Analysis of the diffusion controlled growth of cobalt silicides in bulk and thin film couples
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P. Gas Laboratoire de Metallurgie associe au CNRS-URA443, Faculte des Sciences et Techniques de Saint Jerome, Case 511, 13397 Marseille-Cedex 20, France
F.M. d'Heurle IBM Research Laboratory, P.O. Box 218, Yorktown Heights, New York 10598 (Received 11 May 1994; accepted 13 December 1994)
The solid state reaction between Co and Si has been studied in bulk diffusion couples between 850 and 1100 °C. At the scale of the observations made, the three phases Co2Si, CoSi, and CoSi2 are found to grow simultaneously, according to diffusion controlled kinetics. The results are analyzed in term of the Nernst-Einstein equation that directly relates diffusion fluxes to the free energy changes driving the formation. The growth rates obtained for C0S12 at high temperatures, in the present bulk samples, are compared with those determined by others in thin films, at much lower temperatures. The comparison requires that attention should be paid to two factors. The first one is that the laws of growth are slightly different for a phase growing simultaneously with two other ones (bulk) and one phase growing alone (thin films). The second factor is the grain size of the various samples, which varies with the temperature of reaction. Once this is done, excellent agreement is obtained between the two sets of measurements. Moreover it is shown that knowing the grain size, it is possible to calculate quite accurately the growth rate from the respective isotope diffusion coefficients both for lattice and grain boundaries of Co and Si in CoSi 2 .
I. INTRODUCTION Phase formation by solid state reaction (reactive diffusion1) is a fascinating research subject both for its fundamental aspects and for its numerous applications in different domains ranging nowadays from classical metallurgy (e.g., "hot dip" galvanization2) to microelectronics technology.3 Different processes affect phase formation during solid state reaction (SSR); a general treatment should include nucleation, interfacial reactions, and diffusion.45 At the risk of over-simplifying and discarding the thermal formation of silicon dioxide6 or some metallic phases,7 one can distinguish two stages. Nucleation phenomena and interfacial reactions affect the initial stages of phase formation while diffusion controls the subsequent growth. Apparently diffusion is easily analyzed. However many points of diffusion controlled growth still need to be clarified. The abundant literature on the growth of silicide thin films8-9 illustrates clearly this last statement. Most of the kinetics of the metal/silicon reactions are controlled by diffusion processes but there exist (i) an apparent differ1134
J. Mater. Res., Vol. 10, No. 5, May 1995
http://journals.cambridge.org
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ence in behavior between thin films and their equivalent bulk couples, such as the absence of certain phases10 or the general observation of sequential diffusion controlled (DC) formation in thin films instead of the simultaneous DC formation often reported for bulk couples, and (ii) relatively high rat
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