ANALYSIS OF THE DIFFUSION-CONTROLLED GROWTH OF COBALT SILICIDES IN BULK AND THIN-FILM COUPLES

The solid state reaction between Co and Si has been studied in bulk di ffusion couples between 850 and 1100 degrees C. At the scale of the ob servations made, the three phases Co2Si, CoSi, and CoSi2 are found to grow simultaneously, according to diffusion controlled kinetics. The r esults are analyzed in term of the Nernst-Einstein equation that direc tly relates diffusion fluxes to the free energy changes driving the fo rmation. The growth rates obtained for CoSi2 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 th at attention should be paid to two factors, The first one is that the laws of growth are slightly different for a phase growing simultaneous ly with two other ones (bulk) and one phase growing alone (thin films) . The second factor is the grain size of the various samples, which va ries with the temperature of reaction, Once this is done, excellent ag reement is obtained between the two sets of measurements, Moreover it is shown that knowing the grain size, it is possible to calculate quit e accurately the growth rate from the respective isotope diffusion coe fficients both for lattice and grain boundaries of Co and Si in CoSi2.