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at hand. Ideally, the substrate should give mechanical support but not interact with the film except to provide sufficient adhesion, and in many cases, the provision of a template for atomic ordering. In practice, however, the substrate exerts considerable influence on film characteristics. The search for viable substrate materials is an active area of research.

Issues in Substrate Selection The importance of substrate selection in dictating all aspects of film growth, from ease or feasibility of growth to film

quality, cannot be overemphasized. In some materials systems, the best films that can currently be produced are grown on materials that have serious limitations. Significant breakthroughs are often required to identify and produce materials that satisfy all of the requirements that films and many of their applications pose.

Global Issues in Substrate Selection A number of issues are critical in substrate selection for thin-film growth, regardless of the details of the film to be produced. These are listed in Table I, together with the materials properties they affect. One of the first issues that must be dealt with in determining the suitability of a substrate for a thin film is the chemical compatibility of the two materials. Ideally, this means that no chemical reactions between the film and substrate can exist, as shown in Figure la. If, on the other hand, reactions occur, significant interdiffusion between film and substrate can exist, leading to a blurred interface and/or inclusions of undesired atoms or compounds in both film and substrate, as shown schematically in Figure lb. These considerations require close examination of the composition of

Table 1: Global Issues in Substrate Selection. ^ \ ^ ^

Chemical Compatibility

ThermalExpansion Match

Maximum Processing Temperature

X

X

Reacted Layer at Interface

X

X

X

Impurity in Film

X

X

X

Impurity in Substrate

X

Film Adhesion

X

EFFECT

ISSUE ^ " " ^ \ ^

Film Buckling/Cracking

Substrate Cleanliness

Substrate Homogeneity

Substrate Thermal Stability

Buffer Layers

X

X

X X

X

X X

X X

Film Microstructure Film Composition

Surface Quality

X

X

X

X

X X

Film Morphology

X

X

X

X

X

Film Uniformity

X

X

X

X

X

X

X

X

X

X

Electrical Properties

MRS BULLETIN/APRIL 1995

X

X

35

Substrate Selection for Thin-Film Growth

substrate materials and their possible chemical reactions with the film. In general, one is likely to encounter fewer problems with chemical compatibility if the film and substrate are composed of chemically similar species. For example,

oxide substrates are more likely to tolerate the growth of high-temperature superconducting materials than are non-oxide materials. Semiconductors, on the other hand, are more likely to be chemically compatible with other semiconductors.

Figure 1. Chemical compatibility is one of the most important factors in determining the suitability of a substrate for a particular thin film. If, as shown in (a), there is no chemical interaction between the two materials, the interf