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Preface
8 }3 _: w9 s0 c0 F" B7 `2 ?' vWithin a period of a few decades, the ¯eld of materials science and engineering. j1 E0 J5 ]" o# F' k
has emerged as a focal point for developments in virtually all areas1 W$ c6 m8 Y k% G: N4 Z% k) A
of engineering and applied science. The study of thin film materials has
5 i1 B# f; Q J: t0 w1 Jbeen one of the unifying themes in the development of the ¯eld during this
5 V! v, v9 x3 ^( Aperiod. As understood here, the area encompasses ¯lms bonded to relatively
. X) [4 H7 `4 d( C k' h# Athick substrates, multilayer materials, patterned ¯lms on substrates
+ J6 y+ k$ L+ gand free-standing ¯lms. Signi¯cant advances in methods for synthesizing) v" h2 k, k* ?0 T1 u& P
and processing these materials for ever more speci¯c purposes, as well as in
: O7 f# h* W/ G! d5 F! H( Ninstrumentation for characterizing materials at ever diminishing size scales,8 x \' I% X7 f$ `6 a/ Z" a
have been key to modern engineering progress.4 ]$ E$ J- q4 G& @" A
At the dawn of the 21st century, the United States National Academy
6 @$ t5 Y |- ^" ^# h3 bof Engineering reported the outcome of a project intended to identify the+ ?: \5 t" J! ~0 S- L
twenty most signi¯cant engineering achievements of the preceding century.
/ _2 G$ D7 s) y' uIt is evident from the list compiled that achievements of the second half7 n, ?" m) q1 B' R- }0 Q
of the twentieth century { electronics, computers, health technologies, laser
5 U3 E$ l$ W8 C7 S' ?; Wand ¯ber optics, for example { were all based on the creative and e±cient
+ X7 I0 _% A# S: `: n& @exploitation of materials; thin ¯lm materials represent a major component: h- I0 x7 k/ }; f9 p5 u( L4 J N$ D
of this advance in materials technology. In fact, the impact of advances
. B7 l }- O$ \$ x* W, X0 I* ?% s1 r* `in the specialized uses of materials was so pervasive in the achievements) U1 T" t; F" c# `7 _* x& p
being recognized by the Academy that the development of high-performance
8 A7 Y9 I) D# T8 Z. jmaterials itself was included as one of the most signi¯cant achievements.
' B L1 u, Z8 C2 [; I$ Q( GThe goal of this book is to summarize developments in the area of thin4 s3 |2 @- ~- `7 I
¯lm materials that have occurred over the past few decades, with emphasis, g3 ]7 x% S8 M1 L
on the generation of internal stress and its consequences. Internal stress can
" ]9 P4 G* {8 Cinduce a variety of undesirable consequences including excessive deformation,% Q) H) d' B5 ^- z" A
fracture, delamination, permanent deformation and microstructural
9 U$ P# J$ j2 I7 ?8 u0 t5 r; jalterations. In spite of these possibilities, thin ¯lms have been inserted: R; t$ }: T6 S8 {; I) t. X
into engineering systems in order to accomplish a wide range of practical6 \7 g$ i0 s( G @" f- j. ]& j# a
xv$ g5 u& p! ~6 R$ x
xvi Preface
; g% L Z, {2 \* ^7 S2 Lservice functions. Among these are microelectronic devices and packages;5 h! o8 l6 a0 y$ ^' H, P0 d
micro-electro-mechanical systems or MEMS; and surface coatings intended2 b# x4 S0 ?- m0 t6 S A; V! f, z! M
to impart a thermal, mechanical, tribological, environmental, optical, electrical,
; Y% E+ N( H$ D: P# f% K$ X emagnetic or biological function. To a large extent, the success of this' M% V# [ A6 Y9 n. U) ^
endeavor has been enabled by research leading to reliable means for estimating
' E: \ ?# r: m0 ?stress in small material systems and by establishing frameworks in
- m& k2 w, W7 dwhich to assess the integrity or functionality of the systems. The prospect
4 ^9 {, v1 `0 j3 S+ N0 A9 C- L. gfor material failure due to stress continues to be a technology-limiting barrier,: B' a$ |# [. E
even in situations in which load-carrying capacity of the material is not# G! T1 A" A3 m7 e
among its primary functional characteristics. In some circumstances, stress
2 z/ Q* k: u) c) Whas desirable consequences, as in bandgap engineering for electronic applications
1 T8 ]5 G) q8 g# c2 j/ wand in the self-assembly of small structures driven by stored elastic2 R" l! v- I. d5 g! `- o0 F& Q4 {
energy. It is our hope that the information included in this book will be
8 D2 M& v) l" M; ?+ suseful as an indicator of achievements in the ¯eld and as a guide for further
' [, ?8 W/ Q" B. { hadvances in a number of new and emerging directions.2 T M2 G# @% G/ `
The ¯rst chapter is devoted largely to a discussion of the origins of
4 i% f1 `8 {/ h) d6 R$ Nresidual stress in thin ¯lm materials and to identi¯cation of relationships' M8 q" @% x/ g$ O) J; Z
between processing methods and generation of stress. The consequences of. p; ?) J2 q; V5 r8 ^8 l6 [
stress are discussed in subsequent chapters, with the presentation generally3 d& }1 J% D9 [
organized according to the size scale of the dominant physical phenomena
& ^9 k0 I; }" g, w8 cinvolved. Overall deformation of ¯lm-substrate systems or multilayer structures9 `8 ^: u# C" n; \" b
are considered in Chapters 2 and 3. This is followed by examination
4 [# u5 s: W; i. I z/ x; ]5 ]& uof the general failure modes of fracture, delamination and buckling of ¯lms1 [6 D3 a& W( N3 a% _
in Chapters 4 and 5. The focus then shifts to a smaller scale to discuss
0 G3 M. J3 B$ {1 Q7 Yconditions for dislocation formation in Chapter 6 and inelastic deformation( j; h7 s4 j+ s# B5 y
of ¯lms in Chapter 7. Finally, the issues of stability of material surfaces6 L1 T8 w: w8 m
and evolution of surface morphology or alloy composition are considered in7 u' e* l8 Q$ ^+ @- X
Chapters 8 and 9. The consequences of stress in thin ¯lms is linked to the
4 G p& n. ?0 [5 k% d4 ?! J% X( rstructure of the ¯lm materials wherever possible.
& H/ f. Y6 f" \) `0 @It is recognized that each of the principal topics covered in the book/ _' e" P6 |, i l
could itself be developed into a substantial monograph, but the goal here
3 D$ U4 s8 `5 l( `6 Fis not the exhaustive treatment of a topic of limited scope. The area is
/ @, N" w1 G9 V' C1 ~3 |( N# yinherently interdisciplinary, and the intention is a provide a comprehensive+ f) n6 C' J; F, v
coverage of issues relevant to stress and its consequences in thin ¯lm materials.3 j6 v* t6 ]; U$ j
Adoption of this approach meant that many choices had to be made! E( P9 D/ a! A6 `, @1 z
along the way about depth of coverage of speci¯c topics and balance among
" N1 H: G, l f; f$ i1 t/ j4 Adi®erent topics; we hope that the readers will judge the choices made to be
# G" }7 d5 {7 h8 s6 rreasonable. The main purpose of the book is the coherent presentation of2 t; o7 r+ b$ i; {
the sound scienti¯c basis for describing the origins of stress in ¯lms and for& n; v* p" j7 w- c: y% L: u/ I3 j
anticipating the consequences of stress in defect formation, surface evolution
) n6 R# ]" U% n, [- @Preface xvii% @2 M* O! t; q. g: Z! f5 f
and allied e®ects. Many references to original work are included as a guide$ W) ]( l# P2 u' [0 a- C
to the archival literature in the area. In addition, the fundamental concepts
6 p) L9 W% s' Q; wdeveloped are made more concrete through implementation in sample calculations
* i8 Z+ d5 N0 C3 {* r, S# q7 Band through discussion of case studies of practical signi¯cance. The; C5 c9 j1 }6 j
description of experimental methods, results and observations is included' t, L( M" r# q+ x0 H' K$ V
as an integral part of developing the conceptual structure of the topics examined.
8 \. N5 ~1 r7 g: AEach chapter concludes with a set of exercises that further extend
y' ~$ M& G" ~9 N( A; t Athe material discussed, and which can challenge newcomers to the area at$ X; R! [# y' Z1 a2 Z4 X+ z' a
applying concepts. It is our hope that, with this structure, the book will$ ^4 V1 Y+ o/ m& Y
serve as a research reference for those pursuing the area at its frontiers, as a
Q7 `% u1 t& W0 j' k \useful compilation of readily applicable results for practicing engineers, and4 y t( r1 x6 |: j
as a textbook for graduate students or advanced undergraduate students$ Z0 ~& L/ d, ~
wishing to develop background in this area.& t* {1 L8 Q: {" }' a
The idea for the book grew out of a course on thin ¯lms that has& b8 _: }/ P; S9 Q& \
been o®ered for students in solid mechanics and materials science at Brown4 Y. c- ]6 z- W& Y3 J7 g) m; U
University since 1992, as a natural outgrowth of emerging research activity
4 c! P- \( w/ H, x8 m3 ` Sin the area. We are grateful to the many students, postdoctoral research
6 m5 h# c. P" s+ [ e1 e" rassociates and colleagues who attended these lectures and whose enthusiasm, A% F Y7 a3 q; F# B. S
gave this project its initial impetus.9 W& ~& ]1 f9 g% i
We are also grateful to many colleagues who have contributed in various
( p9 c/ k) c3 Q T) F2 v- y% @+ U: vways to the preparation of this book. We particularly thank John
$ h2 N8 \: O) c$ m1 uHutchinson who used a draft of parts of the book for a course for graduate0 m% x- }$ G5 @: O0 A; t
students at Harvard and MIT, and who provided valuable feedback on this
1 ^ n1 j/ i8 ?6 z) g6 Ematerial. Both John Hutchinson and Bill Nix kindly shared with us their
. f$ m$ x, @/ i- n8 V1 ] oown course materials on thin ¯lms. Several colleagues read drafts of various6 H6 B, g5 @! A: E$ E# X, I
sections of the book and o®ered helpful recommendations; they include
( m; U9 ]& S1 _3 J6 zIlan Blech, Eric Chason, Ares Rosakis, Vivek Shenoy and Carl Thompson.
( z% t* M3 I$ B& X9 f2 K5 @Several graduate students who took courses based in part on draft chapters,
8 g* }5 m% \* O" `particularly Yoonjoon Choi and Nuwong Chollacoop, suggested a number of' \% q& H& N1 H# {! Q3 x3 ^2 Y9 J
clari¯cations and improvements in the presentation. Finally, we are grateful
( N1 C/ I( Y1 j4 Y4 A# n( q0 j- ^to the many colleagues who provided ¯gures and micrographs from their
4 X$ C+ `# V2 r" X- I2 T, @own work; in these cases, acknowledgments are noted along with the included( r8 B3 Y6 d8 b9 \+ ?4 i
material. Tim Fishlock at Cambridge University Press o®ered us, v1 e6 b( a* v* ~# |4 Y: `
considerable °exibility in the formulation of the scope of this book and in, s5 L( t8 @' P. n. w
the preparation of the document.
) w* [+ l. Z. I8 U& p- uLBF is grateful to the Materials Research Science and Engineering
7 U! n! ^8 b. l. K O* I, t) kCenter, funded by the National Science Foundation at Brown University,+ L# F: W2 a; `2 N
for long-term support of research in the general area of thin ¯lms and for
% f- v2 Z+ f4 } e0 y8 Mthe collaborations fostered through the Center. He is also thankful to the
# l$ l& [! x7 k$ y' v: ADivision of Engineering and Applied Sciences at Caltech for hosting a sabxviii$ A" C- k$ c' M% b* E5 V* s2 n
Preface
& F" B1 ?! V! q5 c& hbatical leave; the kind hospitality and congenial environment a®orded an
9 N) B- f. |1 jopportunity for pursuing the book writing project at its early critical stage.
7 X6 r6 u ^$ q% ?- ]SS is grateful to the Defense University Research Initiative in NanoTechnology,
' _( I* ^7 t( Q3 rfunded by the O±ce of Naval Research at MIT, and the Programme on
; ~+ c }4 L3 W) UAdvanced Materials for Micro and Nano Systems, funded by the Singapore-
M3 n3 m+ d* ]" C) @- y% A4 n. @MIT Alliance, for their ¯nancial support for research in the areas covered
& B* ] ^0 e4 ]+ f J' N1 ~* Nby the book.( }2 X* h; X% G5 y# {) S
A project of this magnitude would not have been possible without
: x0 C) \7 p% Othe support and encouragement of the members of our families. We are( k+ g% D5 I+ M1 c1 F* b
extremely grateful for their enduring patience and understanding during3 s/ s* @- h6 b4 Z: W
our long hours of immersion in this project over the past several years.+ A) d5 m6 V+ d8 m9 G( f% T' P5 I/ ~
L. B. Freund and S. Suresh
7 e/ F, l) \5 B4 E6 ZJanuary 2003 |
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