Low temperature reactions of thin layers of Mn with Si
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Reactions between manganese thin films and silicon substrates, annealed at relatively low temperatures (38 Ni/Si, 6 and Mo/Si. 39 For example, when a Pt-Si couple6 was
J. Mater. Res., Vol. 6, No. 7, Jul 1991
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L. Zhang and D. G. Ivey: Low temperature reactions of thin layers of Mn with Si
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Annealing Time (min) FIG. 7. Change in average diffusion fluxes with annealing time. Both fluxes decrease with time, but Mn flux decreases faster than Si flux.
Ni2Si, which grows until all the Ni is consumed.6'8'11'45'46 The second phase, NiSi, begins to form and grow by consuming Ni2Si, at higher annealing temperatures, until Ni2Si completely disappears. The last phase to form is NiSi2, which forms by consuming NiSi. It is noteworthy that Ni 3 Si 2 , which exhibits the most extensive growth in bulk couples,46 is absent in thin film couples. The results presented on manganese silicide formation in this paper appear to be contrary to accepted behavior for thin film silicide growth, i.e., single layer growth. Three silicide phases were found to coexist and two of these phases, MnSi and Mn 5 Si 3 , were found to grow simultaneously. A detailed model is proposed to explain this growth behavior and this model is then compared to existing models. A. Mechanisms of manganese silicide formation
annealed in a UHV system, the diffusion coefficient was about one order of magnitude higher than that for a Pt-Si couple annealed in a system with a poorer vacuum. In another experiment with Ni-Si couples, it was suggested that oxygen diffused through the Ni film to the silicide/metal interface and prevented Ni transport.6 Studies have been done on Pt/Si and Mo/Si thin film couples38'39 in which the metal films were intentionally doped with oxygen. In the Pt/Si system, PtSi formed between Pt2Si and Si in the presence of unreacted Pt,38 whereas PtSi does not usually form until all the Pt is consumed. Similarly, for the Mo/Si system, Mo 5 Si 3 forms and grows between MoSi2 and Mo before Mo is totally consumed.39 It is worth noting that Pt is the major diffuser in Pt/Si couples, while Si is the major diffuser in Mo/Si couples. The diffusivities of both moving species are decreased by oxygen, and this reduced flux induces the second phase to form. IV. DISCUSSION It is well known that, in bulk diffusion couples, all the phases predicted by the equilibrium phase diagram will grow simultaneously, except for some exceptional absences of individual compounds.40"43 In thin film couples, however, these phases tend to grow sequentially; i.e., they grow one by one, instead of several or all of them growing together as in bulk couples. Moreover, some phases which are observed in bulk couples do not show up in thin film couples.8'11'44"^7 Because of the difference in growth behavior, the growth process in bulk couples is referred to as multiple phase growth, while
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