薄膜资料(Thin film Material)连续送第3部分! Introduction and Overview

xx_912 · 发表于 2006-3-11 19:12:44

1
Introduction and Overview
Thin solid ¯lms have been used in many types of engineering systems and
have been adapted to ful¯ll a wide variety of functions. A few examples
follow.
¡ Great strides in thin ¯lm technology have been made in order to
advance the rapid development of miniature, highly integrated electronic
circuits. In such devices, con¯nement of electric charge relies
largely on interfaces between materials with di®ering electronic properties.
Furthermore, the need for thin materials of exceptionally high
quality, reproducible characteristics and reliability has driven ¯lm
growth technology through a rapid succession of signi¯cant achievements.
More recently, progress in the physics of material structures
that rely on quantum con¯nement of charge carriers continues to
revolutionize the area. These systems present new challenges for materials
synthesis, characterization and modeling.
¡ The use of surface coatings to protect structural materials in high
temperature environments is another thin ¯lm technology of enormous
commercial signi¯cance. In gas turbine engines, for example,
thin surface ¯lms of materials chosen for their chemical inertness,
stability at elevated temperatures and low thermal conductivity are
used to increase engine e±ciency and to extend signi¯cantly the useful
lifetimes of the structural materials that they protect. Multilayed or
continuously graded coatings o®er the potential for further progress
in this e®ort.
¡ The useful lifetimes of components subjected to friction and wear due
to contact can be extended substantially through the use of surface
1
2 Introduction and Overview
coatings or surface treatments. Among the technologies that rely on
the use of thin ¯lms in this way are internal combustion engines, arti
¯cial hip and knee implants, and computer hard disks for magnetic
data storage.
¡ Thin ¯lms are integral parts of many micro-electro-mechanical systems
designed to serve as sensors or actuators. For example, a piezoelectric
or piezoresistive thin ¯lm deposited on a silicon membrane
can be used to detect electronically a de°ection of the membrane in
response to a pressure applied on its surface or by an acceleration of
its supports. Devices based on thin ¯lm technology are used as microphones
in hearing aids, monitors of blood pressure during exercise,
electronically positioned thin ¯lm mirrors on °exible supports in optical
display systems, and probes for detecting the degree of ripeness
of fruits.
Numerous other technologies rely on thin ¯lm behavior. An immediate
observation that follows from the foregoing list is that the principal
function of the thin ¯lm components in these applications is often not structural.
Consequently, load carrying capacity may not be a principal consideration
for design or material selection. However, fabrication of thin ¯lm
con¯gurations typically results in internal stress in the ¯lm of a magnitude
su±cient to induce mechanical deformation, damage or failure. A tendency
for stress-driven failure of a thin ¯lm structure can be a disabling barrier to
incorporation of that ¯lm into a system, even when load carrying capacity
is of secondary importance as a functional characteristic. The presence of
an internal stress in a thin ¯lm structure may also in°uence the electrical
or magnetic properties in functional devices.
This chapter provides an overview of commonly used deposition and
processing methods for the synthesis and fabrication of thin ¯lm and such
structures. This is followed by a discussion of the fabrication of small volume
structures, where examples of the basic steps involved in lithography,
surface micromachining, bulk micromachining, and molding processes are
considered in the context of the manufacture of microelectronic devices,
as well as small structures encountered in the development of micro-electromechanical
systems (MEMS) and nano-electro-mechanical systems (NEMS).
Attention is devoted to the e®ects of processing on the nucleation and growth
of monocrystalline and polycrystalline thin ¯lms on substrates, the evolution
of ¯lm microstructure in polycrystalline ¯lms, and the generation of internal
stresses through processing. In the chapters that follow, consequences of
1.1 A classification of thin film configurations 3
stress in thin ¯lm and multilayer materials is studied, organized according
to the deformation or failure phenomena which can be induced by internal
stress.
1.1 A classification of thin film configurations
As a guide for the development and application of concepts that are useful
for describing mechanical behavior of solid thin ¯lm and multilayer materials,
it is convenient to classify structures in terms of their geometrical
con¯gurations and the nature of constraint on their deformation imposed by
their surroundings. For this purpose, con¯gurations are classi¯ed in terms
of the relative extent of the solid bodies in three orthogonal directions, with
the orientation of the reference coordinate system being dictated by the con-
¯guration. The degree of constraint is determined by the interaction of the
thin ¯lm structure with other deformable solids to which it may be attached,
or with which it may otherwise be in contact. The former situation requires
compatibility of deformation, while the latter requires some restriction on
its motion. The categories of con¯guration are termed ¯lm (or layer), line
(or wire), and island (or dot); the categories of constraint are termed uncon-
¯ned, partially con¯ned and fully con¯ned. These classes are illustrated in
Figure 1.1. There is nothing fundamental about such a classi¯cation scheme,
but its adoption can facilitate understanding of the ranges of applicability
of the various ideas in the ¯eld.
With reference to Figure 1.1, a structure of an extent that is small in
one direction compared to its extent in the other two directions is termed
a thin ¯lm; in structural mechanics, such con¯gurations are identi¯ed as
plates or shells. The quali¯er `small' as used here means that the largest
dimensions are at least twenty times greater than the small dimension, and
more commonly are hundreds of times greater than the small dimension. A
structure that has small extent in two directions compared to its extent in
the third direction is termed a line or wire; such con¯gurations are usually
identi¯ed as rods or bars in structural mechanics. Lastly, a structure that
has small extent in all three directions, compared to the dimensions of its
surroundings in this case, is termed an island or a dot.
Concerning the degree of constraint on deformation, a small structure
is said to be uncon¯ned if the boundaries associated with its thin dimensions
are free to displace without restriction. On the other hand, it is said
to be fully con¯ned if all boundaries associated with its thin dimensions are
constrained against deformation. In virtually all cases, the constraint at a
boundary is due to another material which shares that boundary as a com4
Introduction and Overview







Fig. 1.1. A categorization of small volume structures in terms of their general shapes
and levels of constraints. A ¯lm is often called a two-dimensional structure, a line
is called a one-dimensional structure and a dot a zero-dimensional structure, but
this terminology is not standard and it varies signi¯cantly among di®erent technical
specialities.
mon interface. The structure is said to be partially con¯ned if displacement
of its boundaries associated with some, but not all, directions of thinness
are unconstrained.
The classi¯cation matrix in Figure 1.1 includes some illustrations of
varying degrees of con¯nement and thinness. As a speci¯c example, consider
a layer of a SiGe alloy 1 ¹m in thickness, which is deposited on a 1cm by
1 cm area of a Si substrate that is 0:5mm thick. This con¯guration results
in a partially con¯ned thin ¯lm structure. A stripe of copper with a square
cross-section that is 0:5 ¹m on a side and a length of 5mm deposited on
a relatively thick Si substrate is a partially con¯ned line. If the surfaces
of the Si substrate and the wire are then completely covered over with a
1 ¹m thick coating of SiO2 to electronically isolate the wire, the structure
becomes a fully con¯ned line. An InAs quantum dot being formed by stress
driven surface di®usion is a partially con¯ned island. If this con¯guration is
then covered over by a blanket deposit of AlAs, the ¯nal structure is a fully
con¯ned island or quantum dot con¯guration.
The idea of the classi¯cation scheme is intended only as a conceptual
guide. There are many situations in which the structure exhibits behavior
represented by more than one entry in the categorization matrix in Fig1.1
A classification of thin film configurations 5
ure 1.1. For example, suppose that a thin ¯lm bonded to a relatively thick
substrate supports a compressive stress. This is a partially con¯ned thin ¯lm
con¯guration. If the stress becomes large enough in magnitude, the ¯lm will
tend to buckle by debonding from the substrate over some portion of the
interface and then de°ecting away from the substrate over this portion. The
region of the buckle becomes an uncon¯ned ¯lm but the remainder of the
¯lm is partially con¯ned.
The categorization of thin ¯lm systems as summarized in Figure 1.1 is
based on relative physical dimensions and makes no reference to any length
scale re°ecting the underlying structure of the material. There is usually a
heirarchy of such length scales associated with a material of given chemical
composition, and the scales can depend on the processing methods used to
form the material structure. For example, for a polycrystalline ¯lm, intrinsic
length scales include the size of the atomic unit cell, the spacing of crystalline
defects, and the size of the crystal grains, at the very least. Thus, a further
subcategorization of ¯lm con¯gurations based on a comparison of the small
dimension of the thin structure to the absolute length scale characteristic of
the constituent material must be considered.
When the thickness of the ¯lm is small compared to that of the substrate
(typically by a factor of 50 or more), it represents a mechanically thin
film. In this case, the ¯lm material either has no intrinsic structural length
scales, as in the case of an amorphous ¯lm, or the ¯lm thickness is much
larger than all the characteristic microstructural length scales such as the
grain size, dislocation cell size, precipitate or particle spacing, diameter of
the dislocation loops, mean free path for dislocation motion, or the magnetic
domain wall size. Such structures, typically tens or hundreds of micrometers
in thickness, are deposited onto substrates by plasma spray or physical
vapor deposition, or layers bonded to substrates through welding, di®usion
bonding, explosion cladding, sintering or self-propagating high temperature
combustion synthesis. This de¯nition, of course, holds only when the size
scale of the microstructure is small compared to the ¯lm thickness. The
continuum mechanics approach to be presented for the analysis of stress,
substrate curvature and fracture in such mechanically thin ¯lms applies to
a broad range of practical situations.
When the small dimension of the material structure is comparable to
the characteristic microstructural size scale, the ¯lm is considered to be a
microstructurally thin film. Most metallic thin ¯lms used in microelectronic
devices and magnetic storage media are examples of microstructurally thin
¯lms, where the ¯lm thickness is substantially greater than atomic or molecular
dimensions. Although the ¯lm thickness normally includes only a few
6 Introduction and Overview
microstructural units in these cases, the plane of the ¯lm has dimensions signi
¯cantly larger than the characteristic microstructural size scale. The mechanical
properties of these ¯lms are much more strongly in°uenced by such
factors as average grain size, grain shape, grain size distribution, and crystallographic
texture than in the case of mechanically thin ¯lms. Grain to grain
variations in crystallographic orientation as well as crystalline anisotropy of
thermal, electrical, magnetic and mechanical properties also have a more
pronounced e®ect on the overall mechanical response of microstructurally
thin ¯lms. The mechanisms and mechanics of microstructurally thin ¯lms
are considered extensively throughout this book. A microstructurally thin
¯lm can be patterned into lines or stripes on the substrate surface, in which
case the cross-sectional dimensions of each line are comparable to the microstructural
unit dimension. For single crystal ¯lms epitaxially bonded to
relatively thick substrates, the only microstructurally signi¯cant dimension
is the lattice spacing. Consequently, such ¯lms can usually be treated as
microstructurally thin ¯lms even though the ¯lm thickness may be as small
as several times the atomic unit cell dimension. Such structures are studied
in Chapter 6.
Atomically thin films constitute layers whose thicknesses are comparable
to one or a few atomic layers. An adsorbed monolayer of gas or impurity
atoms on a surface is an example of an atomically thin layer. Here the
mechanical response of the thin layer is likely to be more in°uenced by interatomic
potentials and surface energy than by macroscopic mechanical
properties or by micromechanisms of deformation.
1.2 Film deposition methods
Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are
the most common methods for transferring material atom by atom from
one or more sources to the growth surface of a ¯lm being deposited onto
a substrate. Vapor deposition describes any process in which a solid immersed
in a vapor becomes larger in mass due to transference of material
from the vapor onto the solid surface. The deposition is normally carried
out in a vacuum chamber to enable control of the vapor composition. If the
vapor is created by physical means without a chemical reaction, the process
is classi¯ed as PVD; if the material deposited is the product of a chemical
reaction, the process is classi¯ed as CVD. Many variations of these basic
vapor deposition methods have been developed in e®orts to balance advantages
and disadvantages of various strategies based on the requirements of
¯lm purity, structural quality, the rate of growth, temperature constraints

		自动登录	找回密码
密码			注册

薄膜资料(Thin film Material)连续送第3部分! Introduction and Overview

终身成就奖

原创先锋奖