene carbonate)
Gel
- acidified PYA (polyvinylalcohol)
Polymeric
- Poly-Amps (2-acrylamido-2-methylpropane-sulfonic acid), NAFION
Solid State
- Ta205, Si02, Zr02, Na-,B-alumina, CeF 3, MgF2, LiAIF4, LiaN
• Electrodes:
Transparent electrodes
- ITO (indium-tin-oxide), Sn02, Sn02 (CI, F)
Rear electrodes
- Pt-type metals, e.g. Pt, Rh; Al
Storage electrodes
- Cgraphite (nontransparent), Au (transparent)
• Reflectors (not electrodes):
Metal reflector
- Pt-metals, e.g. Pt, Rh
Dielectric multilayer reflector
- e.g. Si02/Ti02, Si02/Ta205
• Storage compounds:
Transparent
- H 2 0, Li salts with redox anion (e.g. LiI)
Nontransparent
- electrochromic compounds such as W03, NiO, IRTOF, etc.
Reservoirs
The reservoir, or storage layer, of an electrochromic system stores the chargebalancing ion of the electro chromic reaction when the electro chromic layer is
in its bleached or, in some cases, in its coloured state. The storage layer is
electrically neutral at any time because of the principle of electroneutrality;
therefore, it also stores the counter charge of the ion, i.e., the reducing or
5.4 Electrochromic Devices
263
oxidizing charge involved in the electrochromic reaction. Thus, the element
bringing about the change of the redox state of the electrochromic layer is
actually stored in the reservoir. In principle, this storage can be accomplished
in five ways, as demonstrated by cathodically colouring W0 3 [5.76J:
(1) The negative charge and the charge-balancing cation are stored in a second electro chromic layer (res). If this material is of the same redox type
as the electrochromic compound (ec), as for example in the symmetric
system [5.67J
HW0 3 (ec)+ W0 3 (res) = W0 3 (ec)+ HW0 3 (res) ,
(coloured)
(bleached) (bleached) (coloured)
(5.15)
its application is limited to reflecting devices where it must be positioned
behind a reflector.
(2) Complementary materials, which are respectively coloured and bleached
simultaneously with the electro chromic layer, are suited for transparent
as well as reflecting systems. Equation (5.16) gives an example, where
the distinction between the electro chromic and the storage layer is rather
arbitrary.
HW0 3 (ec) + NiO(OH) (res)
(coloured)
(coloured)
=
W0 3 (ec) + Ni(OH)2 (res). (516)
(bleached) (bleached)
.
(3) The charge-balancing cation and a polyvalent anion in its more negative
redox state are stored in a gelled solution (res) when the electrochromic
system is in its bleached state, for example,
LiW0 3 (ec) + (Li+ + A -) (res)
(coloured)
= W0 3 (ec) +
(bleached)
(2Li+ + A 2-) (res) ,
(5.17)
and vice versa. Such reservoirs are useful in transparent and reflecting
devices if both redox states of the anion are colourless. However, these
systems exhibit a certain tendency of spontaneous interaction and discharge due to the contact of electro chromic and storing layer.
(4) The reservoir consists of water, which is stored in an electrolyte layer
when the device is in the bleached state. During coloration of the system,
the water is electrolyzed, and the generated hydroxyl or oxide are stored
at an electrode surface (M).
HW0 3 (ec) + HOM (electr.)
(coloured)
= W0 3 (ec) +
(bleached)
H 20 (res) + M (electr.) .
(5.18)
This kind of reservoir is restricted to reflecting electro chromic devices
because of the presence of the nontransparent metal layer.
(5) The reservoir is a hydride-forming metal, which stores hydrogen when
the electrochromic system is in the bleached state [5.70J,
5. Developments at Schott: Selected Topics
264
HW0 3 (ec)
(coloured)
+ M (res) = W0 3 (ec) + HM (res) ,
(bleached)
(5.19)
and vice versa. A practical application of this reservoir, however, has not
yet been reported.
Although the application of these principles yields a large number of electrochromic devices, the development of new storage materials is of great
interest, if not crucial, for the practical application of electrochromism.
5.4.2 Typical Examples of Electrochromic Devices
The following presents typical electrochromic devices and discusses their
advantages and disadvantages. Figure 5.24 shows a reflecting system well
suited for practical applications. The combination of the cathodically colouring electro chromic W0 3 film and the anodically colouring Ni(OHh storage
layer [5 .77] is an innovative choice, which realizes electro chromic devices with
a fairly high coloration efficiency [5.78]. The system contains a transparent
indium- tin oxide (ITO) layer as the front electrode and an integrated aluminium rear electrode, which also acts as the reflector. In addition, Ta205
is a colourless, stable, solid electrolyte with quite a large ionic conductivity
if deposited in the correct manner. Like all reflecting electrochromic devices ,
the system shows a faint double image in the partially coloured state, which,
however , can be eliminated by antireflecting coatings at the front surface of
the substrate glass or by a slightly wedge-shaped front glass plate [5 .68] .
The current-density- voltage curves and the reflectance-voltage curves in
Fig. 5.25 cover the voltage range from -l.OV to +l.5V, in which these
systems are optically active [5.79]. The current-density-voltage characteristic
Coloured
Bleached
: Glass
.' ... .. ... ........ 1---tt'l1t--HftH--
., ................ .---++1-1+--+-1-------,
--i
: ITO
................. l----1rt1+H-1fttt-----l
, .................1----'1+WH-H-i+--
................. I---\t\-t\--t--.,I---i
: ITO
: W03
: Ta20S
: Glass
--1
: Ni(OH)2
................. "'==..I~
................
: Ta20S
: NiOOH
: AI
: AI
: Glue
: Glue
: Glass
: Glass
.. .. .. .. .. .. ...... p.>-"==
................. I -- - - - - - - i
..............
................. ' - - - -- - - - - - '
Fig. 5.24. Reflecting all-solid-state electro chromic device with complementary (i.e. ,
simultaneously colouring or bleaching, respectively) ec (electro chromic) layers and
a reflecting rear electrode
5.4 Electrochromic Devices
265
":'
E 0.4
<.l
«
..§
0.2
c
Z'
·00
[i5
""C
~
0
c
~
5 -0.2
u
1.0+--------------------1
c
-.~
a:::
~
c
~ 0.5
~
B
Q)
'$
a:::
O~--'---'---r--'---'--~~
-1.0
-0.5
o
0.5
1.0
1.5
Voltage U IV
Fig. 5.25. Current-density-voltage and reflectance-voltage curves of the elec-
trochromic system shown in Fig. 5.24
indicates two separate processes. The shape of these curves suggests that the
positive voltage range characterizes the functioning of the nickel hydroxide
film, whereas in the negative region the tungsten oxide film produces the
colour change.
Figure 5.26 displays the change in the hydrogen concentrations of the layers of the system shown in Fig. 5.24 [5.79]. The hydrogen content of the layers
was measured by nuclear reaction analysis (NRA). Its change is distinct in the
bleached and coloured NiO x film, whereas the coloured Hx W0 3 film shows
a concentration gradient perpendicular to the layer, and the bleached W0 3
does not seem to lose an equivalent amount of hydrogen. This observation
indicates a more complicated mechanism for W0 3 colouration than a simple
introduction and extraction of hydrogen although the hydrogen exchange,
according to all other experiments, is the overall process which yields the
colour change.
Figure 5.27 shows a symmetric electro chromic system with two tungsten
oxide layers and an integrated reflector that has no electrochemical function but whose potential is determined by the potential distribution in the
266
5. Developments at Schott: Selected Topics
til
til
ro
(3
o Bleached
1.6
:!:
• Coloured
1.2 -
J:
0.8 -
0.4
O ~~I~:£--'-----~~~~~~~
6500
7000
7500
8000
Energy IkeV
Fig. 5.26. Hydrogen depth profile of the ec device shown in Fig. 5.24 in its coloured
and bleached state as obtained by nuclear reaction analysis (NRA)
Bleached
· .. ...... . ........ .-----\t\tt-~/t#t--,
: Glass
· .... . ... . ........ /---+I+Hr-+-+l-H+--1
· .... . ............ /-----\:tttt-+-ttltt---1
Coloured
: Glass
.... ............. . l----I+IrI+-+-+-: ITO
-I
: W0 3
· . ... . ....... . .... / - - -lffitt+Ht1ft---1
: Electrolyte
: Electrolyte
: Reflector
: Reflector
: Electrolyte
: Electrolyte
.. .. . . . . . .... 1'-=:':":';:"""'''''+''''''''''''''':'':'':':'''''
: Electrode
. .. .... .. , .... , ... 1----------1
. .......... .... ... 1 - - - -- - --1
: W03
.. .. .. ... ... . . 1 - - - - - - - - - 1
Electrode
. .......... . .... . . /---
: Glue
................ .. 1 - - - - - - - - - 1
: Glue
: Glass
: Glass
------1
Fig. 5.27. Symmetric electro chromic system with an integrated, electrically floating, reflecting layer
5.4 Electrochromic Devices
267
cell [5.67]. The reflectivity change is caused by the absorption of the tungsten oxide layer in front of the reflector. The current-density-voltage and the
reflectance-voltage curves in Fig. 5.28 demonstrate a shift of hydrogen ions
between the optically visible electrochromic and the hidden storage half cells,
into which the electrochromic layer system is divided by the reflector, during
coloration and bleaching [5.68]. The reflecting layer must thus be transparent to the transported protons, whose kinetics has been discussed in detail
in [5.76].
Figure 5.29 shows yet another type of reflecting system. It is a laminated electro chromic mirror, consisting of a transparent multilayer combination and a reflector that is deposited at the rear surface of the unit [5.69].
As in Fig. 5.24, electro chromic and storage reactions are complementary and
respectively colour or bleach the two ec layers simultaneously [5.78]. For laminated systems employing complementary W0 3 and Ir0 2 layers, the charge
'f'
E
u
«
..§ 0.1
~
'iii
c
Q)
~
0
~
::J
u
-0.1
0.6
a::
~ 0.4
c
tl
Q)
'\i5
a:: 0.2
o
-2
-1
o
2
Voltage U IV
Fig. 5.28. Current density-voltage and reflectance-voltage curves of the symmetric
electro chromic mirror shown in Fig. 5.27
268
5. Developments at Schott: Selected Topics
Coloured
Bleached
Glass
..................... I-+IH+I--\---fjff+---1
ITO
...... , .......
"
.. .
., .................... r-tltH--+--t--,
: Glass
.' ....................
I-+~r--I-I-----I
: ITO
: NiOOH
Ni(OH~
: EleCtrolYte:." ·· ..
ElectrolYte =
.g!\l~ .......... .
W0 3
:.g!\J.~ ....... ...... .
: Hx W0 3
ITO
: ITO
Glass
. . . .. . . ... .. +----t+Ht1+----t
: Glass
Reflector
: Reflector
... ... .. ....... ... .. ..~~~~~
Fig. 5.29. Reflecting electrochromic device consisting of a transparent electrochromic system and an externally added reflector
density and thus the number of cations consumed by both layers has been
balanced by measuring the charge density of both layers as a function of
the potential referred to the same reference electrode [5.80]. The system can
easily be manufactured because the electrolyte also serves as the glue. A disadvantage of this system is the appearance of phantom images, especially
in the partly coloured state, due mainly to the large distance between the
reflector and the outer surface of the front substrate glass.
A reflecting all-solid-state electro chromic system with a reflecting front
electrode is presented in Fig. 5.30 [5.67]. During coloration, protons are discharged at this (negative) electrode, and the generated hydrogen atoms diffuse through the metal into the W0 3 layer in front of the reflector, and vice
Bleached
: Glass
." ..... ... . ..... .. .. 1--tt1:tthftthf----1
Coloured
..................... ,----ttJI-lt-ir-i'---,
: Glass
: W0 3
: HxW~
:. f(ont. ~I.ec r.......
.
~
:. RefiEiCiing-" -... ~~~~~~I
:. front ~lecJr·
~
:' RefieCiiii~" " "" ~~~~~~I
: Electrolyte
: Electrolyte
: Hx W0 3
: w0 3
: Electrode
: Electrode
-..................... 1 - - - - - - - 1
: Glue
...................... 1 - - - - - - - 1
: Glass
. ................... _-
: Glue
..... .... ..... .... .... 1 - - - - - - - 1
: Glass
. ..................... l...-_ _ _ _....l
. ..................... l...-_ _ _ _....l
Fig. 5.30. "Diffusion-controlled" electrochromic device with reflecting front electrode
5.4 Electrochromic Devices
269
versa. The coloured tungsten oxide film of this so-called "diffusion-controlled
electrochromic system" is thus formed and decomposed purely chemically.
These devices offer the widest choice of materials because the reflector hides
all layers of the system. They are, however, restricted to operation with protons, which are small enough to diffuse through the metal at a sufficient rate.
The kinetics has been thoroughly discussed in [5.76].
Figure 5.31 shows a transparent electro chromic system with a reservoir
characterized by the electrolysis of water [5.67]. The electrolyte serves also
as the reservoir for the water, which is electrolyzed between the ITO and
the only few nanometer thick gold layer, whose surface bonds the generated
hydroxyl or oxide ions [5.81]. The water is regenerated during the bleaching
half cycle. The solid combined electrolyte- reservoir layer can be replaced by a
water-containing, gelled, organic layer, which also serves as the glue keeping
the W0 3 -covered ITO substrate and the gold-covered glass plate together.
Serious disadvantages of such systems are the finite optical absorption of
even very thin gold layers and possible changes in their structure caused by
repeated electrolyses during cycling.
Figure 5.32 presents a transparent electro chromic system with a reservoir
containing polyvalent anions [5.82] which are colourless in both oxidation
states. Serious disadvantages of such devices are spontaneous, although slow,
bleaching caused by a reaction of the lithium tungsten bronze with the less
negatively charged form of the anion, and the UV sensitivity of the organic
compounds used for gelatinizing the reservoir.
Figure 5.33, finally, exhibits a segment of an electro chromic display [5.68].
The electrolyte behind the W0 3 layer contains inert particles, for example
Ti0 2 , which reflect light diffusely, and the coloration of the tungsten oxide
darkens this white background. Addressing electro chromic displays, however,
is more complicated than addressing liquid crystal devices because each elecColoured
Bleached
..... ' ... , '.. , ' .., , ,.. ...----#1----,
: Glass
: ITO
: W0 3
:. H2'O 'EEi lcticiiYte
: ,.. , , .. :=: r~,~~.ryQi(
: Electrode
: Glass
[]
[J
.... , .... , , .. , .... , , .. 1-----+1+-----1
: HxWO:,
:. H2'OEiedr'OiYte'
: . , ,-. .' :=: r~s~.ry9jr ,1----1------1
: Electrode
: Glue
: Glue
: Glass
: Glass
............... 1----+----1
Fig. 5.31. Transparent electro chromic system characterized by an electrolysis of
water
270
5. Developments at Schott: Selected Topics
Bleached
Coloured
... .... .... ... .......... r - - - t t t - - - ,
: Glass
: Glass
: ITO
1
: 2Lt + Redox 2-
J
: Glass
.... . .. ... . . .. .. . ..... . . I - - - - f - H - - - - i
: ITO
: u+ +..............
Redox 1-----11-----1
: ITO
.............. 1-----11-----1
: Glass
....... . ..... . .. . .. . .. ..'----1-----'
Fig. 5.32. Transparent electro chromic system with a reservoir based on an electrolytic redox reaction of a polyvalent anion
Bleached
., ...................... .-----IH1f++1----,
: Glass
: ITO
........................ 1----1..----1
: W03
:. Dffhiselt·ieneclirig
: .e.IE!~r~ ~e......... f - - - - - - - I
: Reservoir
........................ f - - - - - - t
. Electrode
........................ 1 - - - - - - - 1
Coloured
....................... .-----\-+-+--,
: Glass
: ITO
: Hx W0 3
W1. :. biffi.iselK reflectliig
ffi.J :.~I~~.tr~ .~e... ..... -r- - - - ---i
: Reservoir
.. ...................... f - - - - - - t
: Electrode
....................... -1-- -- - - - - 1
:........................
Glue
1-------1
: Glue
:.••.••.••.••..•..•..•..•
Glass
' - -_ _ _ _...J
:. .Glass
. . . . . . . . . . . . . . . . . . . . . .L -_ _ _ _ _- '
Fig. 5.33. Segment of an electrochromic display
trochromic segment forms a battery that must be charged and discharged
independently during each cycle.
5.5 Electron-Sensitive Coatings
Frank- Thomas Lentes
Introduction
Electron-sensitive coatings are used for a variety of applications. Photomasks
for microlithography, for instance, are coated with a layer of electron-sensitive
organic material. However, this type of coating requires physical or chemical
postprocessing after the electron-beam writing process. Therefore, the idea
5.5 Electron-Sensitive Coatings
271
of a directly electron-beam-writeable coating on a glass substrate is very
attractive.
The necessary electron-beam sensitivity of the photomask is defined by
the electron-beam-writing devices employed today and by the economically
tenable maximum writing time. A dose of maximal 100 Il-C/cm 2 20keV electrons is demanded to achieve an optical density (OD) of 2.5 (at 365 and
436 nm wavelengths).
The structured mask must have a high long-time stability: The achieved
optical density is allowed to change over three years by ±10% at most, even
under a light exposure at 436 or 365 nm at an energy density of 105 J /cm 2
cumulated over the lifetime and a power density of 30 m W cm
/ 2 . This is relevant for the dark structures, which must not bleach, as well as for the light
structures, which must not darken upon solarization, etc.
The thickness of the electron-sensitive layer must not exceed 1 Il-m.
Greater layer thicknesses increase the lateral electron scattering in the layer.
The resulting diffuse structure would decrease the process stability during
the imaging of the mask in Ie fabrication.
The mask must not produce scattered light, since this would superimpose
on the image of the structures, thus impairing the contrast and consequently
the image quality and the yield. Integrated over all solid angles, the fraction
of scattered light has to be smaller than 0.1%. This definition analogously
applies also to fluorescence .
From the requirement that scattered light be restricted there results a
requirement for the layer roughness or layer graininess. The layer constituents
must not show any segregations, microcrystallites, etc., which fail to meet the
above requirements for scattered light. The grain of the layer is allowed to
amount to 30 nm at most to ensure a steep lateral contrast course at the
structural edges of OD = 2 to OD = 0.5 within O.lll-m.
With the electron-sensitive layer, the following structures are possible (see
also Fig. 5.34):
rUOUUP
~~~~~~~~~~i~~~~j~~~~!~~~~[~~~~~j~1~~!~i~~! I
.. . ....................
......
..............
_-- ....... ...............
. __ .. ....
.. .. . .................
. .........
... .........
.... ..
......
. .. . , .. , .... .... ...................
Atom ically dispersed
blackening agents
Colloids of 20-30 nm
in d iameter
Multilayer system
Fig. 5.34. Structure of the electron-sensitive layer
272
5. Developments at Schott: Selected Topics
• atomically dispersed blackening agents,
• colloids of 20-30 nm in diameter,
• multilayer systems.
There are different types and generations of photomasks. Their nomenclature is "SDR xxx", where SDR stands for Schott direct (w)rite. The xxx
= 100-lxx series represents masks of the first generation with ion-exchanged
surface films of 3-4 j.!m thickness for application at 436 nm. The xxx = 2002xx series represents masks of the second generation with additively deposited
films of (~ 1 j.!m thickness for application at 365 nm and 436 nm.
5.5.1 Physical/Chemical Principles for Generating
Optical Extinction
Colouring by Change of the Oxidation State
The intensity of absorption bands in the visible to medium UV region of
the spectrum, which is induced by transition metals in the glass and in the
crystals, varies by several orders of magnitude. The wavelength-dependent
variation of the intensity is usually expressed by the extinction coefficient. If
a light beam with intensity fo passes through a glass, the intensity decreases
to f according to the Lambert-Beer law
~
= lO-Ecd
fo
or
log (
'
7) =
E
cd.
(5.20)
(5.21 )
The layer thickness d of the glass is given in cm, the concentration c of the
absorbing component in mol/L, and the molar decadic extinction coefficient
E is given in 1 mol-1cm- 1. The quantity
OD= Ecd
(5.22)
is called optical density OD. With regard to the photomask, the following
requirements must be met:
• The internal transmission of the layer before radiation should amount to
95% at least. This corresponds to an OD of 0.0223. After radiation the OD
must be higher than 2.5 at 365 and 436 nm. During the transition from
one oxidation state to another, the extinction coefficient must therefore
change by the factor of hundred at least. Because a complete conversion of
all transition-metal ions is unlikely (cf. experience with colloidal systems),
the value should amount to at least 500.
5.5 Electron-Sensitive Coatings
273
• In the opaque state, an OD between 2 and 3 is to be reached after electron irradiation. Although a complete conversion is unlikely, theoretically
an OD of about 10 and more can be reached for a given concentration.
Therefore, the product of concentration and extinction coefficient must
amount to at least 105 cm- 1 for layer thicknesses between 0.5 x 10- 4 cm
and 1.0 x 10-4 cm. If, furthermore, the layer is assumed to consist to
100% of oxides of the absorbing species, typical transition-metal concentrations of 50 mol/L and a required minimum extinction coefficient of
2000Lmol- 1 cm- 1 result. However, the layer will contain further elements
that are necessary for producing other desired layer properties; therefore
the maximum concentration of the absorbing component is more likely to
lie between 25 and 10 mol%, and the required values for the extinction
coefficients at 365 and 435nm will be 8000 and 20000Lmol- 1 cm-I, respectively.
Table 5.3 summarizes the optical properties of those transition metals
whose changes in valency for the realization of a photomask of type SDR 200
have been discussed. The data are taken from several scientific papers. In
particular the articles by Smith and Cohen [5.83] and by Bamford [5.84]
already compile a catalogue of spectra that comprises almost all transitionmetal ions.
Table 5.3. Optical properties of transition metals
Element
Valency
c (365nm)
Lmol-1cm- 1
Factor
References
22Ti
3+
4+
3+
4+
5+
3+
6+
2+
3+
2+
3+
2+
3+
5-10
smaller rv 0.5
smaller rv 5
rv 20
greater 20
smaller rv 2-10
200-4000
smaller 0.5
rv 5-100
smaller rv 20
no data
smaller rv 20
no data
smaller rv 10
smaller rv 20
rv 320
rv 0.3
rvlO
[5.83,85]
[5.83]
[5.83-87]
[5.84, 86, 87]
[5.83,84,86]
[5.84,85,88,89]
[5.83, 84, 88, 90]
[5.83, 85, 89]
[5.83-85,87,90]
[5.84,87,90]
23V
24Cr
25Mn
27CO
28Ni
29CU
63Eu
1+
2+
2+
3+
at most 10
100-2000
10-200
rv2
rv 1000
[5.84,85,87,90]
[5.84,85]
[5.83,84]
[5.83,84,87,90]
[5.83]
[5.83]
Note: The extinction coefficients of the transparent species may still contain minor
errors. If a fraction of the ions is implanted into the glass in the opaque valency
state, then the values obtained for the extinction coefficients of the transparent
species are too high.
274
5. Developments at Schott: Selected Topics
In the relevant spectral range only charge-transfer transitions are able
to meet the above-mentioned requirements. With all other types of transitions the extinction coefficients are too small to ensure a sufficiently high
OD at 365 nm for a layer thickness of 1 J..Lm. None of the transitions shows a
sufficiently high change in the OD at 436nm. Moreover, the change in the
extinction coefficient is too small (by a factor of about 1-50).
Colouring by Colloids
A photomask of type SDR 100 uses the formation of extinction bands by
generation of colloidal silver via electron bombardment. Figure 5.35 shows
the calculated optical density at 365 nm as a function of colloidal radius
0.8
-Pd
...... Pt
- --Au
-··_·Ag
0.6
~
'iii
s:::
CII
1:J
iii
(.)
0.4
E-
O
0.0
--- ---
"-"-
0.2
10
0
20
30
40
50
Particle radius Inm
0.8
-Rh
...... Ir
-··-·Os
0.6
- - -Cu
~
'iii
- - - - --.-.:" -
c:
CII
1:J
iii
(.)
0.4
,"~ -"-"-"<~<~'~":::<. ::.:::~ . :::.
--- ---
a0
0.2
..
-"
~-'
0.0
0
10
20
30
40
50
Particle radius Inm
Fig. 5.35. Optical density at 365 nrn as a function of the colloidal radii r for various
noble metals; ml = 1.72, d = 1 J..Lm, p = 1 vol%
5.5 Electron-Sensitive Coatings
275
for several noble-metal colloids, which are assumed to be spherical. A layer
thickness of 1 ~m and a volume concentration of the colloids of 1% were
assumed. Pd and Rh colloids can induce optical densities of 0.6 at most. Ir
and Pt are next best suited for the spectral range in question. With OD >:::: 0.2
at 365 nm, the Ag extinction is only slightly dependent on the colloidal radius
and is a factor of 6 lower than the extinction at 436 nm.
Based on general considerations (Kramers-Kronig relations) and on certain assumptions that are valid for the integral of the optical density, the
following important relation (rule of sums) results:
(5.23)
where Ao is the vacuum wavelength, ml is the refractive index of the surrounding medium (glass), p is the volume concentration of the colloids, d is
the thickness of the colloidal layer and e is the base of natural algorithms.
The integral over the optical density has a material-independent maximum
value at a constant volume percent and a given layer thickness. On the assumption that the form of the extinction spectrum remains unchanged, the
optical density at a certain wavelength can only be enhanced by increasing
the concentration and/or the layer thickness. The thickness of the sensitive
layer is limited to about 1 ~m, however, because otherwise the resolution of
the structures is reduced to an unacceptable level by the scattering of the
electron beam.
An increase of the concentration may broaden the extinction spectrum
and thus may even lead to a reduction of the optical density at certain wavelengths. The calculations for the Ag system made by Quinten [5.91] show
that colloid-colloid interaction at 436 nm reduces the optical density if the
critical distance r c falls below r c >:::: 4 . r. Assuming the Ag colloids to be homogeneously distributed in the glass matrix, the concentration is limited to
about 5 at%. Interestingly, according to the above calculation, the extinction
at 365 nm is nearly independent of the colloidal distances. An increase of the
Ag concentration over a broader range is expected to manifest itself in an
increase of extinction at 365 nm.
Further Approaches
• Colour Centres. Metastable structures forming under the influence of ionizing radiation often exhibit defined absorption bands in the visible and
ultraviolet spectral region. This approach will not be investigated further.
• Interference Layers. Besides changing the extinction of a layer system, it is
theoretically also possible to achieve a variation in contrast appearance by
means of an electron-beam-induced change in the reflection and/or transmission behaviour of an interference system. For this purpose a multilayer
system has to be constructed that shows a high reflection (> 99.7%) in
276
5. Developments at Schott: Selected Topics
the initial state at 365 and 435 nm. By destroying this high reflection via
electron irradiation, a transparent layer can be achieved.
A detailed analysis of the various approaches leads to the conclusion that
the required optical density can only be achieved by using noble metal colloids. In the following the properties and generation of elementary silver colloids will be discussed.
5.5.2 Structural Aspects of Ag-Containing
Electron-Sensitive Layers
The composition and sensibilization of the directly electron-beam-writeable
glasses of the first generation was protected for Wu by several patents
[5.92,93].
The radiation-sensitive glass consists of a glass substrate with an ionexchanged surface coating, which contains water and Ag+ ions in high concentration. By irradiation with highly energetic particles, the ions are reduced
to Ag atoms and the layer finally changes colour in the irradiated parts. Postprocessing such as development or contrast enhancement is unnecessary.
The substrate glass is a water-free, halogen-containing alkali-silicate glass
with oxidic additions to suppress an unwanted spontaneous reduction of silver
ions during the exchange process (so-called RSS agents: red shift suppression)
and to prevent a photoreduction of the diffused Ag+ ions (so-called PI agents:
photosensitivity inhibitor). The most effective RSS agents are oxides of the
transition metals Ti, Ta and Zr; the preferred PI agents are oxides of Ti, Nb
and Y. But all transition metals with 1-4 d-electrons in the atomic state are
suited in principle.
The glass composition must also contain ADAGNS (acid-durability-andglass-network strengthener), that is, agents that increase the acid and alkaline
durability and strengthen the glass network in the hydrated ion exchange
layer and thus help avoid etching of the glass in the aqueous solution. Possible
agents are oxides of AI, Ca, Mg, Pb, Sr and Zn. Additional network formers
such as B 20 3 and P 20 5 may be introduced into the substrate glass, whereby
Zr02 may act as ADAGNS.
Further components such as BaO (up to 35mol%) and Ce02 (up to
0.1 mol%) may be introduced to obtain certain chemical and physical properties (hardness, thermal expansion, absorption in the UV, etc.). Typical
compositional ranges of substrate glasses are given in Table 5.4.
The glasses are melted in platinum crucibles - sometimes also in quartz
and Al crucibles - at 1300-1550 °C for 2-24 h. The substrate glass is preferably melted in chlorine atmosphere to keep the loss of halogens small.
The sensitizing to highly energetic radiation is done by Ag+ ion exchange.
The exchange can take place in an aqueous Ag+ -containing solution at temperatures well above 200°C. This process is preferably carried out in an
5.5 Electron-Sensitive Coatings
277
Table 5.4. Typical compositional ranges of oxide-based
SDR 100 substrates and exchange layers in mol%. RSS: red
shift suppression; PI: photosensitivity inhibitor; ADAGNS: aciddurability-and-glass-network strengthener [5.92,93]
Function
Element
Glass formers
Si
B
Li, Na, K
total
Ti
Al
Ca
Alkali
PI and RSS
ADAGNS
Substrate
Layer
65-75
0-15
12-18
0-10
3-"10
0.5- 5
50-89
Mg
Halogens
Sensibility
Pb
Zn
Cl
F, Br, J
Ag
H
4-15
0.4- 3
0.1-25
5-25
1.2-35
1.2-25
0-10
0-20
0-20
0-20
2-20
0- 6
0- 3
0.01-12
autoclave at pressures above the saturated vapour pressure so that evaporation of water from the solution can be avoided. The favoured depth of the
ion exchange layer lies in the range 1-3 Il.m. The required diffusion time decreases with increasing temperature. It is therefore advisable to choose not
too high a temperature because otherwise the layer thickness is increasingly
influenced by the heating and cooling profile, and process control becomes
more difficult.
The thickness of the ion exchange layer is approximately proportional to
the square root of the diffusion time. The diffusion rate strongly depends on
the substrate composition and can be reduced through variations in the glass
composition, for example by
• applying lower alkali concentrations,
• substituting larger alkali ions by Li, and
• increasing the concentration or adding (at the expense of Si0 2 ) oxides of
the elements Zn, Ti, PbO, Al and Zr.
The diffusion rate is also strongly dependent on the composition of the
aqueous solution and therefore strongly decreases with increasing pH value.
According to Wu, the layer exchanged in the autoclave contains oxidebased Ag+ ions in concentrations of up to 25 mol% which constitute network
modifiers. H+ and H30+ ions of the aqueous solution are likewise exchanged
for alkali ions of the glass; the intensity of the exchanges depends on the
diffusion temperature and on the compositions of the glass and the solution.
The preferred concentrations of the silanol groups or the water molecules lie
278
5. Developments at Schott: Selected Topics
between 0.1 and 6wt%. Typical compositional ranges for the exchange layer
given by Wu are included in Table 5.4.
5.5.3 Experimental Determination of Properties
Spectral Properties
The absorption spectra of a SDR 100 sample are presented in Fig. 5.36 for
different times after starting the radiation exposure. After 44 min irradiation a
total dose of 1200 ~C/cm2 was reached; thereafter the optical density changed
only insignificantly with time.
The spectra show that a broad extinction band occurs in the material
immediately after the beginning of radiation. The colloid formation must
therefore take place within seconds or fractions of seconds. The following
2.5...------------------,
44 min
2.0
~ 1.5
'iii
c:
Q)
"C
~
a
o 1.0
0.5
2min
1 min
0.0L::==;===:;:::==;:==~~~=;===~
400
450
500
550
600
650
700
750
Wavelength Inm
Fig. 5.36. Absorption spectra of SDR 100 material at different times after the
beginning of the electron irradiation; dose 1200 ~C/cm2 after 44 min
5.5 Electron-Sensitive Coatings
279
estimate shows that this should indeed be possible: Under the given experimental conditions about 3 x 10 20 e-h pairs/cm3 are produced within one
minute at an assumed energy loss of 2 keV I!-Lm (at 30 keV bombardment energy) and an assumed necessary energy of 10 eV for the formation of one e-h
pair. The e-h concentration is therefore comparable with the concentration of
the Ag+ ions of about 6 x 1020 cm -3. Due to recombinations of electrons and
holes, only a fraction of the electrons can contribute to the silver reduction.
By means of the OD spectra given in Fig. 5.36 the silver reduction can be
estimated to be about 3% after 1 min. Accordingly, the average separation of
the reduced Ag atoms amounts approximately to J = {l0.74/1.8· 10 18 cm =
7.4 nm. To form a spherical colloid consisting of about 4000 Ag atoms and
having a diameter of 5 nm, the Ag atoms have to travel a mean diffusion path
of about 50 nm.
Unless the diffusion time is known, the diffusion coefficient cannot be
determined. Because it is known neither for AgO nor for Ag+ of the hydrated
layer, the following assumptions have been made:
• The diffusion coefficients of AgO and Ag+ are equal. This assumption is
supported by the fact that a multiple charge reversal from AgO to Ag+ and
back is possible during the diffusion process and the colloid formation, and
that a mean diffusion coefficient is obtained.
• The diffusion coefficient to be applied was experimentally determined in
a Ca-Na silicate glass for Ag+ ions [5.94J: D = Do . exp( -QI RT), where
Do is 1.43 X 10- 6 cm 2 Is and Q is 39.77kJ/mol. The strong hydration of
the glass causes a disruption of the silicate network and a lowering of
the Tg to temperatures below 200°C [5.95J. The resulting higher mobility
of the ions is taken into account in the diffusion coefficient by inserting
into the Arrhenius relation an effective temperature Teff = Tg(CaNaSi) (Tg(hydr) - T). With Tg(CaNaSi) = 780K, Tg(hydr) = 475K, and T =
300 K this yields D ~ 5 X 10- 10 cm2 Is.
Via J = V2· C . tdiff the diffusion time tdiff ~ 25 ms is finally obtained.
This estimate, whilst containing many assumptions concerning the efficiency
of reduction of Ag+ ions and the diffusion coefficients, still gives a first clue
to the time constant of the colloid formation.
With increasing exposure time or increasing electron dose, the optical
density is correspondingly higher. The changes in the spectral distribution
are but weakly developed (compare Fig. 5.36); only after about 20 min does
a slight excess in the red spectral region start to grow there, too.
The interaction of electrons with the Ag-containing coating is described
by the electron energy and the dose; the result of this interaction is the
extinction induced in the coating. The penetration depth, the lateral and
vertical profiles of the optical density and the influence of electron scattering
were calculated and compared with measurements by means of a simulation
program.
280
5. Developments at Schott: Selected Topics
Electron Energy Ranges
In order to check the theoretical predictions on the electron ranges, an extremely deep exchange layer was produced in SDR 100 material. A 10.6-!.tmthick electron-sensitive layer was prepared and bombarded with 10, 20, and
30 ke V electrons. The blackening thus generated was measured in the transversely prepared specimen with the microscope photometer UMSP 80; the
results are listed in Table 5.5. For 20keV energy, which is the energy of the
SDR 100 main application, the difference between the calculated and the measured range is smaller than 10%, which is a very good agreement. Therefore,
the program correctly describes the process of electron energy loss in glass.
The depth profile of the optical density at 436 nm was determined as
follows: SDR 100 was bombarded with 20 keY electrons. According to measurement and calculation (see above), the electrons have a penetration depth
of 4 !.tm. The thickness of the electron-sensitive layer of the specimens was
about 3.5!.tm (optical measurement of the layer thickness). The surface of
the specimen was polished at an angle of 0.57° relative to the surface. For
this angle, a lateral step of 100 !.tm corresponds to l!.tm on the depth scale.
By means of UMSP, a lateral scan of the surface was produced at 436 nm
(Fig. 5.37). The linear dependency of the optical density on the depth shows
that the optical density profile has a box-type form. (The derivative of a
straight line is a constant). The discrepancy between the layer thickness that
was optically determined by RTA (reflectance, transmission, absorbance spectral measurement) and the layer thickness that was derived from the profile
is likely to result from inaccuracies during the polishing process.
According to the theoretical prediction, the optical density should increase
with increasing sample thickness (Fig. 5.38). The deviation of the measurement from the prediction shows that the optical density is not proportional
to the specific energy loss dE / dx.
Optical Density as a Function of Voltage
A simple consideration shows that for a given thickness of sensitive layer
(e.g., 4!.tm) there exists an optimum electron energy which at a given dose
generates the maximum optical density. For electron energies with ranges
Table 5.5. Comparison between calculated and observed ranges of electrons with
various energies in SDR 100
UlkV
10
20
30
RIJ.lm
1.5 ± 0.2
4.3 ± 0.2
2: 10.6
R( theor.) I J.lm
projected
maximal
1.2
4.1
8.4
1.3
4.4
9.0
5.5 Electron-Sensitive Coatings
281
End of Layer
o
50
100
150
200
250
Lateral position (in 11m) corresponds with relative depth scale
Fig. 5.37. Integral optical density measured for a polished SDR 100 sample. The
optical density depth profile is obtained by differentiating the curve: a box-type
profile results
0.0
1.00
2.00
3.00
4.00
5.00
Depth 111m
Fig. 5.38. Theoretical prediction of the optical density depth profile from Monte
Carlo calculations
far below the layer thickness, a comparatively small amount of energy per
electron is deposited in a very limited portion of the available layer. With
increasing electron energy an increasing fraction of energy is deposited in the
sensitive layer. Nearly optimum conditions are achieved if energy range and
layer thickness are equal: The electron energy can be completely converted
into blackening. With further increasing energy, the main part of energy is
deposited in the substrate below the layer without any blackening. Moreover,
the specific energy loss of the electrons is inversely proportional to the electron energy. Therefore the optical density should decrease with increasing
282
5. Developments at Schott: Selected Topics
electron energy. The described facts were simulated with the Monte Carlo
program for electron scattering. The result is included in Fig. 5.39 as theoretical prediction: A distinct maximum is perceivable at about 15 ke V for a
layer thickness of 4 IJ.m. The main assumptions of the simulation were that
the optical density is proportional to the specific energy loss of the electrons
and that a linear relation exists between dose and optical density.
Dependence of Optical Density on Dose
The dependence of the optical density at 436 nm on the electron dose was
investigated by numerous exposures on SDR 100. In Fig. 5.40 the optical
density is shown as a function of electron energy with cumulated dose as
parameter. The maximum is found close to 23 ke V. This experiment also
confirms that the optical density is not proportional to the specific energy
loss of the electrons. The results of 20 keV exposures at a fixed current (10 nA)
are depicted in Fig. 5.41.
Edge Gradient
The edge gradient of patterns, written in SDR 100, is defined as that lateral
distance on the mask at a structural edge over which the transmission at
436 nm falls from SO% to 20%. For structural edges on a Cr mask, values of
about 0.4-0.5 IJ.m are obtained by this measuring specification; they reflect
the resolution limit of the lens of the microscope photometer. On SDR 100
material, values between 1.4 and 2.71J.m are found; computer simulations
predict an edge gradient of 0.S- 1.2IJ.m for layer thicknesses of 3- 4IJ.m .
•
Optical density at 365 nm
Optical density at 436 nm
~ Calculation
~
~
I
~
~
~
~
~
~
~
~
10
15
20
~
25
~
~
30
40
~~
48
Energy IkeV
Fig. 5.39. Relative optical density at 365 and 436 nm and according to theoretical
prediction for various electron voltages
5.5 Electron-Sensitive Coatings
283
2.0. - - - - - - - - - - - - - - - - - - - - - - ,
_____________ 800 ~c cm- 2
E
c 1.5
CD
M
._.- .- .. . , .- .- .- .- - - - - '- 400
"'co"
__- -- .----- ---- - --------
c
- - - -- - --
.~ 1.0
C11
"0
~c
cm-2
200 ~c cm- 2
.............. 100 ~c cm- 2
~
_________....__________ _____
50
o
_ ----- -- --- - ------
25 ~c cm-2
12.5 ~c cm-2
~0.5
~c
cm- 2
0.0 + - - - - - - , - - - - - - , - - - - - - - 1
30
25
15
20
Electron energy IkeV
Fig. 5.40. Optical density as a function of the electron energy with dose as parameter
2.0. - - - - - - - - - - - - - -- - - - - - ,
E
c 1.5
CD
M
"'co"
Z:-
'iii
cQ)
1.0
"0
~
a
0
Fit:
A=-O.177
B =-0.00095
c= 1.84
0.5
O . O.f..r-~~~---~------_._-----'
62.5 500 1000
2000
Dose I~C cm- 2
4000
Fig. 5.41. Measured OD values versus dose and fitted curve; OD (436 nm)
C [1 - exp(A + B) dose]
This edge gradient limits the resolution power of the mask: Structures,
for instance grids within the range of this gradient, intermingle. But the
microlithographic image of bigger structures is also influenced by the edge
gradient: the shallower the edge gradients, the stronger the impact of variations on the process. Investigations (20 kV, various values of beam current,
address size, and spot size) proved a relation between the maximum optical
density of the written patterns and the edge gradient (Fig. 5.42). (The spot
size is a measure of the beam diameter, the address size is the difference in
the digitally controlled positions of the beam when a continuous line is writ-
5. Developments at Schott: Selected Topics
284
3.0.--------------------------------------.
E
.2C
2.5
:@ 2.0
~
OJ
Q)
OJ
"0
LlJ
1.5
1.0 + - - - - - , , - - - - - - . - - - - , - - - - - , - - - - - - - l
0.0
0.5
1.0
1.5
2.0
2.5
Optical density at 436 nm
Fig. 5.42. Edge gradient as a function of the maximum optical density of the
photomask
ten. All parameters are interdependent and have to be adjusted in order to
achieve non-fluctuating optical density along this line).
Sputter Deposition of Ag-Containing Multicomponent Layers
The structure and functionality of thin layers are determined by the coating
technology. Unless a direct modification of the substrate material at the surface is possible, another process is necessary for the deposition of an electronsensitive layer. Sputtering is the preferred technology for the production of
multicomponent layers [5.96].
The development and the preparation of suitable targets are essential key
elements for sputtering. Typical compositions of Ag-containing targets are
given in Table 5.6.
The deposition of electrically non-conducting target materials onto a nonconducting substrate requires a technology known as reactive RF magnetron
sputtering. The presence of the magnetic field (magnetron) increases the deposition rate significantly.
Table 5.6. Target compositions of electron-sensitive layers on borate glass
System / components B2 0 3
Na2 0
BaO Ti0 2
Mo0 3
BBaNa
BBaPbTi
AgB
BAgNaMo
25.0
25.0
12.5
2.0
50.0
47.5
50.0
70.0
30.0
30.0
2.5
PbO
AgN0 3
Agel
2.0
20.0
2.0
37.5
2.0
5.5 Electron-Sensitive Coatings
7.0
285
a)
6.0
Z-
5.0
'iii
c 4.0
Q)
"0
ro
a0
()
3.0
2.0
Specimen 970-2 middle
B.25 at%Ag
2.25 at% CI
1.0
0.0
200
300
400
600
500
Wavelength Inm
700
6.0
BOO
b)
5.0
Z- 4.0
'iii
c
Q)
"0
ro
3.0
0
2.0
a
()
1.0
0.0
200
Specimen 970-0B
7.5 at% Ag, 10 at% Pd
2.5 at% CI
300
400
500
600
Wavelength Inm
700
BOO
Fig. 5.43. Spectral dependence of the optical density of sputtered layers with high
concentrations of (a) Ag and (b) Ag and Pd
The optical density induced by electron-beam writing of sputtered layers
is shown in Figs. 5.43 and 5.44. The addition of Pd eliminates the loss of
optical density of a pure Ag layer at 365 nm.
5.5.4 Modelling of the Generation and the Stability
of Ag Colloids
In the following section, the experimental findings described in the previous
sections will be summarized and the formation of extinction and its stability
will be interpreted.
5. Developments at Schott: Selected Topics
286
3.0.-----------------------,
2.5
Z. 2.0
'iii
c:
Q)
"C
~ 1.5
ao 1.0
Before irradiation
-..-..- After irradiation
0.5
O.O+-------.-------.--------.------l
400
500
600
700
Wavelength Inm
Fig. 5.44. Exposure test with a stepper around 436 nm: optical density before and
after 23 000 exposures is indistinguishable
Formation of Extinction
The extinction spectrum induced by electron bombardment can be attributed
mainly to a reduction of silver atoms and a subsequent formation of Ag
colloids or agglomerates in the sensitized layer. For small doses the typical
change in extinction in SDR 100 is d OD ~ 0.005· ODmax per J.l.C / cm 2. At Agfree hydrated layers no extinction could be induced by electron bombardment.
In the following, the processes leading to extinction will be explained in more
detail and possibilities to enhance the sensitivity of SDR will be discussed.
By electron bombardment, e-h pairs are generated in the exchanged layer.
For 30 keV electrons the energy loss amounts to about 2 keV / J.l.m; hence,
at an assumed energy of formation of lOeV, about 200 pairs per J.l.lli and
incoming electron are produced. The produced e-h pairs may recombine;
this is especially expected to be the case at higher energy losses per path
length, and therefore at higher generation densities. Moreover, the e-h pairs
may be localized by electron and hole trapping. By the trapping of electrons
on Ag+ ions, atomic silver is formed. Subsequently, the AgO atoms generate
colloidal silver; this operation possibly proceeds via a number of intermediate
products such as Agt, Ag+, and Ag2+. In the ESR spectrum, however, these
small molecular silver centres have not been detected.
The generation of Ag colloids by electron bombardment is demonstrated
without doubt by TEM images on cross-sections. One observes spherical silver
colloids of 5 nm in diameter, which are homogeneously dispersed over the
sensitized layer. "Hot-irradiated" specimens, that is, specimens exposed to
temperatures of about 400 DC, show somewhat bigger colloids in the middle
of the layer. The extinction spectra of pure, spherical colloids calculated by
means of the Mie theory, however, are not in accord with the experimental
5.5 Electron-Sensitive Coatings
287
observations. EDX (energy-dispersive X-ray analysis) investigations indicate
that the colloids do not consist of pure silver but are multicomponent; in
bigger colloids, for example, chlorine has been detected. One hypothesis is
that reduced silver settles down at AgCl nuclei already contained in the
layer. Mie calculations on layered Ag deposits at AgCl nuclei show that with
decreasing shell thickness the extinction maximum shifts to the red spectral
range. The broad extinction spectrum and particularly the appearance of a
double band structure may thus be explained by a superposition of extinction
spectra of differently composed colloids.
The efficiency of colloidal silver formation (pure and layered) by electron
bombardment is determined by the probability of electron capture by an
Ag+ ion and by the mobility of the silver in the sensitized layer. To show
possibilities for a more efficient electron capture by Ag+ ions, the following
reactions must be considered in principle:
+c
Generation:
'T
-+ h+
Direct recombination:
h+
-+ 'T .
-+ Ti 3 + ,
(5.25)
-+ Ti 4 + ,
(5.27)
-+Ago ,
(5.28)
-+ Ag+ .
(5.29)
-+ HT,
(5.30)
-+ HC.
(5.31)
-+ HT
(5.32)
Capture processes:
Rearrangement processes:
+ eTi4+ + eTi3+ + h+
Ag+ + eAgO + h+
HC + eHT + h+
HC + Ti3+
HC + AgO
Ti 3 + + Ag+
+ Ti4+ ,
-+ HT + Ag+ ,
-+ Ti 4 + + Ago.
(5.24)
(5.26)
(5.33)
(5.34)
The e-h pairs generated according to (5.24) may partly decay again
through direct recombination according to (5.25). To optimize the sensitivity
of SDR, that is, to enhance the reduction of Ag+ ions, the rate of direct
recombination must be reduced. This can be done by alternative processes
according to (5.26-5.30), where HC denotes hole centres and HT hole traps.
Primarily, Ti 4 +, Ag+, and HF are located in the sensitized layer, which means
that at the beginning of the electron bombardment the processes described
by (5.26,28,31) are dominant. Indeed, ESR signals of Ti3+ and HC as well
as reduced silver (induced optical density, TEM images) are observed in SDR
specimens directly after bombardment. Accordingly, with increasing Ag+ and
HT concentration the sensitivity is expected to increase, whereas Ti4+, being
an interceptor, competes with silver and is therefore expected to decrease the
sensitivity with increasing concentration. An unequivocal correlation has not
been experimentally observed, however.
288
5. Developments at Schott: Selected Topics
Increasing the silver concentration brings about only limited success: For
SDR 100, a maximum optical density of about 1.3 per J.1m layer thickness and
per at% Ag content is observed at 436 nm. Theoretical considerations show
that at concentrations above about 5 at% Ag the extinction spectrum broadens due to colloid-colloid interaction and that the optical density decreases
again at 436 nm. Therefore a maximum possible optical density of about 6.5
per J.1ffi layer thickness can be estimated.
The recombination rate of the e-h pairs according to (5.25) can also
be reduced by suitable irradiation parameters. The induced extinction, for
example, depends not only on the electron dose applied but also on the ratio
of the electron ranges, which is determined by the electron energies, and on
the thickness of the sensitized layer as well as on the dose rate, that is, on
the number of electrons hitting the layer per sand cm 2 .
As mentioned above, the optical density is not induced by single Ag atoms
but by Ag colloids. To achieve high sensibility, the lifetime of the hole traps
HT must exceed the time that is necessary for the colloid formation process via Ag diffusion. HC 1 ,2- and O 2 centres of the silicate network and
the Vk centre CLi are stable hole traps at room temperature. The mobility
of the silver in the sensitized layer strongly depends on the composition of
the glass matrix. Experiments on differently sensitized specimens show that
the sensitivity can be increased by increasing the hydration of the layer (see
Fig. 5.45). Stronger hydration, which is concurrent with the progressive disruption of the silicate network and the lowering of the Tg down to values
below 200°C [5.96] obviously increases the mobility of the monovalent ions.
In the hydrated layer, the diffusion coefficient of silver can be estimated to be
D = 5 X 10- 10 cm2 /s. Consequently, within time scales of less than a second
diffusion paths of > 100 nm can already be covered and even small doses suffice for bigger Ag colloids to form. This also explains the occurrence of a broad
2.0
•
w.
1.6
•
~
.~ 12
CD
"C
iii
,ga.
o
•
•
•
-
0.8
•
0.4
' I·
~
0.0
0.00
0.03
0.06
0.09
0.12
0.15
0.18
[OH]/a.u.
Fig. 5.45. Optical density per unit of deposited energy versus water sorption
References
289
extinction spectrum after 60 s of radiation at a dose rate of 0.5 J..LC / (cm2 s).
The existence of AgCI nuclei possibly enhances and speeds up the colloid
formation under electron bombardment. Moreover, the concentration of hole
traps is enhanced by hydration and thus the reduction of Ag+ to AgO via
electron capture is promoted.
In summary, one can say that the achievable sensitivity is presumably
limited by the spontaneous recombination rate of the e-h pairs.
Stability
According to (5.33), the stability of the generated silver colloids and silver
agglomerations is determined by the lifetime of the generated hole centres HC.
With increasing temperature, the concentration of these centres decreases
analogously to the induced optical density. Above 250 DC these hole centres
are unstable.
The instability of the image is a thermally activated process. Firstly,
an electron-blackened image bleaches out completely at temperatures above
300 DC and secondly, a thermal post-treatment merely causes a shift on the
bleaching curve. The existing hole centres, which stabilize the AgO, can and
will be thermally activated unless a fixation process is performed. The system
is therefore thermodynamically unstable. In contrast, H 2 -reduced specimens
are stable because the silver is reduced "directly", that is, not via the formation of e-h pairs, which makes a stabilization of hole defects superfluous. The
activation energy of the fading was experimentally determined to be 0.8eV
(20 kcal/mol).
By varying the composition of the sensitized layer, the autoclave treatment, and the parameters of the electron bombardment, it was possible to
influence the stability. Increased stability, however, always resulted in decreased sensitivity. According to present knowledge, fading is an inherent
property of the Ag colloid system.
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290
5. Developments at Schott: Selected Topics
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5. Developments at Schott: Selected Topics
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Galileo's "Occhialino" to Optoelectronics, ed. by P. Mazzoldi (World Scientific, Singapore 1993) pp. 14-33
5.79 K. Bange, C. Ottermann, W. Wagner, F. Rauch: "Investigation of reflecting
electrochromic all-solid state devices by nuclear reaction analysis (NRA)",
SPIE 1272, 122-128 (1990)
5.80 S.F. Cogan, RD. Rauh: "The a-W0 3 /a-Ir02 electrochromic system", SPIE
Institute Series IS 4, 482-493 (1990)
5.81 F.G.K. Baucke, B. Metz, J. Zauner: "Elektrochrome Schichtsysteme mit
variierbaren optischen Eigenschaften", Physik in unserer Zeit 18 (1), 21-28
(1987)
5.82 T. Kamimori, J. Nagai, M. Mizuhashi: "Electro chromic devices for transmissive and reflective light control" , Proc. SPIE 653, 2-9 (1986)
5.83 H.L. Smith, A.J. Cohen: "Absorption spectra of cations in alkali-silicate
glasses of ultra-violet transmission", Phys. Chern. Glasses 4, 173 (1963)
5.84 C.R. Bamford: "The application of the ligand field theory to coloured glass" ,
Phys. Chern. Glasses 3, 189 (1962)
5.85 A. Paul: Chemistry of Glasses (Chapman and Hall, London 1990)
5.86 G.J. Kakabadse, E. Vassiliou: "The isolation of vanadium oxides in glasses",
Phys. Chern. Glasses 6, 33 (1965)
5.87 G. Lehmann: "Farben von Mineralen und ihre Ursachen", Fortschritte
Miner. 56, 172 (1978)
5.88 P. Nath, A. Paul, RW. Douglas: "Physical and chemical estimation of trivalent and hexavalent chromium in glasses" , Phys. Chern. Glasses 6, 203 (1965)
5.89 A. Bishay, S. Arafa: "A photochemical reaction induced in borate glasses
containing arsenic and manganese", Phys. Chern. Glasses 6, 134 (1965)
5.90 H. Scholze: Glas: Natur, Struktur und Eigenschaften (Springer, Berlin, Heidelberg 1988)
5.91 M. Quinten: "Optische Eigenschaften inhomogener Materie am Beispiel
aggregierter, kolloidaler Edelmetall-Systeme", Lecture on March 7, 1990
(Mainz, Otto-Schott-Forschungszentrum)
5.92 C. Wu: "High energy beam colored glasses exhibiting insensitivity to actinic
radiation", US Patent 4,567,104 (1986)
5.93 C. Wu: "High energy beam sensitive glasses", US Patent 4,670,366 (1987)
5.94 J. Matousek: "Diffusion of silver in alkali-calcium-silica glasses", Silikaty 12,
89-95 (1968) original in Czech
5.95 RF. Bartholomew: "Water in glass", in Treatise on Materials Science and
Technology, Vol. 22, ed. by M. Tomozawa, R.H. Doremus (Academic Press,
New York 1982) pp. 75-127
5.96 H. Frey, G. Kienel (Eds.): Dunnschichttechnologie (VDI Verlag, Dusseldorf
1987)
6. Products
6.1 The Principle of Interference Filters
Klaus-Dieter Loosen
Interference is a characteristic of the wave nature of electromagnetic radiation. Two or more coherent waves with the same wavelength and equal state
of polarization can superimpose and, depending on the amplitudes of the
electric field and phase relationship, enhance or compensate each other.
Figure 6.1 shows the two limiting cases, where both waves have the same
amplitude of electric field E. The letter t in the figure denotes time.
In principle an interference filter consists of a substrate (e.g., glass) upon
which thin film layers with different refractive indices are deposited. There
may be some absorption within the layers. For the sake of simplicity the effect
of refraction is neglected in Fig. 6.2 and multiple reflections are not shown
either.
l=~~.
I.
I
E
E
+
a)
b)
E
I.
I
E
+
E
Fig. 6.1. (a) Constructive interference, (b) destructive interference
296
6. Products
Incident
radiation
Reflected
radiation
Transmitted
radiation
Fig. 6.2. Schematic diagram showing the
principle of an interference filter
At each interface of two materials with different refractive indices, the incoming radiation is split into a transmitted and a reflected beam according to
the Fresnel laws (some absorption may take place within the thin layers). As
this process is repeated at each interface, numerous superimposing secondary
beams are formed that give rise to interference if they are coherent either in a
constructive or destructive manner. A wide variety of spectral characteristics
with high transmission or high reflection ratios can be achieved by varying
the nature, number, thicknesses and order of the thin film layers.
The basic theory ofthin film filters is explained in detail by Macleod [6.1],
and Dobrowolski provides an overview of optical filters in Chap. 8 of the
Handbook of Optics [6.2]. Additional information is given in the Schott catalogue Interference Filters and Special Filters [6.3].
6.1.1 Spectral Specification of Interference Filters
In many cases interference filters are produced according to customers' specifications. In order to avoid misunderstandings between the customer and
the producer, it is advantageous to define the different quantities that are
useful and necessary for the specification of interference filters. As examples
the characterizations of bandpass and longpass filters are described in more
detail (see also [6.4,5]).
The most important characteristics of bandpass filters (Fig. 6.3) can be
defined by the following values:
6.1 The Principle of Interference Filters
A"1I2
A'1/2
A'1/10
A'1/1000
297
Amax
A
m
A"1/10
A"1/1000
't max
;$
...
~
c:
:Ill
't max /2
~
't max /10
'E
~
't's
1!!
t5
CD
c%
't max /1 000 1-----:.~If4.-------l~
TW
't'S -------------,
-;,..£----i
't'g
AS3
Wavelength "-
Fig. 6.3. Characterization of bandpass filters. The dotted line is a specified upper
limit [6.3]
Tmax:
Am:
Amax:
HW
=
ZW
= ~Al/lO:
TW =
~Al/2:
~Al/lOOO:
Q value:
q value:
TS:
T~, Tfj,
etc.:
Maximum spectral transmittance within the passband
(peak transmittance).
Centre wavelength: If A~/2 and A~/2 are the wavelengths
at which spectral transmittance is Tmax/2, then
Am = (A~/2 + A~/2)/2.
Wavelength at which the filter reaches maximum spectral
transmittance Tmax (peak wavelength).
Half width = width of the transmittance curve at Tmax /2.
Defining T(A~/2) = T(A~/2) = Tmax /2 we have
HW = ~Al/2 = A~/2 - A~/2·
Tenth width = width of the transmittance curve at
Tmax/lO. Defining T(A~/lO) = T(A~/lO) = Tmax/l0 we have
ZW = ~Al/l0 = A~/l0 - A~/lO"
Thousandth width = width ofthe transmittance curve at
Tmax/lOOO. Defining T(A~/lOOO) = T(A~/lOOO) = Tmax/lOOO
we have TW = ~Al/lOOO = A~/lOOO - A~/lOOO'
Q = (tenth width)/(half width)
= ~Al/l0/ ~Al/2 = ZW /HW.
q = (thousandth width)/(half width)
= ~Al/lOOO/ ~Al/2 = TW /HW.
Upper limit for spectral transmittance within the blocking range.
Upper limits for spectral transmittance within blocking
ranges from ASl to AS2, from AS3 to AS4, etc.
298
6. Products
The main characteristics of edge filters (example: longpass filters ; Fig. 6.4)
can be defined by:
TD:
Edge wavelength, whereby the spectral transmittance reaches
a certain specified value, for example T(Ac) = 0.46.
Minimum value of spectral transmittance within the passband.
Minimum values of spectral transmittance within pass bands
from ADl to AD2, from A02 to A D3, etc.
Upper limit for spectral transmittance within the blocking
range.
Upper limits for spectral transmittance within the blocking
ranges from AS1 to AS2 , from AS2 to A S3, etc.
Shortpass filters can be specified in a similar way.
The specifications given above depend on definitions in terms of transmission properties. In an analogous way one can define certain characteristics via
reflection properties.
Interference filters exhibit angular dependence of their spectral characteristics. For incidence angles and cone angles not equal to zero this dependence
must be known, together with the state of polarization. In the following,
two different graphical representations of spectral transmittance or spectral
internal transmittance are used.
The so-called diabatic representation has an advantage over the linear
form in that both the passband and the blocking range can be clearly seen.
In Fig. 6.5 the curve of spectral transmittance of the same bandpass filter is
used in both cases to ensure a proper comparison.
g
...
't'b I -- - - - - -- - - - . . . . : - - ----,-L---,
't'o f-- - - - - -- - - - - I - ---I'--r
Q)
<.J
co
£!
'E
<I)
co
't(1o.cl l - - - - - - - - - - - - - - ! --I.
~
~
u~
en
't'13 1--- - - - - ----:
't's
't'5 F--==--",
Wavelength A
Fig. 6.4. Characterization of edge filters (example: longpass filter) . The dashed
lines are specified upper and lower limits [6.3]
6.1 The Principle of Interference Filters
1.0
2:p
0.8
I Ordinate:
't ("-) in linear scale
Abscissa: "- in linear scale
Q)
()
1==
299
a)
c
ro
:::: 0.6
'Een
c
~ 0.4
~
U
Q)
a. 0.2
C/)
\
0
0.95
2:p
Q)
()
c
:£l
0.90
Wavelength "-
-l
Ordinate: 't ("-) in diabatic scale 1-lg {Ig (1h
Abscissa: "- in linear scale
0.70
("-))}r
b)
,....
'E 0.50
en
c
~
0.30
~ 0.10
U
Q)
a. 0.01
/
C/)
10-4
10-6
/'
\
\
'"'Wavelength "-
Fig. 6.5. (a) Linear and (b) diabatic illustration of spectral transmittance [6.3]
6.1.2 All-Dielectric Filters
The multilayer system of all-dielectric interference filters (ADI filters) consists
of dielectric thin layers, i.e., electrically nonconducting layers. In order to
simplify considerations, the dielectric multilayer system is assumed to be
practically absorption-free in the spectral range in question, and scattering
within the layers or at the interfaces of two materials with different refractive
indices is neglected.
The alternating sequence of two dielectric materials of different refractive
indices is a very important multilayer design; the optical thickness of the
layers is Ao/4 of a given design wavelength Ao. Figure 6.6 shows the calculated
curve oftransmittance ofthe multilayer design glass -11 (HL )H-glass. Hand L
mean materials with a high or low index of refraction and an optical thickness
of Ao/4. The sequence (HL) has to be taken eleven times.
300
6. Products
0.95
2:...
0.90 ''''11 A
V\ j'
Q)
u
c:
~
II
0.70
I
v \/
Design
glass-11 (HL)H-glass
/\
I
I\
/
L
E 0.50
'"
~ 0.30
\
/
c:
V'
~ 0.10
"0
Q)
en0. 0.01
\
10 4
10-6
"-
./'
/
I
Fig. 6.6. AD! multilayer system (dielectric mirror)
AO
Wavelength A.
A region of low transmission is flanked by two regions with disturbed high
transmission. If absorption and scattering are neglected, low transmission also
means high reflection. Therefore such sequences of thin film layers are called
ADI mirrors. They are essential components in the multilayer design of ADI
bandpass filters.
The regions of disturbed high transmission of the mirror design are
smoothed by suitable matching layers with the aid of computer optimization. Smoothing the short-wave transmission region leads to a shortpass filter, smoothing the long-wave transmission region leads to a longpass filter,
and smoothing both transmission regions leads to a band stop filter. Figures 6.7-6.9 show typical filters with so-called hard coatings, which are used
as subtractive filters for colour mixing in equipment for photo finishing.
The curve of spectral transmittance of the mirror design shows that there
is only a restricted region of low transmittance (blocking region); that means,
1.0
2:...
0.8
~
c:
~ 0.6
E
'"c:
£
~
tt
0.4
Q)
0.2
o
400
\
500
600
Wavelength A./nm
700
lJ
800
Fig. 6.7. Shortpass
filter, type cyano [6.3]
6.1 The Principle of Interference Filters
1.0
I
.. 0.8
g
301
r--- ~
~
c:
:Ill 0.6
·e
tI)
c:
~
i
0.4
(If 0.2
I}
o
./
400
500
600
Wavelength A Inm
700
800
Fig. 6.S. Longpass filter, type yellow [6.3]
1.0
. 0.8
g
IN~
(\V
I---... + - -
III
"c:
:Ill 0.6
·e
tI)
c:
~ 0.4
et5
III
ena. 0.2
o
400
r-....
500
./
600
700
800
Wavelength A Inm
Fig. 6.9. Band stop filter, type magenta [6.3]
the blocking regions of ADI shortpass and longpass filters are also restricted.
To gain broader blocking regions, the multilayer design must be modified by
additional sequences of thin film layers, by extra interference filters, or by
suitable optical glass filters.
A fundamental change of the transmittance curve of the ADI mirror is
achieved by replacing the >'0/4 layer in the centre of the design by a layer with
optical thickness >'0/2. The >'0/2 layer is flanked on both sides by mirrors of
the design 5(HL)H. Figure 6.10 shows that a passband of high transmission
appears at design wavelength >'0.
The space between two mirrors separated by an absorption-free spacer
layer is called a cavity. The cavity can be of optical thickness >'0/2 or an
302
6. Products
0.95
2:
...
Q)
u
<:
ro
:t::
0.90
0.70
'E 0.50
<J)
<:
~
0.30
~
0.10
Q)
a. 0.01
(/)
10-4
10-6
r ,
r 71
I
1m\ Y
I
I
I
I
Tr,~
I
\
I
I
I
:1
\
I
I
•
if
!I
:1
II
:
::
I
Ii
Ii
\
\
\.\
,,-
1\
-
/
\
'1
1
I
\
\
/
,
~
I
I
I
~-
1..0
Wavelength A.
Fig. 6 .1 0. ADI mult ilayer systems
integer mult iple thereof. In the case of Fig. 6.10 we have aso-called onecavity ADI bandpass filter. An N -cavity bandpass filter basically consist s of
N one-cavity filters by means o f ap propriate mat ching layers. Figure 6.11
shows t he mult ilayer design of a typical three-cavity bandpass filter. The
layer t hicknesses in t he figure are proportional t o t he o ptcal
i thicknesses.
1)(
Cover glass
1'>1
nH
nL
]
Mirror
Cavity
]
]
Mirror
Cavity
Mirror
Cavity
]
Mirror
Substrate
Fig. 6.11. Design of a ht reecavity ADI bandpass filter
6.1 The Principle of Interference Filters
303
0.95
g
...
0.90
fl
c 0.70
~
I
I
E 0.50 I
I•
III
V
r
VI
c
.§ 0.30
J
~ 0.10
"0
Q)
Co
(/)
0.01
I
500
/
1\
I
700
600
800
Wavelength A./nm
Fig. 6.12. Transmittance curve of a non-blocked ADI bandpass filter [6.3]
The materials are denoted as nH (of high refractive index) and nL (of low
refractive index).
The typical transmittance curve of an ADI bandpass filter in Fig. 6.12
shows the restricted regions of low transmission on both sides of the passband.
In order to extend the blocking ranges, additional interference filters or
optical glass filters are necessary. In many cases metal dielectric filters or
filters with induced transmission are used as blocking filters. These filter
types are described in the Sects. 6.1.3 and 6.1.4.
The number of cavities mainly determines the steepness of the passband
curve, as is illustrated in Fig. 6.13. An increase in the cavity number changes
the form of the spectral transmittance curve from triangular to rectangular.
0.95
g
--- One-cavity
bandpass filter
-.-.. Two-cavity
bandpass filter
- - Three-cavity
bandpass filter
0.90
...
Q)
u
c
CtI
0.70
:I::
'E 0.50
VI
~
1/\1
j
\
.\
I
1\
c
.§ 0.30
~ 0.10
"0
Q)
Co
(/)
0.01
10-4
10-6
-----.
_._' _.-'
--
../
/
i
/1/
.-._/ /
./
\
,
"- -r--______
\'"
\
""
'"
.........
.........
-.-
'- .
Wavelength A.
Fig. 6.13. Influence of cavity number on the characteristics of the passband [6.3]
6. Products
304
ADI bandpass filters with three cavities and typical half widths of 6-10 nm
are standard in analytical techniques (e.g., biomedical analysis, environmental investigations). The required blocking T s outside the passband typically
ranges from 10- 5 to 10- 6 . As these bandpass filters are mostly used in combination with silicon detectors, a blocking range from about 200--1200 nm is
necessary.
The production program comprises line, band and broadband ADI interference filters. Broadband filters can have more than ten cavities. Special
broadband filters are, for example, the FITC (fluorescein-isothiocyanate) filters (Fig. 6.14).
The use of the fluorochrome FITC in fluorescence microscopy for investigating immune reactions requires filters for insertion in the excitation and
emission beams that are adapted for the specific absorption and fluorescence
properties of FITC. The FITC filters enable an excellent separation of the
excitation radiation from the fluorescence which is to be observed.
The steepness of the flanks with edge filters (shortpass and longpass filters) can be enhanced by increasing the number of thin film layers. In many
cases the additional blocking, which is necessary to achieve broadband blocking in the short-wave region with longpass filters, can be done by suitable
optical glass filters (Figs. 6.15, 6.16) that are offered by Schott [6.6].
Unlike spectral transmittance, spectral internal transmittance does not
take into account the reflection losses at the two glass-air interfaces.
In many cases interference filters have user-oriented names, for example
UV mirror, cold mirror, hot mirror, dichroic beam splitter, and so on. UV
mirrors (Fig. 6.17), which are often used at an angle of incidence of 45° , reflect
part of the UV region and transmit visible and infrared (heat) radiation. Cold
mirrors (Fig. 6.18) reflect visible radiation and transmit infrared radiation.
0.95
g
c:::
~
I
('v'''~
.J'--\
~
~
FITCA-40 I
1-·_. Type
Type FITCE-45
0.90
!
0.70
!.'1
!
!
~
E 0.50
en
!!
!!
c:::
~ 0.30
li!
1:)
0.10
CD
enCo 0.01
104
10 ..6
400
i
i
!
500
\
.j
'1
'1
\.
iV
\
\
i
600
700
800
Wavelength A./nm
Fig. 6.14. FITCA-40 (exciter filter) and FITCE-45 (barrier filter) [6.3]
6.1 The Principle of Interference Filters
0.99
"y
.
~ 0.95
~ 0.90 f- ~
c:
III
c:
roE
Q)
-
WG225
WG280
WG295
WG305
WG320
WG335
WG345
-WG360
7
~
305
0 5. 0
E
E 0.10
tl
Q)
0.01
~ 10- 3
10- 5
200
300
400
500
600
700
800
Wavelength i.. Inm
900
1000
1100
Fig. 6.15. UV longpass filter glasses, thickness 2 mm [6.6]
0.99
I I II I 1/1 /II}
I I I,. boN U 1 1 f., [I
.
/""
II; rl/ 11/ /I II I '/I /I
I III
"J:
rJ1
~ 0.95
c:
g 0.90
·E
7 rTfl
III!J
I 'II I III
III
c:
~
roE
I
1
/I
I
rn
I
I
1
17
1,V
7-
V
I--
'j
II/
II I
"7
./
II
I
0.50
Q)
.5
E 0.10
tl
Q)
~~",!:,~",J,J, IJ,J,d, Id,d,od,~",.j
0.01
gg&g&&&&& 88888fififififi fi
~ 10- 3
10-5
200
'"
~.".f!l*~~~~~f2tBt;;~:ot2:g:g~ ~
III
300
m
400
500
600
I
II I
~ #$
$ ,f$
700
Wavelength i.. Inm
800
900
1000
1100
Fig. 6.16. Longpass filter glasses, thickness 3 mm [6 .6]
The counterpart of the cold mirror is the hot mirror (Fig. 6.19) or heat
reflecting filter, which has high transmission for visible radiation, whereas a
part of the infrared spectrum is reflected. Because the region of high reflection
is restricted , a hot mirror is not always the optimum choice. If, for example,
a tungsten halogen lamp is used, the infrared emission is only partly reflected
because the emission characteristic of this type of lamp is similar to blackbody
radiation with a colour temperature of about 3000 K. In this case it is better
to use heat absorbing filters (Fig. 6.20) or a combination of heat absorbing
and interference filters. The lower transmission on moving toward the red
region is a disadvantage of heat absorbing filters .
Dichroic beam splitters divide the visible spectrum into two complementary colours by transmission and reflection. The angle of incidence is usu-
306
6. Products
1.0
Ir--
:$
..., 0.8
~
c
g
·E
45°
-
I
7
1
0.6
C/)
c
~ 0.4
~
t5
Q)
fli 0.2
o f'\
l/
A
300
400
500
Wavelength A Inm
Fig. 6.17. UV mirror
1.0
(\
:$
..., 0.8
~
Q)
c.>
c
g 0.6
·E
C/)
c
.§
~
t5
Q)
fli
0.4
0.2
o
\AA
400
1.0
~t"'
r-..
~
600
500
.I/\rv-.,
I
lJ
700
Wavelength A Inm
800
900
Fig. 6.18. Cold mirror
~
:$
0.8
...,
~
c
g
·E
0.6
C/)
c
~ 0.4
~
t5
Q)
fli
0.2
o
400
600
"- 800
Wavelength A Inm
)
1000
1200
Fig. 6.19. Hot mirror
(heat-reflecting
filter), both sides
coated
6.1 The Principle of Interference Filters
307
0.99
:$
...
Q)
u
0::::
-
7r::. I':
0.95
.......
......
............
C-:.7
~ f'.-.-- p
,g! 0.90
'E
~rr
CI)
0::::
/I.
.§
r::::
~ ~ "'-..
f-/
~
~~~~
"""" " .......
"'-" "'-
"iii
E 0.50
.sa
.5
"iii 0.10
15
Q)
S-
0.01
10-3
10-5
200
..........
'
......
"-
............
r--..
.......
r-....
............ '-.......
II
........
71
300
400
500
600
700
800
kG4
kG:!
-
kG1_
kG3_
kGS
900
1000
1100
Wavelength A Inm
0.99
...
B
0::::
0.95
:§ 0.90
'E
CI)
0::::
.§
"iii
E 0.50
~
"iii 0.10
15
,~ 0.01
w
10-3
101~00
KG4_ KG2
___ KG 1
KG3
KG5
1800
\
--r::::::::::
~
~
2600
Wavelength A Inm
.......
3400
4200
Fig. 6.20. Heat-absorbing filter glasses, thickness 2 mm [6.6]
ally 45°. Colour effect filters (the design is mainly that of longpass, shortpass,
band stop and bandpass filters) are used for coloured illumination purposes.
So-called conversion filters decrease or increase the colour temperature of light
sources. With ADI design it is also possible to construct achromatic beam
splitters over a restricted wavelength region with different transmission and
reflection ratios.
The spectral characteristics of interference filters depend on the angle of
incidence and on the cone angle. Furthermore, the behaviour is sensitive to
the polarization state of the radiation. With increasing angle of incidence, the
centre wavelength or edge position is always shifted towards shorter wavelengths. If the beam is parallel and the angle of incidence Q: not too large
(the acceptable range of Q: depends on the filter in question), the degree of
shift ~A is given approximately by the formula
308
6. Products
(6.1)
For a certain filter and a given state of polarization of the radiation, K
can be taken as approximately constant. The interdependence of spectral
characteristic and state of polarization at angles of incidence other than zero
can be used to construct thin film polarizers.
6.1.3 Metal Dielectric Filters
The first metal dielectric interference filter (MDI filter) to be used as a
bandpass filter was constructed by Geffcken [6.7]. In contrast to ADI bandpass filters, the mirrors of a MDI bandpass filter consist of thin, partially
transmitting metal layers separated by essentially absorption-free dielectric
spacer layers (cavities). The optical thicknesses of the cavities determine
mainly the spectral position Al of the passband with the longest wavelength.
Further transmission maxima are obtained approximately at AN = AdN
(N = 2,3, ... ). This can only be an approximation for passbands of higher
order, because the refractive indices and the absorption depend on the wavelength (Fig. 6.21).
In contrast to ADI bandpass filters, MDI bandpass filters with comparable bandpass characteristics have broader inherent blocking ranges. Because
of the absorption of the metal layers, they cannot attain the same maximum
transmission that is achieved by ADI filters. Undesirable passband orders
with MDI filters are eliminated by suitable additional blocking filters. MDI
bandpass filters are manufactured especially for those regions of the ultraviolet where blocking the ADI bandpass filters with additional interference
filters or optical glass filters is very difficult or even impossible. Figure 6.22
shows the typical transmission curve of a three-cavity MDI line filter.
0.90
:$
"" 0.70
~
c:
,gJ 0.50
.~ 0.30
~
~ 0.10
tl
~ 0.01
\
\
1\
c:
rJ
CJ)
10-4
10-6
300
\
\
\
'-
\
500
........
700
--
900
/
Wavelength A Inm
)
I
1\
1100
"'" ----
1300
Fig. 6.21. Two-cavity MDI bandpass filter (non-blocked) [6.3J
6.1 The Principle of Interference Filters
309
0.90
~
... 0.70
~
~ 0.50
.~ 0.30
c
~
I f\
ti
1/ \
~ 0.10
~ 0.01
\
(/)
/
10-4
./
10-6
200
\
/""'
300
400
Wavelength A Inm
500
600
Fig. 6.22. Three-cavity UV MDI line filter [6.3]
Furthermore, MDI line, band and broadband filters (Figs. 6.23-6.25) are
produced for the visible and near-infrared wavelength regions. MDI line and
band filters are widely used as blocking filters with ADI bandpass filters.
A linear variable interference filter is a filter whose centre wavelength
changes nearly linearly along a geometric filter coordinate. The basic design
is mostly that of a MDI line or band filter. The variation of the centre wavelength along one coordinate is achieved by especially depositing the spacer
layers (cavities) in a wedge-shaped form. The greater the optical thickness
of the spacer layer, the longer the centre wavelength. Additional blocking of
unwanted wavelengths is done by suitable optical glass filters. Linear variable
interference filters can for example be used as essential components for the
construction of simple monochromators.
0.90
~
... 0.70
~
~ 0.50
.~ 0.30
1\
I
c
~
- 010
~
.
ti
~ 0.01
(/)
10-4 /
10-6
700
/'
)
I\
'"" ........
800
900
Wavelength A Inm
1000
Fig. 6.23. Two-cavity MDI line filter [6.3]
1100
310
6. Products
0.90
:$
.... 0.70
fl
(\
~ 0.50
.~ 0.30
i
0.10
2i
0.01
\
c
I \
~
rn
10-4 1//'
10-6
700
/
/
'"
.........
800
900
Wavelength;' Inm
1000
1100
Fig. 6.24. Two-cavity MDI band filter [6.3]
0.90
:$
.... 0.70
flc
£!
0.50
I
.~ 0.30
i
0.10
2i
0.01
c
/
~
rn
/
10-4
10-6
700
I
I
0
\
\
\
/
800
\
~
--
900
Wavelength;' Inm
1000
1100
Fig. 6.25. Two-cavity MDI broadband filter [6.3]
6.1.4 Induced-Transmission Filters
Filters with what is known as "induced transmission" represent a mixture
of MDI and ADI design. The term was coined by Berning and Turner [6.8].
"Induced transmission" means increasing the transmission of absorbing thin
layers by suitably matched absorption-free ADI systems.
Figure 6.26 shows the calculated transmission curve of a two-metal-layer
induced-transmission filter centred at about 600 nm. Induced-transmission
filters are frequently used as blocking filters with ADI bandpass filters.
6.2 A Universal Transducer for Optical Interface Analytics
311
0.90
~
.. 0.70
~
~ 0.50
.~ 0.30
c
~
~ 0.10
ti
~ 0.01
CJ)
10-4 "10-6
400
/
/ \
\
600
"
800
1000
Wavelength), Inm
1200
1400
Fig. 6.26. Two-metal-layer induced-transmission filter (non-blocked)
6.2 A Universal Transducer for Optical Interface
Analytics: Transducer Design and Concepts
for an Economical Mass Production
Burkhard Danielzik, Wolfgang Ehrfeld, Christof Fattinger, Martin Heming,
Holger Lowe, Andreas Michel, Frank Michel, Norbert Oranth, Jiirgen Spinke
Introd uction
The optical, real-time detection of molecular interactions at a sensor surface
by monitoring refractive index changes has become an accepted analytical
method in biochemical research [6.9-11]. These interactions may result from
adsorption, desorption, or binding of molecules from a sample medium at the
sensor surface. The sensor monitors a change in phase velocity or absorption
of an optical surface wave induced by a change in surface-attached mass loading. This surface wave can be a surface plasmon propagating at the surface
of a thin metallic film or a waveguide mode propagating in a thin dielectric
waveguide of high refractive index.
Basically, the surface wave scheme relies on the fact that the propagation
of an excited waveguide mode or surface plasmon mode is affected by the interaction with the molecules in the immediate vicinity of the sensor surface. In
refractive index sensing, low-refractive index molecules from the sample solution are replaced by higher-index biomolecules and thereby change the phase
velocity of the surface wave. In optical absorption sensing, biomolecules with
optical absorption bands, which may be tagged by a dye molecule (molecular marker), replace the low-adsorption molecules from the solution. The
molecular interactions are limited to the evanescent field of the sensor, which
results in a highly surface-sensitive detection.
312
6. Products
The availability of optical transducers has stimulated the development of
a new analytical methodology in biochemistry. Key features of this methodology are the label-free and real-time detection of biomolecular interactions
at the sensor surface. An application of this method is the analysis of biospecific interactions in terms of binding constants and affinities by means of an
optical sensor chip with a biochemically modified surface. Typical biospecific
interaction pairs are for example drugs and proteins, antibodies and antigens,
hormones and receptors, DNA and proteins, or DNA and DNA.
Surface modification with biomolecules turns the optical transducer into
a biosensor. The specificity of such a biosensor is entirely governed by the
properties of the biological component because this is where the target analyte interacts with the sensor. The sensitivity of the device, however, depends
on both the biological component and the transducer because there must be
a significant interaction with the analyte and a high efficiency of subsequent
detection of this reaction with the transducer. For example, capture molecules
such as antibodies can be immobilized on the sensor surface, permitting the
detection of antigen (analyte) binding of the capture molecules and making
direct affinity sensing (immunosensing) feasible.
The main advantages of direct affinity sensing by optical transducers compared to conventional technologies are:
• Label-free detection, often without purifying the interact ant in advance.
Conventional assay technologies such as radioactivity assays (RIA), enzyme
assays (EIA) or fluorescence assays (FIA) are all based on labelling one of
the biomolecular interaction partners. This procedure requires separate
process steps such as adding of conjugate and washing.
• Real-time monitoring follows association and dissociation processes, providing a kinetic description of the interactions that is seldom available from
other techniques.
In the last three years the first commercial systems aimed at the application of optical transducers to biomolecular interaction analysis have become
available. These include:
• the surface plasmon resonance sensor (SPR) [6.12,13] of Pharmacia Biosensor,
• the resonant mirror sensor (RM) [6.14,15] of Fisons Applied Sensor Technology, and
• the resonant integrated grating coupler sensor (IGC) [6.16, 17] of Artificial
Sensing Instruments.
Most application work has been done with the Pharmacia SPR systems.
The plasmon resonance sensor is a glass chip with a thin Au coating (40nm);
the optical system monitors the plasmon resonance angle in reflection. To
achieve a high analytical sensitivity, it is necessary to perform the highprecision optical angle measurement: The sensor chip is contacted by an
6.2 A Universal Transducer for Optical Interface Analytics
313
immersion gel to a prism fixed inside the system. Thus the interface between
the sensor chip and the analytical system is quite complex.
The other systems use waveguiding modes in low-loss dielectric waveguides. Both sensors are based on a glass substrate; for the RM this consists
of a Ti0 2-coated prism, for the lGC it is a Ti0 2 or Ta205 waveguide on a
microstructured planar glass plate.
All commercial systems use sensors not designed as low-cost disposable
devices. Because they are all based on glass substrates, integrating a glass
sensor and a fluid cell as a disposable device is rather complicated. As demonstrated by commercially available systems, the direct affinity sensing scheme
has many useful applications in pharmaceutical research, quality control and
life sciences. Furthermore, the detectable concentrations are sufficient for direct immunosensing applications in diagnostics and environmental monitoring. For these applications, economical disposable sensor devices and their
simple and reliable link to the detection system are a must.
We will explain the design, the operational principle, and the fabrication
of a universal optical sensor platform, whose characteristics include
• state-of-the-art analytical performance,
• compatibility with "design to cost" fabrication technologies, and
• well-defined and reliable link to the detection system.
The process technology used is capable of a totally integrated in-line fabrication from raw material to the sensor device. The transducer is based on a
microstructured polymer substrate with a dielectric waveguide coating. The
optical microstructure is the "bidiffractive grating", which serves as input
and as output port for coupling and decoupling light beams to and from
the planar waveguide. Key properties of the Ti02 waveguide coating are a
high refractive index and the compactness of the film, which yield high sensitivity and low drift, respectively. The optical microstructure is transferred
from a metallic master stencil to polymer substrates of typically 100-150 mm
in diameter by hot embossing; the waveguide coating is deposited by a lowtemperature microwave plasma-impulse chemical vapour deposition (PlCVD)
process. The surfaces of these sensor platforms can be chemically modified
with an application-specific sensing layer and cut into single sensor chips.
Each chip is combined with a polymer cell to form a disposable sensor.
The sensor principle and the transducer design will be described in detail
in Sect. 6.2.2, while in Sect. 6.2.3 the fabrication process, the technology, and
examples of sensor performance will be given.
6.2.1 Optical Transducer
Requirements
The requirements for the optical transducer are summarized in Table 6.1,
with the implications for sensor design, selection of materials, and production
314
6. Products
Table 6.1. Requirements for the optical transducer and their implications for sensor design, selection of materials, and production processes
Requirements
sensor design
Implications for:
material
process
Interface to analytical system
Transparent substrate
• Optics
- Coupling scheme
without mechanical contact
Grating coupler for
input/output coupling
into/from waveguide
- Positioning-independent device
Translationally invariant grating structure
- Background-free
optical detection
scheme
Bidiffractive grating
structure
• Fluid handling
- Integration of
fluid cell and sensor chip
Tight (non-detachable)
connection of fluid cell
and sensor chip to an
integrated device
Micro-stucturable
polymer substrate,
PC, PMMA
Structure replication from
master to substrate
Matched polymeric
materials for sensor
substrate and fluid
cell
Standard
connection
processes from
polymer
technology
Analytical system
• Low susceptibility to temperature
shifts
Reduction of temperature shifts on sensor signal by differential measurement of
at least two parallel
propagating waveguide
modes
• Mechanical
stability
Input/output coupling
scheme insensitive to
small angle deviations
• Sensor
sensitivity
High differences in
refractive index of substrate and waveguide,
high-refractive waveguide on low-refractive
substrate
High-refractive waveguide material, lowrefractive substrate
material, optional lowrefractive intermediate
layer between substrate and waveguide,
monomode waveguide,
thickness adjusted to
max. sensitivity
Low-temperature waveguide
deposition
processes
• Sensor stability
Low porosity of waveguide
Amorphous waveguide material of high
density
Plasmaassisted
waveguide
deposition
processes
• Surface
chemistry
Reactive groups for
surface modification
and immobilization
techniques
High surface density of
OH groups, chemical
and thermal stability
6.2 A Universal Transducer for Optical Interface Analytics
315
processes. The aspects of sensor design will be discussed in detail in this
section, the selection of materials and process aspects in Sect. 6.2.3.
An analytical system based on an optical waveguide sensor basically requires a simple interface between the read-out apparatus and sensor device.
Therefore an optical coupling scheme without mechanical contact and with
high positioning tolerances of the sensor must be chosen. This can be achieved
by grating couplers, especially with a translationally invariant symmetry
of the grating structure, where coupling is independent of the position on
the sensor surface. For background-free detection, a multi-diffractive grating structure separates reflected beams from diffracted signal beams; the
bidiffractive coupler is the preferred type [6.18].
For economical sensor production, the grating structure has to be replicated from a master into the surface of a polymeric material. This polymer
substrate can be directly connected to the fluid cell, forming an integrated
sensor device, whereby the sensitive optical surface is protected by the cell.
In a bidiffractive grating coupler sensor, the measurement scheme is a
differential one, with detection of coupling angle difference from two waveguide modes, propagating parallel in the waveguide. Therefore temperature
shifts are reduced, and the differential angle measurement is insensitive to
small deviations in angle between sensor device and read-out apparatus.
In addition to a simple and stable interface between the analytical system
and the sensor chip, sensitivity is important. Because the sensitivity of waveguide sensors is strongly dependent on the refractive index [6.19], a highrefractive film has to be deposited onto the low-refractive polymer substrate.
Furthermore, high density and compactness of the waveguide are essential
for surface probing [6.19,20]; otherwise the sensitivity is limited by diffusion
into the micropores of the waveguide.
Theory
The operational principle of the bidiffractive grating coupler [6.21,22] is
shown in Fig. 6.27 in a cross-sectional view. Figure 6.27a shows the coupling mechanism and Fig. 6.27b the angle configuration. The bidiffractive
structure is located on top of the sensor substrate, consisting of two fundamental space harmonics, and is realized by the superposition of two gratings
G a and Gb.
When a laser beam impinges on the microrelief and meets the resonance
condition for one of the gratings, a surface wave propagates along the waveguide. This mode is continuously decoupled by the bidiffractive structure,
whereby the two fundamental frequencies decouple the waveguide mode into
different directions with angles 0: (via G a ) and (3 (via G b ), both in transmission and reflection. Because one of these angles differs from the direction
of the transmitted or reflected beam, a background-free out coupled beam
results. The continuous decoupling along the propagation of the waveguide
316
6. Products
a)
Transmitted
beam
Surface wave
excited via G a
Oecoupled
beams in
transmission
I
I
Waveguide
Substrate structured
with bidiffractive
grating. consisting of
grating Ga . Gb
Incident
beam
Reflected
beam
Via G a Via G b
Decoupled
beams in
reflection
b)
Waveguide modes
TEo.
™O
Waveguide
Substrate structured
with bidiffractive
grating. consisting of
grating Ga . Gb
Incident
beam TE
Incident
beamTM
Oecoupled beam TE
Fig. 6.27. Bidiffractive grating coupler: (a) coupling mechanism, (b) angle configuration for sensor device
mode causes it to extinguish after a certain propagation distance, characterized by the so-called "leakage distance" [6.21,22].
For a differential measurement, the above-described coupling scheme has
to be realized for two waveguide modes simultaneously. Figure 6.27b shows
the coupling of two modes with different polarization, TE and TM, into the
6.2 A Universal 'Transducer for Optical Interface Analytics
317
waveguide. The TE mode, incident from the left, is coupled via Gb, whereas
the TM mode, incident from the right, is coupled via G a ; their angles of
incidence both meet the resonance conditions (a for TE and f3 for TM).
sin f3 = NTM -
~
Aa
,
(6.2)
with grating periods Aa, Ab and laser light at wavelength A.
The decoupling operates at angles where the assignment of gratings a and
b to the TE and TM mode is reverse to that of the incoupling scheme, i.e.,
TE via Ga and TM via Gb:
.
A
sm<PTE = NTE - Aa '
sin <PTM
= NTM - AAb '
(6.3)
with decoupling angles <PTE and <PTM.
It has to be emphasized that the bidiffractive grating does not predefine
the lateral position of the waveguide coupling on the sensor surface. Because
of this translational invariance, any position along the sensor surface may be
chosen for measurement, which means that the sensor is self-positioning.
The possibility of varying the two grating periods independently is an
important feature of the bidiffractive grating. If the refractive index and the
thickness of the waveguide are known, the out coupling angles <PTE and <PTM
can be adjusted to appropriate values. The angle difference (<PTE - <PTM)
between the beams decoupled from the two modes can be determined by
• the transformation of the outcoupled beams into foci in the focal plane of a
lens, where (<p TE - <PTM) is proportional to the lateral distances within the
focal plane and can be detected by means of a diode array or a positionsensitive detector;
• the interference of the orthogonally polarized TE and TM beams by means
of a linear polarizer with 45° orientation. The spatial period of this interference pattern is proportional to the angle difference (<PTE - <PTM) between the beams decoupled from the two modes and can be evaluated by
methods well-known from Fourier transform spectroscopy. This new interferometric detection scheme offers very high sensitivity; it is described in
detail in [6.24,25].
The sensitivity of optical waveguides for interfacial mass detection relies
strongly on the characteristics of the field distribution in the waveguide. The
adsorption or the binding of molecules from the sample solution within the
penetration depth of the evanescent field changes the effective refractive index
of the waveguide mode. For high-refractive waveguides, the evanescent field is
concentrated near the sensor surface, giving high sensitivity. The differential
detection scheme of the bidiffractive grating-coupler sensor relies on the fact
that the sensitivity for the orthogonally polarized modes differs distinctly.
Sensitivities calculated according to [6.19], given in Fig. 6.28, demonstrate
the strong dependence on the refractive index of the film. Figure 6.28a shows
318
6. Products
0 . 8 y - - - - - - - - - - - - = = = = = = = = = i l a)
- T E 2.4
TEO
™O
- -TE 2.2
0.7
- - - TE 1.8
-TM 2.4
IE 0.6
.... - .....
- -TM 2.2
c:
/
- - - TM 1.8
0.5
,
t
/
~ 0.4
~
~
'00
/
""
'" :-.. , ."
.. .. .. -, -~ $...........,
0.3
c:
~ 0.2
~
,
,
" ....
---.-.'
.......
......
............... :.-............... .
.......
..::...
--:.......--
0.1
0~4L~Y4~~~~~~~~~~~~~~
o
50
100
150
200
250
300
Film thickness Inm
0.8,------------------------------------.
0.6
I
~-~
-2.4
- -22
™O - TEO
--~
E 0.4
c:
'700.2
.....
-.
>.
~
:;:;
'00
~
/
/
0
/
-0.2
C/)
-0.4
1
I
/
b)
1.8
....
.' . .. ................ --:-~ ..~-
/
/
Film thickness Inm
Fig. 6.28. Sensitivity !5N~~E/TM) /!5tadlayer for adsorption of an adlayer of biomolecules at the waveguide surface, for film material with refractive indices nfilm
of 1.8 (Si02/Ti02), 2.2 (Ta205), and 2.4 (Ti02). (a) Sensitivity for TEo and TMo
mode and (b) differential sensitivity for (TMo - TEo
the data for refractive indices nfilm of 1.8 (Si0 2/Ti0 2), 2.2 (Ta205), and
2.4 (Ti0 2) in comparison. Figure 6.28b displays the differences in sensitivity
between the TEo and TMo mode. In a Ti0 2 film with a refractive index
nfilm of 2.4, the difference can be as high as 60% of the overall TEo or TMo
phase change. Therefore, the differential phase detection scheme with two
orthogonally polarized modes is chosen for high sensitivity measurements.
For a Ti0 2 waveguide with a thickness tfilm of 140 nm, for example, the
penetration depth of the evanescent field into an aqueous cover medium is
65 nm for the TEo mode and 82 nm for the TMo mode (further parameters:
6.2 A Universal Transducer for Optical Interface Analytics
319
substrate nsub = 1.5, cover medium n super = 1.33, at HeNe wavelength
A = 632.8nm).
The sensitivity of the waveguide sensor for interfacial mass detection can
be optimized by adjusting the film thickness according to calculations in the
plots in Fig. 6.28. High-refractive Ti0 2 films with an optimum thickness tfilm
of about 140 nm for example may have the following design: bidiffractive grating periods Al = 320 nm and A2 = 366 nm; operation at HeNe wavelength of
A = 632.8nm results in incoupling angles a = 16.5° for TEo and (3 = -11.7°
for TMo. The decoupling angles <P(TE/TM) range between 0.5° and 4.5°, with
differences <PTE - <PTM between 0.5° and 1.5°; these differences can be adjusted by tuning the film thickness in the nm-range, without significantly
affecting the sensitivity.
In terms of mass resolution, the sensitivity can be calculated from the
(TE/TM)
.
phase change o<p = kONeff
~x along the propagatlOn of the surface
mode in the x direction: The mass loading or at the interface is proportional
to the adlayer thickness tadlayer and the excess concentration of biomolecules
in the adlayer over that of the homogeneous sample solution Cadlayer. The
refractive index of the adlayer depends on the biomolecule concentration
according to [6.23]
or =
[Cadlayer]tadlayernadlayer = nsuper
dn
+ dc [Cadlayer]
,
(6.4)
with dn/dc = 0.18 mL/g for typical biomolecules such as proteins.
The phase shift of the optical transducer changes the mass loading
given by
or,
(6.5)
with sensitivity S denoted in (mrad·mm/pg)
(TE/TM)
Sadlayer
=
oN (TE/TM)
5:t
U
adlayer
~~
---""'---- k
nadlayer - nSllper
.
(6.6)
In biochemical applications, the optical transducer is operated with a liquid
sample medium. The compactness of the waveguiding film is very important,
because the guided modes detect changes in the refractive index that are
caused by changes in the interfacial mass loading as well as those caused by
changes in the film density. The diffusion of water molecules into and out
of the film volume changes the density and the refractive index of the waveguiding film. Drift-free sensor operation requires a stable refractive index of
the waveguiding film in order to gain a high resolution for the detection of
interfacial mass loading.
An additional feature of the above-described detection scheme is the measurement of absorption. When optically absorbing molecules are bound or
320
6. Products
adsorbed at the waveguide surface, the attenuation of the waveguide mode
is increased. The excitation of only one of the surface modes is necessary for
determining the adsorption changes. The attenuation can be derived from
the signal of the diode-array detector in the sensor image plane.
The translational invariance of the sensor platform allows the operation in
parallel measurement channels, associated with different lateral positions on
the sensor surface. These channels can be addressed either simultaneously by
an array of optical read-out units, or sequentially by a single unit. The surface
elements corresponding to different measurement channels can be sensitized
separately. Thus, screening experiments on the selectivity of analyte binding
for panel testing are feasible.
6.2.2 Materials and Processes
The selection of materials and production processes for the transducer device
has been focused on an economical process suitable for in-line mass production. Process steps are:
•
•
•
•
structuring of the polymer substrate by a thermal hot-embossing process,
dielectric waveguide coating,
biochemical functionalization of the sensor surface, and
preparation of the individual sensor chip and integration with a fluid cell.
Requirements
First of all, a master structure of the bidiffractive grating is generated by
standard holographic photoresist processing. The photoresist surface structure is then copied onto a hard and durable metallic surface, including the
replication of several stencils from the metallic master structure. For an economical grating production, the substrate material has to be a thermoplastic
polymer; polycarbonate (PC) and PMMA, for example, are substrates wellknown from the fabrication of integrated-optical components [6.26]. PC was
chosen because of the significantly higher glass transformation temperature
Tg compared to PMMA (145 DC versus 100 DC) and because of the long-term
stability of the microoptical structures [6.27]. Regarding the waveguide coating process, the highest possible substrate temperature should be chosen for
the deposition of high-refractive and dense layers. The limited substrate temperature compared with glass necessitates a low-temperature plasma-assisted
deposition process.
As already mentioned, high refractive index and compactness (or high
density) are important features of the waveguide material. Ti0 2 , having one
of the highest refractive indices among dielectric materials, satisfies these
requirements. The sensitivity of Ti0 2 waveguides to surface mass loading is
typically optimized at a thickness tfilm of 140 nm.
6.2 A Universal Transducer for Optical Interface Analytics
321
Integrated-optical components made from PC are usually fabricated by
hot embossing; commercial roll-embossing systems are now available [6.26].
However, a flat-bed hot-embossing scheme is advantageous for the in-line
capability of the production process. For the optical sensor platform, wafer
formats analogous to silicon wafers are used. This permits the use of commercially available standard equipment and accessories from silicon wafer
processing, especially handling systems.
Mastering Ni
The resist masters were holographically fabricated on polished ZNK 7 glass
substrates in 6" format by Carl Zeiss (Oberkochen). The well-known CD process for the replication of shallow bidiffractive gratings cannot be transferred
to the metallization of photoresist material. The metallization of the resist
master with silver by a wet chemical process, followed by the electrodeposition of nickel and the removal of the silver moulding and embossing lead to
an intolerable loss in accuracy of the grating profile.
In our case, the resist master is metallized with nickel by a PVD process,
which is carried out in a Balzers BAK 650 PVD chamber. The appropriate
vacuum is about 1 x 10- 5 mbar, the Ni layer thickness about 60nm. A reinforcement of these PVD layers with nickel by electroplating is very easy if
the current density starts at a low level and increases slowly to the working
point. Due to island growth, PVD layers thinner than 60 nm are not always
closed. They tend to blow up during the electrolysis because of their very
high ohmic resistance.
For the electrolytic reinforcement, the metallized photoresist master is
placed in a special carrier. The master is adjusted on a ground plate and
contacted with springs. During the electroplating process, the metallized
photoresist and the contact springs are covered with nickel. To reduce the
unequal metal distribution at the edges of the master, they are covered. The
current density distribution is nearly uniform.
We use a modified electrolyte based on nickel sulfamate salts, for example:
• 100 giL nickel (400 giL nickel sulfamate-dihydrate), 40 giL boric acid, and
2mL/L wetting agent, fluortenside FT 248 (2%) (Bayer Leverkusen),
• temperature T is 50 DC, the pH value is 3.8, both permanently controlled,
• anodically depolarized nickel rounds in a titanium mesh,
• filtration permanently 0.2 !lm at a flow rate of nearly 600 Llh,
• current density 1 A/dm 2 and deposition rate 12.3 !lm/h.
By taking into account the applicable current density of 1 A/dm 2 and all
contact pads, a total current of 2.4 A results. Within an exposure time of
24 h, the reinforcement is about 280 !lm. After they are removed from the
electrolyte, the stencil and the master stick together strongly. An enforced
separation leads to a bent stencil, which is unusable. Separation by thermal
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6. Products
shock (e.g., dipping in liquid nitrogen) is also unsuitable because splinters
from a damaged glass substrate may cause scratches in the stencil surface. A
suitable way to remove the stencil from the glass master is to put the package
into water for a few minutes. With a careful light pressure on the edges, the
stencil splinters off. Sometimes small mussel-shaped splinters break off from
the glass master and adhere to the surface of the stencil. These splinters can
be removed by a jet of deionized water without scratching the stencil surface.
On the surface of the stencil there is always some residue of the photoresist. To dissolve this, the stencil is dipped into a remover (AZ 100, Hoechst)
at room temperature. The stencil is rinsed with isopropanol and deionized
water and then dried in a jet of nitrogen.
From the practical point of view, hardness, internal stress, and microroughness of the nickel stencil are of interest:
• A hard stencil surface is necessary for an economical replication process
that guarantees a high number of microstructured PC wafers per stencil
without reducing the quality of the surface.
• Low internal stress provides very flat metallic stencil plates, which enable
quick and easy handling procedures during stencil production and embossing (e.g., changing the stencil in the embossing tool).
• Low microroughness of the stencil surface is a key feature for the replication
of optical waveguide substrates. A significant increase of the microroughness versus the master structure will result in a strong attenuation of the
waveguide mode causing deterioration of sensor operation.
The hardness of electroplated nickel stencils is about 220 HV(5). Without any additives to the nickel electrolyte, the hardness cannot be increased.
Hardness-increasing additives or alloys, for example cobalt, inevitably increase the internal stress in the stencil. To produce stress-free stencils of
reproducible quality, the electrolyte must be permanently controlled. Slight
internal stress deviations during the electrodeposition cannot be detected
with conventional contractometer methods.
To detect internal stress, we proceed as follows: Silicon wafers are coated
with photoresist and processed in the same way as the real resist master. The
electroplated nickel layer with a thickness of about 100 J.lm is removed from
the silicon wafer and put on a plane plate. No bending of the nickel layer
should be detectable. This method is highly sensitive and can be used for
quality control in a production line.
The surface roughness increased only slightly during the transfer of the
photoresist master structure into a nickel stencil. Experiments addressing
surface roughness were done by means of unstructured masters and stencils
and measurements with an optical profiler (ZYGO Maxim 3D, vertical resolution 0.6 nm RMS, 2D scan with area 400 x 400 J.lm2, lateral resolution
1.3 J.lm). The resist surfaces were processed as the structured ones, by replacing the holographic irradiation by a uniform intensity. The resulting surface
6.2 A Universal Transducer for Optical Interface Analytics
323
roughnesses were 0.9 nm RMS for resist surfaces and 1.1-1.4 nm RMS for Ni
surfaces.
The uniformity of grating depth replication from the resist surface to the
Ni surface structure was checked by quantitative measurements of the optical
diffraction efficiency. Figure 6.29 shows the results as grating depth variation
versus position for the wafer centre C and eight positions along a circle of 4/1 in
diameter on a 5/1 wafer format. The grating diffraction efficiency is a function
of the grating depth and the optical constants of the surface; therefore the
results in Fig. 6.29 are plotted in arbitrary units because of the very different
optical properties of Ni metal and resist. For the 4/1 diameter quality area,
the standard deviation of depth uniformity is 5%.
The accuracy of the grating constant replication is very good; the grating
constants of resist and Ni structures differ by less than 0.2 nm. Hence, changes
in grating periods due to the electroforming process are negligible.
Economical production necessitates a stencil replication process that enables the manufacture of more than five stencils from one resist master. By a
further electroplating process, a mother-to-daughter replication of the nickel
stencil is done. Analogous to the CD process, an electroformed nickel stencil,
the mother, is replicated to new stencils, the daughters, by a further electroforming process (mother-to-daughter replication). The daughter stencils,
being much cheaper than the mother stencil, are used as embossing tools.
After the mother stencil is removed from the photoresist master, it is cleaned
and then dipped into a solution of potassium dichromate (c = 2%) for one
minute. The nickel surface of the stencil is passivated. This step is necessary for cutting off the daughter stencil from the mother after the second
electroplating process. Replication by electroforming is highly accurate: The
1.5
1.4
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~
$
en
en
1.3
1.2
..§ 1.1
Z
.s::::
~ 0.9
-c
g> 0.8
:;:::;
~
(9
0.7
0.6
0.5
C
2
3
4
5
6
7
8
Position number
Fig. 6.29. Grating depth variation of the Ni stencil relative to the resist master.
Depth versus position for wafer centre C and eight positions along a circle of 4" in
diameter on a 5" wafer format
324
6. Products
differences between mother and daughter stencil amount to less than 5% for
grating periods and less than 0.2 nm for grating depth.
Replication
For the replication of the grating structure from the stencil to the PC wafer
substrate, a hot-embossing process in flat-bed geometry has been developed.
The unstructured PC substrate is heated to temperatures above the glass
transition point, and then the stencil is replicated into the polymer material
by pressure. The aim is to transfer the high-quality optical grating structure
of the stencil quickly into the PC surface.
The embossing machinery consists of three main components, depicted
schematically in Fig. 6.30:
• custom-designed embossing tool,
• pressing machinery,
• heater/cooler for thermal cycling.
A key component of this machinery is the custom-designed embossing
tool; pressing and thermal cycling are done with commercially available
equipment.
The mould cavity is operated under vacuum in order to avoid the formation of bubbles or voids due to air enclosed between substrate and stencil
(pressure < 50mbar during closing of the mould). Because PC can absorb
up to about 0.5% of water from the ambient air, the substrate wafers are
dried under vacuum (T = 150°C for t > 1 h) and stored under dry nitrogen before embossing. For thermal cycling, the top and bottom plate of
the mould are manufactured with flow channels for liquid heat-transfer media. The computer modelling of the heat flow inside the mould is important
for achieving high and uniform heating and cooling; achieved heating rate:
Heating I cooling
~...._
Liquid metal
Structured stencil
Unstructured stencil
Heating I cooling
~...._
Fig. 6.30. Hot-embossing system with tool, press and heater/cooler
6.2 A Universal Transducer for Optical Interface Analytics
325
50 Klmin, achieved cooling rate: 30 K/min. Because temperature differences
within the wafer surface may give rise to non-uniform grating depths and
birefringence, the temperature uniformity has to be better than ±3 K in the
temperature range from 80 to 180 DC.
An important task of the flat-bed hot-embossing machinery is the generation of a uniform pressure distribution within the wafer area. Slight deviations
from parallel-plate geometry already lead to the formation of a wedge-shaped
slit between stencil and substrate, which induces macroscopic material flow
during embossing. To ensure the uniformity of pressure, the embossing tool
is equipped with a flexible element, mostly consisting of rubber material.
However, this measure proved insufficient for the replication of optical sensor
platforms. Because the depth of their grating structure is only 4 nm, depth
deviations must be kept in the sub-nm range. The results for the uniformity
of grating replication, depicted in Fig. 6.31, illustrate why rubber cannot be
used: shown are the grating depth versus position for wafer centre C and
eight positions along a circle of 4/1 in diameter on a 5/1 wafer. The grating
depth was determined by a quantitative measurement of the optical diffraction intensity, analogous to the Ni stencils. The depth scale in Fig. 6.31 is
normalized to the resist master structure.
The depth distributions for two examples of replications done with rubber sheets indicate significant variations of grating depth within the sample.
These variations are strongly dependent on the rubber material and the material history. Figure 6.31 shows the differences in depth and uniformity generated by the use of two different rubber materials. With rubber type A, an
average depth of 30% and a standard deviation of 15% are gained, whereas
::::i 0.9
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,.
c
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~
(9
,
/
/
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'"
,. ,.
~ 0.3
/
/
-.
. .... ........ .......
3
4
,.
0.1
0
C
2
5
Position number
6
7
8
Fig. 6.31. Grating depth uniformity for different types of flexible elements: liquid
metal GaIn cushion and two types of rubber material. PC substrate thickness 1 mm,
5" wafer format, grating depth versus position for wafer centre C and eight positions
along a circle of 4" in diameter on a 5" wafer format
326
6. Products
with rubber type B an average depth of 80% and a standard deviation of
35% are achieved. Similar effects concerning depth and uniformity can be
observed when a new rubber sheet is used.
Because of the very small grating depths, even slight inhomogeneities and
structural relaxations of the rubber material can induce the observed behaviour. To solve the problem of depth uniformity, an embossing tool with
an integrated liquid cushion was developed. In this approach, a liquid element
compensates for wedge errors. It is basically free of structural relaxation effects. Because the mechanical compressibility of the liquid is low, the embossing pressure is uniformly applied. The liquid is filled into a cavity with the Ni
stencil forming one wall and thus operating as a membrane supported by the
liquid. A mixture of Ga and In is used for matching the thermal expansion
of the liquid material to the cavity materials. The thermal expansion coefficient can be adjusted by the concentrations. As is evident from Fig. 6.31,
the GaIn cushion makes for higher grating depth and much better uniformity
than a rubber cushion. The average depth is 83% of the master structure,
the standard deviation of uniformity measurement is 5%.
The required surface quality of the wafer backside is determined by the
sensor operation, whereby stray-light and wave-front distortions of the beams
passing through the substrate have to be minimized. A reliable and simple
solution for the substrate support is an unstructured stencil. Although this
high back surface quality is not necessary for the optical transducer operation,
the Ni stencil support has some advantages compared to polished metallic or
vitreous materials:
• Glass or fused silica are available in excellent surface quality, but tend to
slow down the process because of the low thermal conductivity. Furthermore, mechanical breakage of the support can damage the microstructured
stencil.
• Polishing metallic surfaces to meet the quality demands requires special
machinery and long processing times.
From a polished glass plate, several unstructured stencils can be manufactured and then replicated analogously to structured ones. Surface defects
are crucial for the sensor operation and the lifetime of the stencils involved
(for front and back surface). Therefore substrate conditioning, embossing tool
and press were operated under class 100 laminar flow.
Compared to grating depth variations, the grating period is uncritical.
The replicated grating periods on the PC are by 0.7% smaller than the stencil
periods. This difference is caused by the different thermal expansion coefficients of the Ni stencil and the PC, leading to a shrinkage of the PC material
upon cooling down from glass transition temperature to room temperature.
The observed shrinkage values are very stable and vary by less than 0.5 nm;
hence shrinkage can be compensated for by properly designing the master
grating periods.
6.2 A Universal 'Transducer for Optical Interface Analytics
327
Waveguide Coating
The plasma impulse chemical vapour deposition (PICVD) process developed
by Schott allows the deposition of high-refractive Ti0 2 waveguide layers on
PC substrates in a low-temperature process. Although Ti0 2 seems to be ideally suited for waveguides, high waveguide losses proved to be notoriously
problematic in depositing Ti0 2 waveguide films by PVD methods [6.28].
These losses are due to the presence of several stable oxygen-deficient phases.
The extinction coefficients of Ti0 2 films deposited by various methods were
characterized by photothermal deflection spectroscopy [6.29]. None of the
films exhibited extinction coefficients below 1.5 x 10- 5 , which corresponds to
a waveguide loss of more than 10 dB/cm.
The PICVD process enables the deposition of monomode Ti0 2 waveguides with losses well below 10 dB/cm, even on PC polymer substrates. The
PICVD process is explained in detail in Sect. 5.3; therefore only a concise
description will be given here. For the deposition of Ti0 2 , TiC1 4 and O 2 are
used as reactants in the following process:
• These gases are fed at pressures P of about 1 mbar to a parallel plate
reactor through a gas distributor plate.
• Film deposition starts by igniting a plasma with a 2.45 MHz microwave
pulse, which forms highly reactant fragments like TiO, facilitating a heterogeneous reaction to Ti0 2 at the substrate surface.
• After every microwave pulse, exhaust gas is replaced by fresh reactant gas
during the pause.
The following advantages of the pulse mode are especially relevant for waveguide coating on PC:
• The excitation of a highly energetic plasma without heavy thermal load on
the substrate yields a high film density.
• The uniformity problems, which are inherent in all CVD processes (see
Sect. 3.2), are reduced because after the ignition of the plasma all the
material is deposited during the pulse.
• The incorporation of impurities is negligible due to the gas exchange during
the pulse pause. (The typical Cl content is less than 1%, as determined from
RBS measurements.)
To characterize the waveguiding properties, glass and PC-on-glass substrates were coated with the same process parameters as the PC wafers.
Thus a distinction between the effects induced by the material and by surfaces roughness became possible. Polycarbonate layers of 2 ~m thickness were
prepared by spin-coating PC on glass (AF 45, DESAG). The waveguide losses
of Ti0 2 films deposited on this PC/glass substrate are shown in Fig. 6.32a,
and those of Ti0 2 directly coated on glass in Fig. 6.32b. The attenuation
curves were measured by scanning an optical fibre over a short distance
328
6. Products
100
a)
2.3 dB/cm
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10
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en<..l
n02 on PC/glass
1
0
5
10
15
Position /mm
20
25
30
100
b)
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.!!!
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g
-c
~
10
£lro
Ti0 2 on glass
<..l
en
__~--~~--~--__-1
30
15
20
25
Position /mm
1+-~--~--~~--~--
o
5
10
Fig. 6.32. Waveguide losses of Ti02 films at HeNe wavelength (.>. = 632.8 nm)
(a) deposited on a 2-J.Lm-thick PC layer on glass, (b) deposited on glass
(1-2 mm) along the light streak of the guided modes that had been excited by
a prism coupler. The attenuations ofthe TEo modes are 2.3dB/cm for Ti0 2
on PC/glass, and 2.4 dB / cm for the glass substrate. Because the waveguiding
properties differ but slightly, the surface degradation of the polycarbonate under the influence of the excited plasma is negligible. The observed attenuation
lengths are mainly due to scattering losses by surface and interface roughnesses of the waveguiding film. For a 620-nm-thick multimode Ti0 2 film, the
waveguide losses are reduced to less than 0.2 dB/cm [6.31] because for multimode waveguides this surface scattering effects are much weaker than for
monomode waveguides.
6.2 A Universal 'fransducer for Optical Interface Analytics
329
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40
Fig. 6.33. Uniformity of (a) film thickness and (b) refractive index for a line scan
of length L = 100 mm across a 5" PC wafer
Figure 6.33 shows the uniformity of the film thickness and of the refractive
index of the Ti0 2 waveguide coatings on 5" PC wafers. Over a 4" quality
area, the film thickness tfilm varies by less than 2.2 nm PV, and the refractive
index nfilm by less than 0.017 PV.
The compactness of the Ti0 2 films was checked by analytical and optical
measurements, both experiments performed with waveguides deposited on
AF 45 glass with the same process parameters as employed for PC:
• The hydrogen depth profiles were measured after exposure to humidity stress testing by means of the nuclear reaction analysis (NRA) technique [6.30].
330
6. Products
• The optical detection of the phase changes in the waveguide layers, occurring when the ambient air was changed from dry to humid nitrogen gas,
showed the diffusion of water molecules into the waveguide [6.20].
The incorporation of hydrogen into the film volume was forced by a humidity stress test, where the sample is exposed to air with RH > 95% at
temperatures varying between 30°C and 60 °C for ten days. (This test according to MIL 81OE, 507.3, Proc. III is a well-known accelerated humidity
test for optical components.) The refractive index of the waveguide was found
to be unchanged after the test. Figure 6.34 shows the H/Ti ratio versus depth
before and after testing. Within the film volume, this ratio is < 0.01, indicating a very high density. Higher ratios are only observed near the surface and
the substrate/film interface; they are generated by surface-adsorbed water.
Typical values for Ti0 2 film deposited by various methods are < 0.01 for reactive low-voltage ion plating, about 0.05 for sputtering and 0.1 for reactive
evaporation [6.30].
With an optical common-path interferometric scheme (called "deltainterferometer"), a real-time monitoring of the water absorption in the micropores and at the interface is feasible [6.20]. The waveguiding modes are
excited by butt-face coupling, and the phase difference is detected after travelling a common path. Within this path, controlled changes from dry to
humid nitrogen atmosphere above the waveguide surface are performed. This
method is described in [6.20,32,33] in detail. The long-term drift stability
under continuous flow of dry nitrogen gas is 0.1 mrad/mm/min, limited by
adsorption and residual impurities.
0.30r--------------------,
0.25
0.20
o
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t:
::I:
t,
-
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As deposited
.... After 10 days climate testing
..
0.10
0.05
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6.6
6.7
6.8
6.9
6.4
6.5
6.3
Energy /MeV
t
Film surface
t
Film/substrate
interface
Fig. 6.34. Environmental stability: depth profiles of H/Ti ratio before and after
accelerated climate testing according to MIL 81OE, 507.3, Proc. III
6.2 A Universal Transducer for Optical Interface Analytics
331
The biochemical modification of the sensor surface requires various processing steps, for example:
• cleaning to achieve a high surface density of OH groups,
• silanization with a monolayer of functionalized silane (e.g., octenyltrichlorosilane), and
• oxidation of the double bonds to provide reactive groups for covalent protein immobilization [6.32,33].
Although the details of surface modification are very application-specific, a
broad range of pH and temperature changes are common features. Silanization, in particular, is done at elevated temperatures well above RT.
Because the thermal expansion of the oxidic Ti0 2 waveguide and the
polymer substrate differs significantly (ape = 6.8 x 10- 5 , arutile = 7.2 to
9.9 x 10- 6 for Ti0 2 bulk material), intrinsic film stress has to be very carefully
adjusted to compressive stress by tuning the deposition process parameters
(see Sect. 4.3.1). This intrinsic stress has to compensate for the tensile stress
that is thermally induced by the high expansion coefficient of the substrate
during thermal processing. Tensile film stress can result in the growth of
cracks, starting from local defects and causing the waveguiding properties of
the film to deteriorate.
The resistance of the optical sensor platform to chemical and thermal load
was checked by several accelerated tests, which simulate typical processing
conditions:
• acidic and basic resistance for 1 h at pH = 1.1 and pH = 12, respectively,
in a NaCI-based buffer solution at 60 DC,
• hydrophilization for 15 min in concentrated H 2 S0 4 , and
• silianization with HMDSO (hexamethyldisiloxane) under reduced pressure
« 50mbar) for Ih at 80 DC.
No changes in the refractive index were observed under these conditions,
and no cracks or film delaminations appeared. Therefore a great variety of
standard biochemical surface modification procedures are feasible.
Packaging
All fabrication steps of the optical sensor platform, including the biochemical
functionalization, can be done on the wafer format. After this processing is
finished, the wafers are cut into sensor chips. The chip dimensions are scaled
according to the dimensions of the effective sensor area; for a 0.3 x 5 mm 2
sensor area, for example, chip dimensions of 3 x 8 mm 2 can be used to provide
edges for contacting and sealing the fluid cell; the production yield is about
280 chips per 5" wafer.
Due to the translational invariance of the grating structure, changes of
the chip dimensions have implications only for the cutting process and the
332
6. Products
cell geometry, but not for the wafer fabrication process. For example, several
independent flow channels can be integrated into one chip to provide reference
measurements or simultaneous measurements of several parameters.
Sensor Performance
The experimental evaluation of the sensor performance requires the operation
of a complete analytical system, consisting of
• sensor device with integrated chip and fluid cell,
• fluidics for feeding sample and buffer solutions, and an
• optical read-out unit.
The effective sensor area is defined by the width and length of the excited
waveguide mode, the width being determined mainly by the laser spot size,
the length by the leakage distances due to continuous decoupling through the
grating. This interaction length was adjusted to a leakage distance 1/0; of
1.2-1.8 mm, with a grating depth tgrating of 3-4.5 nm. For this configuration,
the intensity of the decoupled beams is typically 1-2% of the incident laser
intensity. The sensor chip was integrated into a fluid cell, which defined a flow
channel of about three times the effective sensor area (width 1 mm, length
5 mm) and a thickness of only 40 f.lm; the fluid cell contained a sample volume
of 0.2 f.lL. For measurements under liquid flow the cell was operated at typical
flow rates of 2-100 f.lL/min.
An optical read-out unit operating at a ReNe wavelength of 632.8 nm
was used to read the decoupled beam in reflection by means of the interferometric detection scheme. The TE-polarized and TM-polarized incoupling
beams were weakly focused into the sensor surface plane with a beam diameter of 0.3 mm. This focusing has the advantage of a greater width of the
coupling resonance curve; thus coupling is maintained even for small changes
of the effective refractive index, which are due to the absorption or binding
of molecules at the sensor surface, and angle tuning can be avoided.
The sensor performance was characterized by the determination of the
response to a change in refractive index of the cover medium. This change
induces a phase shift b¢6.x; the mass coverage br, corresponding to this
phase shift, can be calculated directly from the equations given in Sect. 6.2.2
above.
Figure 6.35 shows the phase shift due to refractive index changes induced
by replacing water by a water-glycerol mixture: (a) for 1% and (b) for 0.1%
glycerol concentration. The sensor drift during about one hour is less than
0.12mrad/mm/min. From the phase changes detected, the detectable mass
loadings are calculated to be 160 pg/mm2 and 18 pg/mm2. Therefore the
sensor platform is capable of a resolution in the 1-2 pg/mm2 range.
6.2 A Universal Transducer for Optical Interface Analytics
333
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2000
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1500
1000
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500
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200
400
600
800
1000
1200
Time Is
400 , - - - - - - - - - - - - - - - - - - , b)
350
1:E
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Ul
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300
250
200
150
100
50
O+-~--~_,--._~~_.--.__r--r_-r--~~
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200
400
600
800
1000
1200
Time Is
Fig. 6.35. Phase shift of the sensor for changing the cover medium from water to
water-glycerol mixtures. Glycerol concentration (a) 1% and (b) 0.1%
Summary
The design and production technology of a universal optical transducer for
surface-selective molecular detection have been described. This transducer
provides high sensitivity to molecular interactions at the surface, and the
additional chemical surface functionalization selects specific biochemical interaction pairs.
Both the optical sensor platform fabrication as well as the biochemical
functionalization steps allow in-line production. The sensor platform consists of a waveguide film on a PC substrate, designed as disposable device.
334
6. Products
A state-of-the-art analytical performance comparable to commercial, glassbased, non-disposable sensors has been demonstrated.
The transducer design is based on a substrate, microstructured with a
translation-invariant bidiffractive grating, and an amorphous Ti0 2 waveguiding film of high density. The bidiffractive grating is a superposition of two
uniform gratings with different periods, which provide position-independent
input and output light coupling. Incoupled light propagates in a commonpath interferometer scheme along the waveguide, thereby interacting with
analyte molecules at the surface. By optically monitoring the phase shift induced by analyte binding, a measure of the interfacial mass loading is gained.
A unique feature of the bidiffractive coupling scheme is the decoupled light
that is free of background radiation. By independently adjustable grating
periods, coupling angles of TEo and TMo can be set to appropriate angular positions matching various detection schemes. Mass loading is detected
by monitoring small difference angles of decoupling. Due to the differential
and common-path measurement scheme, angular shifts common to TEo and
TMo, induced by small tilts and thermal shifts of the transducer, cancel to
first order.
The replication of the master grating structure onto a PC substrate via
hard metal stencils and hot embossing of the microrelief into the PC surface
is important for a cost-effective production of the transducer. The transfer
of microstructured shallow bidiffractive gratings of about 5 nm in depth from
the resist masters to the Ni stencils and the replication of Ni stencils have
been demonstrated. A set of stencils for the PC-structuring process can be
produced from every resist master. The stencil surface microrelief structure
can be transferred to the PC surface by a hot-embossing process, optimized
for fast process cycles as well as for uniform distribution of grating depth and
period.
Sensitivity and drift stability of the transducer are provided by an amorphous Ti0 2 waveguiding layer. The high refractive index of about 2.4 limits
the evanescent field of the waveguiding mode to a region of about 60-80 nm
next to the transducer surface; therefore highly sensitive interfacial mass detection becomes possible. The high density of the waveguide ensures low drift.
Only plasma-assisted coating technologies such as PICVD can accomplish the
deposition of dense and high-refractive layers on polymer substrates.
The sensor platform can be mechanically cut into single sensor chips that
can be integrated into a fluid cell to form a disposable device. The analytical
performance of the sensor device was shown to be comparable to commercial systems based on non-disposable substrates, having a resolution of 12 pg/mm2 for interfacial mass loading. The optical transducer that we have
presented meets all requirements for an economical, mass-producible, stateof-the-art-performance, disposable direct affinity sensor.
6.3 Laser Coatings
335
6.3 Laser Coatings
Wolfgang Rupp
6.3.1 Lasers
After the first laser - a ruby laser - had been developed by Maiman [6.34]
in 1960, there was rapid progress in laser technology. In 1961, the first ReNe
laser was already working, and only three years later CO 2 and Ar+ lasers
had been developed. The numerous efforts to construct lasers that emit at
various wavelengths and are suitable for diverse applications are reported in
greater detail in some excellent reference works [6.35-37].
To understand the function of a laser one has to take a closer look at the
meaning of the acronym "laser", which describes the physical process called
"light amplification by stimulated emission of radiation". Upon interaction
with light, an atomic system can undergo three main processes involving the
transition of electrons between lower and upper energy states. Figure 6.36
shows the possible transitions for the simplest case of a two-level system,
consisting of a lower and an upper energy state with energies El and E 2 ,
respectively. According to Bohr's relation, the energy difference E2 - El is
equivalent to hf, where h is Planck's constant and f is the frequency of the
light photon involved in the process. The three processes are:
• Spontaneous emission is a transition from the excited upper state with
energy E2 to the lower state El accompanied by the emission of a photon.
According to Bohr's relation, the frequency of the photon is (E2 - Ed/h.
Stimulated emission
hf
2hf
hf
Absorption
Fig. 6.36. Absorption, stimulated and spontaneous emission in a two-level system
6. Products
336
This process is a statistical function of space and time in which there is no
phase relationship between the individual emission processes: The emitted
photons are incoherent.
• Absorption is the excitation of the atomic system from the lower level El
to the upper level E 2 , initiated by an incoming photon with frequency
f
=
(E2 - E1)/h.
• Stimulated emission is the transition from the upper to the lower state,
triggered by an incoming photon with suitable frequency according Bohr's
relation. This process yields two photons with identical frequencies, phases,
and directions of propagation. Hence the stimulated emission is an amplification of light constituting the basic process for laser operation.
An essential requirement for the amplification process is population inversion, which means that there are more atoms in the upper than in the lower
state. This inversion can be achieved by suitable excitation mechanisms (optical, electrical, etc.), which are said to "pump" the system. An additional
condition for laser oscillation is the optical feedback, normally achieved with
two mirrors one of which is highly and the other partly reflecting at laser
wavelength.
The many materials in which laser action can take place can be usefully
classified according to their aggregate states. Table 6.2 provides a short survey
of standard laser materials [6.35]; the strongest and therefore most important laser lines are given in brackets (wavelength in nm). The abbreviation
Nd:YAG, for example, means that an yttrium aluminium garnet crystal is
doped with laser-active neodymium ions.
Host materials often used for solid-state lasers are:
•
•
•
•
•
•
YAG yttrium aluminium garnet,
YAP yttrium aluminate,
YLF yttrium lithium fluoride,
GGG gadolinium gallium garnet,
GSGG gadolinium scandium gallium garnet,
glasses: silicate glasses, phosphate glasses.
Depending on the laser material and the pumping process, a laser can
operate continuously (cw = continuous wave) or in a pulsed mode, which is
characterized essentially by pulse duration, pulse energy, and repetition rate.
6.3.2 Laser Applications
Laser light has some specific properties that make it attractive for many
applications, but not every type of laser exhibits these properties to the same
degree. In contrast to classical light sources such as electric light bulbs, a laser
emits light in one distinguished direction defined by the optical axis of the
laser resonator. In the TEMoo mode (TEM = transverse electromagnetic
mode), corresponding to a Gaussian intensity distribution, the divergence
6.3 Laser Coatings
337
Table 6.2. Standard laser materials (main wavelengths in nm)
Crystals, glasses
Solids
Semiconductors (laser diodes)
Nd:YAG (1064, 1319)
Nd:YLF (1047, 1053)
Nd:YAP (1064, 1319)
Nd:GGG (1064)
Nd:GSGG (1064, 1335)
Nd:silicate glass (1061)
Nd:phosphate glass (1053)
Ro:YAG (2080)
Er:YAG (2940)
Cr:Ab03 = ruby (694)
In GaAlP (600-700)
GaAlAs (800-900)
In GaAs (1000-1300)
In GaAsP (1000-1500)
Liquids (dyes dissolved in solvents)
Gases
Stilbene (400-440)
Coumarine (410-600)
Rhodamine 6G (570-610)
Rhodamine 800 (770-820)
ArF (193)
KrF (248)
Ar+ (488, 514)
Kr+ (647)
ReNe (543, 632)
CO 2 (9400-9800, 10500-10800)
of the beam is determined only by diffraction. Focusing such a beam leads
to the smallest possible spot sizes given by 1.27 X .\ x f / D, where .\ is the
wavelength of the light, f is the focal length of the focusing lens, and D is
the diameter of the beam. The excellent directionality of the laser radiation
is used for the accurate alignment of optical systems or for inter-satellite
communication over long distances. The ability to realize very small spot
sizes is exploited for example in bar code scanners, compact disc systems and
printing systems. Spot sizes of some 10!-Lm can also lead to very high power
densities, which in turn are necessary in materials processing for drilling,
cutting or welding different materials. The lasers most frequently used for
these high-power applications are the Nd:YAG and CO 2 lasers with power
levels of up to 2 and 20 kW, respectively.
The main applications of lasers in medicine, especially in surgery and
ophthalmology, are in the lower and medium power range (1-100W) [6.38].
The suitability of a particular laser type in medicine is determined by the
interaction process of the laser light with the tissue to be treated. In therapeutic medicine there are four basic modifications that laser radiation can
bring about in biological tissue:
• thermal coagulation and ablation, mostly done with continuous-wave CO 2
lasers, Nd:YAG lasers, and Ar+ (retina coagulation) lasers,
• non-thermal ablation, a typical application of UV (excimer) lasers,
338
6. Products
• photochemical disruption and lithotripsy, an application of pulsed lasers
(Nd:YAG for microsurgery of ocular media),
• photochemical interactions such as photodynamic therapy of cancer.
Many spectroscopic and analytic methods, for example absorption, fluorescence, or Raman spectroscopy, require laser light with a small spectral
line width t5f, corresponding to a high temporal coherence lk = e/2 x t5f
[6.37]. For pollution control, the LIDAR (light detection and ranging) technique is a very helpful tool; it permits the detection of gas concentrations
smaller than 1 ppm at distances greater than 10 km [6.39]. When targeting a
moving object, the reflected light is Doppler shifted by 2 x v x f /e, where
v is the velocity of the object, e the speed of light, and f the frequency of
the laser used. The advantage over classical radar systems in the microwave
regime is the more than three decades higher frequency shift that increases
the sensitivity to velocities and velocity changes [6.40]. Of course there are
many other applications of lasers in fields such as metrology, military, eq,ergy
technology, microelectronics, entertainment and art; a detailed description of
all applications is beyond the scope of this work.
6.3.3 Carl Zeiss Lasers
Laser activities at Carl Zeiss started soon after the first lasers had been
developed in the early sixties; in the meantime a great store of knowledge
has accumulated in the field of lasers and laser optics. Consequently, lasers
play an important role in many products and processes of Carl Zeiss. Two
lasers will be described in detail later on and others will be mentioned in a
short summary.
The laser activities at Carl Zeiss cover the whole spectral region from
the ultraviolet through the visible and near-infrared to the far-infrared. ArF
excimer lasers, for example, are used to engrave glasses, and KrF excimer
lasers (with a wavelength of 248 nm) are employed in the development and
production of lenses for semiconductor lithography. In the visible spectrum,
Ar+, HeNe, and diode lasers are in use. Ar+ lasers are mainly applied in
ophthalmologic systems, in laser scanning microscopes, and in the production
line of holographically generated gratings, whereas HeNe lasers and visible
laser diodes are chiefly used as guiding beams in diverse systems and in
interferometry.
Today the main laser systems in the near-infrared and mid-infrared spectral region are the laser diodes at about 800-900 nm, the Nd:YAG laser at
1064 nm, and other rare-earth lasers such as the holmium laser at 2000 nm
and the erbium laser at about 3000 nm. At Carl Zeiss some of these laser
types are represented in the VISULAS and OPMILAS series, standing for
the ophthalmological and surgical applications, respectively. In ophthalmology, laser diodes and the Ar+ lasers are used for the coagulation of the retina,
6.3 Laser Coatings
339
and the Q-switched Nd:YAG laser is used for microsurgery of the anterior
and intermediate ocular media.
In order to realize an optimum optical system many parts have to be ideally combined. This can be exemplified by two laser systems of Carl Zeiss, one
of which is a prototype laser in the developmental stage and the other a fully
developed laser. The first is a diode-pumped frequency-doubled Nd:YAG, the
second a radiofrequency-excited waveguide CO 2 laser.
The presently prevailing strong tendency towards laser diodes results in
more efficient and compact systems. In the long run, the diode-pumped
frequency-doubled Nd:YAG laser at 532 nm has the potential to partly replace the Ar+ laser used on the main line at 514 nm.
The good spectral overlap of the laser diode radiation and the absorption
bands of the Nd:YAG leads to a very efficient laser system, whereas only a
small portion of the emission spectra of a conventional Xe pump lamp or Kr
pump lamp can be used for the excitation of the laser crystal [6.36]. Because
most of the pump light energy of these classical light sources is converted
into heat, the efficiency is low. Another advantage of using laser diodes as
pump sources for solid-state lasers is the ability to focus most of their light
energy into a small volume inside the laser crystal that can be matched to
the laser-active volume.
The object of the work performed at Carl Zeiss on diode-pumped solidstate lasers was to establish a basis for systems in the range of small to
medium power levels « 10 W), with the field of medicine (ophthalmology) as
the primary focus of attention. The implementation of such a system requires
very sophisticated optical components, especially optical coatings. Figure 6.37
shows an example of an elaborate optical design of a diode-pumped frequencydoubled Nd:YAG laser emitting at 532nm [6.41].
The laser rod is end-pumped from both sides with two laser diodes. To
minimize the number of optical surfaces, one side of the laser rod is directly
used as high-reflecting laser mirror that is provided with an appropriate coating. This highly reflective coating for 1064 nm must at the same time have
a high transmission for the pumping radiation at 808 nm. In order to use
both directions of the frequency-doubled light, a folding of the resonator is
installed. The intracavity folding mirror is a key optical component of this
arrangement because the following specifications must be simultaneously met
to achieve efficient frequency doubling:
• high reflectivity at 1064 nm,
• high transmission at 532 nm,
• high transmission at 808 nm.
To minimize the optical losses inside the resonator, the focusing lens and
the nonlinear crystal (for example KTP) must be antireflection-coated for the
fundamental as well as for the second harmonic wavelength; normally both
reflectivities are lower than 0.1%. All the coatings used in this system have
340
6. Products
2
1
2 3
4
4
2 3
2
Fig. 6.37. Optical design of a diode-pumped frequency-doubled Nd:YAG laser.
1 laser diode, 2 collimating system, 3 ),,/2 plate, 4 polarizing cube, 5 focusing lens,
6 cylindrical lens, 7 output coupler, 8 end mirror, 9 detector, 10 filter, 11 >../4 plate,
12 frequency doubler [6.42]
been optimized with respect to losses, damage threshold, optical specifications, and long-term stability of the coatings [6.43].
When pumped with about 11 W of laser-diode radiation, the output powers are about 5 W at 1064 nm and 1.5 W at 532 nm, which is sufficient for
coagulation of the retina, for example.
The main applications of the CO 2 laser are materials processing, medicine
and metrology. Problems in materials processing arise primarily from the high
power used in these systems; they will be described later on. In the field of
microsurgery the power level is up to 100 W continuous wave. At Carl Zeiss
these lasers are represented in the OPMILAS series with two dc-excited CO 2
lasers emitting 25 and 50 W, respectively.
Because of the good transparency of the atmosphere at 10 !-lm, the CO 2
laser is often used for range finding and laser radar systems. Especially at low
signal levels it is possible to use the heterodyne detection characterized by
mixing the signal beam with that of a local oscillator on the detector [6.37].
The advantage of this technique is a reduction of the signal frequency to
the substantially smaller difference frequency of the two beams, the so-called
intermediate frequency. The absolute frequency stability of the local oscillating laser is an essential condition for this technique. In order to meet this
requirement, Carl Zeiss developed CO 2 lasers with some specific design features [6.44].
6.3 Laser Coatings
341
For the selection of a special line within the broad CO 2 emission spectrum, a ruled grating in Littrow arrangement is used as the end mirror of
the resonator. Combined with an efficient radiofrequency excitation and a
ceramic waveguide design, the short-time frequency stability is better than
20 kHz. The output mirror is made of ZnSe with dielectric coatings produced
in-house. Possible reflectivities range from 0.1 to 99.8%. The coating materials ThF 4 and ZnSe guarantee a residual absorption in the coatings of less
than 0.2% with a damage threshold greater than 500kW /cm 2 in continuous wave operation. The output powers of these lasers range from 100 m W
to lOW.
The hard seal technology and the consistent met al/ ceramic design ensure
a durable functioning for more than 10000 h and a resistance to environmental effects such as heat, vibration or shock. As indicated above, these systems
are applied in interferometry, heterodyne reception, injection locking, vibration analysis, laser radar and LIDAR.
6.3.4 Requirements on Laser Coatings
In the early stages of laser development, evaporated metal films were used as
mirror coatings, but because of their inherent absorption they have been replaced by dielectric coatings consisting of stacks of thin films with alternating
high and low refractive indices. The number of materials used for evaporation
is restricted by the following essential requirements: mechanical and chemical
durability, optical transparency for antireflecting and partial-reflecting coatings, suitable index of refraction and good evaporation in high vacuum. The
spectral region of high transparency and the index of refraction for the most
frequently used dielectric thin film materials are summarized in Chap. 4 of
this volume and in [6.45].
Another important feature influencing the laser operation is the quality of
the optical materials. In addition to a very good homogeneity of the material
itself, an extremely smooth surface, with roughnesses not exceeding some
tenths of the wavelength >., is required. >./20 at 632 nm for example means
a surface roughness smaller than 30 nm. Depending on the type of laser and
the spectral region, numerous optical materials in various forms can be used;
for example crystals, glasses, laser-active crystals and nonlinear crystals.
Normally lasers consist of two mirrors and the laser medium itself, which
in the case of a solid-state laser is a laser rod, for example. In order to avoid
Fresnel reflection losses, the end faces of the rod should be covered with very
good antireflection coatings for the respective laser wavelengths. To extract
a certain amount of laser power from the resonator, one of the two mirrors is
partially, the other highly reflecting at the appropriate wavelength. Because
any loss inside the resonator reduces the usable optical power, the reflectivity
of an antireflection coating should be below 0.1 %, whereas the reflectivities of
high reflectors should be above 99.5%, ideally 99.9%. These extreme demands
342
6. Products
on the quality of coatings are important for medium-gain and low-gain lasers
such as most continuous wave lasers, but are not so crucial for high-gain lasers
such as pulsed Nd:YAG lasers. Here, on the other hand, the peak powers of
the laser pulses are very high, especially inside the resonator, because only a
certain amount of the intracavity power is used externally.
Besides reflection losses, which normally dominate, scattering and absorption also contribute to the total loss in an optical system. Scattering is
an essential problem in laser gyroscopes where two light waves are counterpropagating.
For a good performance, the scattering of a gyroscope mirror should be
less than 50 ppm. In order to form a laser beam, several optical elements come
into question, for example telescopes, beam dividers, polarizers, lenses or mirrors. The low damage threshold of the antireflection coatings and the change
of the optical properties by thermal effects are two disadvantages of lenses
in high-power applications. The small residual absorption of the substrate
material and the coating leads to a local heating. Because of the poor heat
conductivity, an inhomogeneous temperature profile is generated, which then
changes the form, the refractive index, and consequently the focal distance
of the lens. These problems are less serious with mirror optics, which can be
made of materials with higher heat conductivity (copper, molybdenum) and
can be cooled from the back plane.
Beam splitters serve to divide a laser beam into two (or more) beams with
a determined relation of intensities. An application, for example, is the use of
one laser as beam source for several working stations. Normally such beam
splitters are realized with dielectric coatings, but one has to pay attention to
the polarizing effects of these coatings when used at angles of incidence not
equal to zero.
Many lasers emit linearly polarized light, which is undesirable for some
applications in the field of materials processing. The size of the cutting section, for example, depends on the direction of polarization relative to the
direction of feed. When working with circularly polarized light, this direction
of preference is eliminated. Most circular polarizers are reflection mirrors with
special coatings shifting the phase of the laser beam by A14.
The experience in the field of optimized laser optics gathered by Carl
Zeiss over many years is summarized in the final report of a funded BMFT
project [6.46].
The quality of coatings not only depends on excellent reflectance values
or on low scattering and low absorption, but also on a high durability under
different environmental conditions. Damage of laser mirrors or other laser
components may occur either internally or at the surface of the component
due to a number of intrinsic or extrinsic factors. Absorption, colour-centre formation and a variety of nonlinear processes (self-focusing, electron avalanche
breakdown, etc.) are intrinsic, whereas impurities, material and surface defects and surface contamination are extrinsic. Normally surface damage is the
6.3 Laser Coatings
343
more serious problem because it occurs at irradiance levels well below those
that cause bulk damage. Multilayer dielectric thin films in particular are
very sensitive to laser radiation influenced mainly by factors such as residual
stress, substrate cleanliness, purity, uniformity and composition.
Additionally, the coating design plays an important part: generally multilayer coatings fail in the high-index component. The damage thresholds of
high-reflectance coatings are typically a factor of two higher than those of
antireflection coatings. This difference is attributed to the fact that antireflection coatings have their maximum field intensity at the substrate surface
where the coating is most vulnerable because of surface imperfections, imbedded polishing materials and poor thin film structure. A hierarchy of materials
for dielectric coatings arranged in order of decreasing damage resistance includes the following materials [6.36]: ThF 4 , Si0 2, MgF 2, Ah03, CaF 2, Zr02,
Ti0 2, SiO, LiF, MgO, Ce02, ZnS.
Besides the material parameters, the laser radiation itself determines the
damage threshold of an optical component. These laser parameters are wavelength, energy, pulse duration, beam size, transverse and longitudinal modes.
Typical peak power damage thresholds in the visible and near-infrared
regime for optical materials such as glasses, crystals or coatings for laser
pulses with 1 ns pulse length are [6.36]:
•
•
•
•
antireflection coating
5 GW /cm 2
multilayer dielectric coating 10 GW / cm2
surface damage
10 GW/cm2
bulk damage
50 GW/cm 2 .
As can be gathered from this data set, dielectric films are generally the
weakest elements of any laser system.
The damage threshold of most laser materials can be estimated from the
empirical scaling law [6.47]
(6.7)
El is the reference energy damage threshold for a pulse duration of 1 ns,
and tl is the pulse duration. Provided that all other factors (spot size, beam
quality, etc.) are kept constant, this scaling law is valid for pulse durations
in the range of 0.1-100 ns.
When designing lasers that include nonlinear crystals, one has to consider
the very different bulk damage thresholds of these materials (1064 nm, 10 ns):
• KDP (potassium dihydrogen phosphate):
6 GW /cm2
•
•
•
•
BBO (beta barium borate):
LBO (lithium barium borate):
KTP (potassium titanyl phosphate):
KNb0 3 (potassium niobate):
10
GW /cm 2
19 GW /cm 2
1 GW /cm 2
0.01 GW / cm 2.
344
6. Products
6.4 Cold-Light Reflectors
Lars Bewig, Thomas Kupper, Roland Langfeld
Introduction
A special type of mirror that simultaneously exhibits a high reflectivity in
the visible range of the spectra and a high transmission in the near-infrared
region is called a cold-light reflector (CLR).
In the middle of this century, CLR were applied in cinema projectors for
the first time. In the early days of cinema, the projectors had been equipped
with high-power incandescent lamps or carbon arcs. The high power of these
illuminations (3100 and 4500 K respectively) resulted in an undesired heating
of the film material. The application of heat filters such as water-filled glass
cuvettes or IR-absorbing special glass filters (e.g., BG 19 or BG21 by Schott)
reduced this problem. Despite intensive cooling, the absorption led to a temperature rise in the filters that limited the maximum allowable illumination
power.
The discovery of the interference-optical effect of dielectric layers [6.48-52]
enabled the design of spectral-selective mirrors. The application of welldesigned cold-light mirrors resulted in improved projectors with reduced heating of the film material and increased intensity of light [6.53,54].
6.4.1 Requirements and Design
Typical requirements for a cold-light reflector are
• high reflectivity of more than 95-98% in the visible range of the spectrum
(400-700 nm),
• high transmission in the near-infrared (800-2500 nm),
• scattering and absorption in the layers < 1%,
• high reproducibility of the colour temperature (in reflection),
• high thermal stability of the layers (> 250-400 DC),
• lifetime of up to 5000 h.
A typical CLR consists of between 19 and 31 alternating layers of two
dielectric materials with different refractive indices. For a given stack of alternating layers with quarter-wave optical thickness, the reflectivity and its
spectral range depend on the ratio of the refractive indices of the materials
used (for further details see e.g. [6.55]).
Practical available materials with low refractive indices are MgF 2 or Si0 2
(nJ = 1.35 and 1.46 respectively); Ti0 2 and ZnS (nh = 2.28 and 2.4 respectively) are high-index materials. In applying these materials, two stacks have
to be combined to achieve the required high reflectivity in the broad range
from 400 to 700 nm.
6.4 Cold-Light Reflectors
345
Figure 6.38a shows a typical CLR design. The resulting spectrum, given in
Fig. 6.38b, is a compromise between the optical requirements on the one hand,
and the necessity to reduce the complexity and to increase the reproducibility
in production on the other hand. In practice, between 19 and 31 alternating
layers are applied.
A reduced number of layers will simplify the deposition process and lower
the costs. Among others, the following procedures have been proposed for
achieving this:
• building a stack of Si (an IR-transparent semiconductor) with reflectionincreasing dielectric layers,
• combining several Si layers with alternating dielectric layers [6.56], and
• using the induced transmission of certain metal layers within a dielectric
stack [6.57].
Until now, none of the numerous suggestions has been introduced in production.
3.0
a)
x 2.5
<Il
"0
.5
<Il
:B
~
0::
2.0
Glass
substrate
1.5
1.0
o
1000
500
1500
2000
2500
Depth Inm
100
r'
y\
b)
80
e::
~
c:
60
~
<Il
'$ 40
0::
20
o
250
\
500
750
1000
'-- v---
~
1250 1500
1750
2000
Wavelength Inm
Fig. 6.38. (a) Refractive index versus depth for a typical cold-light mirror design,
(b) typical spectrum (reflectance versus wavelength) of a cold-light mirror
346
6. Products
Three types of dielectric system are commonly used:
(1) MgF 2 and ZnS (known as soft CLR) ,
(2) Si0 2 and ZnS (semi-hard CLR),
(3) Si0 2 and Ti0 2 (hard CLR).
Due to the smaller difference in refractive indices of system 3 compared to
system 1, the hard system requires about 50% more layers for a given spectral
reflectivity than system 1. This leads to an economic advantage of the soft
and semi-hard systems over the hard systems which is enhanced by the fact
that for widespread processes (reactive evaporation) the deposition rates of
oxides are very small. However, hard systems are superior in chemical and
physical stability and have longer lifetimes than soft systems.
6.4.2 Applications
Today, cold-light mirrors are applied in many optical instruments. In all types
of projectors (for films and slides, in microfiche projectors and in overhead
projectors), CLR reduce the heating of the projected medium. As part of illumination systems (e.g., in operating theatres of hospitals), as special lamps for
dental applications and as part of variable traffic signs, where cold light with
well-defined colour temperature is required, dielectric mirrors are superior
to metallic reflectors. In optical instruments such as fibre-optical endoscopes
and in illumination for microscopy, CLR protect sensitive tissue from heat
during inspection.
Usually plates or parabolic reflectors made of heat-resistant and temperature-change-resistant glass (borosilicate glass), with diameters ranging from
a few cm to more than 30 cm, are used. Si0 2 -ZnS systems are widely applied;
hard CLR systems are used where long-term stability and reliability have
priority over cost issues. With the introduction of the halogen lamp with a
50-mm-diameter reflector for business and private application, the cold-light
mirror has become a 100-million-pieces/year product with a market that is
still growing. The development of halogen lamps rated for grid voltage (llOV
in the US and 230 V in Europe) instead of 12 V bulbs requires reflectors with
diameters of up to 130 mm. New technologies such as metal-halide discharge
bulbs allow one to produce smaller lamps with higher power density: The
demand for hard CLR is growing. The cold-light mirror will continue to be
a key element in future illumination systems.
6.4.3 Processes
Usually CLR are produced by electron-beam evaporation in high vacuum.
Vacuum chambers up to 3 m in diameter are used to increase the throughput. A typical 1.6 m vessel produces 270 reflectors with 50 mm diameter
in 105 min [6.58J; modern equipment with vessels of 2.5 m diameter achieve
6.4 Cold-Light Reflectors
347
throughputs of 600 pieces per hour. Special techniques such as ion-assisted
deposition have been developed to increase the layer density and the stability [6.59] . Sputtering techniques for the deposition of CLR have been proposed, but have not been successfully commercialized [6.60] . In 1993, AuerSOG introduced the plasma impulse CYD (PICYD) process, developed at
Schott for the production of 50 mm cold-light reflectors for halogen lamps
(for details on the PICYD process see Sect. 5.3). Advantages of the PICYD
process such as
• simplicity (no high vacuum required),
• stability ("digital" deposition overcomes in-line process control), and
• high deposition rate of several 100 nm/min for oxides
result in hard-coated CLR with a superior lifetime of up to 5000 h and excellent heat stability (up to 450°C without any changes in the reflection spectrum) compared to conventional Si0 2-ZnS systems - at the same production
cost!
Figure 6.39 is the view looking into the PICYD coater in an opened position. All the reflector substrates sit on their individual vacuum sealings, thus
Fig. 6.39. View into the opened multistage PICVD coater for cold-light reflectors
348
6. Products
forming the mini-recipients. A combined vacuum, exhaust and gas supply system is connected from the bottom. The microwave antennae at the top are
moved down over the reflectors in the deposition phase of the process. One
complete process cycle requires several minutes. Each station can be individually adjusted to optimized process parameters as determined in single-station
laboratory or pilot equipment, which are then computer-controlled. The yield
of the coating and the up-time of the equipment are both excellent.
6.4.4 Glass-Ceramic Reflectors with SCHOTT PI Coating®
Thomas Kupper, Christoph Moelle, Lars Bewig
The Schott market segment Digital Projection requires reflectors with high
optical performance and durability. This market has been served by Schott
Auer for years, with strongly increasing production volumes. Driven by customer demands, the technical progress in this area is characterized by minimization of the digital projector units combined with increased lumen output.
Therefore heat management has become one of the most important developmental tasks.
In earlier projectors, UHP (ultra-high pressure) arc lamps with 120 W in
95 mm reflectors were operated; today, arc lamps with up to 270 Ware operated in reflectors with diameters of less than 65 mm. The Schott borosilicate
glass SUPRAX® 8488, which is used for glass reflectors, enables high operation temperatures. Nevertheless, for small-sized reflectors with high wattages
the thermal load becomes so high that the temperature limits are approached.
Problems are frozen tensions, caused by thermal expansion, and crack formation or even softening of the glass, caused by fast heating and cooling.
The solution to all these problems would be a low-expansion material
with high melting point and high glass transition temperature. Consequently,
Schott started developing reflectors made of glass ceramics, a zero-expansion
material.
To optimize the heat management, further specific features are required.
Figure 6.40, for instance, shows reflectors with different grooves for fan cooling.
The typical geometry of UHP arc lamps requires very deep reflectors (see
Fig. 6.41). As a result, the aspect angles are very large for projective coating
methods such as physical vapor deposition, and the coating quality on the
steep flanges is therefore rather poor. For this type of reflectors, SCHOTT
PI Coating® is particularly advantageous. The coatings, deposited in a CVD
process, are highly durable and brilliant.
6.4 Cold-Light Reflectors
349
Fig. 6.40. Reflectors with grooves for fan cooling
Implications of Material Properties on SCHOTT PI Coating®
In this microwave-driven CVD process the reflector serves as:
1. part of the vacuum chamber,
2. microwave window, and , of course, as
3. substrate for the deposition process.
Concerning item 2, it is important to know the dielectric values of glass
ceramics at 2.45 GHz, the microwave frequency of the coating process. In
initial tests, well-established Schott glass ceramic materials (Robax® and
Ceran®) have been compared with borosilicate glass and quartz. Temperature measurements showed higher microwave absorption in the glass ceramic
materials. This effect appeared to be even stronger in the final glass ceramic
composition especially developed for reflector applications. The objectives of
our materials development were high infrared transmission, good hot-forming
properties, and compatibility with existing patents.
The adjustment of the SCHOTT PI Coating® process to this new substrate material was focused on carefully balancing the microwave power required for the deposition and heating of the material due to material absorption. The dependence of material absorption on temperature was found to be
nonlinear. Because microwave absorption is ruled by the phase composition
350
6. Products
Fig. 6.41. Reflectors for UHP arc lamps
of the glass ceramic material, the ceramization process has to be reproducible
and well controlled for time and temperature.
The fully cerami zed material, which is a high-quartz solid-solution glass
ceramic, shows the same microwave absorption behaviour as borosilicate glass
or unceramized material, the so-called green glass (Fig. 6.42).
Referring to item 3, the reflectors need to have perfect surface quality in
the reflection area. In search of a new substrate material, early investigations
also considered the use of sintering processes. With grain diameters of about
50 I.l.m, the quality yielded by this technology is insufficient for optical surfaces
without additional polishing.
Another development approach addresses the thermal mismatch, i.e. the
difference between the thermal expansions of the substrate and the deposition
materials. Because coated reflectors are subject to enormous temperature
variations, the matching of thermophysical properties is important for the
control of internal stress. Especially tensile stress above a certain limit generates cracks in the multilayer coating. Figure 6.43 shows the thermal expansion
coefficients for three different substrate/layer combinations. Si0 2 and Ti02,
the well-established coating materials of the SCHOTT PI Coating® process,
have widely different thermal expansion coefficients. The thermal stress gen-
6.4 Cold-Light Reflectors
351
90
Microwave
.l!l 80
·c
::l
70
~
60
-e
c
--0--- absorption
0
'E.. 50
0
If)
.0
40
co
(J.)
> 30
co
~
eu
~
20
10
0
Glass
Glass ceramics
Substrate material
h-quartz s.s.
glass ceramics
Fig. 6.42. Microwave absorption of glass, glass ceramics, and high-quartz solidsolution glass ceramics
"I
100
::.::: 90
l()
I
0
~
0
0
80
C
70
(J.)
·0
ti=
60
(J.)
0
u
c
Low-index material
o High-index material
t;, Substrate
¢
50
0
·iii
40
c
co
a. 30
x
(J.)
ro 20
E
(J.)
.c
I-
10
0
0
8488/Si02lTi02
0
Glass ceramicl
Glass ceramicl
Si02/Nb20S
Si02/Ti02
Material combination
Fig. 6.43. Thermal expansion coefficients for three different substrate/layer combinations
erated by this difference is reduced by the borosilicate glass substrate, whose
thermal expansion coefficient lies just between those of the deposition materials. In the second column of Fig. 6.43, the combination of glass ceramic
substrate with the above-mentioned coating materials is shown. The thermal
expansion coefficient (a) of Ti O 2 is now 20 times higher than that of the
substrate material or of Si0 2 . Currently we are optimizing the parameters of
the SCHOTT PI Coating® process in order to reduce internal thermal stress
and cracks in the coating. An alternative way for optimization is shown in
352
6. Products
the right column. Here, the high-index material Ti0 2 is replaced by Nb 2 0 5 •
The refractive index of Nb 2 0 5 is similar to that of Ti0 2 (which is important for layer thickness and coating times), whereas its thermal expansion is
comparable to glass ceramics and Si0 2 .
Durability Specifications
Coated reflectors for digital projection must pass the following tests:
•
•
•
•
•
•
•
boiling water test
tape test
operation humidity test
acid and alkaline wash test
oven water test (up to 30 cycles)
long-term high-temperature test, 450°C for 1 h
ultra-high temperature test, 700°C for 10 min
Optical Specifications
For effective separation of light and IR radiation, the coating design has to
guarantee high IR transmission. The IR cut-off line in the spectral emission
from the reflector can be optimized for steepness by optimizing the coating
thickness distribution on the reflector surface from the neck to the rim area
to compensate for varying incident angles of the light. A typical measured
reflectance spectrum is shown in Fig. 6.44.
To achieve high luminous output and excellent color reproduction, the
coating design has to be calculated for highest reflection in the complete
110
100
~
!:..
90
Q)
80
c
70
u
~Q)
-------------------,\
r .. _ .. - .. _ .. _ .. - .. _ .. _ .. _ .. _ .. -"-" _ .. _ .. _ .. _ .. ,
.
.
\
60
Upper limit
~
50
~
40
Spectral reflectance
Lower limit
q::
\
Q)
c. 30
(/)
20
\
\
\
\
\
\
\
\
\
\
\
\
\
'---
10
O~~--r_.__.--.--,_,--,__,~r_.__.~
3504004505005506006507007508008509009501000
Wavelength Inm
Fig. 6.44. Typical reflectance spectrum
6.5 Automotive Rear-View Mirrors
100
90
~
353
-...... _"- . _... -... _......\
~-----------------------------------,
I
I
I
I
I
I
80
lSc: 70
B
I
I
I
I
I
\I
R
G
I
I
~ 60
E
CJ)
c: 50
~ 40
Qi
c:
c:
III
.s:::
(,)
I
I
I
I
I
I
I
I
I
I
I
I
I
I
30
20
10
0
'-
420
460
500
540
580
Wavelength Inm
620
660
Fig. 6.45. Example for spectral performance of blue/green/red separation
visible spectral range from 400 to 750 nm. In digital projectors this spectral
range is subdivided into the RGB (red - green - blue) spectra, which are
treated separately in color management and picture generation (Fig. 6.45).
Coating of Polymer Reflectors with SCHOTT PI Coating®
In order to extend coating competencies to substrate materials other than
glass or glass ceramics, an evaluation of high temperature polymer materials
has been carried out. For applications such as cold-light mirror reflectors,
high IR transmission and high thermal and mechanical stability are required
properties of the substrate. Polymer reflectors are of interest because they
offer more degrees of freedom regarding geometric design (e.g., snap fit functionality), weight and safety in comparison with glass. The SCHOTT PI
Coating® allows very precise control of the substrate temperature and yields
coatings of highest quality, generated by microwave pulses.
6.5 Automotive Rear-View Mirrors
Falko v. Unger
Automotive mirrors have been an industrial safety product per regulation for
a long time. World-wide about 50 million cars are produced annually, most of
which are equipped with three mirrors. The regulation 71/127/EWG or ECE
R46 specifies all the significant properties for the 15 million cars produced in
Europe, including the 5 million cars produced in Germany.
354
6. Products
6.5.1 Specifications for Automotive Mirrors
Automotive mirrors provide an indirect view, i.e., they assist the driver's orientation in traffic. The traffic behind can be properly observed with exterior
and interior rear-view mirrors only if the panes and mirrors combine to give
images of sufficient size and brightness. If the rear window of a private car,
for instance, is very dark, even a highly reflective interior rear-view mirror
cannot produce a bright image in the driver's eyes. The specifications for
automotive mirrors are therefore coordinated with the other motorcar parts.
An automotive mirror consists essentially of the mirror glass and the housing
- today usually made of plastic - and of pivoted and hinged parts, heating,
mounting, etc. We will describe the mirror glass, which is responsible for the
optical quality.
Private cars and lorries must have indirect visual fields which can be taken
in with both eyes (binocular) by a standard-sized person in standard position.
For the left wing mirror of private cars today a visual field of ~ 2.50 m at
10 m distance from the driver's eye is demanded. Because almost all the wing
mirrors of private cars are fixed to the door, sizes of about 150 x 100 mm 2
result for flat mirrors. Car designers, who try to optimize the drag coefficients
(cw values), often use spherical-domed mirror glasses, which admit of smaller
dimensioning because they, being convex mirrors, scale down the image; or the
mirror size is kept constant and the visual field is enlarged via the convexity
of the glass. Radii of convexity below 1200 m are not permitted for private
cars - except for aspherical mirrors, which have an enhanced convexity in the
exterior part and thus reduce the blind angle (Fig. 6.46).
Besides size and convexity, the reflectance of mirror glasses is additionally
specified. To match the spectral sensitivity of the eye (visible), the standard
illuminant A must be reflected with PYA ~ 40%. This requirement is met by
metal oxide mirrors with PVA :::::: 45-55%, chrome mirrors with PVA "" 55% or
65%, and by silver-coated mirrors with PYA:::::: 90%. These values are derived
from the spectral reflectance (Fig. 6.47) for the human eye.
Flat mirror
Spherical mirror
Aspherical mirror
Fig. 6.46. Field of view of different wing mirrors (bird's-eye view)
6.5 Automotive Rear-View Mirrors
355
100
80
CD
0
c:
~
60
I--+--f----f----I-----+------j
~17~:::::~:::---_+---_I_--__l
CD
q::
~
~
1:5
CD
c.
en
Silvered mirror
(second surface)
Front surface chrome
Second surface chrome
40
Second surface
" N - - - - ; blue mirror
20
Front surface
interference mirror
400
500
600
700
800
Wavelength Inm
~------~vr---------J
UV
Visible
Fig. 6.47. Spectral reflectance of various rear-view mirrors
Other metals such as aluminium or nickel are not applied on account
of technical difficulties in production or because of their insufficient wear
resistance. At night, when the spectral sensitivity of the eye is shifted and
the pupils are widely dilated, the driver is often dazzled via the windscreen
or via all mirrors. Mirror reflectances of about 50% ensure good visibility
during day and night. Blue mirrors, as used by several German car producers,
guarantee a better contrast and are also preferred for physiological reasons.
The best optical image is provided by front surface mirrors because only
one mirror surface reflects one single image. Second surface mirrors such as
the silver mirror, in contrast, produce two images: The first is generated
by the material interface air/glass, and the second by the material interface
glass/silver. Even though at perpendicular reflection the intensity ratio of
the glass surface relative to the silver surface with 4:90 is less distinct, the
double reflection still is very annoying. If there are drops of water or dirt on
the surface, the flaws are doubled because of the inclination of the mirror and
thus the driver is doubly troubled. On account of its better optical properties,
the front surface mirror would have been fully accepted a long time ago, were
it not for a small difference in vulnerability: neither the chrome mirror nor
the blue metal oxide mirror according to Schott patent reach the hardness of
glass.
Just a brief remark on UV light, which can excite fluorescence in the eye,
whereby contrast (and focus) are reduced. If this is made the criteria for
the given mirror coatings, the mirror which reflects no UV light is always
superior.
356
6. Products
Automotive mirrors must also meet physico-chemical requirements. DIN
tests, for example CASS 50021, and heat tests specify the lower limiting
values for corrosion resistance.
6.5.2 Manufacturing Process
Today, automotive mirrors are produced by three different processes:
• chemical deposition for silvered mirrors,
• sputtering process for, e.g., chrome mirrors,
• sol-gel process for metal oxide mirrors.
For the production of blue second surface mirrors, often two of the above
processes are combined.
Chemical Deposition
This process is used for the production of silvered mirrors. Today, plane mirrors with float dimensions of about 3 x 6 m 2 can be silver-coated. If convex
sheet glass is to be silver-coated, raw glass of 530 mm in diameter or about
600 x 600 mm 2 in size has to be produced beforehand by a hot forming operation at about 700 ac. The resulting initial products, the so-called calottes,
then can be silver-coated. Sheet glass coating, in particular, is carried out in
a horizontal automatic process: After the wet cleaning of the glass surface first with brushes and washing agents, then with demineralized water - there
follows the activation of the glass surface in a wet process with a tin-chloride
solution and a rinsing with demineralized water. By spraying an ammoniacal
silver-nitrate solution and by using a reductive (e.g., dextrose), silver is deposited at the glass surface with up to 10 m feed per minute and forms a thin
smooth layer, which is reinforced by a layer of copper that is wet chemically
deposited from blue copper sulphate and zinc particles.
After drying and twofold lacquering, an infrared drying process is carried
out. For environmental reasons and because of their insufficient corrosion
resistance, silver coatings have become increasingly unpopular with the motor
industry.
Sputtering Process for Chrome Mirrors
Today, chrome mirrors with about 40-nm-thick chrome coatings are massproduced in magnetron sputtering equipment. Plane glass of up to rv 3 x 6 m 2
in size is coated in vertical position in a continuous operation. For curved
calottes horizontal coating equipment is generally used; they pass on the
substrate under one or more cathodes at a speed of more than 2 m/min. The
layer thickness is controlled via transmission measurement; at a transmission
TVA ~ 2% the highest reflection is reached. The sputtering process is preceded
6.5 Automotive Rear-View Mirrors
357
by an extensive cleaning and drying process; after rinsing with super-clean
water and infrared drying, the glass is finally ready for coating.
Sputtered chrome coatings show remarkably high reflectances: Front surface mirrors reach a reflectance of PYA = 68%, second surface mirrors
PYA = 58%. Sputtered metal coatings are very dense, have a good adhesion,
and are sufficiently scratch-resistant. These qualities are due to the high energy of the sputtered atoms. Backside chrome coatings are often additionally
protected against scratching and corrosion in humid environments by a coat
of lacquer. From the environmentalist's point of view the coating process as
well as the product and its waste disposal are virtually free of problems.
Sol-Gel Process for Metal-Oxide Mirrors
Interference mirrors have several metal-oxide layers of alternating refractive
index with thicknesses in the AI4 and A/2 range. World-wide known is the
interference mirror according to Schott patent with three surface layers of
high-refractive and low-refractive oxides and a blue, contrast-enhancing reflecting colour:
Ti0 2 rv 50nm ~ AI4 with a refractive index of n = 2.1,
Si0 2 rv 80 nm ~ AI4 with a refractive index of n = 1.45,
Ti0 2 rv 50 nm ~ A/ 4 with a refractive index of n = 2.1.
The single layers are obtained by means of the sol-gel process in a dipping
and slow drawing process from an alcoholic solution and are strengthened by
burn-in. The three-layer system is deposited on large plane glass panes; it
can be subsequently bent if convex mirrors are to be produced. The double
coating produced in dipping is destroyed at one side by an enamelling so
that an optically pure front sided reflection results. Other blue layer systems
include, for example, Ti02 and Sn02 with chrome on glass.
Two different types of backside interference mirrors with likewise blue
reflection are in use:
• glass
ing),
• glass
+ Si0 2/Ti0 2 + Si0 2 + Ti0 2 + Cr + lacquer
+ Cr + Sn02 + Ag + Cu + lacquer
(dipping and sputter-
(sputtering).
Whether the future belongs to the costlier blue mirrors is up to the customers
and the buyers.
Further Developments
In reviewing the changes that exterior mirrors have undergone in the course
of the years, the increasing number of functions becomes evident: from the
simple, of course adjustable, reflection device of silver, the development progressed to reduction of dazzling, adjustability from inside (at the beginning
mechanical, then electrical), heat ability, folding-down mechanism, and finally
358
6. Products
Reflector
-tir-__'-- Electrochromic
solution
Transparent = :1:===1+-- 1-1
Glass
ITO layer
with reflector
1.2 V
Front glass
Fig. 6.48. Electrochromic mirror system
to the automatic regulation of reflection via sensors, which for inside and outside mirrors is sometimes already offered as standard equipment; the product
name is EC (electro-chromatic) mirror. Upon sudden dazzling from behind
via the rear-view mirrors, the reflectance is reduced from "-' 70% to about
10% within 3- 6 s. An agreeable effect! The market has been captured by an
American mirror type (Fig. 6.48). It consists of two glasses with transparent
electrically conductive coatings - so-called ITO (indium- tin oxide) coatings and an EC solution in between, which can be darkened by a voltage of "-' 1.2 V.
At the backside of the second plate there is a conventional silver coating as a
reflector. More advanced mirrors of this type now include a metallic reflector,
for example made of Rh/Cr, on the inside of the back glass.
Up to now, neither the LCD mirrors nor the video systems with the
displays installed in the dashboard have succeeded.
Exterior mirrors increase the air resistance of a car and hence the fuel
consumption! Therefore a mirror system consisting of two prisms or Fresnel
lenses and a mirror in the car door attracted attention for being aerodynamically much more favourable.
The opportunities to sell coated sheet glass for automotive mirrors are
still good, provided that the supplier is able to cope with cost pressure or
can offer a high-tech product.
General Reading
H. Anders: Dunne Schichten (Wissenschaftliche Verlagsgesellschaft, Stuttgart
1965)
G. Schottner, G. Abersfelder: Fahrzeugverglasung (Expert, Renningen 1995)
6.6 Large Area Sol-Gel Dip Coatings
359
6.6 Large Area Sol-Gel Dip Coatings
Eckart K. Hussmann
Today the majority of coated products are manufactured by physical vapour
deposition processes (PVD) such as evaporation, sputtering, or ion-assisted
techniques. Chemical vapour deposition (CVD) methods have gained increasing importance in the last decade and led to new applications. Apart from the
"mainstream" techniques PVD and CVD, Schott has developed the sol-gel
coating technique and extended it especially to oxidic multilayer coatings on
very large panes.
6.6.1 Historical Background
For more than 50 years Schott has been developing techniques to deposit metallic and dielectric thin films. Many different methods were investigated. Besides vacuum evaporation [6.61] and chemical vapour deposition techniques [6.62] especially sol-gel dip coatings were systematically examined. This research was carried out mainly by Geffcken and
Schroder [6.62-67] and was further extended by Dislich [6.68,69], Arfsten [6.70] and Hussmann [6.71]. Geffcken also invented the three-layer antireflective systems [6.72]. His important contributions to the theory and
design of interference layer systems are extensively reviewed in Sect. 5.1 of
this volume.
In 1953 Schott started to sell dip-coated interference layer systems for
many different applications. Table 6.3 lists the most important products.
Table 6.3. Sol-gel dip-coated products by Schott
1959
Automotive rear view mirrors
1964
Antireflective coatings Mirogard®
Deutsche Spezialglas AG,
Griinenplan
Deutsche Spezialglas AG,
Griinenplan
Schott, Mainz
Schott, Mainz and
Deutsche Spezialglas AG,
Griinenplan
Schott, Mainz
Sunshielding coatings Calorex®
Antireflective coatings Conturan®
(CEF: contrast enhancement filters
for cathode ray tubes)
Antireflective coatings Amiran®
1987
(shop windows 3.08 x 3.75m2 )
Antireflective coatings Conturan®
1988
Schott, Mainz
(CEF electrically conductive)
1987
Dichroic filters on large panes
(3.08 x 3.75 m 2 ) for architectural purposes Schott, Mainz
1965-1992 Multilayer interference filters: cold mirrors, Schott, Mainz
heat mirrors, beam splitters,
dichroic filters, etc. (up to 1 m 2 )
1969
1985
360
6. Products
6.6.2 Sol-Gel Dip Coating Process
In Sect. 3.3 the sol~gel process ~ especially the chemistry ~ is described in detail. Therefore this section will give only a brief overview of the features most
relevant to the production of coatings, with special emphasis on multilayer
coatings on large panes.
Coating Solutions
The coating solution contains compounds from the elements that later form
the transparent oxide film. These organometallic compounds should readily
dissolve in an adequate solvent and build polymolecules. When the solvent
evaporates, the compounds should form gel-like films on the substrate with
no tendency to crystallize. The gel film is transformed by drying and heating
into a solid homogeneous transparent oxide film. High bonding strength to
the substrate is achieved. No cracks or turbidities are allowed. The solution
should have a small contact angle to the substrate to ensure good wettability.
Furthermore, long-time stability of the solution is required. There are many
elements forming transparent oxide layers, for example AI, In, Si, Ti, Zr, Sn,
Pb, Ta, Cr, Fe, Ni, Co and other rare-earth elements. Mixed oxides such as
Ti02~Si02 can be produced. Any refractive index in the range 1.45 < n <
2.30 can thus be realized. With amorphous and crystalline brookite and rutile,
the composition of the glass substrate and the duration of baking are highly
influential. By incorporating metal colloids into Si0 2 or Ti0 2 matrices, the
films can exhibit a wide range of absorption. The properties of these oxide
films can be influenced by the composition of the atmosphere during baking.
Baking in air or in a reducing gas atmosphere can lead to different oxidation
stages. Silicon dioxide, zirconium dioxide, and thorium dioxide can be used
for interference filters in the UV region because they are transparent there.
Thorium oxide, being radioactive, is no longer used at Schott.
Coating Procedure
Sol~gel coatings can be applied by spinning, lowering and dipping. For architectural coatings only the dipping technique is appropriate. It is the most
economical method for large substrates.
The liquid film, adhering to the surface of the substrate, is lifted up with
the substrate, runs down and partly flows back into the cuvette. At the
same time the solvent evaporates in this "flow zone" . Finally these competing
influences reach an equilibrium, and the film takes on a constant thickness
along the lifting direction.
The thickness of the layers is therefore mainly a function of the lifting
speed and the viscosity of the solution. Because the viscosity changes with
temperature, one has to keep the temperature constant. The thickness of the
substrate and the angle of inclination between the substrate and the surface
6.6 Large Area Sol-Gel Dip Coatings
361
of the liquid both influence the thickness of the layers to a minor degree. It
should be mentioned that the edges of dip-coated plates show small irregular
zones of 5-10 mm in width, especially at the lower edge. These "trouble zones"
have to be cut off.
After coating, the substrates are heated up to 100-180°C. Most coatings
are then sufficiently stabilized so that later on the final firing can be performed
in larger batches in an intermittently heated electric furnace at 400-500°C.
The bonding strength between different coating materials is usually of the
same order as that between layers and glass substrates, and in some cases
it is even considerably higher. Multilayer interference coatings are therefore
easily producible by the dip coating process. The various oxides diffuse into
each other at imperceptible rates. The interfaces between the layers remain
sharply defined. The statistical surface roughness of a film is not carried over
to the surface of successive layers.
Peculiarities of the Dip Coating Process
Both sides of a pane are coated simultaneously by dipping. It is even easier
to coat both sides of a plate instead of only one side. The vacuum methods
coat one side preferentially. Recently a method has been developed to coat
large panes on one side only.
Another advantage is the good homogeneity of the dip-coated layers.
There is no great difference between coating small or large panes. The dip
coating process is very well suited for applying interference layer systems
onto the inside and outside of tubes of various diameters and shapes. Here
the vacuum process requires considerable effort. Dip coatings can be hard and
scratch-resistant and very resistant to environmental influences. Normally the
dip coating process is confined to oxide layers. Metal films, for example, are
difficult to produce. Thickness and refractive index cannot be monitored directly. These properties can be measured only after baking the dip-coated
layer. Vacuum processes are favoured here.
6.6.3 Dip Coating Facility
At Schott the sol-gel coatings are produced mainly by the dipping procedure.
Figure 6.49 shows a scheme of a sol-gel dip coating facility. Schott operates
many different facilities with sizes from 30 x 50 cm 2 up to 3.21 x 3.75 m 2 for
architectural coatings. Depending on the size and type of glass (float glass,
optical glasses, borosilicate glasses), the washing process varies. Small substrates are usually cleaned in ultrasonic washing facilities. Large substrates
are washed under rotating brushes with cleaning agents. The coatings are
heated up to 400-500 °C to convert them into transparent oxides. Both continuous ovens and batch ovens are employed.
In Fig. 6.50 the sequence of the different production steps is shown. For
multilayer coatings the sequence has to be repeated as necessary. Usually
362
6. Products
Control station
Oven (400-500 0c)
-200°C
Fig. 6.49. Scheme of a sol-gel dip coating facility
Dip
coater
E
E
E
Fig. 6.50. Production steps in dip coating
each layer has to be burned in separately. Some types of coating can be mildly
dried at about 200 ce before the next layer is applied and then both can be
burned in together at 400-500 ce. This considerably reduces the production
time because the firing is the most time-consuming step of the sequence given
in Fig. 6.50.
The oven of the production facility for very large panes (3.21 x 3.75m2 )
consists of six independently controlled chambers. Between adjoining chambers are doors. The panes move from chamber to chamber, staying in each
one for several minutes. Three chambers are used to heat up the pane to the
required temperature. In the remaining three chambers the panes are cooled
down; this procedure is necessary to avoid breakage by thermal stresses.
6.6 Large Area Sol-Gel Dip Coatings
363
6.6.4 Influence of Process Parameters on Properties
of the Coatings
Spectral reflectance, transmittance and absorptance depend on the properties
of the individual layers of a multilayer system, i.e. on thickness, refractive
index, absorption and scattering characteristics. They have to be maintained
to within small tolerances in order to meet the required optical specifications.
For the complete system the following properties are significant: mechanical
strength, chemical stability, coating defects (pinholes, voids) and internal
stresses.
The sol-gel dip coating process does not allow direct monitoring of the
optical properties when the coating emerges from the solution. These properties can be judged only after baking the layer. Modern vacuum coating
facilities are different; they employ optical measuring systems. Therefore a
reliable production of optical multilayer interference coatings is only possible
if all the parameters that influence the properties of the dip-coated layers are
kept constant. This will be discussed in detail.
Washing Procedure
Depending on the type of glass (float glass, optical glass or borosilicate glasses
such as Pyrex® or Tempax®), the washing procedure has to be adapted. For
example, float glass sometimes has small tin particles on the surface situated
opposite to the tin bath of the float process. When a dip coating is applied
onto this side, these tin particles often prevent an undisturbed coating. The
liquid film breaks up and a circular uncoated zone appears around the tin
particle. Therefore a prewashing step has to be introduced for float glass in
order to remove the tin particles. During some washing procedures, sodium
ions may be leached out of the glass surface. Since during the firing process a
chemical reaction takes place between the first coating on the substrate and
the glass, all surfaces must be identically prepared. To provide surfaces of
constant "quality", the washing procedure must be stabilized.
Drying
The washed panes can be dried either by infrared heating or by blowing off
the remaining water. The accumulation of dust on the freshly cleaned surfaces
must definitely be prevented. Sometimes it is even necessary to free the dried
surfaces from static electricity.
Dipping Solution
A good dipping solution is one that provides the best compromise between
lifetime, concentration, wettability, required optical and "durability" parameters and costs. Large panes require large volumes of dipping solution (2000 L).
364
6. Products
It is too costly to dispose of such large quantities every week. The average
lifetime should exceed one month. The concentration should be chosen in such
a way that high lifting speeds bring about the required film thickness. This
speeds up the production. The viscosity of the solution is one of the major
factors governing the lifting speed. Because the viscosity strongly depends
on the temperature of the solution, it is mandatory to keep the temperature constant. In normal production some solution has to be replaced. Each
coating event consumes a certain amount of solution, depending on the size
of the plate. In addition, the solvent of the solution partly evaporates and
has to be replaced. Most dipping solutions tend to react with water from the
surrounding atmosphere. This too has to be monitored closely. Each solution
can only withstand a certain water content.
Atmosphere in the Drawing Chamber
The air in the drawing chamber should be constantly exchanged to remove the
evaporating solvents of the emerging coating. The humidity and the temperature of the air must be maintained at exactly the required values. Depending
on the type of solution, 4-15 gjm 3 absolute humidity are required. Laminar
air flow is essential for uniform, cloudless coatings. Only filtered dust-free air
can be used.
Drawing Machine
The mechanism used to lift the panes out of the solution has to provide and
maintain the required lifting speed without any vibrations or oscillations.
The lifting speed is in the range of 2-15 mmj s.
Substrate
The cleaned substrate should have the same temperature as the dipping solution. In this way turbulences or temperature changes in the solution are
avoided. After they have rested for a while, the panes are drawn out of the
solution. Air bubbles in the solution that stick to the substrate surfaces have
to be removed. Some glasses (e.g., float glass) have surfaces with slightly
differing properties that in turn may cause small differences in the optical
properties of the coatings directly adjacent to the two substrate surfaces.
Parameters Influencing the Coating Thickness
Besides the refractive index, the thickness is the most important attribute of
a coating. Therefore all the relevant parameters are summarized once more:
• concentration, viscosity and temperature of the solution,
• lifting speed,
6.6 Large Area Sol-Gel Dip Coatings
•
•
•
•
365
thickness of the substrate,
humidity of the atmosphere,
angle of inclination between the substrate and the surface of the liquid,
firing process.
Firing Process
Many parameters influence the drying at 200°C and the firing process at
400-500°C. Not only the final temperature is of importance. Chemical reactions between the glass and the first coating involve diffusion processes.
Therefore the rate of temperature increase and the baking time at maximum
temperature strongly influence the properties (thickness, refractive index) of
the coating [6.66]. Another parameter is the composition of the atmosphere
in the oven. Reducing compounds (forming gas) in the atmosphere tend to
generate sub oxides or even metal clusters. The humidity of the atmosphere
can affect the properties of the coatings too, especially during the drying
process at about 200°C.
6.6.5 Accuracy of the Dip Coating Process
As outlined above, many parameters influence the properties, in particular
the optical properties, of a multilayer coating. If these parameters are kept
within appropriate limits, highly consistent multilayer coatings can be produced. Even on large panes the optical properties are uniform over the whole
area. Figure 6.51 shows the refractive index and the thickness of a Ti0 2 coating on a pane of 3.21 x 3.75 m 2 in size, measured at nine positions. The mean
values and standard deviations are:
thickness d m = (65.53 ± 0.69)nm'
refractive index nm = 2.224 ± 0.003.
Another example is shown in Fig. 6.52. The variation of x and y (tristimulus values of the CIE chromaticity diagram) of a dichroic filter illuminated
with a black body radiation source of 3200 K is very small. This result is
valid for different positions on a single plate (60 x 50cm2 ) as well as for
different plates of a larger set (25 panes). The dip coating process is very stable. Changes in the lifting speed to compensate for alterations of the dipping
solution (evaporation) are very small.
Switching from one product to another is easily done. Other thicknesses
and sequences of layers can be realized instantly by an experienced operator
of the dip coating facility.
6.6.6 Solar Control Coatings: Calorex®
The outer covering of larger buildings - windows, roofs, spandrels - can be
made from a huge variety of materials, for example glass, metal, natural
366
6. Products
=
d 64.9 nm
n = 2.226
65.6 nm
2.221
65.1 nm
2.223
65.3 nm
2.225
65.9 nm
2.221
64.6 nm
2.229
66.4 nm
2.221
66.7 nm
2.219
65.3 nm
2.226
E
3.75m
Mean values:
dm
nm
=(65.53 ± 0.69) nm
= 2.224 ± 0.003
Fig. 6.51. Refractive index and thickness of a titanium dioxide coating measured
at nine points on a large pane
stones, etc. Today glass can be used for any part of the hull and some buildings are even glazed completely. Modern energy management of such buildings demands certain properties from the glazings, especially from windows.
Most high-rise buildings are air-conditioned to heat or cool the building and
to provide fresh air. Windows should therefore have good thermal-insulating
properties and should provide an appropriate balance between lighting properties and retention of solar radiation, which can put a heavy load on the
air-conditioning. Figure 6.53 illustrates these properties of a window and its
performance. Most windows consist of two or more panes which are sealed to
one another to form so-called insulating glass units.
The surfaces of the panes are numbered, beginning from the outside. Usually position 1 and/or 2 of a sunshielding glass is coated with thin interference
layers. Part of the incoming solar radiation is either reflected or absorbed
by a properly designed interference layer system. The absorbed radiation is
transformed into heat, which ultimately goes to the outside and inside of
the building, warming it. Not only the coatings, but also the glass itself can
absorb. To improve the thermal-insulating properties, the heat transfer by
radiation exchange between surface 2 and 3 can be strongly reduced if one of
the surfaces 2 or 3 has a low emissivity. Heat transfer by conduction can be
reduced if the insulating glass unit is filled with gases of lower thermal COnductance (e.g., argon, krypton, xenon) than air. The coatings On surface I,
6.6 Large Area Sol-Gel Dip Coatings
367
0.9
520
0.8
0.7
550
505
0.6
500
0.5
~
~
>-
0.4
0.3
0.2
0.1
0.0
0
0.2
0.4
0.6
0.8
x (2°)
Fig. 6.52. Variation of the tristimulus values x and y (CIE chromaticity diagram)
of a dichroic filter
2, or 3 can be chosen in such a way that the different parts of the spectrum of solar radiation are influenced differently. In other words, reflectivity,
transmissivity and absorptivity in the UV region in the visible part and in
the IR region of the spectrum can be different. The German standard DIN
67507, "Light transmittance, radiation transmittance and total energy transmittance of glazings" [6.73], describes these relationships quantitatively. The
European Committee for Standardization CEN is presently preparing a corresponding standard (pr EN 410) [6.74]. Since 1969 Schott has been producing
and selling sunshielding glasses under the trade name of Calorex® (English
trade name: Irox®). Glass panes are coated on both sides with titanium dioxide (usually position 1 and 2 of an insulating glass unit) by the sol-gel dip
coating process. The high refractive index of titanium dioxide results in a
high reflectivity. It is possible to introduce absorption into the titanium dioxide layers by finely dispersed palladium particles [6.75]. Being much smaller
than the wavelengths of light, these particles cause no scattering. The degree
368
6. Products
Luminous and radiometric quantities
T
R
= transmission
= reflection
A = absorption
= secondary heat emission
[towards the inside]
qo = secondary heat emission
qi
T
[towards the outside]
A=A(o) +A(i)
qo
qi
T + R + A =00%
1
Fig. 6.53. Principle of a sunshielding window
of absorption can be chosen arbitrarily. The coatings can be applied to tinted
glasses (grey, bronze, green), yielding a great variety of differently coloured
types. Table 6.4 gives an overview of the different types of Irox® products.
By changing the thickness of the titanium dioxide layer, the spectral position of the maximum of the reflectivity can be changed. The colour of the
reflected light is then altered to bluish (types B) or to golden (type NG).
Figure 6.54 shows for example the transmissivity T (upwards scaled) and the
reflectivity R (downwards scaled) of Irox® type IRA 1. As the wavelength
scale is distorted in such a way that each unit of length corresponds to equal
units of solar energy, the area under the transmittance curve T represents the
solar energy that is transmitted. The spectral distribution of the solar energy
follows CIE 20 [6.76]. Correspondingly, the reflected energy is represented by
the area above the reflectivity curve (R). The absorbed energy is represented
by the shaded area between the R curve and the T curve.
As mentioned above, Irox® oxide coatings do not possess the low emissivity that is necessary for providing the insulating glass unit with the required
good thermal-insulating properties. Sputtered coatings in most cases have interference layer systems with thin semi-transparent metal layers. They have
the required low emissivity. Irox® therefore must be combined with so-called
low-e (low-emissivity) glass. Today these low-e-coated glasses are readily
6.6 Large Area Sol- Gel Dip Coatings
369
Table 6.4. Light transmittance, total heat gain and shading coefficient of different
Irox® types
Irox® type
Light transmittance %
Total heat
gain %
Shading
coefficient
Outer pane Irox® 6 mm
Inner pane float 6 mm
AO
Al
BZ
Bl
55
42
57
45
57
50
63
55
0.66
0.57
0.73
0.63
Outer pane Irox® 6 mm
Inner pane low-e glass* 6 mm
AO
Al
BZ
B 1
45
34
49
38
41
32
44
36
0.47
0.37
0.51
0.41
* These figures refer to a typical, silver-coated neutral low-emissivity glass.
Total heat gain: direct transmittance of solar radiation and secondary heat transfer.
Shading coefficient: compared to a single 3-mm-thick float glass (= 1)
available at low costs. The sol-gel coatings of Irox® are hard, scratch-resistant
and very resistant to mechanical and environmental influences. These properties are mandatory because one layer is situated in position 1. In contrast, sputtered layer systems containing metal layers can be situated only
in position 2 or 3 inside the insulating glass unit. Such coatings are not stable with respect to mechanical and environmental influences. Irox® with its
coatings in position 1 has a unique brilliance, demonstrated in numerous
100
90
80
::$!
70
c:
0
·iii
60
~
·E 50
tJ")
c: 40
--
"'"
tJ")
~
f-
20
0
---
)
0
~v
f-
10
20
30
40 ~c:
0
,.--
l.--/
50 ti
Q)
60 ~
a::
70
80
I
0.4
I.
ectlon
Absorption
/
30
10
--
Re
fI
Transmission -
I
0.5
0 .6
0.7
Wavelength
0 .8
1 .0
I
1.2 1.5 2.0
90
100
I~m
Fig. 6.54. Reflectivity R , transmissivity T of Irox® A 1. The wavelength scale is
"energy equalized" for solar radiation
370
6. Products
buildings [6.77-79]. Its neutral-coloured reflection was extended to spandrels,
giving architects the opportunity to design buildings with all-glass exteriors.
lrox® can be fully tempered, heat-strengthened, and even bent; all these
processes can be carried out after coating without affecting the integrity or
stability of the coating. These advantages make Irox® the ideal choice for the
production of spandrels and curtain walls.
Moreover, Irox® is particularly well suited for structural glazings [6.80].
The reflective coating on each face hides the supporting substructure. The
coatings are fully compatible with silicone material and as the coating must
not be removed for double glazed units it is an ideal glass for this purpose.
6.6.7 Antireflection Coatings
Boundaries between media of different refractive indices (e.g., glass surfaces
adjacent to air) partly reflect the incoming optical radiation. The higher the
difference of the refractive indices, the higher is the proportion of the reflected
part. The quantitative relationship is given by (angle of incidence equals zero)
nl =
n2 =
R=
1.00 (air),
1.52 (glass),
0.0426.
(6.8)
Float glass with a refractive index of n = 1.52 reflects about 4% at each
surface. A single pane, having two surfaces, already reflects about 8%. This
loss is in most cases of minor importance. Much more annoying is the unwanted reflection that obstructs the view through glass panes such as cover
glasses on instruments, pictures and showcases. The reflection of the glass is
often brighter than the light coming from the item exhibited behind the glass.
Single or multiple interference layer systems can reduce these reflections.
Figure 6.55 shows the spectral reflection curves of a two-layer and a fourlayer antireflection coating in the visible region compared with uncoated glass.
The dip coating process is very well suited because the coatings are applied
onto both sides of the glass pane automatically.
Since 1964 Schott has been selling three-layer antireflection coatings under the trade name Mirogard®, produced at the Deutsche Spezialglass AG
(Grunenplan) [6.82]. Today Mirogard® is provided on glass panes of up to
1.22 x 1.77 m 2 in size, with thicknesses of 2-8 mm. Mirogard® is primarily
used for picture framing, especially in museums, and for showcases.
Under the trade name Amiran® [6.83], Schott has been selling antireflection coatings on very large panes (3.09 x 3.75 m 2 ) since 1987. This three-layer
system follows the design invented by Geffcken in 1940 [6.72]. A quarterwave layer (nd = A/4, A = 550nm) of a material with a medium refractive
index is deposited directly on the glass surface and covered with a thick layer
(nd = A/2) of a high-index material. The last layer is again a quarter-wave
layer, but of a low-index material.
6.6 Large Area Sol-Gel Dip Coatings
10
I
8
e... 6
c
------~-----------
0
tlQl
c;::
Ql
0::
4
2
\
o
\
,
400
--------------- -----------
~
'-
-.
I
Two-layer system
_._._._.- Four-layer system
----------- Uncoated glass
~
~
371
/
"'"
.-.-.-.-.-. ~
500
._.-.....LV/
- 600
----/.
/l
/
I
i
./
...
700
780
Wavelength Inm
Fig. 6.55. Spectral distributions of the reflectivity of a tw(}-layer and a four-layer
antireflection coating as compared with the reflectivity of one surface of the pane
The high-index material of Amiran® is titanium dioxide, the low-index
material is silicon dioxide, and the medium-index material is a mixture of
silicon dioxide and titanium dioxide. Sol-gel dip coating displays here one of
its advantages, namely the possibility of generating materials of any refractive
index between n = 1.45 (Si0 2 ) and n = 2.25 (Ti0 2 ) by mixing dip coating
solutions of silicon dioxide and titanium dioxide.
Figure 6.56 shows the spectral reflectance and transmittance of the
Amiran® coating in the visible region. As can be seen, the reflectivity is
reduced to less than 1%. In the "green" region, the spectral curve indicates
10
8
~
e...
c
0
6
:0
Ql
c;::
Ql
0::
4
\
2
o
380
~
480
-
580
Wavelength Inm
V
/
../
680
780
Fig. 6.56. Spectral distribution of the reflectivity and transmissivity of Amiran®
372
6. Products
a slight increase of reflectivity, which has been purposely induced to give
Amiran® a greenish tint instead of an unpleasant purple colouring. Amiran®
is preferentially used for glazing shop windows. The visibility of the goods
on display is dramatically improved. This effect is even more pronounced for
shop windows glazed with insulating glass units (two or more panes). The
showroom behind Amiran® can be illuminated with fewer lamps, thus saving
energy. It should be mentioned that Amiran® has a lower UV transmittance
than uncoated float glass. This is an advantage for shop windows because
many goods on display, for example leather and textiles, bleach under the
influence of UV radiation.
Amiran® is produced on clear, tinted or low-absorption float glass of
3.75 x 3.09 m 2 in size with thicknesses of 4-10 mm. It is available as two-side
coated or one-side coated product. The one-side coated panes are worked up
into laminated safety glass.
There are many other applications of Amiran®, such as glazing in TV
studios or panoramic windows in restaurants which provide an unobstructed
outside view at night.
6.6.8 Dichroic Coatings
Today, the film industry, television, and theatre employ very large lighting
installations with lamps that often lack the required spectral radiation distribution. Specially designed filters, placed in front of such lamps, convert the
radiation into the spectral distribution necessary for the illumination of the
sets. Tinted foils, which are often used, soon perish under the heavy radiation
load.
Dichroic interference filters are a superior alternative. The unwanted part
of the radiation is reflected and the usable radiation passes the filter. Such
dichroic filters were produced by dip coating on very large panes (3.09 x
3.75 m 2 ) for decorative, architectural purposes. Their effect differs widely
from that of stained glass windows. For example, the shining wealth of colours
of stained glass windows in old and modern churches cannot be seen from
the outside, whereas dichroic filters display vivid colours independent of the
perspective. In technical terms, the spectrum is divided into parts that are
reflected and parts that are transmitted. The coloured reflections have a
unique appearance and can be used for decorating large buildings. A broad
range of colours can be realized. Smoothly changing colours across the front
of the building or more spectacular effects of abruptly changing colours can
be produced. For example, an obelisk in front of a multi-storey building in
Berlin has been glazed with dichroic filters ·manufactured by the dip coating
process at Schott [6.84]. The panes are coated on both sides with a sevenlayer system. Titanium dioxide layers and silicon dioxide layers are alternately
coated on top of each other, whereby the first and the last layer consist of
titanium dioxide. In daylight the panes shine yellow-golden; at night they
6.7 Schott Type I Plus® Containers for Pharmaceutical Packaging
373
give off a light blue. The white light of the incandescent lamps inside the
obelisk is transformed into blue.
6.7 Schott Type I Plus® Containers
for Pharmaceutical Packaging
M arlen Walther
Introduction
For decades tubular glass containers have been widely used for packaging
of pharmaceutical products. High quality standards and performance requirements are met; processing and filling procedures are well established
and validated. According to the International Pharmacopoeia, different glass
compositions may be used for different applications. In general, injectable
solutions require the highest standard for packaging because the drug is injected directly into the body, bypassing some of the human safety systems.
Only type I glass containers manufactured from borosilicate glasses may generally be used for parenteral packaging [6.85]. However, even highly resistant
borosilicate glass containers cannot be considered to be completely inert; the
interaction of container surface and content during sterilization and storage
must be taken into account: Leaching of alkali ions (especially sodium) out of
the glass surface may result in an undesired pH-shift in water for injections of
up to 1-2 pH-units [6.86] during storage. Containers that have undergone surface treatment, for example with (NH4hS04, show a significantly decreased
alkaline extraction [6.87], but the long-time stability of these modified surfaces is still a problem. For special applications, even minor traces of leached
aluminium ions may interfere with highly effective protein-based drugs and
thus cause degradation and lessen the effectiveness of the drug.
In contrast, plastic materials show different container/content interactions. Organic extract abIes are the major problem of plastic containers, even
if they are approved for food contact. In addition, the insufficient diffusion
barrier properties of plastics against water vapour, oxygen and carbon dioxide significantly limit the shelf life of drugs packed in plastic containers. Even
larger molecules may diffuse through the plastic container walls (e.g., certain
components of the adhesive material to fix the labels).
To satisfy the future demand for highly inert glass containers for new
drugs, we have developed a coating technology to produce a container with a
very pure and highly stable diffusion blocking layer on its inner surface that
can be sterilized, processed and filled in exactly the same way as standard
vials.
374
6. Products
6.7.1 Selection of Coating Technology
None of the technologies using organic layers meet the requirements of high
temperature depyrogenation, where operating temperatures of 300°C and
more are reached in the sterilizing tunnel.
Inert inorganic materials can fulfil this requirement, but care must be
taken that the layer adhesion is sufficient. Si0 2 from liquid phase [6.88] may
be deposited by using an HSiF 6 solution saturated with Si0 2 . This process
generates stable layers with good barrier properties; however, it includes the
use of hydrofluoric acid, which requires a very high safety standard for fabrication and a technology that completely excludes HF residues in the coating
and in the container. On the other hand, validation of the coating technology as a batch process is difficult and the deposition rates for layers showing
sufficient barrier properties are very low.
In general, a gas phase process (CVD) seems most attractive for coating
vials. This process can coat three-dimensional surfaces with a variety of oxide materials. Thermally activated CVD requires temperatures of more than
800°C [6.89], which may already result in a modification of the glass substrate. With plasma-enhanced CVD (PECVD), the temperature level can be
shifted to values below 200°C, leaving the substrate almost unchanged. In
the large volume batch reactors usually employed for PECVD coating applications, many bottles are coated simultaneously. This results in a precursor
gradient in the coating chamber. High gas flow rates have to be used to
overcome this problem, resulting in a bad utilization of the precursor.
Plasma Impulse CVD Coating Technology
A plasma impulse CVD (PICVD) process has been chosen to coat the Schott
Type I plus® vials. This technology combines the advantages of the gas phase
technology, which allows the coating of three-dimensional geometries, with
the advantages of low-pressure plasma to achieve low temperatures during
coating. Using the pulsed mode of the plasma CVD process, we can achieve
an efficient gas exchange in the vials with uniform layer properties over the
entire coated surface. The process is carried out in a single station, where
every bottle is coated individually. This allows a precise control of the process and minimizes the upscaling problems when transforming the process
to large-scale fabrication. Choosing microwaves as energy source, we achieve
high coating rates of more than 500 nm/min, so that this concept may become
economically interesting. Figure 6.57 represents the coating station schematically: Key elements are the vacuum system, which allows a fast pump down
to the 1 mbar region, the microwave supply, which uses standard components
of the 2.45 GHz technology, and an efficient, computer-controlled gas flow
system to apply the coating. The reactor has been optimized to coat various
sizes of bottles with only minor changes. Additionally, the plasma process,
6.7 Schott Type I Plus® Containers for Pharmaceutical Packaging
Microwavepower
Substrate
375
Microwave
generator
Process
control
1 mbar Vacuum
technology
Gas outlet
Fig. 6.51. Individual PICVD coating station for pharmaceutical vials (schematic)
with its capability to etch materials, allows a final cleaning of the surface
with an oxygen plasma prior to the coating process.
6.7.2 Layer Structure
The layer structure of a coated vial is shown in Fig. 6.58b. The inner surface is
coated by a thin oxide layer, with a typical thickness of 50 to 500 nm. Oxides
of Ti, Si, or Ta are used as coating materials. This layer acts as a diffusion
blocking layer, preventing an ion exchange between the glass and a chemically
sensitive drug inside the vial. The interaction process is schematically shown
in the inset of Fig. 6.58a, which displays the process without a diffusion
barrier, and Fig. 6.58b, where the ion exchange is suppressed by a diffusion
barrier.
6.7.3 Process Control
Process validation of the fabrication of a pharmaceutical product has become
increasingly important in ensuring the highest quality level. A desired 100%
control of the coated vials is impossible because such thin Si0 2 layers are
invisible. But the plasma process itself offers the characteristic optical line
spectrum as in situ process control. In contrast to a batch fabrication process,
where a lot of vials are coated simultaneously, the PICVD process coats
every vial individually, and every fabrication step can be closely monitored.
Figure 6.59 shows the temporal evolution of the optical line spectrum emitted
during a Si0 2 deposition in a PICVD reactor. It is dominated, by the various
oxygen emission lines, mainly in the spectral range 500- 900 nm. At 425 nm the
characteristic line of SiO clearly shows up in the spectrum, resulting from the
376
6. Products
b)
a)
Water and leached
components
Glass
(a) uncoated surface
(b) Si02-coated surface
Fig. 6.58. Scheme of the coating system, displaying the function as a diffusion
blocking layer. (a) Interaction of solution with glass matrix. (b) Prevention of the
interaction by a diffusion blocking layer
deposition mechanism of Si0 2 from the gas phase: The dynamic behaviour of
this line, with a sharp rise at the beginning and a decay after some tenths of
a millisecond, indicates that SiO is formed homogeneously in the gas phase,
diffuses to the substrate and oxidizes to Si0 2 at the surface [6.90j. Thus the
emission line at 425 nm can be taken as an indicator for Si0 2 deposition.
3000
2500
-'='
·iii
2000
~Oxygen
emission lines
<::
.s
EO
I
1500
1000
500
o
216.499
1.2
325.1281
439.8416
555.2898
663.4399
Wavelength Inm
778.756 889.0623
Fig. 6.59. Temporal evolution of the optical emission during pulsed Si02 deposition
in a PICVD reactor
6.7 Schott Type I Plus® Containers for Pharmaceutical Packaging
377
Figure 6.60 shows the principle of the in situ detection of the SiO emission line used for process control: The time dependence of the spectral line
at 425 nm with the typical decay, prior to the end of the microwave pulse,
indicates the deposition mechanism mentioned above. In addition, the figure displays a grey area, which marks the control window chosen to verify
a correct Si0 2 deposition. A monitor circuit detects pulse length and pulse
height outside of this window as errors to ensure proper deposition of every
pulse. By counting the deposition pulses, the layer thickness can be controlled
to within 0.1 nm, so that uncoated or partly coated vials can be sorted out
immediately.
6.7.4 Characterization
Parenteral products should be sterilized in the final container, for example the
vial, if the drug withstands the sterilization process. Sterilization is done by
autoclaving at 121°C for 30 min. This autoclaving procedure is also employed
to test the containers, using prolonged time intervals [6.91] . Only drugs that
cannot withstand this procedure (e.g. , protein drugs) may be filled aseptically
into presterilized vials. The sterilisation of the empty vial is usually done in a
heat tunnel at approximately 300°C for 10 min, to ensure the depyrogenation
of the vial. These two processes impose a major stress on the layer.
To verify the proper function of the coated containers, the Schott Type I
plus® containers are sterilized in a heat tunnel prior to additional test procedures and are finally sterilized in an autoclave. Using the procedure of ISO
4802 Part 2 [6.92], we measured the sodium content of each vial with atomic
absorption spectroscopy (AAS) after an accelerated autoclaving time of 6 h.
Precautions had to be taken to avoid any contamination due to the ultra-low
sodium content of the solution during the filling and measurement procedure.
Table 6.5 displays the results for uncoated and coated bottles.
Detection
window
Light emission
Lower Limit
---
Microwave pulse
Time /a.u.
Fig. 6.60. Temporal evolution of the spectral line in the plasma at 425 nm
378
6. Products
Table 6.5. Na content after autoclaving with water for injection
Time
1h
6h
R6-DlN vial
Si0 2-coated vial
0.8-1.2 ppm
::; 0.01 ppm
2-3 ppm
::; 0.01 ppm
The increased autoclaving time is used to simulate a shelf life of more
than three years. However, this accelerated test strongly depends on the drug
and has to be verified for every different solution in the vials. Figure 6.61
shows a SIMS profile of the coated surface without stress test. The layer
thickness of the sample was 190 nm in this test. As was to be expected,
the layer only contained Si0 2 . The substrate below shows the typical lines
of the different components of borosilicate glass such as AI, Ba, Ca, etc.
The interface between substrate and layer is equally sharp for all elements,
indicating the low substrate temperatures during the deposition of the layer
with no diffusion processes.
Figure 6.62 shows a SIMS profile of various coated vials with different
chemical stress tests. The sodium line, which is only present in the substrate
glass, was chosen as indicator for the location of the layer. Autoclaving with
di-water at 121°C for 6 h or with 0.04 mol hydrochloric acid for 1 h does not
affect the layer; this result is consistent with our Na measurement above. By
using a NaOH solution with pHlO, a distinct etch rate of 40 nmjh autoclaving
can be measured. The etch rates of alkaline solutions on glass are known to
be much higher, which proves the good stability of the layer. These results
demonstrate that the stability of a glass type I is exceeded by far.
1000 ~--------------------------------.
900
S
800
- AI
- Na
- Si
- Ca - Sa
700
~Cl 600
'iii
C/)
500
::!: 400
U)
300
200
100
O ~~~~~~~~~~.-~,,~~
o
50
100
150
200
Analysis depth Inm
250
300
Fig. 6.61. SIMS profile of a vial coated with a 190-nm-thick Si02 layer
6.7 Schott Type I Plus® Containers for Pharmaceutical Packaging
379
300.--------------------------------,
250
Q)
~rg 200
~
~ 150
without test
01
'u;
-
~ 100
1i5
50
di-H20
NaOH
K3P04
o ~~~~~~~~~._~_.~~
o
50
100
150
200
250
300
Analysis depth Inm
Fig. 6.62. Na line of SIMS profiles after different chemical tests
When a 0.4 mol K 3 P0 4 solution is used, the layer is completely removed
after 1 h autoclaving. If injection solutions with such a strong, complex forming behaviour are to be used, the layer stability has to be monitored closely.
6.7.5 Formation of Glass Particles
In some cases the formation of glass particles in drug solutions is observed for
high-pH solutions. However, under severe storage conditions even water for
injection can lead to the formation of glass particles. The formation process
may start at sections of an ampoule or vial that are formed by heat, for
example at the neck or at the bottom [6.93]. During the local heating process
prior to the forming step, volatile components (e.g., alkali borate) tend to
evaporate from the surface. These fumes may redeposit at colder parts of the
container, resulting in a surface that is much less stable than the bulk glass
and may become vulnerable to water attack. The glass particles are easily
observable with a collimated white light source, penetrating the vial from
the side. By turning the vial, the glass particles can be seen as "blinking"
bright spots in the solution. In our study, a classification has been made from
class I (no visible glass particles) up to class VI (numerous large particles)
under illumination with a cold-light source and a magnifying glass. Again, we
compared the coated glass vials with uncoated Fiolax® vials of glass type I.
The results in Table 6.6 indicate a significantly lower glass particle formation
with the Si0 2 diffusion blocking layer. The joint test results of 64 individual
vials are given, where none of the coated vials showed more than class II.
In fact, the Na detection presented earlier suggests no particle formation at
all. The break-down of even small areas of the layers should be immediately
detected as an increased Na line in the AAS signal during analysis for sodium.
380
6. Products
Table 6.6. Glass particle classes for chemical treatment of glass
NaOH(l h/121 0c)
Fiolax® vials
Si02-coated vial
Uncoated vial
I-II
III
I-II
IV
Conclusion
A new pharmaceutical container with a diffusion blocking layer has been
developed that allows packaging of extremely sensitive drugs. The stability
of the layers was proven by accelerated autoclaving tests, which indicated that
a complete blocking of the alkali release from the vials had been achieved.
The PICVD process used for the coating technology is precisely controllable
and can be scaled up to large-scale production without problems.
6.8 Ophthalmic Coatings
Michael Witzany
Introduction
Since 1959 the Carl Zeiss Company has been producing ophthalmic thin-film
coatings. During this period the eyeglass industry has introduced a variety
of new substrate materials, which have repeatedly posed great challenges
to coating technology. One of the biggest problems had to be solved when
plastic substrates were introduced. They, unlike mineral substrates, could not
be handled by drawing on the experience gained from high-precision optics.
Completely new, unique approaches to ophthalmic coating had to be found.
6.8.1 Market Review
The great effort and expense invested in developing these technologies has
proved necessary in hindsight. Today, plastic substrates represent a considerable fraction of the market, a fraction that has continuously increased over
the last seven years - and this in the traditional country of mineral eyeglasses
(see Fig. 6.63). There are, of course, good reasons for this development. Plastic substrates are in some respects superior to mineral substrates, especially
where weight, safety and aesthetics (colouring) are concerned.
Weight. In comparing the weight of two rimmed eyeglasses of identical optical
efficiency (dioptre), the advantages clearly lie with the plastic substrate.
6.8 Ophthalmic Coatings
381
Example:
Punktal® (W 70) - 4.0 dpt 10.5 g
Clarlet® (CR 39) - 4.0 dpt 7.6g.
Safety. With regard to safety, the plastic substrate is again at a definite
advantage. The so-called falling-ball test according to DIN or FDA (USA)
clearly proves the higher resistance to fracture of the plastic eyeglasses.
Aesthetics. With plastic substrates of superior quality, colouring is almost
standard today. Eyeglasses are subject to fashion trends. Therefore eyeglass
manufacturers as a rule offer more than 100 different colourings, which are
produced with water-soluble textile colours in a so-called dipping process.
This method also enables the production of a wide variety of gradual colours
and tricolour colourings.
6.8.2 Layer Systems
In 1904 Dennis Taylor filed a patent, based on the findings of Fraunhofer
(1817), which never gained universal importance [6.94]. In contrast, the
patent filed by Dr. Smakula at Carl Zeiss Jena in 1935 was very successful [6.51]. Originally employed exclusively for the production of high-precision
lenses (which were mainly used for military purposes), the patented method
was also adopted for ophthalmic coatings on mineral substrates in the late
1950s. When plastic substrates were introduced later on, new evaporation
materials had to be found. The following short chronologies (Tables 6.7 and
6.8) offer a rough survey of the development of ophthalmic coatings [6.95,96].
The Carl Zeiss Company now employs more than 60 different production
processes for manufacturing a broad range of products.
mineral multifocal glasses
____ organic multifocal glasses
O +-----.-----.-----.-----,-----r---~
1990
1991
1992
1993
Years
1994
1995
1996
Fig. 6.63. Market trends for silicate and plastic progressive glasses from 1991- 1996
382
6. Products
Table 6.1. Coatings on mineral eyeglasses
1959
1964
1974
Since 1982
Since 1990
1992
1994
1995
MgF2 coating on crown glass
Punktal® ET, single-layer coating
Sun protection coating on crown glass
inhomogeneous films according to Dr. Anders
Umbra Punktal® (35/65/85)
Multilayer coating on crown glass
Punktal Super ET®, four-layer coating
Multilayer coatings on high-index materials
(e.g., BaSF64) six-layer coatings
Optimization of multilayer coatings on all materials
Punktal SL Super ET®, six-layer coatings
New Umbra technique with AR coating
Umbra Gold ET® (mixture: metal/Si02)
Super Filter ET®, eight-layer coatings
Introduction of a hydrophobic coating on all
Super ET® products (plasma polymerization)
highly reflecting sun protection glasses
Punktal SL Cool Blue®
Table 6.8. Coatings on plastic eyeglasses
1974
1983
Since 1986
Since 1991
1992
1996
AR coatings on plastic eyeglasses (CR 39)
Clarlet ET®
Multilayer coatings on plastic eyeglasses (CR 39)
Clarlet Super ET® , seven-layer coating (thick Si0 2 film)
Introduction of hard coatings on CR 39 (dip coating)
Clarlet Hart® combined with all AR coatings
Introduction of a high-index plastic material (n = 1.6)
and adjusted AR coating
Hard coating on plastic bifocal glasses (plasma polymerization)
Optimization of multilayer coatings on plastic eyeglasses
Carat® and Carat Filter®
hard coating + Super ET® /Super Filter ET® + clean coat
(plasma polymerization)
highly reflecting plastic sun protection glasses
Clarlet Cool Blue®
6.8.3 Processes
Hard Coating Processes for Plastic Substrates
Physical Vapour Deposition (PVD) Processes. PVD processes are classical
evaporation processes which have been available for hard coatings since the
early 1970s. Silica (Si0 2 ), which is very hard and abrasion proof, but also
very brittle, is used as evaporation material. The processes of the first gener-
6.8 Ophthalmic Coatings
383
at ion were very problematic (layer delaminations). Silica hard coatings with
acceptable adhesion were not successfully produced until the early 1980s.
Layers with thicknesses from 1.4 to 1.8 J.Ull were deposited. Brittleness continued to be a problem. These processes are experiencing a revival today.
The so-called plasma-assisted evaporation of the silica layer was adopted for
ophthalmic coatings. This technology originated from high-precision optics
where it is useful in keeping constant certain optophysical parameters. The
employment of ion or plasma sources is meant to eliminate a serious disadvantage of silica hard coatings - their extreme brittleness. The success, however,
is limited. In spite of the employment of this technology, cracks quickly develop under heat load. The old disadvantages also remain with regard to
mechanical load (see Fig. 6.64a-d) [6.95,97].
Chemical Vapour Deposition (CVD) Processes. In CVD chemical vapour deposition processes, layers are deposited from the chemical vapour phase.
Plasma polymerization, a method applied by several companies, is one of
them. Unlike classical evaporation, which works in a pressure range of
p ~ 10- 4 hPa, these processes are performed at pressures of 10- 2 hPa <
p < 1 hPa. CVD processes are carried out in a vacuum chamber which is
pumped down to the pressure range of 10- 3 -10- 4 hPa. Then a gas mixture
consisting of a carrier gas (usually argon) and a monomer gas is let in through
a mixing chamber, whereby the pressure rises to 10- 2 -1 hPa. The mixture is
injected into the vacuum chamber via gas showers which ensure a uniform
distribution of the gas. Hexamethyl disiloxane (HMDS) is frequently used
as a monomer gas today. A plasma is ignited between the electrodes, i.e.,
electrical energy is supplied and causes an organic-inorganic hard layer to be
continually deposited in the so-called plasma chamber. The properties of this
layer very much resemble those of lacquer hard coatings. Disadvantages unavoidable with silica hard coatings can be ruled out in this case by choosing
adequate process parameters [6.97].
Wet-Chemical Processes. Such processes have been employed for about ten
years. After cleaning and pretreatment, the plastic eyeglasses are coated with
liquid lacquer (so-called polysiloxane lacquers), which is then cured at temperatures above 100°C [6.95]. The curing process causes a chemically stable
bonding of eyeglass and lacquer that prevents a peeling of the lacquer layer
in practice. The observation of optimum process parameters is required. Lacquer hard coatings have a high abrasion resistance. Unlike silica hard coatings
they are not brittle but visco-elastic. This clearly shows in Fig. 6.64a-d [6.97].
Recently, so-called nanocomposite lacquers have also come into use. Here mineral particles ofthe order of 10-100 nm are added. Two processes, dip coating
and spin coating, are used [6.97,98].
Antireflection Coating Process
Today, antireflection coatings are usually produced by PVD processes [6.95].
High-index and low-index materials are evaporated in succession. If a silica
384
6. Products
a)
b)
Fig. 6.64. Magnified photograph of a scratch on CR 39 with (a) a silica hard
coating (1)/1982, (b) a lacquer hard coating (1)/1982, (c) a silica hard coating and
AR (integrated layer/group 8)/1995, (d) a lacquer hard coating and AR (optimized
process; e.g. , Carat® /group C)/1995
6.8 Ophthalmic Coatings
385
coating is used as the hard layer, the antireflection coating can be deposited
in the same process or in the same facility. Figure 6.65 shows the principle
of an evaporation process by means of an electron-beam gun. As already
mentioned, ion or plasma sources may also be employed in this process. Such
techniques are also applied to wet-chemically produced hard layers and CVD
hard coatings [6.95,97].
Surface Protective Coatings
Recently there has been an increasing tendency to finish the layer stacks of
plastic glasses too by depositing a hydrophobic protective coating, which is a
very thin, optically inactive layer. Such layers may be deposited by dipping,
evaporation or plasma polymerization. At Carl Zeiss, ophthalmic coatings
are carried out in three-chamber coating facilities. In the third chamber the
hydrophobic protective coating is deposited via plasma polymerization (see
Fig. 6.66) [6.97].
In Table 6.9 the basic procedure steps of the above-described technologies
are listed and contrasted with a classical AR coating on CR 39 (group E).
Processes of the S group are also called integrated processes because the silica
layer and the AR layer are produced in the same facility. In the processes
of the C or C' group, in contrast, each single procedural step is optimized
separately [6.97] .
Fig. 6.65. Schematic representation of an evaporation process by
means of an electron-beam gun
386
6. Products
Fig. 6.66. Three-chamber evaporation facility for Carat® (process of group C)
Table 6.9. Today's coating technologies for plastic eyeglasses
Group E
Group S
1 Plastic eyeglass (0) Plastic eyeglass (0)
Group C'
Group C
Plastic eyeglass (0)
Plastic eyeglass (0)
2 -
Si02 hard coating (a) Lacquer hard coating Plasma polymerization
ion-assisted (lAD)
(o/a)
(o/a)
or plasma-assisted
3 AR coating (a)
with hydrophobic
protective layer
AR coating (a)
ion-assisted (lAD)
with hydrophobic
protective layer
o
Optimized AR
coating (a)
with hydrophobic
protective layer
Optimized AR
coating (a)
with hydrophobic
protective layer
= organic material; a = inorganic material; lAD = ion-assisted deposition
6.8.4 Requirements on Ophthalmic Coatings
Optical Requirements
In speaking about the demands placed on ophthalmic coatings, naturally
the optical requirements are of prime concern. "Why apply an ophthalmic
coating?" is the question to be answered.
In practice several disturbing reflections may develop on eyeglasses.
• Backward concave-sided reflections are particularly annoying for car
drivers, especially when driving in twilight or at night.
• Interior or cornea reflections also impair the wearer's view by drastically
reducing the contrast via multiple images, ghost images, etc.
• Front convex-sided reflections indirectly concern the wearers of eyeglasses
by influencing their appearance. Nearly reflection-free glasses support communication by facilitating visual contact.
6.8 Ophthalmic Coatings
387
In summary, ophthalmic coatings, by eliminating disturbing reflections,
provide a better view and more safety and convenience, and have a favourable
aesthetic effect on the wearer's appearance [6.97].
Mechanical Stability
Mechanical stability is usually determined by two different test methods today. The so-called diamond method shows the reaction of the layers to individual injuries and practically characterizes the layer. A thin diamond is
drawn over the coated glass with a defined load and at a defined speed. The
load at which the scratch becomes visible (to the naked eye) is recorded.
Not only the individual injuries of the layer, but a variety of small flaws resulting from use are disturbing in practice. The latter induce light scatter on
the glasses. This phenomenon is examined by means of the Taber scratch test.
A rubber disk, into which corundum grains of defined size and distribution
are embedded, is rotated over the glass in two runs of ten passes each. Then
the light scattering additionally induced by this procedure is determined with
a set-up of optical measuring instruments. This is a very adequate method
to reconstruct practical results [6.99]. Numerous other durability tests exist,
but cannot be dealt with in detail in this context. Some producers apply
scratch tests with steel wool, spikes, or other similarly exotic materials. Such
tests are only meaningful if they in some way or another reflect practical
results. A broad variety of tests developed over more than 30 years of experience with thin ophthalmic coatings are based on comparative analysis of
worn eyeglasses. The meaning of such laboratory tests is often hardly comprehensible to non-experts. Therefore we will draw a comparison based on
the only really meaningful and easily comprehensible "test", the everyday use
of eyeglasses. For several years worn eyeglasses have been analysed. Based on
these practical results, the quality standards reached by the above coating
technologies are compared in Table 6.10. With antireflective eyeglasses made
from mineral glass, complaints concerning the coating are very rare, i.e., they
meet a very high quality standard. Currently, hardly anybody will claim this
to be true for plastic eyeglasses [6.97]. A detailed consideration and analysis
of such figures, however, yields unambiguous and highly interesting results.
Lacquer-hard-coated and antireflective plastic eyeglasses reach very high
quality standards if special lacquers are used and if these lacquers are optimally integrated in the production process. This requires much experience in
technology and production.
All new technologies must endeavour to optimize these high standards.
But how are the widely differing rates of complaint to be explained? We
suppose that the main reason is the sequence of application of the coating
materials. Whenever two materials with the same properties (i.e., inorganicinorganic/MgF 2-crown glass) are joined the results are excellent. This means
that for plastic (organic) eyeglasses a technology must be applied as a "buffer"
to the inorganic AR coating which suits both the organic and the inorganic
388
6. Products
Table 6.10. Comparison of the figures of complaint for different products (as far
as already reasonably determinable) standardized on CR 39 + AR coating
Type of product
1
2
3
4
5
6
Ratio of complaint
Synthetic + AR coating
Synthetic + Si02 silica hard coating + AR coating
(techniques hardly used today)
Synthetic + Si02 silica hard coating + AR coating
both plasma-assisted (S)
Synthetic + lacquer hard coating (conventional)
Synthetic + lacquer hard coating + AR coating
adhesion-optimized (C)
Mineral glass + AR coating
1*
0.6*
< 0.6 (expected)
0.09*
< 0.09 (expected)
0.06*
* This analysis has been performed over comparable time periods from 1981 till
now.
properties. This is done for instance in the production of Carat®, where adjusted lacquer hard coatings are used. They contain organic OH compounds
and inorganic SiO x compounds. Thus an optimum bonding between the plastic eyeglass, the lacquer hard coating and the AR coating is achieved, which
sets the standard for all future technologies [6.97].
6.8.5 Production of Eyeglasses at Carl Zeiss
As is partly known, the production philosophy of the Ophthalmic Division
at Carl Zeiss differs fundamentally from that of its competitors. Almost all
prescription glasses for the European market and the major part of the serial
glasses are coated in the central manufacturing plant in Aalen. More than
50000 glasses are coated each day (see Tables 6.11 and 6.12).
The main advantages of our centralized manufacturing are cost reduction
and central technology service. The argument that decentralization means
better service (because being closer to the customer shortens the time of
delivery) has been largely invalidated by better shipment logistics, and thus
the production of prescription glasses has been concentrated in Aalen. Of the
formerly existing 13 decentralized prescription shops in Europe there remain
only four (London, Paris, Oslo, and Zurich).
Table 6.11. Production figures and equipment
Facility
Pieces per run
Equipment
developed by
18 A 1100
10 ACE
7 B 12
15 B 10
130
64-90
22-36
32-38
Leybold
Carl Zeiss
Carl Zeiss
Carl Zeiss
6.9 IR-Reflecting Multilayer Films for Energy-Efficient Lamps
389
Table 6.12. Shares of AR-coated glasses (Carl Zeiss)
Substrate glasses
Per cent
Plastic eyeglasses
Mineral eyeglasses
Superior-quality plastic eyeglasses (e.g., Clarlet Gradal Top®)
Percentage of AR-coated eyeglasses in total
88
69
93
75
In 1992 another Carl Zeiss manufacturing plant for eyeglasses was built
in Mateszalka, Hungary, producing ophthalmic coatings in 3 B 10, 4 B 6, and
2 B 12 facilities. The B 6 and B 10 facilities, apart from producing single-layer
and two-layer coatings for Carl Zeiss serial products, are mainly employed
for competitors' products, i.e., contract coating is carried out [6.96].
Prospects
The quality of ophthalmic coatings has reached a very high standard today.
Thus future technologies will mainly aim to reduce costs and production
time. The latter is very important in view of the large amount of prescription
coatings to be newly produced each day (over 60% of the orders for plastic
glasses). This means that the technologies of interest are those that enable
short coating times at reduced costs and the integration into our material
handling system (1-2 glasses per run). One technology that may qualify is
the so-called PICVD process developed by Schott. Carl Zeiss and Schott
Glaswerke will cooperate in testing the production capability over the coming
years. The first results are quite promising. The possibility to manufacture
widely different variants of optical or abrasion-resistant coatings with a single
technology will prove to be an advantage.
6.9 IR-Reflecting Multilayer Films
for Energy-Efficient Lamps
Hrabanus Hack, Torsten Holdmann
6.9.1 The Principle
Most of the radiated energy of incandescent or halogen lamps is emitted in the
near-infrared rather than in the visible. This makes these lamps inherently
inefficient.
Numerous improvements in efficiency by means of thin films have been
described in patents and scientific papers over the last four decades. The most
effective approach is an interference heat reflector that comprises at least 30
6. Products
390
layers and transmits nearly 100% of the visible light while reflecting up to
80% of the radiation in the wavelength region between 750 and 1800 nm
back to the filament (see Fig. 6.67). The reflected IR energy is absorbed
by the filament, heats the filament up and in this way reduces the power
consumption of the lamp.
To use the benefit of such a multilayer IR-reflecting system, the lamp
geometry has to be configured so as to return as much IR energy as possible
to the filament after only one reflection. In the ideal case, the yield is maximal
for a point source and a spherical reflector. A real short axial filament requires
an ellipsoid as an reflector. For long filaments (e.g., in floodlight lamps), a
tubular geometry of the bulb is appropriate. In all cases a careful centering
of the filament is critical for maximizing the efficiency gain.
6.9.2 Materials
An interference filter needs two different materials for the multilayer system
with refractive indices as widely different as possible.
Suitable high-index materials and their refractive indices are
2.1-2.7
Ti0 2
2.1
Ta205
Nb 20 5 2.35
1.9
Y2 0 3
2.9-2.15
Zr02
Si02, with a refractive index of 1.45, is the most favourable low-index material.
Further material selection criteria are:
100.---------~~,~,---------------------,
"
r:::
o
80
~
60
~
'6
--filter transmission
"'" ----·coil radiation
'" ,
r:::
o
·00
.!!1 40
,,
,,
""
""""
E
tJl
....................
r:::
~
I-
20
O+---r-~---,---,---r--,---,---r-~~
350
550
750
950 1150 1350 1550 1750 1950 2150
Wavelength Inm
Fig. 6.67. Typical transmission IR filter
6.9 IR-Reflecting Multilayer Films for Energy-Efficient Lamps
•
•
•
•
391
stability and transparency at lamp operating temperatures of up to 800°C,
resistance to oxidation, reduction and moisture,
thermal expansion coefficients matched to the substrate,
high deposition rate and low precursor cost (processing costs).
6.9.3 Deposition Processes
The parameters of the deposition process strongly influence the film properties, which generally differ from the properties of the bulk material. Depending on the processing methods, the refractive index, transparency, microstructure and optical scattering, and adhesion and internal stresses may
vary strongly. One of the most critical factors is the internal film stress. Lifetime and durability of a multilayer system are mainly determined by this
property. To obtain the optimum performance, each layer must be deposited
on the entire substrate surface with a maximum deviation of 5% from the
nominal layer thickness. This precision is necessary to achieve the optimum
efficiency gain and to avoid colour effects in the emitted light.
The deposition of IR-reflecting interference films is done by several significantly different processes. Among those, physical vapour deposition (PVD),
dip coating and chemical vapour deposition (CVD) are extensively described
in the literature [6.100~102].
By using an evaporation process, GE and OCL! successfully developed a
coating with an efficiency gain of about 30%. The interference system consists of Ta205/Si02 layers and is characterized by an excellent temperature
stability [6.103].
6.9.4 IR-Reflecting Coating by PICVD
Plasma impulse chemical vapour deposition (PICVD), which is described in
Sect. 5.3, allows the production of very dense and stable films with a high
precision and a high deposition rate [6.104]. Especially Ti0 2 layers with a
refractive index as high as 2.45 can be prepared by using the cheap TiC14 precursor. So this method is well suited for depositing an effective IR-reflecting
interference system at low cost.
A multilayer coating based on Ti02/Si0 2 has been developed for halogen
lamps. The main developmental challenges were
• to reduce the internal film stress for achieving a high coating stability and
a long lifetime,
• to realize a uniform coating on a small ellipsoidal lamp bulb,
• to deposit many absorption-free layers with low light scattering and highly
accurate thicknesses.
In spite of its many layers the resulting system is clear and has a transmission of up to 102% as compared to an uncoated lamp. This increase of
392
6. Products
transmission is due to the fact that the coating works as an antireflection coating in the visible for the outer surface of the lamp. In combination with an
IR reflection of more than 75% in the wavelength range from 780 to 2200 nm,
this results in an efficiency gain of 30% or more. Owing to the high uniformity
of the coating, the light of this lamp has the same colour coordinates as that
of the uncoated lamp.
6.9.5 Possible Lamp Configurations
The IRC technology can in principle be applied to all incandescent and halogen lamps. Due to the small size of the outer bulb, halogen lamps have a
potentially good geometry factor and are therefore expected to show the
best energy saving effect.
Flood lighting lamps (Fig. 6.68) were the first halogen line voltage lamps
to demonstrate the IRC technology commercially.
The main effect has been to allow a substantial energy reduction; for
example,
• a 400-W lamp can replace 500-W with same luminous flux,
• a 250-W lamp can replace 300-W with even higher luminous flux.
For the European market, the low-voltage cool-beam lamps (Fig. 6.69)
are even more important than the flood lighting lamps. Special geometries of
the burner are necessary to yield a reasonable efficiency gain.
In the case of dichroic lamps, the following substitutions are possible:
• a 35-W lamp can replace 50 W with the same phototechnical data;
• a 50-W lamp can replace 75 W or 65 W with the same phototechnical data.
In conclusion, a coating with a multilayer IR-reflecting interference system
deposited by the PICVD method has been developed. The performance of
"" " ,t" r"',, "" 'I""'"'''' rr".",
t ,.,"'"
'I'
It ' "
Fig. 6.68. Flood lighting lamp
;..
,....~
~
c::::
c::::
~
t-...
'"
r-.
Fig. 6.69. Low-voltage cool-beam lamp
6.10 Coatings on Plastics with the PICVD Technology
393
halogen lamps equipped with this coating is comparable with that of uncoated
halogen lamps of the next higher level of wattage.
6.10 Coatings on Plastics with the PICVD Technology
Markus Kuhr, Stefan Bauer, Uwe Rothhaar, Detlef Wolff
Introduction
During the past few years, plastics have substituted glass products in many
optical applications where low weight, breaking strength, and easy and flexible formability are of major importance [6.105]. However, plastics are well
known to have drawbacks in their chemical resistance, have inferior gas and
water barrier properties and show most often a very poor scratch resistance.
These drawbacks of polymers can be overcome by applying an appropriate
coating onto the substrates. The greatest challenge is the sustained adhesion
of the layer systems to the substrate [6.106], even after hard environmental
tests such as temperature cycling and various climate storage procedures.
Schott has pioneered the work on adhesive optical coatings on plastics in
developing a highly integrated scratch-resistant, easy-to-clean antireflection
coating on ophthalmic lenses made from the duroplast CR39 (see [6.107] and
Sect. 6.8). For the first time these coatings have shown that it is possible to
deposit all functional coatings in a single process chamber with one and the
same coating technology.
In this context the surface modifications induced by plasma treatment
of polymers must be taken into consideration, because of their great impact
on topography, wettability and hence on the adhesion of thin films, especially those deposited by plasma-assisted methods [6.108,109]. In contrast to
most inorganic materials, polymer surfaces are very sensitive to radicals, ions
and photons generated within the plasma. No universal modification after
a certain plasma treatment will occur, because the result often depends on
the polymer type [6.106,110], the fabrication process or the admixture of
additives [6.111].
In this section we report on our recent advances in producing high-quality
anti-reflection and scratch-resistant coatings on different polymer substrates,
such as PC (polycarbonate) and PMMA (polymethyl methacrylate). The
subdivisions reflect the various steps in the process chain up to the final
product.
6.10.1 General Experimental Procedure
All depositions have been carried out in a PICVD reactor that has been
closely adapted to the substrate shapes and sizes. The experimental setup in
its crucial components is described in [6.112]. The deposition system enables
394
6. Products
the uniform coating of an area of more than 150 cm 2 • The substrates are vertically mounted in the process chamber to be coated from front and rear sides
in one and the same process without the need of being turned. No substrate
movement is necessary to achieve a uniform anti-reflective (AR) and antiscratch (AS) coating. SiOxCyH z layers are deposited from an organometallic
precursor, hexamethyldisiloxane (HMDSO), Ti0 2 layers are obtained from
titanium chloride (TiCI4 ) with the addition of oxygen. Stoichiometry, composition and properties of the silicon and carbon containing layers can be
varied in a wide range by using different precursor concentrations with respect to the total gas flow. The concentrations of HMDSO and TiCl4 used
in this study covered a wide range from 100% of precursor material to below
1%.
X-ray photoelectron spectroscopy (XPS) measurements have been used to
characterize process-induced changes in the chemical composition of the nearsurface region of substrates as well as the stoichiometry of the deposited films.
The XPS spectra are obtained by utilizing a Physical Electronics PE 5500
XPS-System employing Mg-Ka (1253.6eV) X-rays. The detection angle can
be switched between 30° and 70° to the plane of the surface. The shift in
energy due to the insulating character of the samples has been corrected by
calibrating against the (C-C)- and (C-H)-carbon peak which was fixed at
284.6eV.
For the secondary ion mass spectrometry (SIMS) analyses a reflect ron
time-of-flight instrument (TOF-SIMS IV) was used, simultaneously equipped
with Ga+ (analysis) and Ar+ /0+, Cs+ (sputtering) ion guns.
All scanning electron microscopy (SEM) pictures were performed with a
commercially available LEO-Gemini 1550 microscope equipped with an additional EDX detector(Voyager). To avoid sample charging during the analyses,
a thin gold film was sputtered onto the polymer surface.
6.10.2 Substrate Cleaning
In contrast to conventional optical glasses there is much more variability
in chemical composition of polymers. Due to different fabrication processes
(e.g., extrusion or injection molding) and the lower environmental stability
of polymers compared to glass, surface states of the polymers are not always
well defined after fabrication. Moreover, injection molding, the most promising process for the production of high-quality optical parts with complex
shapes, often needs mold release agents based on polysiloxane compounds
to promote the release of the substrates from the machining tool. Unfortunately, these mold release agents lead to an almost complete loss of adhesion
of functional coatings subsequently deposited onto the plastic substrates. It
is therefore indispensable to avoid using any kind of release agents for injection molding or to find a feasible way of cleaning the substrates before
coating, respectively. Utilizing a specially developed cleaning process that is
based on heated basic and acidic baths and ultrasonic assistance, we were
6.10 Coatings on Plastics with the PICVD Technology
395
able to remove any organic and inorganic contaminations from the PMMA
surface. Figure 6.70 shows TOF-SIMS spectra in the range between 140 and
300 amu of a typical contaminated PMMA sample before and after cleaning
with the above-mentioned process. From this figure it is clear that there is
no silicon oil contamination (masses 147 and 207) left on the surface after
our cleaning process. However, as can be seen from Fig. 6.71, AFM investigations reveal that the cleaning process causes enhanced surface roughness
and changes the surface chemical composition of the PMMA. The surface
roughness of the samples shown in Fig. 6.71 is almost an order of magnitude
higher after cleaning. In order to circumvent any potential problems with
these modified surfaces, the substrate preconditioning and the deposition of
the subsequent adhesion-promoting layer on top of the PMMA substrate has
to be individually adapted to the substrate type. Procedures appropriate for
this purpose will be described in the following.
6.10.3 Preconditioning - Interaction of Plasma with the PMMA
Surface
Plasma-PMMA interactions, the subject on which we will henceforth concentrate, have been intensively investigated [6.106,113,114], because PMMA
is a very common material for optical applications. Several mechanisms described in these studies, for example surface cleaning, etching, cross-linking
of the near-surface layer, and decomposition, depend on a variety of plasma
parameters. The combination of all these processes, which often occur simultaneously, affect the adhesion properties.
According to recent investigations, the positive plasma ions were found to
easily decompose the ester group (COOCH 3 ) in PMMA, probably through a
(I)~
coO
::l ::l
o E
'::rn
.EO)
1-"2o~
3000
2000
135
j139 147
1000
0
'2 14000
~:c 12000
::l ::l
o E 10000
'::rn 8000
.EO) 6000
~~ 4000
2000 1111 .LI.
0
150
200
I
?H3
I
I
CH3
250
CH3'S(O'st
CH3-Si-O-Si+
CH/11'CH
CH3
CH{ 'cH 3
/
CH3
SiwOel147
150
300
3°'8(°
3
Si-Oe1207
200
J
L
.1.
250
300
Fig. 6.70. Comparison of the TOF-SIMS spectra (140-300amu) between clean
(upper spectrum) and contaminated (lower spectrum) PMMA surfaces
396
6. Products
Ra: 17.0 nm
Rms: 21 .3
b)
Fig. 6.11. AFM measurement of the surface roughness of an as-molded (a) and
an ultrasonic-bath cleaned (b) PMMA plate
neutralization process [6.106, 115]. This side-chain scission leads to a decrease
of polar components and consequently to a reduced ability of strong bond
formation between the surface and an inorganic film.
In a first series of measurements, PMMA samples were bombarded with
a raster-scanned lkeV Ot-ion beam and subsequently analyzed with XPS.
Figure 6.72 depicts the C-ls core level spectra of the virgin PMMA surface
and after ot -bombardment with an ion dose of 1 x 10 16 cm -2. The comparison reveals a distinct decrease of the polar groups (C-O, O=C- O) within
the information depth of the respective photoelectrons (a few mono layers )
associated with ion exposure. Although the bombardment with "keV" ions
does not accurately simulate the ion-surface interaction during plasma treatWithout ion bombardment
••• After 1 keV (10 16 cm- 2 0 2)
-----=-I
~.......:-;-.-;;t;.'"
. ---------------------------------------.~.
290
285
Binding energy leV
Fig. 6.12. CIs detail XPS spectra before (-) and after (- - -) IkeV
bombardment
ot
ion
6.10 Coatings on Plastics with the PICVD Technology
397
ment, the extent of modification even for the low ion dose applied gives an
idea about the severity of this decomposition process.
A second interesting phenomenon is the influence of the plasma-emitted
UV and VUV (vacuum ultraviolet) radiation on the stability of the PMMA
surface. The photon absorption by PMMA in this wavelength region can
initiate a photo fragmentation reaction resulting in side chain (ester group)
and/or main chain scission [6.116, 11 7]. Figure 6. 73 illustrates the transmission of non-irradiated and 254 nm irradiated PMMA samples (dose: 1.2 x 10 17
photons cm- 2 ). Obviously the absorption between 250nm and 500nm increases after irradiation. These changes in transmission are accompanied by
a noticeable brownish appearance.
With regard to the ion- and photon-induced damages described, efforts
were focused on finding "optimum" plasma parameters for substrate conditioning and the early stages of film deposition, i.e., the creation of chemical
functionalities (e.g., oxygen-containing groups) and mechanical strengthening
without degradation of the polymer. The relative amount of a certain plasma
species depends on the equilibrium conditions between formation and recombination processes (mainly wall recombinations in the plasma chamber).
The respective probabilities of generation obey an Arrhenius-like law with a
characteristic proportionality to exp( - Ex/k Te). Here Ex denotes the specific activation energy of the respective process and Te being the electron
temperature (k: the Boltzmann constant). As a rule of thumb the Ex values
necessary for ionizations are significantly higher than for dissociation and of
course the same relation is valid for the excitation of high and low electronic
states. Hence, a small fraction of ions and highly excited species, which is a
1.0
..---- ----'-
0.8
c
0
·iii
rJl
,,
,,,
0.6
rJl
~
I-
I
,i.
·E
c
,/
0.4
,
,,I
J.
0.2
"Without irradiation
;' - - - After irradiation (approx. 94 mJ/cm 2)
:'
(approx. 12 x 1016 photons/cm 2 at 254 nm)
0.0 +------r-~-.-~-.-~-.-~-.-~-.--I
200
300
400
500
700
600
800
Wavelength Inm
Fig. 6.73. UV-VIS transmission curves of untreated (e) and 254-nm-irradiated (&)
PMMA
398
6. Products
major precondition for reduced decomposition, can readily be established in
a "low electron energy" plasma.
From this point of view the Schott-specific PICVD conditions lead to
typical k Te values of only 2 eV, which seem to be highly suitable for polymerrelated applications. To verify this assumption, PMMA (type 8N of Rahm,
Germany) plates were O 2 plasma treated in a PICVD system for different
time periods. After this procedure, the samples were taken out of the vacuum
chamber and kept at room temperature for two hours before contact angles
were measured (probe liquid: distilled water). In Fig. 6.74 these angles are
depicted as a function of plasma pulse time (product of pulse time and number
of pulses = 2s) for different sets of microwave power and gas pressure. Each
data point represents an average value of three single measurements. The
contact angle decreases for prolonged pulse times reaching a minimum value
of 37° at 4 ms. At further increased pulse times the temperature load exceeds
a critical limit, leading to local melting of the PMMA surface. Nevertheless,
contact angles even smaller than 20° were achieved under optimized operating
conditions. Especially in comparison with an unmodified specimen (i'::j 77°),
the enhanced wettability provides an unequivocal evidence for the increase
of polar groups on the PMMA surface.
As can be concluded from the XPS-Cls spectra in Fig. 6.75, the fraction
of "oxygen-bonded" carbon is indeed significantly higher in the near-surface
region of the plasma-treated PMMA surface. Although the XPS characterizations were done two days after exposure, the proportion of the oxygencontaining chain segments like C-O or O=C-O is still increased within the
topmost atomic layers.
80
-e- p
=0.2 mbar power =10%
--e-- p =0.2 mbar power =60%
---A- P =0.7 mbar power =10%
--v- p =0.7 mbar power =60%
75
~
<Il
Ol 50
I::
III
13
.l!l
I::
0
u
45
40
v
0
2
3
4
Pulse time Ims
Fig. 6.74. Contact angle as a function of plasma pulse time for different sets of
working pressure and MW power
6.10 Coatings on Plastics with the PICVD Technology
399
Without plasma treatment
••• After 0 2+ plasma treatment
C-O
O=C-O
~"
, ,
,
,, ,
I
-_ ..... "
I
,, ,
290
285
Binding energy leV
Fig. 6.15. Changes of the CIs detail XPS spectrum of the PMMA surface under
O 2 - plasma treatment
Considering the results shown in Figs. 6.74 and 6.75 and the correlation
of the pulse-to-pause ratio with the plasma heating, thence the controllable
substrate temperature, the PICVD method provides a promising technique
for the pre-conditioning of PMMA surfaces.
6.10.4 Adhesion of the Layer System
Polymers with different chemical structures, such as PC or PMMA, differ
completely in their capability of being coated. Some of the polymers containing benzene rings in their chemical structure like most of the polycarbonates
are quite easy to coat. PMMA, in contrast, is an inherently challenging candidate as far as coating with inorganic functional coatings is concerned. Some
of the reasons are mentioned in Sect. 6.10.3. Many reports about vacuumbased methods enabling cross-linking of inorganic layers to PMMA surfaces
are quoted in the literature [6.113,114]' but so far none of these methods
has been industrially applied for the production of environmentally stable
hard coatings on PMMA. The proposed methods, for example, plasma pretreatment with inert, noble (e.g., nitrogen, helium, argon), or reactive (e.g.,
ammonia (NH3)) gases did not improve the adhesion significantly [6.106,114].
On the contrary, they modify or even destroy the PMMA surface. Moreover,
a modification of the effective refractive index of the material has been observed [6.113]. Frequently used adhesion-promoting layers such as chromium
or aluminum oxides have been tested, but no improvement was found [6.114].
One approach, the activation of the PMMA surface with a water/argon
plasma has been described in the literature to be successful in improving
the adhesion of Si0 2 layers to the PMMA. Unfortunately, it turned out that
environmental stability could not be guaranteed [6.113].
400
6. Products
In our studies we used a different approach, which probably could only
be realized by a CVD method. Following a pre-conditioning as described
in Sect. 3.2, an organic adhesion-promoting layer is deposited next to the
PMMA surface. Several difficulties have to be mastered:
•
•
•
•
•
adaptation of the layer thickness to the subsequent layer system,
uniformity of the layer,
correct composition of the layer,
choosing the "best" precursor substance,
evaluating the "best" substrate temperature.
Many experiments have been carried out to find the "best" window of
PICVD-deposition parameters such as process pressure, microwave power,
gas flows, substrate temperature, etc. Statistical design of experiments has
been used as a valuable tool for experimental "efficiency" . Figure 6.76 shows a
response plot of a series of experiments in which process pressure, microwave
power, and precursor concentration in the carrier gas O 2 have been varied.
The desired "quality" has been chosen as the adhesion of the complete layer
system after thermal cycling tests from -40°C to +85 °C for eight hours.
From such experiments the parameters, which have a major influence on the
adhesion properties of the layer, have been deduced: microwave power introduced into the plasma, precursor concentration in the process gas and
the process pressure. The exact values of the process parameters are Schottproprietary; generally speaking, low microwave power, HMDSO concentrations well above those needed for stoichiometric Si0 2 layers, and comparably
high pressures are needed to produce highly adhesive layers. Figure 6.77 depicts a SIMS sputter-time profile of a typical adhesive layer. The figure shows
a clipping of a complete spectrum that denotes the PMMA substrate on the
right, followed by the adhesive layer, and part of the subsequent anti-scratch
(AS) layer on the left. As can clearly be seen, the adhesive layer has a higher
Microwave power 1%
Fig. 6.76. Response surface plot of a DOE to find optimal process parameters for
the adhesive layer on PMMA
6.10 Coatings on Plastics with the PICVD Technology
401
105
10 2
--e-- 0 2
--+- Si
--e-- SiC
101+-~---------.------------.-----------~
500
1500
1000
2000
Sputter time /s
Fig. 6.77. TOF-SIMS spectra of an adhesive layer on a PMMA 8N substrate
carbon content than the AS layer. Moreover, from the shown spectrum the
gradual transition to the AS layer becomes obvious.
It has also been found that gradients in the composition of the adhesive
layer could be useful for adapting the polymer to the hard functional coating in terms of thermal expansion coefficient. However, the exact design of
the adhesion-promoting layer depends on the substrate polymer as well as
on the thickness and functionality of the subsequent layer system. A further aspect is the growth mechanism of the partially organic adhesive layer,
because generating structures in the dimension of the visible light implies
strong light scattering after depositing the thick functional anti-scratch coating (> 2!-lm in thickness). A special method has been developed for inhibiting
undesired columnar growth in the scratch-resistant layer, which follows the
adhesion-promoting layer. This topic will be the subject of a forthcoming
publication [6.119].
6.10.5 Scratch Resistance
The low scratch resistance of polymers compared to inorganic materials (microhardness of less than 1 CPa) seriously limits their application in optical
components exposed to mechanical attack (scratches generate light scattering). Therefore, a scratch protection is frequently used for transparent optical
components.
Often an additional anti-reflective surface is needed. It usually consists of
metal oxides such as Si0 2 or Ti0 2 with typical optical designs of 4-6 layers.
A mechanical indentation leads to a typical fringed damage of these hard and
brittle thin films if they are directly deposited onto the soft plastic surface.
The scratch-protective layer has to be arranged underneath the AR film stack
for optical reasons and in order to support the AR stack statically. Moreover,
402
6. Products
it builds the elastic composite to the plastic substrate. The functionality of
such anti-scratch layers for optical applications should not be compared to
common wear-protective layers such as TiN, WC, or diamond-like carbon
(DLC) with a microhardness > 20GPa [6.120]. Generally, plastics are protected by lacquer or organic silicon composites. Depending on the applied
technology, the maximum reachable hardness is that of glass with < 8 GPa.
To avoid thermal damage of the substrates, the deposition of thin films
on plastics must be done at low temperatures (typically < 100°C). As, for
example, described by Thornton's model [6.121], the morphology of the films
strongly depends on the ratio between the substrate temperature (Ts) and
the melting temperature of the film-forming material (TM). For ratios Ts/TM
lower than 0.3, which are achieved for Si0 2 layers on plastics (Ts = 375 K,
TM(Si0 2 ) = 1975 K, Ts/TM = 0.19) a columnar morphology is predicted (see
Fig. 6.78).
This morphology shows a good stability against indentation due to the
ability of the columns to react by a mutual dislocation without plastic deformation [6.122]. The film hardness on the other hand is influenced by the
column size. Columns in general lead to a higher hardness, therefore films
with columnar structures have a better resistance to scratches than films
made at higher temperatures without any columns.
The SEM picture in Fig. 6.79 shows an edge of a PMMA sample coated
with a typical film stack by PICVD. The expected columns can be clearly
identified. The scratch resistance of these films is sufficient for most optical
applications. They show for instance a scratch resistance measured with an
Erichsen Tester (model 318) of more than ION.
In addition to the good scratch resistance, the columns induce a positive
secondary effect. The intrinsic stress of films deposited by plasma-enhanced
methods can partly relax between the columns. This stress relaxation reduces
the tendency towards film delaminations.
Fig. 6.78. Thornton's model for thin-film morphology
6.10 Coatings on Plastics with the PICVD Technology
403
Fig. 6.79. SEM picture of the columnar growth of a PICVD anti-scratch layer
In optical films the columnar morphology causes a crucial disadvantage,
as briefly mentioned in Sect. 6.10.4. Whenever the size of the voids between
the columns or the size of the dome-shaped column tops has dimensions in the
range of the wavelength, light is scattered and the samples look hazy [6.119].
In addition to the film structure, the stoichiometry influences the hardness significantly. An addition of organic molecules, for instance, changes the
modulus of elasticity. The elastic properties of SiOxCyH z films are adjustable
in a wide range by varying the carbon content. To achieve good scratch resistance, the hardest layers of the stack must be on the top. In Si0 2-based
anti-scratch systems or in AR systems this could be the top Si0 2 layer or
the interference film stack itself. To avoid the typical fringed damage, the
hard and brittle top layers must be supported by a stable base layer. Thick
SiOxCyH z films with low carbon contents are suited for this purpose. With a
sufficient thickness they will protect the whole stack against damage induced
by normal indentation. But for tangential load the shear stresses also have to
be balanced. This is why an adhesive interlayer is used to increase the film
adhesion.
A further improvement is obtained by a gradual transition of the organic
component from the adhesive layer containing much carbon to nearly Si0 2
stoichiometry near the surface. This enables a continuous reduction of the
shear stresses and there are no additional interfaces of different elasticity
were a pile-up of microcracks could cause film delamination.
Mechanical and Optical Performance
As described in Sect. 6.10.5, an AR coating often completes the functional
coating on polymers. A multilayer system of Ti02 with a refractive index of
404
6. Products
n550 = 2.1 and Si0 2 with a refractive index of n550 = 1.46 can be used for
a lot of applications. The number of layers, the thickness of the individual
layers, and the optical design defines the performance of the multilayer stack
(e.g., filter functions, mirror functions, anti-reflective functions) [6.123].
Underlying thick anti-scratch films (several j.tm in thickness) influence
the optical spectrum, because in general the polymer substrate and the antiscratch layer have different refractive indices. This leads to a modulation
of the reflection spectrum as depicted in Fig. 6.80 (simulation). Using our
PICVD technology and silicon-organic precursor materials it is possible to
vary the refractive index of the anti-scratch layer from n = 1.46 up to n >
1.53 (Fig. 6.81) [6.112]. Another method, published by Schiller et al. , is to
create coatings with gradients in the refractive index in order to minimize
the differences between the refractive index of the plastic substrate and the
subsequent layers [6.112] . The result of such procedures is a fairly smooth
optical spectrum, as can be seen in Fig. 6.80.
Usually ophthalmic coatings have an "easy-to-clean" top layer to enhance
the cleaning performance of eyeglasses. This top layer is a polymeric layer
from 100% HMDSO precursor concentration and has a contact angle of more
than 90 0 . Hence, it is very easy to clean the surface of eyeglasses from particles or even fingerprints [6.123].
With the three basic functionalities created by the PICVD process, it
is possible to produce a lot of different product functionalities on several
plastic materials, including PMMA substrates with different shapes and sizes.
However, many additional properties are needed for such products, which
are described in customer-related specifications. From business to business
and from product to product these properties can vary considerably. Several
10
Refractive index substrate:
1.59
Refractive index anti scratch layer: 1.47
9
8
Reflection uncoated substrate ~
7
~ 6
c
Only antireflection filter (AR)
Scratch-resistant layer (AS) and AR
Index-matched AS and AR (N : 1.57)
0
5
is
Q)
~ 4
0:::
3
2
o t-~~~~~~~~~~~
350
400
450
500
550
600
650
700
750
800
Wavelength Inm
Fig_ 6_80. Spectra of PMMA sample with AR coating and AR/ AS coating (simulation)
6.10 Coatings on Plastics with the PICVD Technology
405
1.58
E
c:
0
LO
~
c:
SiOxCyH z
Si02
1.55
1.52
1.49
1.46
1.43
1.40
100
95
90
85
75
80
70
65
60
02 content 1%
Fig. 6.81. Variation of index of refraction with oxygen admixture during deposition
of SiOxCyHz layers
specifications from different customers have been clustered and summarized
in Table 6.13.
The most important property for applications in optics and optical sensor
technology is of course the optical performance itself. Low visible reflection
AR coatings, mirrors, or difficult filter functions on plastics have to be developed. Scratch resistance and/or critical environmental conditions are not as
important as in the other business segments.
The basic application is once again the eyeglass made from CR 39 plastic
substrates. The required multilayer system has to fulfill a highly demanding specification: anti-reflective functionality combined with sufficient antiscratch performance (thickness of the anti-scratch layer in the range of 121J.m, classification "low" in Table 6.13), good adhesion, resistance to different solvents, heat resistance, and easy-to-clean top layer [6.95]. Very similar
requirements exist for products for the metrology industry.
A completely new class of properties is required for products for the automotive industry. In addition to the above-summarized tests in all automotive
specifications a defined resistance to solvents has to be guaranteed. The most
challenging tests, however, are the temperature cycling from -40°C up to
+85 °C and humid storage at elevated temperatures. These tests simulate the
the environmental conditions that the coatings will be subjected to in cars in
summer and winter at different parts of the world and thus yield information
about their lifetime. As a result of our experiments we are convinced that to
produce coatings with thick anti-scratch layers (1-2IJ.m) and sufficient temperature cycling performance, the anti-scratch layer will have to be organic
or at least partly organic. It is possible to use different siloxane-based lacquers or CVD technologies with organic precursors such as PICVD. Over the
406
6. Products
Table 6.13. Summary of specifications of multilayer coatings on plastics for various
applications
Optics
Ophthalmics
Automotive components
Optical
sensoric
devices
Mobile
phones
Measuring
systems
Optical performance
filter,
AR,
mirror
AR
AR
filter,
AR,
mirror
AR
AR
Adhesion
grid &
tape
grid &
tape
grid &
tape
grid &
tape
grid &
tape
grid &
tape
Anti-scratch
and abrasion
low
+
+
low
++
+
Temperature
change
+ 85°C + 85°C
-40°C
+85°C
+85°C
(-40°C) (-15°C)
+85°C
+85°C
Climate tests
humid
humid,
condensed
water
humid
humid
boil test
humid
Chemical
solutions
acids,
acids,
fats, bases,
bases,
salt water salt water
Easy to clean no
yes
yes
no
(yes)
no
Material
CR39,
PC
PMMA,
PC
PMMA,
PC,
COC
PMMA,
PC
PMMA,
PC
PMMA,
PC,
COC
acids,
bases,
fats
last two years such coating systems have been developed and the resulting
samples have passed temperature shock tests according to ISO 9022 with PC
and PMMA substrate materials. However, using PMMA is rather difficult
because, among other things, the thermal expansion coefficient of PMMA
(7-8 x 10- 5 K- 1 ) differs by more than an order of magnitude from inorganic
bulk materials such as Ti0 2 (8-9 x 10- 6 K- 1 ) and Si0 2 (~ 0.5 X 10- 6 K- 1 ).
At temperature differences between -40°C and +85 °C stresses between substrate and coating systems in the range of several hundred MPa can be induced. Pure inorganic anti-scratch coatings are not able to compensate such
big differences, and the results are very strong cracks or the complete peel-off
of the layer system after such test procedures [6.119]. The specifications of
the mobile phone industry seem to be most challenging. Here one has to cope
with automotive specifications in combination with an improved scratch resistance, well beyond that for ophthalmics (classification "++" in Table 6.13).
Layer thicknesses between 2 and 5 11m of SiOxCyH z are necessary to obtain
such performances. Schott has also succeeded in developing coatings, which
pass all the tests for the telecommunication industry.
6.11 Optical Multilayers for Ultra-Narrow Bandpass Filters by PICVD
407
Fig. 6.82. Examples for anti-reflective and anti-scratch coatings on customerrelated products
The different optical coatings are part of high tech products from different businesses. R&D groups worldwide think about new applications and
products for optics, ophthalmics, automotive industry, metrology, sensor technology or telecommunication applications (mobile phones, etc.). Two examples for AR coating or anti-scratch coatings from our group are depicted in
Fig. 6.82.
Up to now the biggest market for optical coatings on plastics is the antireflective coating system for ophthalmic lenses. A multilayer system of 4- 6
layers from Ti0 2 and Si0 2 guarantees a broadband anti-reflective system
between 380nm and 670nm [6.95]. New applications and products are for
instance a similar anti-reflective coating system for car-radios, speedometer
covers, mobile phones, etc., on different kinds of plastic substrates, or antireflective coatings for special laser wavelengths and in the near-infrared spectrum. Using the PICVD process, Schott is capable of producing the required,
highly sophisticated coatings on polymer substrates, especially on PMMA.
6.11 Optical Multilayers for Ultra-Narrow Bandpass
Filters Fabricated by PICVD
Stefan Bauer, Lutz Klippe, Uwe Rothhaar, Markus Kuhr
Introduction
Over the past few years Schott has developed its PICVD process into a
multifunctional tool for mass production of high-quality optical coatings. To
use PICVD also for the production of ultra-narrow bandpass filters (FWHM
< 1 nm), for which there is a growing demand (e.g., application in dense
408
6. Products
wavelength division multiplexing (DWDM) systems), difficult problems had
to be solved.
The biggest challenges were:
• identification of a suitable high-refractive index (H-Iayer) material (so far,
Ti0 2 has been used in all PICVD processes as H-Iayer material, but its
intrinsic stress behavior makes Ti0 2 ill-suited for very thick (» 10 Il-m)
multilayers) ,
• selection of a stable and volatile precursor species allowing the deposition of
high-quality films, i.e., films with high density, low absorption, low stress,
low surface roughness, good stability at high humidity, etc.,
• high-precision thickness control. In conventional PICVD processes the film
thickness is determined only through process parameters and deposition
time. The production of ultra-narrow bandpass filters, however, definitely
requires optical in-situ monitoring (see e.g. [6.124,125]). For this purpose,
a special optical monitoring system had to be provided.
Conventional PVD technology utilizes Ta205 as the standard H-Iayer material for ultra-narrow bandpass filters. Its higher refractive index would make
Nb 20 5 the better choice, but problems in deposition processes prevent it from
being widely used.
This study compares Nb 20 5 and Ta205 layers deposited by the PICVD
technique and gives an evaluation of the properties of niobia layers synthesized from either chlorine or metal-organic sources (Table 6.14). For the
preparation of multilayer cavity filters, Si0 2 deposited from HMDSO (hexamethyldisiloxane) was selected as low-index (L-Iayer) material.
Table 6.14. Experimental conditions for the deposition offilter coatings by PlCVD
Film material
Precursor
Pressure
Total gas flow
Precursor flow
Substrate temperature
Microwave plasma
Deposition rate
Thickness control
Substrate material
L-layer: Si02
H-layer: Nb20 S or Ta 20s
Si02: hexamethyldisiloxane (HMDSO)
Nb 20 S : NbCl s , Nb-tetraethoxy-dimethylaminoethoxide
Ta20s: TaCls
15 ... 30 Pa, typically 20 Pa
150 ... 600 sccm, typically 500 sccm, transport gas: O2
1-10% of total flow
200-300°C
2.45GHz, power (avg.): 500W,
pulse frequency: 10-100 Hz
Si02: 50 ... 500nm/min
Nb20 S : 50 ... 300nm/min
Ta 2 0s: 90 ... 300nm/min
optical transmission monitoring, spot size: 2 mm 0
Borofloat®, BK7, F7 (all Schott), WMS-13 (OHARA)
6.11 Optical Multilayers for Ultra-Narrow Bandpass Filters by PICVD
409
6.11.1 Experimental Procedure
Figure 6.83 shows an outline of the experimental setup. The process gas is
supplied by a specially designed gas delivery system, where flow rates are
precisely defined by mass flow controllers. The oxygen/precursor gas mixture
is transported through a heated line into the process chamber. In order to
avoid condensation of precursor vapor, the complete recipient is heated to
temperatures above 120 ce. Inside the chamber, plasma is excited from a
microwave field induced by two synchronously pulsed generators, which are
coupled to the process chamber through separate waveguides. The substrate
is fixed onto a heated holder. The film thickness is controlled in situ by a
monitoring system, based on optical transmission measurement. The operation of similar monitoring systems is described in the literature [6.125].
The refractive indices and extinction coefficients of the single layers are
measured ex situ with the help of a rotating analyzer spectroscopic ellipsometer (VASE) from Woollam company.
The narrow-band filters are characterized by a fiber-based optical setup,
which utilizes a white-light ASE source and an optical spectrum analyzer.
The filters are positioned on a gimbal mount, which guarantees a precisely
defined vertical incidence of the light beam that is formed by a fiber-based
GRIN optic. The experimental conditions are listed in Table 6.14.
Process gas
delivery system
Microwave
White-light
source
(monitoring)
Detection unit
(monitoring)
Gas inlet nozzle
Reaction chamber
Substrate holder
(heated)
Substrate
Microwave
Fig. 6.83. Sketch of PICVD system for the deposition of high-precision interference
filter multilayers
410
6. Products
6.11.2 Results and Discussion
All niobia and tantala single layers deposited from chloride precursors were
clear, free of cracks and stable in humid environment, even at a thickness of
» 10 Jl.m. Optical properties have been investigated by ellipsometry. Alternatively, for non-absorbing films the refractive indices were determined from the
monitoring signal. Refractive indices at 1550 nm of niobia films were in the
range of 2.20-2.27, whereas for tantala films values near 2.10 were typically
determined. In terms of optical properties, niobia films showed no drawbacks
in comparison to tantala. Hence, niobia has been selected as H-material.
Secondary-ion mass spectrometry (SIMS) sputter time profiles in Figure 6.84 show that Si02 films contain a low level of carbon, whereas Nb 20 5
layers have incorporated chlorine. The level of C within niobia and CI in silica
is close to the background level and hence not significant. The Cllevel in niabia films has been quantitatively determined by XPS (X-ray photoelectron
spectrometry) measurements to be in the range of 0.1-0.5 at%, depending on
the deposition conditions. The level of carbon contamination within Si0 2 has
not been precisely determined, but can be estimated to be lower than 1 at%.
Niobia films deposited from an organic source show lower refractive indices (n = 2.0-2.2) and, depending on the process conditions, absorption
occurs. Moreover, these films tend to develop cracks and are sensitive to humid climate. Thus, due to evidently superior film properties, NbCl5 has been
chosen as preferred precursor.
The quality of multilayer systems for narrow bandpass filters strongly
depends on a low level of surface as well as interface roughness. The SEM
cross section of an H-L-multilayer cavity structure in Figure 6.85 illustrates
a low level of defects and abrupt interfaces. However, WLI measurements
107
106
105
::J
~
Z- 104
'iii
I::
.l!l
..!: 103
102
101
0
2
3
4
Sputter time Ih
Fig. 6.84. SIMS sputter time profiles of a Nb205/Si02 multilayer system
6.11 Optical Multilayers for Ultra-Narrow Bandpass Filters by PlCVD
411
Fig. 6.85. Scanning electron microscopy (SEM) cross section of an Nb 2 0 5 /Si0 2
multilayer
indicate a significant surface roughness, which among other factors appears
to be correlated to the deposition speed (see Figure 6.86).
Figure 6.87a displays a transmission spectrum of a 3-cavity filter (200 GHz
channel spacing) deposited in a process time of 6 h. In comparison, an equivalent filter deposited in 4 h (see Figure 6.87b) shows a significantly higher peak
loss. The decrease of filter transmission at high deposition rates is very likely
caused by surface roughness, which leads to scattering losses. More experimental data is required to further clarify the correlation between deposition
rate, surface roughness, and filter loss.
However, as can be seen from the transmission spectra depicted in
Fig. 6.87, the PICVD filter coatings meet the demands of telecom applications. Typical 200 GHz-DWDM applications require insertion losses of less
than 0.8dB [6.126] .
It is evident, that temperature stability of the center wavelength is particularly important for the application of narrow bandpass filters . By picking a
suitable substrate material the requirement of a low thermal wavelength shift
can be fulfilled. The linear coefficient of thermal expansion (CTE) is a major
selection criterion for the substrate material. A detailed report on this topic
0.05
0.05
IJm
IJm
-0.03
0.0474
o
mm
-0.03
0.0474
o
mm
Fig. 6.86. Measurement of surface roughness by WLl (white-light interference
microscopy). (a) Single-cavity filter deposited in 90 min, roughness Ra ~ 0.4 nm),
(b) equivalent filter deposited in 45 min, roughness Ra ~ 0.8 nm
412
6. Products
1545
1547
1551 a)
1549
1545
ca -5
ca -5
c
-10
c -10
:~
-15
~
o
E
1551 b)
o
:~ -15
E
~ -20
-25
1---25
~
1549
~
~ -20
I--
1547
O+-----~--~-L~---L__.
O+-----~--~~~----L-_,
~
-30~---+~--r-+--+~----~
-30L-----~--~~~~----~
Wavelength A. Inm
Wavelength A. Inm
Fig. 6.87. Transmission spectra of 3-cavity filters deposited in a process time of
(a) 6h, peak loss = O.6dB, and (b) 4h, peak loss = O.8dB, respectively
is given in the literature [6.127]. In the present study the thermal wavelength
shift behavior of filters deposited by PICVD has been investigated.
Experimental data depicted in Figure 6.88 indicate, that the optimum
CTE value of the substrate is between 10 x 10- 6 K- 1 and 12 x 10- 6 K- 1 .
This result compares well with previous findings for Ti0 2/Si0 2 multilayers
as well as for Ta205/Si02 multilayers made by ion-assisted deposition [6.127].
Apparently, neither the choice of deposition technology nor the type of Hlayer material influence the thermal wavelength shift behavior significantly.
10
Borofloat®
8
1
6
..e-
4
BK7
E
0
s:I--
•
2
F7
0
-2
WMS-13 •
0
2
4
6
8
10
12
a 110-6
Fig. 6.88. TWD (thermal wavelength drift) of the filters center-wavelength as a
function of the thermal expansion coefficient of the substrate material (Borofloat®,
BK 7, F 7, and WMS-13)
References
413
Conclusions
A PICVD process has been developed for the deposition of high-quality
Nb 20 5 and Ta205 films and Nb 20 5/Si0 2 multilayers. To the authors' knowledge, this is the first successful approach to the production of high-precision
multi-cavity optical filters by any kind of CVD or plasma CVD process. The
deposition rates significantly exceed the values achieved by PVD technologies. The quality of the PICVD multilayers, which is ensured by high-precision
optical in-situ monitoring and very precise process control, meets extremely
high demands such as specified, for example, for ultra-narrow bandpass filters. Results show a low level of losses by either scattering or absorption.
However, experimental data indicate enhanced surface roughness for high
deposition rates. This mechanism needs further investigation. The thermal
wavelength shift behavior of PICVD Nb 20 5/Si0 2 multilayers compares well
with literature values reporting on multilayers made by PVD technologies
and employing Ta205 or Ti0 2.
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List of Contributors
Olaf Anderson
Schott Glas, Mainz 1
[email protected]
Joachim Disam
Schott Glaswerke, Mainz 1
[email protected]
Klaus Bange
Schott Spezialglas GmbH, Mainz 1
klaus. [email protected]
Wolfgang Ehrfeld
IMM Institut fur Mikrotechnik
Mainz GmbH,
Carl-Zeiss-Str. 18-20,
55129 Mainz
Friedrich G.K. Baucke
Kaiserstr. 36,
55116 Mainz
formerly at Schott Glas, Mainz 1
Stefan Bauer
Schott Glas, Mainz 1
[email protected]
Wolfram Beier
Schott Glas, Mainz 1
wolfram. [email protected]
Christof Fattinger
F. Hoffmann-La Roche AG,
Grenzacher Str. 124,
CH-4070 Basel
Dirk Gohlke
formerly at Schott Glas, Mainz 1
Hrabanus Hack
Schott Glas, Mainz 1
[email protected]
Lars Bewig
Schott Auer GmbH,
Hildesheimer Str. 35,
37581 Gandersheim
lars. [email protected]
Martin Heming
Schott Lithotec AG
Otto-Schott-Str. 13
07745 Jena
Burkhard Danielzik
Schott Glas, Mainz 1
[email protected]
Torsten Holdmann
Schott Glas, Mainz 1
[email protected]
1
HattenbergstraBe 10, 55122 Mainz, Germany
422
List of Contributors
Eckart Hussmann
Kleiststr. 4,
55270 Oberolm,
formerly at Schott Glas, Mainz 1
Ulrich J eschkowski
Schott Displayglas GmbH, Mainz 1
Lutz Klippe
Schott Glas, Mainz 1
[email protected]
Dieter Krause
Schott Glas, Mainz 1
retired
dieter [email protected]
Thomas Kiipper
Schott Auer GmbH,
Hildesheimer Str. 35,
37581 Gandersheim
[email protected]
Markus Kuhr
Schott Spezialglas GmbH, Mainz 1
[email protected]
Roland Langfeld
Schott Glas, Mainz 1
roland [email protected]
Frank-Thomas Lentes
Schott Glas, Mainz 1
frank- [email protected]
Klaus-Dieter Loosen
Schott Glas, Mainz 1
klaus-dieter [email protected]
1
Holger Lowe
IMM Institut fur Mikrotechnik
Mainz GmbH,
Carl-Zeiss-Str. 18-20,
55129 Mainz
Katharina Liibbers
Schott Glas, Mainz 1
[email protected]
Andreas Michel
IMM Institut fur Mikrotechnik
Mainz GmbH,
Carl-Zeiss-Str. 18-20,
55129 Mainz
Frank Michel
IMM Institut fur Mikrotechnik
Mainz GmbH,
Carl-Zeiss-Str. 18-20,
55129 Mainz
Wolfgang Mohl
Schott Glas, Mainz 1
[email protected]
Christoph Moelle
Schott Auer GmbH,
Hildesheimer Str. 35,
37581 Gandersheim
[email protected]
Hansjorg Niederwald
formerly at
Carl Zeiss,
Carl-Zeiss-Str. 22,
734470berkochen
HattenbergstraBe 10, 55122 Mainz, Germany
List of Contributors
Norbert Oranth
F. Hoffmann-La Roche AG,
Grenzacher Str. 124,
CH-4070 Basel
Clemens Ottermann
Schott Spezialglas GmbH, Mainz 1
[email protected]
Uwe Rothhaar
Schott Glas, Mainz 1
[email protected]
Wolfgang Rupp
formerly at
Carl Zeiss
Carl-Zeiss-Str. 22,
734470berkochen
J iirgen Spinke
F. Hoffmann-La Roche AG,
Grenzacher Str. 124,
CH-4070 Basel
1
Alfred Thelen
RoBkopfstr. 13,
60439 Frankfurt /Main
Falko von Unger
Pestalozziweg 20,
31073 Delligsen
formerly at
Deutsche Spezialglas AG,
Hiittenstr. 1
31073 Griinenplan
Marten Walther
Schott Glas, Mainz 1
marten. [email protected]
Michael Witzany
formerly at
Carl Zeiss,
Turnstr. 27,
73430 Aalen
Detlef Wolff
Schott Glas, Mainz 1
detlef. [email protected]
HattenbergstraBe 10, 55122 Mainz, Germany
423
Sources of Figures and Tables
We are indebted to the following editors and authors, respectively, for the kind
permission to reproduce copyrighted materials.
Material
Source
Original Publisher
Fig. 1.1
[1.8]
Fig. 3.9
[3.84]
Fig. 3.10
[3.84]
Fig. 4.38
Fig. 5.1
[4.346]
[5.2]
Fig. 5.7
[5.49]
Fig. 5.8
[5.51]
Fig. 5.11
[5.49]
Fig. 5.15
[5.49]
Fig. 5.16
[5.49]
Fig. 5.18
[5.51]
Fig. 5.19
[5.51]
Fig. 5.23
[5.63]
Fig. 6.58
[6.97]
Fig. 6.59
[6.97]
Fig. 6.60
[6.97]
Optical Society of America,
1613 19th Street, N.W., Washington DC 20009, USA
Deutsche Glastechnische Gesellschaft,
Mendelssohnstr. 75-77, 60325 Frankfurt, Germany
Deutsche Glastechnische Gesellschaft,
Mendelssohnstr. 75-77, 60325 Frankfurt, Germany
Academic Press, Inc., Orlando, FL 32887-6777, USA
Deutsche Glastechnische Gesellschaft,
Mendelssohnstr. 75-77, 60325 Frankfurt, Germany
Marcel Dekker Inc.,
270 Madison Avenue, New York, NY 10016, USA
Fuji Technology Press Ltd., Dai-ni, Bunsei Bldg. 1-11-7,
Toranomon Minatu-ku, Tokyo 105, Japan
Marcel Dekker Inc.,
270 Madison Avenue, New York, NY 10016, USA
Marcel Dekker Inc.,
270 Madison Avenue, New York, NY 10016, USA
Marcel Dekker Inc.
270 Madison Avenue, New York, NY 10016, USA
Fuji Technology Press Ltd., Dai-ni, Bunsei Bldg. 1-11-7,
Toranomon Minatu-ku, Tokyo 105, Japan
Fuji Technology Press Ltd. Dai-ni, Bunsei Bldg. 1-11-7,
Toranomon Minatu-ku, Tokyo 105, Japan
Elsevier Science S.A.,
P.O. Box 564, 1001 Lausanne, Switzerland
Elsevier Science NL, Sara Burgerhartstraat 25,
1055 KV Amsterdam, The Netherlands
Verlag optische FachverofIentlichung GmbH,
Postfach 10 44 43, 69034 Heidelberg, Germany
Verlag optische FachverofIentlichung GmbH,
Postfach 10 44 43 69034 Heidelberg, Germany
Verlag optische FachverofIentlichung GmbH,
Postfach 10 44 43, 69034 Heidelberg, Germany
426
Sources of Figures and Tables
Material
Source
Original Publisher
Table 4.3
[4.17]
Table 6.9
[6.97]
Table 6.10
[6.97]
Springer-Verlag Heidelberg, Berlin,
Postfach 105280, 69042 Heidelberg, Germany
Verlag optische Fachveroffentlichung GmbH,
Postfach 10 44 43, 69034 Heidelberg, Germany
Verlag optische Fachveroffentlichung GmbH,
Postfach 10 44 43, 69034 Heidelberg, Germany
Index
absorptance 99,363
accelerated humidity test 330
adhesion 121,399
ADI see filter; all-dielectric
(interference)
Amiran® 370
analytical methods 99
annealing 6,149,166,202
applications
- automobiles 3
- buildings 3
- cold-light mirror 346
- displays 3, 13
- eyeglasses 3
- instruments 3
- laser in materials processing 340
- laser in medicine 337
- lighting 3
- lightning 389
- optical communication 3
- pharmaceutical packaging 3
Auger electron spectroscopy (AES)
126
automotive mirror 353, 356
bandpass filter 28, 253
- 3-cavity 411
- ultra-narrow 407
beam splitter 342
Berlin contest 1991 40
bidiffractive grating 315, 334
biosensor 312
Brewster angle 108
buffer layers 33
business strategy 17
Calorex® 367
Carat® 388
characteristic matrix 25, 27
characterization of thin films 99
- acronyms of methods 99
charge-transfer transition 274
Chebyshev synthesis 29,30,45,47
chemical vapour deposition (CVD)
59,383
- laser 60
- low-pressure (LPCVD) 60
- plasma 60,61
- plasma impulse (PICVD) 60,374
- plasma-assisted (PACVD) 60
- plasma-enhanced (PECVD) 60
- thermal 60
Clarlet® 381
coater
- batch 11
- in-line 11
- load-lock 11
- single-piece 12,347
coating 1
- anti-scratch (AS) 394
- absorption 253
- Ag-containing 276, 279
- antireflection (AR) 34,35,37,40,
229,339,341,370,385,392,394
- conductive 113
- design 343
- dipping 8
- electro chromic 7
- electron-sensitive 270
- environmentally stable 29
- hard 7
- hydrophobic 385
- large-area 9
- low-e 15
- magnetron sputtering 10
- materials 2
- multilayer 201,227,390
- ophthalmic 380
- plastics 393
- solar control 365
- spinning 9
- spraying 9
- sputtering 8, 9
428
Index
- technology 237
- thermal evaporation 8
- ultimate-quality 9
coevaporation 237,238
cold mirror 304
cold-light reflector 344,353
complex reflectance 231
complex transmittance 231
composition 114, 125
condensation 67,76
corrosion resistance 356
costs, fixed and variable 12
CR 39 (duroplast) 393
crystal structure 4
crystallinity 108, 114
CVD see chemical vapour deposition
damage threshold 250, 341-343
defect functions 45
delamination 124, 331
dense wavelength division multiplexing
(DWDM) systems 408
density 4, 104
deposition
- parameter 259
- processes 9
- rate 259
- technologies 46
design
- computer-aided 239
- flip-flop 15, 19,38
-- "design-to-go" method 252
- - layers 251
- methods 23
- needle method 15, 38
- numerical 37,42,45
- of coatings 15
- process 13
- product 13
- synthesis 24
dichroic beam splitter 304
diffusion barrier 373
digital projection 348
dip coating 75
- film thickness 76
- laser annealing 156
- modelling 77
- thermal treatment 153
edge filter 28, 32, 36, 39
elastic moduli 120
electro chromic see also coating, layer
- bleaching 186
- colouration 180, 186
- device 259
- display 261,269
- materials 262
- mirror 267, 358
- processes 180
- system 194,268
- - reflecting 263
-- transparent 259,263,269
electrochromism 199
electron spectroscopy for chemical
analysis (ESCA) 127,174,177
ellipsometry 109
equivalent
- index 26
- layers concept 231
- thickness 26
evaporation 237
- by electron beam 52,346
- energy-enhanced 53
- non-reactive 51
- process temperatures 54
- reactive 53, 238
- substrate temperature 52
- thickness uniformity 52
extinction coefficient 4, 106, see also
optical constants
- colloid formation 278, 288
- colour centres 275
- noble-metal colloids 275
- of Ti0 2 5
- stability 289
- transition metals 273
film see also coating
- growth 65
-- model 62
- stability 242,271
filter
- all-dielectric 299
- angular dependence 298
- bandpass 14,35,296,302,308
- blocking 308
- diabatic representation 298
- dichroic 372
- edge 242,304
- induced-transmission 310
- longpass 296
- metal dielectric 308
- multi-cavity bandpass 241
- short pass 298
flip-flop see design
Fourier series see theory
fractionation 58
Index
morphology 403
multilayer see coating, theory
gas chromatography (GC) 72
GIXR (grazing incidence X-ray
reflectometry) 105,161,172
glass ceramic materials 349
graded index films 254
grain boundaries 108
grating coupler 315
hardness 118
heat treatment 80
hot isostatic pressing (HIP)
hot mirror 41,304
hydrolysis 67, 72, 76
impurity 107
indium-tin oxide see ITO
insertion losses 411
interfacial bonding 121
interference see also filter
- constructive 296
- destructive 296
- mirror 357
interference filter 237,295
- IR-reflecting 390
- narrow band 236
ion exchange in an autoclave
ion-assisted deposition (lAD)
IRC technology 392
lrox® 367
ITO 113,264
Kaufman ion source
Nb 2 05 352,408
neutral
- beam splitters 29,35
- filter 236
- reflection 229
Ni(OHh 264
- bleaching 186
- colouration 182, 186
- density 176
- deposition 176
- extinction coefficient 176
- hydrogen content 177
- refractive index 176
- stoichiometry 177,188
- structure 188
nickel oxide see Ni(OH)2
NMR (nuclear magnetic resonance)
69
NRA (nuclear reaction analysis) 105,
127,174,177,265
88
optical
- coating design 15
- constants of Ti02 4
- density 272,280
- - electron dose 282
- sensor platform 313, 321
-- fabrication 313
- - fabrication of bidiffractive grating
321
- transducer 312,313
oxidation state 129
277
54
55
laser calorimetry 112
laser mirror 339
layer see also coating
- buffer 387
- electrochromic 259
- hydrated 286
- semi-transparent metal 368
Levenberg-Marquardt algorithm
light scattering 70
linear tube reactor 243
Lorentz-Lorenz theorem 108
low-voltage ion plating 56
429
110
matching layers 37
MDI see filter; metal dielectric
(interference)
merit function 35
microstructure 9
minus filters 29
Mirogard® 370
modelling extinction of colloids 285
pass band 27
PC (polycarbonate) 393
phase thickness 24
photochromism 199
photomask 271
- edge gradient 282
- electron range 280
- exposure time 279
- resolution 283
photothermal deflection 112
physical vapour deposition (PVD) 51,
see also evaporation, sputtering
PICVD 243,327,375,391, see also
process; plasma impulse CVD
- stability 251
pinholes 108,363
plasma CVD equipment 64
430
Index
PMMA (polymethyl methacrylate)
393
polarization 33
porosity 53, 55, 248
post-processing 13, 15
preconditioning 13
precursor 60, 243
process
- additive 8
- control 375
- CVD 10
- dip coating 76
- dipping 357, 360
- evaporation 46
- ion plating 239, 242
- LPCVD 10
- magnetron sputtering 356
- neutral solution 8
- performance ratings 10
- PICVD 10,347
- plasma polymerization 385
- pulsed plasma CVD 46, 48
- sol-gel 67,359
- spin coating 78
- spinning 230
- spraying 230
- sputtering 46
- stability 252
- subtractive 8
- thermal coating 83
- wet-chemical 383
product
- added value 17
- basic coating 17
production costs 12
properties
- anisotropies 108
- film 16,63
- macroscopic 99, 104
- microscopic 99, 125
- of thin dielectric films 4
- process 16
- substrate 16
pulsed microwave plasmas
- PICVD, 2.45 GHz 65
Punktal® 381
Puttick grid 17
PVD see physical vapour deposition
quarter-wave stack
Raman spectroscopy
rate control 237
28
72, 132, 191
rear-view mirror 353, 354
reflectance 33,99,227,363
refractive index 4, 106, see also
optical constants
- of Ti0 2 5
replication 324
ripple 30, 38, 241
rugate filter 254
Rutherford back scattering (RBS)
128,140,174,177
SAXS see small-angle X-ray
scattering
scattering 4
scratch resistance 401
Sellmeier expression 109
sensor 311
- chip 312,332
- real-time 312
- sensitivity 312,319,334
- stability 334
sheet resistance 113
shifted periods 28
silica see Si02
silicon oxide see Si0 2
single-piece technology 17
Si02 111,127,135,343,344357374
375,391
'
,
,
- absorption constant 254
- blocking of diffusion 169
- density 160
- deposition 246
- dopants 161
- extinction coefficients 165
- impurities 161
- refractive index 160,163
- suboxides 160
Si02 350,401,408
small angle X-ray scattering (SAXS)
71,134,191
sol-gel see also process
- atmosphere 364
- chemistry 66
- condensation reaction 67
- dip coating 76, 359
- drying 82
- firing process 365
- fractal objects 73
- hydrolysis 67
- incorporating metal colloids 360
- method 67
- porosity 79
- process 360
- reaction kinetics 68
Index
- shrinkage 81
- solution 360, 364
- thermal processing 80
spin coating 77
sputtering 57
- diode sputtering 57
- ion beam sputtering 58
- magnetron sputtering 58
- targets 284
stability 16, see also film, PICVD,
process
stain formation 226
stoichiometry 5,53,56,63,107,253
stop band 27,231,232
stress 4, 202
- compressive 115, 165, 242, 331
- intrinsic 114
- relaxation 165
- tensile 115, 148
substrate
- bending 117,118
- cleaning 101,361
- high-index 229
- low-index 229
- plastic 381
- polymer 315,320
- surface 101
- - cleaning 14
surface
- composition 14
- condition 14
- contamination 14
- leaching 373
- plasmon 311
- polished 101
- roughness 101,102,114,341
- staining 103
Ta2 05 128, 135,264,313
- density 172
- deposition 171
- extinction coefficient 171
- oxidation state 174
- refractive index 171
Ta2 05 408
Taber scratch test 387
theory
- approximate solutions 24
- bidiffractive grating 315
- characteristic matrix 25
- effective interfaces 32
- equivalent layer 26
- equivalent layers 25,29,45,47
431
- exact solutions 24
- Fourier series 227
- multilayer 230, 235
- numerical methods 35
- optical thin film 24
- reflectance 24, 229
- shifted periods 36
- transmittance 27
thermal coating 83
- flame spraying 84
- high-velocity oxy-fuel gas spraying
84
- laser-spray coating 87
- plasma spraying 86
thermal expansion mismatch 256
thermal toughening 15
thermochromism 198
thickness 108,111,364
thin film see also coating
- analysis 100
- density 105
- mechanical properties 114
- optical properties 106
- polarizer 236
- properties 99
- structure 130
Ti02 5,105,111,127,135,313,343,
344,357,391
- crystal structure 132, 136, 148
- density 142
- deposition 247,327
- depth profiles 139, 154
- extinction coefficient 147,327
- extinction coefficients 157
- optical constants 4
- oxidation state 141
- refractive index 144,157,365
- roughness 153
- waveguide 313,318
Ti02 350,394,401,408
titania see Ti02
titanium oxide see Ti02
transmission 4
transmittance 33,99,363
- absorption edge 107,113
Tucson contest 1995 41
tungsten bronze 269
tungsten inert gas hardfacing (TIG)
89
tungsten oxide see W03
UV mirror
304
water, adsorption and desorption
104
432
Index
waveguide mode 311
waveguiding structures 108
wide-angle X-ray scattering (WAXS)
71
W03 111,135,263,265
- bleaching 193,197
- colouration 193,197
-
crystallographic structures 191
deposition 189
hydrogen content 198
injection of electrons 197
substoichiometric fragments 191
X-ray diffraction of Ti02
6
