Diffusional Phase Transformation under Induced Thermal Stress
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DIFFUSIONAL PHASE TRANSFORMATION UNDER INDUCED THERMAL STRESS. E C ZINGU AND B T MOFOKENG,
Department of Physics, South Africa
Medunsa,
0204,
ABSTRACT When thin films are deposited on substrates or when compound films are formed through interdiffusion of multi-film structures, intrinsic stress develops in the various films. Thermal mismatch between the expansion coefficients of the substrate and films in multi-film structures gives rise to extrinsic stress at elevated temperatures By using Si and rolled Al foil substrates supporting the same multi-film structure SiO2 /Si/Co, the effect of extrinsic stress on interdiffusion of thin films is isolated. Silicide growth is found to be inhibited (delayed) when formed on Al substrates compared to that formed on Si substrates. The delay in silicide growth is ascribed to delamination caused by large tensile stress prior to silicide formation. The growth rate of Co 2 Si is found to be similar on both Al and Si substrates INTRODUCTION Thin films deposited on substrates are usually in a state of stress which has been established during the deposition process or during the subsequent annealing or cooling. The stress in the film is said to be a combination of intrinsic stress (i.e. stress due to the deposition process, and phase or structural transformations of the deposited film) and extrinsic stress (i.e. thermal stress due to the thermal expansion difference between the film and the substrate). At temperatures well above or below the temperature of deposition thermal stress usually dominates. [I] In multi-layered films the thermal stress in the individual films is independent of the physical properties of the interposing or covering layers. [2] Consider the thin film structure Al/SiO2 /Si/Co or Si/SiO2 /Si/Co in which SiO2 , Si and Co films have been deposited sequentially on Al and Si substrates respectively. Each of the three films will be in a state of thermal stress at elevated temperatures due to the difference between the film expansion and that of the substrates. See Table I for the expansion coefficients and the nature of the stress for various films and substrates. The nature of the stress (i.e. tensile or compressive) and its magnitude will be determined by the difference between the expansion coefficient of the film and the substrate At temperatures around 400*C Co and Si interdiffuse to form Co 2 Si. The aim of this investigation was to determine the effect which thermal stress in the Co and Si films has on the formation of Co 2 Si. In order to vary the degree of thermal stress in the films, different substrates namely Si and Al were used
Mat. Res. Soc. Symp. Proc. Vol. 230. ©1992 Materials Research Society
146
Table I
Film
: Expansion coefficient and the nature of the various films
Substrate
0 (xl0-6 K-') (film)
L. (xlO- 6 K-x) (substrate)
stress
in
Nature of Stress (±4000C)
Co Si SiO2 Co Co Si SiO2
Si Si Si Al Al Al Al
12 2,3 0,5 12 12 2,3 0,5
2,3 2,3 2,3 22 22 22 22
Compressive 0 Tensile Tensile_ Tensile Tensile Tensile
Co 2 Si Co 2 Si
Si
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