GROWTH OF INTERMEDIATE PHASES IN CO/SI DIFFUSION COUPLES - BULK VERSUS THIN-FILM STUDIES

Bulk diffusion couples of Co/Si were annealed at 800, 900, 1000, and 1050-degrees-C for times ranging from 24 h to 1 month. The growth rates of the three intermediate phases Co2Si, CoSi, and CoSi2 and the concentration profiles across the couples were determined by optical microscopy and electron probe microanalysis, respectively. Using these data and the data reported in the literature at lower temperatures, the interdiffusion coefficients of Co2Si, CoSi, and CoSi2 were obtained as a function of temperature. The activation energies obtained were 140, 160, and 190 kJ/mol (or 1.45, 1.66, and 1.97 eV) for Co2Si, CoSi, and CoSi2, respectively. The generally small interdiffusion coefficient of CoSi2 and its high activation energy cause the growth rate of COSi2 to be extremely small at low temperatures. Using the interdiffusion coefficients of Co2Si, CoSi, and CoSi2 extrapolated to low temperatures, the growth rates of Co2Si, CoSi, and COSi2 in thin-film Co/Si couples were predicted. The predictions were made by numerically solving the diffusion equations with boundary conditions appropriate for thin-film couples. Good agreement was obtained between the calculated values and nearly all the experimental data reported in the literature using different configurations of thin-film couples. Although the methodology was applied successfully to predict the growth of Co2Si, CoSi, and CoSi2 sequentially in thin-film Co/Si couples, it is equally applicable to any binary thin-film diffusion couple.