Mechanic and Dielectric Properties: Advances in Research and Development

403 Pages·1993·7.162 MB·English

by  Maurice H. Francombe and John L. Vossen (Eds.)

Contributors to This Volume
SCOTT A. BARNETT MAURICE H. FRANCOMBE
CLAES-GÖRAN GRANQVIST STANISLAV KADLEC
S. V. KRISHNASWAMY B. R. MCAVOY
JlNDRICH MUSIL JIRI VYSKOCIL
Physics of Thin Films
Advances in Research and Development
MECHANIC AND DIELECTRIC PROPERTIES
Edited by
Maurice H. Francombe
Department of Physics
The University of Pittsburgh
Pittsburgh, Pennsylvania
John L. Vossen
John Vossen Associates
Technical and Scientific Consulting
Bridgewater New Jersey
y
VOLUME 17
ACADEMIC PRESS, INC.
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Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
SCOTT A. BARNETT (1), Department of Materials Science and Engineering,
Northwestern University, Evanston, Illinois 60208
MAURICE H. FRANCOMBE (145, 225), Department of Physics, The University
of Pittsburgh, Pittsburgh, Pennsylvania 15260
CLAES-GÖRAN GRANQVIST (301), Physics Department, Chalmers University
of Technology and University of Gothenburg, S-412 96 Gothenburg,
Sweden
STANISLAV KADLEC (79), Institute of Physics, Czechoslovak Academy of
Sciences, Na Slovance 2, 180 40 Praha 8, Czechoslovakia
s. v. KRISHNASWAMY (145), Westinghouse Electric Corporation, Science
and Technology Center, 1310 Beulah road, Pittsburgh, Pennsylvania
15235
B. R. MCAVOY (145), Westinghouse Electric Corporation, Science and
Technology Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235
JiNDRiCH MusiL (79), Institute of Physics, Czechoslovak Academy of
Sciences, Na Slovance 2, 180 40 Praha 8, Czechoslovakia
jiRi VYSKOCIL (79), Institute of Physics, Czechoslovak Academy of Sci-
ences, Na Slovance 2, 180 40 Praha 8, Czechoslovakia
ix
Preface
In Volume 17 of Physics of Thin Films we present reviews on five
topics that effectively complement and extend upon chapters contained in
Volumes 15 and 16 of this series. Thus, mechanical aspects of thin
films—and their critical role in applications—were discussed in Vol. 16,
by Reddy, who dealt with recent developments on self-diffusion and
electromigration in relation to integrated circuit needs, and by Woj-
ciechowski and Mendolia, who addressed fracture and cracking phenom-
ena in the context of composite structural materials. The first article in
the present volume, by Scott A. Barnett, "Deposition and Mechanical
Properties of Superlattice Thin Films," discusses the growth, charac-
terization, and mechanical behavior of films comprising multilayers
primarily of metal and refractory metallic compound components.
The observed enhancement of elastic and hardness properties in
these interesting new materials, relative to the properties of single-
component layers, has been a subject for much theoretical speculation
and experimental research. Barnett's treatment describes techniques
for controlled synthesis of these structures, their structural and
mechanical evaluation (hardness, tensile strength, etc.), and analysis
of the properties of metal/metal, metal/ceramic, and ceramic/
ceramic superlattices.
The second chapter, titled "Hard Coatings Prepared by Sputtering and
Arc Evaporation," by J. Musil, J. Vyskocil, and S. Kadlec, continues the
theme of mechanical properties, but in this case mainly of compound
layers chosen for their refractory character and intrinsic hardness. The
materials suitable for these applications usually possess high melting
points — hence the focus in this article on sputtering and arc evaporative
approaches. The hard coatings of greatest interest and utility embrace
binary compounds displaying metallic (TiN, TaB , etc.), covalent (B C,
2 4
C(diamond), etc.) and ionic (A1 0 , Zr0 , etc.) properties, both in
2 3 2
single- and multiphase form. The various roles of multilayer and gradient
XI
Xll PREFACE
structures, and of film crystallinity, crystal orientation, and morphology,
in influencing properties such as hardness, bonding to the substrate,
coating smoothness, and friction behavior also are discussed.
One of the most successful applications of thin crystalline films of polar
dielectric materials has been as high-frequency transducer structures for
signal processing devices utilizing bulk (BAW) or surface (SAW) acoustic
waves. The unique role played by piezoelectric films in such devices is
reviewed in the third article, by S. V. Krishnaswamy, B. R. McAvoy,
and M. H. Francombe. This technology has been made possible primarily
by two factors: (a) the slow velocity of acoustic waves corresponds to
wavelengths that (for UHF to microwave frequencies) require transducer
thicknesses in the 0.1 to 10 μπι range, i.e., dimensions most easily
achieved by thin-film growth; and (b) highly oriented films of the strongly
piezoelectric wurtzite-type structures such as ZnO and A1N can be grown
readily at low temperatures by simple magnetron sputtering processes.
Functionally, the resulting devices resemble strongly those based upon
magnetostatic wave (MSW) technology, as described by Adam et al. in
Volume 15 of Physics of Thin Films. However, several of the acoustic
devices are now amenable to much greater size reduction than their
MSW counterparts, and they can be integrated directly into Si and GaAs
microwave integrated circuits. The authors illustrate the exciting poten-
tial of this field with examples of SAW correlators and convolvers,
miniature low-noise frequency sources, and channelizer filter banks using
arrays of thin-film acoustic resonators, and discuss new applications, e.g.,
in the area of solid-state sensors.
The fourth chapter, "Ferroelectric Films for Integrated Electronics,"
by M. H. Francombe, complements the preceding article on microwave
acoustics, at least in relation to materials and certain applications.
However, the hysteretic (polarization vs. field) behavior, pyroelectric
response, and unusual electro-optic characteristics of ferroelectrics open
up a much wider range of potential applications for which the availability
of high-quality thin-film structures offers significant advantages. This
review demonstrates, for example, that for uncooled pyroelectric IR
imagers, oriented or epitaxial ferroelectric films of PbTi0 -based com-
3
positions offer superior performance and can be integrated directly into
silicon circuits. Similarly, transparent epitaxial films of PLZT solutions
and of Bi Ti 0 provide a new technological base as modulators,
4 3 12
switches, and displays in a variety of integrated optic applications. By far
the most significant effort in this field at present involves the development
of ferroelectric films for high-performance semiconductor-based
memories. Both discrete (ferroelectric) capacitor memory cells and
PREFACE Xlll
"monolithic" ferroelectric field effect transistor (FEMFET) structures
have been studied and have already been demonstrated successfully in
functioning silicon memory arrays.
The fifth and final chapter, ''Electrochromic Tungsten-Oxide-Based
Thin Films: Physics, Chemistry and Technology" by C.-G. Granqvist
complements the preceeding chapter in that it describes the underlying
physics and chemistry of another type of optical switching device.
Electrochromic devices can be used to modulate diffuse reflectance,
specular reflectance, luminous transmittance, or solar transmittance, and
therefore have numerous potential applications as nonemissive displays,
variable reflectance mirrors, and "smart windows" that can control light
levels in buildings and/or provide energy efficiency by minimizing heating
and cooling. Electrochromic devices involve four or five thin-film layers,
but at least one must be the "active" layer, that is one into which light
ions can be injected and from which they can be extracted using voltage
pulses to produce persistent changes in the optical properties of the layer.
Tungsten oxide is the most widely studied and used active layer for these
devices. This chapter reviews the methods of deposition, electrochemical
and physical characterization techniques, and the optical properties of
these films, along with a survey of the other layers required to form a
complete electrochromic device.
M. H. Francombe
J. L. Vossen
Deposition and Mechanical Properties
of Superlattice Thin Films
SCOTT A. BARNETT
Department of Materials Science and Engineering,
Northwestern University, Evans ton, Illinois
I. Introduction 2
II. Deposition Techniques 3
III. Characterization 8
A. X-Ray Diffraction 10
1. Sinusoidal Composition Modulation 12
2. Square-Wave Composition Modulation 14
3. Numerical Calculations 15
4. Crystal Structure 17
B. Transmission Electron Microscopy 17
C. Other Techniques 22
IV. Deposition Mechanisms, Structure, and Stability 22
A. Crystal Structure and Lattice Relaxation 23
1. Structure and Morphology 23
2. Coherency Strains 24
B. Nucleation and Layer Morphology 30
C. Interdiffusion 35
1. Linearized Diffusion Equation Approach 35
2. Chemical Interaction Effects 36
3. Strain Effects 40
4. Large-Amplitude Composition Modulations 41
D. Summary 42
V. Mechanical Property Measurements 42
A. Elastic Moduli and Constants 42
B. Hardness and Yield Strength 46
VI. Elastic Properties 47
A. Experimental Results 48
B. Theoretical Predictions 54
1. Supermodulus Effect 54
2. Elastic Anomalies 56
VII. Mechanical Strength and Hardness 59
A. Experimental Results 60
1. Metal/Metal Superlattices 60
2. Ceramic/Ceramic Superlattices 63
3. Metal/Ceramic Superlattices 64
1
Copyright © 1993 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-533017-0
2 S. A. BARNETT
B. Theoretical Predictions 65
1. Small-Period Superlattices 66
2. Large-Period Superlattices 68
C. Comparison of Experiment and Theory 70
VIII. Conclusions 71
Acknowledgments 73
References 73
I. Introduction
In this article, the deposition, structure, chemistry, and mechanical
properties of artificial superlattice thin films are described. Artificial
superlattice thin films, or simply "superlattices," are broadly defined as
thin films formed by alternately depositing two different components to
form a layered structure. A variety of terms have been used to describe
thin films with different types of composition modulations. In this article,
however, the term superlattice will be applied to any artificially layered
thin films.
Interest in superlattices first arose in the 1940s. DuMond and Youtz (I)
appear to be the first to report the fabrication of a thin film with an
artificial composition modulation. They attempted to deposit Cu/Au
superlattices for use as x-ray mirrors, and succeeded in observing x-ray
reflections attributable to the composition modulation. Since that time,
thin-film superlattices of a wide range of materials have been investigated
both as media for studies of basic processes, such as diffusion, and as a
means for altering materials properties for a range of technological
applications. Examples of properties that have been studied in detail and
reviewed recently include x-ray reflection (2), neutron reflection (3),
semiconductor optical and electrical properties (4), superconducting
behavior (5,6), magnetic properties (5, 7), and metallic electrical
properties (5). One aspect of superlattices that has been the subject of
great interest and controversy, but has not been reviewed in detail, is
mechanical properties. This is the main topic of the present chapter.
Elastic properties have been a main focus of research on metallic
superlattices since the first report of the so-called supermodulus effect
(#), a dramatic (100-400%) enhancement in elastic modulus observed
only for specific superlattice modulation wavelengths A~2nm. This and
similar results for other metallic superlattices led to considerable effort to
explain the effect. However, there is also considerable controversy based
on doubts concerning the elastic modulus measurement techniques and
the lack of experimental confirmation from other groups. Recent
SUPERLATTICE THIN FILMS 3
attempts to reproduce the supermodulus effect using the same materials
but different measurement techniques have failed. Experiments on other
metallic superlattices using different measurement techniques showed
much smaller (—10%) changes in elastic constants, usually of opposite
sign to the supermodulus effect. At present, several issues regarding
superlattice elastic properties remain unresolved.
Superlattice plastic behavior was of interest even before the first
supermodulus effect report. Palatnik et al. (9) reported an increase by a
factor of ~2 in the hardness H of Cu/Fe superlattices with decreasing
layer thickness. Subsequent investigations have shown substantial in-
creases in hardness, yield strength, or tensile breaking stress in both
metal and transition-metal nitride superlattices. A few explanations of
the strengthening effect have been proposed, including differences in
layer dislocation line energies, coherency stresses, and dislocation block-
ing similar to that observed in bulk polycrystalline materials.
Mechanical property investigations have centered on metal and met-
allic superlattices; hence, these materials will constitute the focus of the
chapter. The currently available data on mechanical properties will be
presented with the hope of clarifying the current understanding of both
elastic and plastic properties. This discussion is in Sections VI and VII.
For completeness, the growth (Section II), structure characterization
techniques (Section III), chemical and structural order (Section IV), and
mechanical property measurements (Section V) of superlattices will also
be described. Section VIII is a brief conclusion.
II. Deposition Techniques
Sputter deposition and evaporation have been used extensively to
fabricate metallic superlattices. These techniques are described in more
detail later. There has also been considerable recent interest in electro-
deposition of metallic superlattices (10), where layering is achieved by
sample rotation and alternately varying the deposition potential. Chemi-
cal vapor deposition (CVD) can be used as well, assuming that the
substrate temperatures are low enough to avoid intermixing, but there
are no reports in the literature of CVD of metallic superlattices.
Superlattice sputter deposition and evaporation systems are similar to
conventional multisource deposition chambers. One additional concern
for superlattices is contamination at the interfaces, due to the vacuum
system residual gas background pressure, since deposition is often