CRITERION FOR SILICIDE FORMATION IN TRANSITION METAL-SILICON DIFFUSION COUPLES

In this paper, a criterion for silicide formation in metal-silicon dif fusion couples, based on the rate of change of free energy, referred t o here as the free energy degradation rate (FEDR), has been developed from a kinetic model for silicide formation, In the kinetic model, sil icide formation is divided into three steps: diffusion of the predomin ant diffuser (or moving reactant) to the reactive interface, followed by release of the less mobile species (non-moving reactant) from its l attice and intermixing with the moving reactant, and finally formation and growth of the silicide phase. It has been shown that the free ene rgy change due to silicide formation in a diffusion couple can be dete rmined by examining the free energy change of the reaction region (or reactive interface) located between the growing silicide and the non-m oving reactant phase. The free energy degradation rate per unit area o f a given reaction region can be expressed as a sum of three contribut ions, each corresponding to one of the three steps. Each term is a pro duct of a thermodynamic flux and a driving force. These fluxes and dri ving forces have been examined individually; by analyzing how they cha nge with time, it is shown that when a number of possible reactions co mpete with one another in a reaction region, there always exists a rea ction that will result in the largest FEDR in this region, It is also shown that the largest FEDR leads the system to a relative minimum fre e energy state that is most stable compared with any other energy stat e at a given instant. Based on these results, a criterion for silicide reactions has been proposed. During silicide reaction in a reaction r egion of a metal-Si diffusion couple, there are always a number of pos sible reactions competing with one another. The reactions which result in the largest FEDR will actually occur. This criterion combined with the new kinetic model has been successfully applied to predict silici de formation in 15 metal-Si systems.