The Materials Science of Thin Films 2010 pptx

The
Materials Science
of
Thin Films



I

I

I

The
Materials Science
of

Milton Ohring
Stevens Institute of Technology
Department of Materials Science and Engineering
Hoboken, New Jersey

Academic Press
San Diego New York Boston
London Sydney Tokyo Toronto


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Library of Congress Cataloging-in-Publication D t
aa
Ohring. Milton, date.
The materials science of thin f l s / Milton Ohring.
im
cm.
p.
Includes bibliograpbical references and indcx.
ISBN 0-12-524990-X (Alk. paper)
1. Thin films. I. Title.
TA418.9.T45oQ7
1991
620'.44-&20

91-9664

CIP
Printed in the United States of America
99 00 01 02 03 M V 1 1 10 9 8 7


+

Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi
...
xiii

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xvii

Thin Films - A Historical Perspective

........................

xix

Chapter 1

A Review of Materials Science . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3. Defects in Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
1.4.
1.5.
1.6.
1.7.
1.8.

Bonding of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermodynamics of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . .
Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14
21
33
40
43
43
46

Chapter 2

Vacuum Science and Technology . . . . . . . . . . . . . . . . . . . . .
2.1. Kinetic Theory of Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2. Gas Transport and Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49
49
55
V


vi

Contents

2.3. Vacuum Pumps and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Excercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

62
75
77

Chapter 3

Physical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
3.2. The Physics and Chemistry of Evaporation . . . . . . . . . . . . . . . . . . . 81

3.3. Film Thickness Uniformity and Purity . . . . . . . . . . . . . . . . . . . . . .
87
3.4. Evaporation Hardware and Techniques . . . . . . . . . . . . . . . . . . . . .
96
3.5. Glow Discharges and Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.6. Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
109
3.7. Sputtering Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
118
3.8. Hybrid and Modified PVD Processes . . . . . . . . . . . . . . . . . . . . . .
132
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
140
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
144
Chapter 4

Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . .
147
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4.2. Reaction Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
149
4.3. Thermodynamics of CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
155
4.4. Gas Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
162
167
4.5. Growth Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6. CVD Processes and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .
177

Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
190
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
193
Chapter 5

Film Formation and Structure . . . . . . . . . . . . . . . . . . . . . . . . 195
5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
195
5.2. Capillarity Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
198
5.3. Atomistic Nucleation Processes . . . . . . . . . . . . . . . . . . . . . . . . . .
206
5.4. Cluster Coalescence and Depletion . . . . . . . . . . . . . . . . . . . . . . .
213
5.5. Experimental Studies of Nucleation and Growth . . . . . . . . . . . . . .219
223
5.6. Grain Structure of Films and Coatings . . . . . . . . . . . . . . . . . . . . .
5.7. Amorphous Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
234
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
243
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246


Contents

vii

Chapter 6

Characterization of Thin Films . . . . . . . . . . . . . . . . . . . . . . . 249
6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
6.2. Film Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
252
6.3. Structural Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
265
6.4. Chemical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

275
300
305

Chapter 7

Epitaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
7.2. Structural Aspects of Epitaxial Films . . . . . . . . . . . . . . . . . . . . . .
310
7.3. Lattice Misfit and Imperfections in Epitaxial Films . . . . . . . . . . . . . 316
7.4. Epitaxy of Compound Semiconductors . . . . . . . . . . . . . . . . . . . . . 322
7.5. Methods for Depositing Epitaxial Semiconductor Films . . . . . . . . .331
7.6. Epitaxial Film Growth and Characterization . . . . . . . . . . . . . . . . .339
7.7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
350
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
353

Chapter 8

Interdiffusion and Reactions in Thin Films . . . . . . . . . . . . . 355
8.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
8.2. Fundamentals of Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
357
8.3. Interdiffusion in Metal Alloy Films . . . . . . . . . . . . . . . . . . . . . . .
372
8.4. Electromigration in Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . .
379
8.5. Metal-Semiconductor Reactions . . . . . . . . . . . . . . . . . . . . . . . . .
8.6. Silicides and Diffusion Barriers . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7. Diffusion During Film Growth . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

385
389
395
398
401

Chapter 9

Mechanical Properties of Thin Films . . . . . . . . . . . . . . . . . .403
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
403

9.1.
9.2.

9.3.
9.4.
9.5.

Introduction to Elasticity. Plasticity. and Mechanical Behavior . . . . . 405
Internal Stresses and Their Analysis . . . . . . . . . . . . . . . . . . . . . . .
413
Stress in Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
420
Relaxation Effects in Stressed Films . . . . . . . . . . . . . . . . . . . . . . 432


viii

Contents

9.6. Adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

439
446
449

Chapter 10

Electrical and Magnetic Properties of Thin Films . . . . . . . 451
10.1. Introduction to Electrical Properties of Thin Films . . . . . . . . . . . . 451
Conduction in Metal Films . . . . . . . . . . . . . . . . . . . . . . . . . . . .
455

Electrical Transport in Insulating Films . . . . . . . . . . . . . . . . . . . 464
Semiconductor Contacts and MOS Structures . . . . . . . . . . . . . . . 472
Superconductivity in Thin Films . . . . . . . . . . . . . . . . . . . . . . . .
480
Introduction to Ferromagnetism . . . . . . . . . . . . . . . . . . . . . . . . .
485
Magnetic Film Size Effects - M, versus Thickness and
Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
489
10.8. Magnetic Thin Films for Memory Applications . . . . . . . . . . . . . . 493
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
505

10.2.
10.3.
10.4.
10.5.
10.6.
10.7.

Chapter 1 1

Optical Properties of Thin Films . . . . . . . . . . . . . . . . . . . . . . 507
11.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
507
11.2. Properties of Optical Film Materials . . . . . . . . . . . . . . . . . . . . . .
508
11.3. Thin-Film Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
524

11.4. Multilayer Optical Film Applications . . . . . . . . . . . . . . . . . . . . .
531
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
542
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544
Chapter 72

Metallurgical and Protective Coatings . . . . . . . . . . . . . . . . . 547
12.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547
12.2. Hard Coating Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 551
12.3. Hardness and Fracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561
12.4. Tribology of Films and Coatings . . . . . . . . . . . . . . . . . . . . . . . .
570
12.5. Diffusional, Protective, and Thermal Coatings . . . . . . . . . . . . . . .580
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
585
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587
Chapter 13

Modification of Surfaces and Films . . . . . . . . . . . . . . . . . . . 589
13.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589
13.2. Lasers and Their Interactions with Surfaces . . . . . . . . . . . . . . . . .591


Contents

ix

13.3. Laser Modification Effects and Applications . . . . . . . . . . . . . . . . 602
13.4. Ion-Implantation Effects in Solids . . . . . . . . . . . . . . . . . . . . . . . 609

13.5. Ion-Beam Modification Phenomena and Applications . . . . . . . . . . 616
624
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
626
Chapter 1 4

Emerging Thin-Film Materials and Applications . . . . . . . . . 629
14.1. Film-PatterningTechniques . . . . . . . . . . . . . . . . . . . . . . . . . . .
630
14.2. Diamond Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635
14.3. High-T, Superconductor Films . . . . . . . . . . . . . . . . . . . . . . . . .
641
14.4. Films for Magnetic Recording . . . . . . . . . . . . . . . . . . . . . . . . . . 645
650
14.5. Optical Recording . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.6. Integrated Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
654
14.7. Superlattices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 661
14.8. Band-Gap Engineering and Quantum Devices . . . . . . . . . . . . . . .669
14.9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
678
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 678
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681
Appendix 1

Physical Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

685

Appendix 2

Selected Conversions . . .
index . . . . . . . . . . . . . .

.......................

687

. . . . . . . . . . . . . . . . . .689



Foreword

It is a distinct pleasure for me to write a foreword to this new textbook by my
long-time friend, Professor Milt Ohring.
There have been at least 200 books written on various aspects of thin film
science and technology, but this is the first true textbook, specifically intended
for classroom use in universities. In my opinion there has been a crying need
for a real textbook for a long time. Most thin film courses in universities have
had to use many books written for relatively experienced thin film scientists
and engineers, often supplemented by notes prepared by the course instructor.
The Materials Science of Thin Films, a true textbook, complete with
problems after each chapter, is available to serve as a nucleus for first courses
in thin film science and technology.
In addition to his many years of experience teaching and advising graduate
students at Stevens Institute of Technology, Professor Ohring has been the
coordinator of an on-premises, M.S. degree program offered by Stevens at the
AT&T Bell Laboratories in Murray Hill and Whippany, New Jersey. This

ongoing cooperative program has produced over sixty M.S. graduates to date.
Several of these graduates have gone on to acquire Ph.D. degrees. The
combination of teaching, research, and industrial involvement has provided
Professor Ohring with a broad perspective of thin film science and technology
and tremendous insight into the needs of students entering this exciting field.
His insight and experience are quite evident in this textbook.

John L. Vossen

xi



+

Preface

Thin-film science and technology play a crucial role in the high-tech industries
that will bear the main burden of future American competitiveness. While the
major exploitation of thin films has been in microelectronics, there are
numerous and growing applications in communications, optical electronics,
coatings of all kinds, and in energy generation and conservation strategies. A
great many sophisticated analytical instruments and techniques, largely developed to characterize thin films and surfaces, have already become indispensable in virtually every scientific endeavor irrespective of discipline. When I
was called upon to offer a course on thin films, it became a genuine source of
concern to me that there were no suitable textbooks available on this unquestionably important topic. This book, written with a materials science flavor, is
a response to this need. It is intended for
Science and engineering students in advanced undergraduate or first-year
graduate level courses on thin films
2 . Participants in industrial in-house courses or short courses offered by
professional societies

3. Mature scientists and engineers switching career directions who require an
overview of the field.
1.

Readers should be reasonably conversant with introductory college chemistry and physics and possess a passive cultural familiarity with topics commonly treated in undergraduate physical chemistry and modem physics courses.


xiv

Preface

It is worthwhile to briefly elaborate on this book’s title and the connection
between thin films and the broader discipline of materials science and engineering. A dramatic increase in our understanding of the fundamental nature of
materials throughout much of the twentieth century has led to the development
of materials science and engineering. This period witnessed the emergence of
polymeric, nuclear, and electronic materials, new roles for metals and ceramics, and the development of reliable methods to process these materials in bulk
and thin-film form. Traditional educational approaches to the study of materials have stressed structure-property relationships in bulk solids, typically
utilizing metals, semiconductors, ceramics; and polymers, taken singly or
collectively as illustrative vehicles to convey principles. The same spirit is
adopted in this book except that thin solid films are the vehicle. In addition,
the basic theme has been expanded to include the multifaceted processingstructure-properties-performance interactions. Thus the original science
core is preserved but enveloped by the engineering concerns of processing
and performance. Within this context, I have attempted to weave threads of
commonality among seemingly different materials and properties, as well as to
draw distinctions between materials that exhibit outwardly similiar behavior. In
particular, parallels and contrasts between films and bulk materials are recurring themes.
An optional introductory review chapter on standard topics in materials
establishes a foundation for subsequent chapters. Following a second chapter
on vacuum science and technology, the remaining text is broadly organized
into three categories. Chapters 3 and 4 deal with the principles and practices of

film deposition from the vapor phase. Chapters 5-9 deal with the processes
and phenomena that influence the structural, chemical, and physical attributes
of films, and how to characterize them. Topics discussed include nucleation,
growth, crystal perfection, epitaxy, mass transport effects, and the role of
stress. These are the common thin-film concerns irrespective of application.
The final portion of the book (Chapters 10-14) is largely devoted to specific
film properties (electrical, magnetic, optical, mechanical) and applications, as
well as to emerging materials and processes. Although the first nine chapters
may be viewed as core subject matter, the last five chapters offer elective
topics intended to address individual interests. It is my hope that instructors
using this book will find this division of topics a useful one.
Much of the book reflects what is of current interest to the thin-film research
and development communities. Examples include chapters on chemical vapor
deposition, epitaxy, interdiffusion and reactions, metallurgical and protective
coatings, and surface modification. The field is evolving so rapidly that even
the classics of yesteryear, e.g., Maissel and Glang, Handbook of Thin Film


Preface

xv

Technology (1970) and Chopra, Thin Film Phenomena (1969), as well as
more recent books on thin films, e.g., Pulker, Coatings on Glass (1984), and
Eckertova, Physics of Thin Films (1986), make little or no mention of these
now important subjects.
As every book must necessarily establish its boundaries, I would like to
point out the following: (1) Except for coatings (Chapter 12) where thicknesses
range from several to as much as hundreds of microns (1 micron or 1
pm = lop6 meter), the book is primarily concerned with films that are less

than 1 pm thick. (2) Only films and coatings formed from the gas phase by
physical (PVD) or chemical vapor deposition (CVD) processes are considered.
Therefore spin and dip coating, flame and plasma spraying of powders,
electrolytic deposition, etc., will not be treated. (3) The topic of polymer films
could easily justify a monograph of its own, and hence will not be discussed
here. (4) Time and space simply do not allow for development of all topics
from first principles. (Nevertheless, I have avoided using the unwelcome
phrase “It can be shown that . . . ,” and have refrained from using other
textbooks or the research literature to fill in missing steps of derivations.) (5) A
single set of units (e.g., CGS, MKS, SI, etc.) has been purposely avoided to
better address the needs of a multifaceted and interdisciplinary audience.
Common usage, commercial terminology, the research literature and simple
bias and convenience have all played a role in the ecumenical display of units.
Where necessary, conversions between different systems of units are provided.
At the end of each chapter are problems of varying difficulty, and I believe a
deeper sense of the subject matter will be gained by considering them. Three
very elegant problems (Le. 9-6, -7, -8) were developed by Professor W. D.
Nix, and I thank him for their use.
By emphasizing immutable concepts, I hope this book will be spared the
specter of rapid obsolescence. However, if this book will in some small
measure help spawn new technology rendering it obsolete, it will have served a
useful function.
Milton Ohring



Acknowledgments

At the top of my list of acknowledgments I would like to thank John Vossen
for his advice and steadfast encouragement over a number of years. This book

would not have been possible without the wonderfully extensive intellectual
and physical resources of AT & T Bell Laboratories, Murray Hill, NJ, and the
careful execution of the text and figures at Stevens Institute. In particular Bell
Labs library was indispensable and I am indebted to AT & T for allowing me
to use it. My long association with Bell Labs is largely due to my dear friend
L. C. Kimerling (Kim), and I thank him and A T & T for supporting my efforts
there. I am grateful to the many Bell Labs colleagues and students in the
Stevens Institute of Technology/Bell Labs “On Premises Approved Program
(OPAP),” who planted the seed for a textbook on thin films. In this regard
D. C. Jacobson should be singled out for his continuous help with many
aspects of this work. The following Bell Labs staff members contributed to
this book through helpful comments and discussions, and by contributing figures, problems, and research papers: J. C. Bean, J. L. Benton, W. L. Brown,
F. Capasso, G. K. Celler, A. Y. Cho, J. M. Gibson, H. J. Gossmann, R. Hull,
R. W. Knoell, R. F. Kopf, Y. Kuk, H. S. Luftman, S. Nakahara, M. B. Panish,
J. M. Poate, S. M. Sze, K. L. Tai, W. W. Tai, H. Temkin, L. F. Thompson,

L. E. Trimble, M. J. Vasile, and R. Wolfe. I appreciate their time and effort
spent on my behalf.
Some very special people at Stevens enabled the book to reach fruition.
They include Pat Downes for expertly typing a few versions of the complete

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xviii

Acknowledgments

text during evenings that she could have spent more pleasantly; Eleanor
Gehler, for kindly undertaking much additional typing; Kamlesh Pate1 for his

professional computerized drafting of the bulk of the figures; Chris Rywalt,
Manoj Thomas, and Tao Jen for carefully rendering the remainder of the
figures; Mehboob Alam and Warren Moberly for their computer help in
compiling the index, and drafting the cover, respectively; Dick Widdicombe,
Bob Ehrlich, Dan Schwarcz, Lauren Snyder, and Noemia Carvalho for many
favors; Professor R. Weil for helpful comments; Profs. W. Carr, H. Salwen,
and T. Hart for their expert and generous assistance on several occasions;
Professor B. Gallois and G. M. Rothberg for support; and those at Stevens
responsible for granting my sabbatical leave in 1988. My sincere thanks to all
of you.
Lastly, I am grateful to several anonymous reviewers for many pertinent
comments and for uncovering textual errors. They are absolved of all responsibility for any shortcomings that remain.
This book is lovingly dedicated to Ahrona, Avi, Noam, and Feigel, who in
varying degrees had to contend with a less that a full-time husband and father
for too many years.


Thin Films - A
Historical Perspective

Thin-film technology is simultaneously one of the oldest arts and one of the
newest sciences. Involvement with thin films dates to the metal ages of
antiquity. Consider the ancient craft of gold beating, which has been practiced
continuously for at least four millenia. Gold’s great malleability enables it to be
hammered into leaf of extraordinary thinness while its beauty and resistance to
chemical degradation have earmarked its use for durable ornamentation and
protection purposes. The Egyptians appear to have been the earliest practitioners of the art of gold beating and gilding. Many magnificent examples of
statuary, royal crowns, and coffin cases which have survived intact attest to the
level of skill achieved. The process involves initial mechanical rolling followed
by many stages of beating and sectioning composite structures consisting of

gold sandwiched between layers of vellum, parchment, and assorted animal
skins. Leaf samples from Luxor dating to the Eighteenth Dynasty (1567-1320
B.C.) measured 0.3 microns in thickness. As a frame of reference for the
reader, the human hair is about 75 microns in diameter. Such leaf was
carefully applied and bonded to smoothed wax or resin-coated wood surfaces
in a mechanical (cold) gilding process. From Egypt the art spread as indicated
by numerous accounts of the use of gold leaf in antiquity.
Today, gold leaf can be machine-beaten to 0.1 micron and to 0.05 micron
when beaten by a skilled craftsman. In this form it is invisible sideways and
quite readily absorbed by the skin. It is no wonder then that British gold
beaters were called upon to provide the first metal specimens to be observed

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Thin Films - A Historical Perspective

in the transmission electron microscope. Presently, gold leaf is used to deco-

rate such diverse structures and objects as statues, churches, public buildings,
tombstones, furniture, hand-tooled leather, picture frames and, of course,
illuminated manuscripts.
Thin-film technologies related to gold beating, but probably not as old, are
mercury and fire gilding. Used to decorate copper or bronze statuary, the cold
mercury process involved carefully smoothing and polishing the metal surface,
after which mercury was rubbed into it. Some copper dissolved in the
mercury, forming a very thin amalgam film that left the surface shiny and
smooth as a mirror. Gold leaf was then pressed onto the surface cold and

bonded to the mercury-rich adhesive. Alternately, gold was directly amalgamated with mercury, applied, and the excess mercury was then driven off by
heating, leaving a film of gold behind. Fire gilding was practiced well into the
nineteenth century despite the grave health risk due to mercury vapor. The
hazard to workers finally became intolerable and provided the incentive to
develop alternative processes, such as electroplating.
The history of gold beating and gilding is replete with experimentation and
process development in diverse parts of the ancient world. Practitioners were
concerned with the purity and cost of the gold, surface preparation, the
uniformity of the applied films, adhesion to the substrate, reactions between
and among the gold, mercury, copper, bronze (copper-tin), etc., process
safety, color, optical appearance, durability of the final coating, and competitive coating technologies. As we shall see in the ensuing pages, modem
thin-film technology addresses these same generic issues, albeit with a great
compression of time. And although science is now in the ascendancy, there is
still much room for art.

REFERENCES
1. L. B. Hunt, Gold Bull. 9, 24 (1976).
2. 0. Vittori, Gold Bull. 12, 35 (1979).
3. E. D. Nicholson, Gold Bull. 12, 161 (1979).


I
Chapter I

A Review of
Materials Science

1.I.INTRODUCTION
A cursory consideration of the vast body of solid substances reveals what
outwardly appears to be an endless multitude of external forms and structures

possessing a bewildering variety of properties. The branch of study known as
materials science and engineering evolved in part to classify those features that
are common among the structure and properties of different materials in a
manner somewhat reminiscent of chemical or biological classification schemes.
This dramatically reduces the apparent variety. From this perspective, it turns
out that solids can be classified as typically belonging to one of only four
categories (metallic, ionic, covalent, or van der Waals), depending on the
nature of the electronic structure and resulting interatomic bonding forces.
Similar divisions occur with respect to the structure of solids. Solids are
either internally crystalline or noncrystalline. Those that are crystalline can be
further subdivided according to one of 14 different geometric arrays or lattices,
depending on the placement of the atoms. When properties are considered,
there are similar simplifying categorizations. Thus, materials are either good,
intermediate, or poor conductors of electricity, and they are either mechanically brittle or can easily be stretched without fracture, and they are either

1


2

A Review of Materials Science

optically reflective or transparent, etc. It is, of course, easier to recognize that
property differences exist than to understand why they exist. Nevertheless,
much progress has been made in this subject as a result of the research of the
past 50 years. Basically, the richness in the diversity of materials properties
occurs because countless combinations of the admixture of chemical compositions, bonding types, crystal structures, and morphologies are available naturally or can be synthesized.
In this chapter various aspects of structure and bonding in solids are
reviewed for the purpose of providing the background to better understand the
remainder of the book. In addition, several topics dealing with thermodynamics and kinetics of atomic motion in materials are also included. These will

later have relevance to aspects of the stability, formation, and solid-state
reactions in thin films. Much of this chapter is a condensed adaptation of
standard treatments of bulk materials, but it is equally applicable to thin films.
Nevertheless, many distinctions between bulk materials and films exist, and
they will be stressed where possible. Readers already familiar with concepts of
materials science may wish to skip this chapter; those who seek deeper and
broader coverage should consult the bibliography for recommended texts on
this subject.

1.2. STRUCTURE
1.2.1. Crystalline Solids

Many solid materials possess an ordered internal crystal structure despite
external appearances that are not what we associate with the term crystalline-Le., clear, transparent, faceted, etc. Actual crystal structures can be
imagined to arise from a three-dimensional array of points geometrically and
repetitively distributed in space such that each point has identical surroundings.
There are only 14 ways to arrange points in space having this property, and the
resulting point arrays are known as Bravais lattices. They are shown in Fig.
1-1 with lines intentionally drawn in to emphasize the symmetry of the lattice.
Only a single cell for each lattice is reproduced here, and the point array
actually stretches in an endlessly repetitive fashion in all directions. If an atom
or group of two or more atoms is now placed at each Bravais lattice point, a
physically real crystal structure emerges. Thus, if individual copper atoms
populated every point of a face-centered cubic (FCC)0lattice whose cube edge
dimension, or so-called lattice parameter, were 3.615 A, the material known as