Thermodynamic and kinetic study of solid state reactions in the Cu-Si system

It has been shown that significant changes in the course of solid state rea ctions can be realized by decreasing length scale, temperature, or by varyi ng parent microstructures. In the case of the formation of Cu3Si by interdi ffusion of Cu and Si, previous research has shown that over a large tempera ture range reaction rates are determined by the rate of grain boundary diff usion of Cu through the growing Cu3Si phase. We have examined the effect of replacing crystalline Si with amorphous Si (a-Si) on these solid state rea ctions, as well as the effect of decreasing the temperatures and length sca les of the reactions. Multilayered thin film diffusion couples of Cu and a- Si were prepared by sputter deposition, with most average composite stoichi ometries close to that of the equilibrium phase Cu3Si. Layer thicknesses of the two materials were changed such that the modulation (sum of the thickn ess of one layer of Cu and a-Si), lambda, varied between 5 and 160 nm. X-ra y diffraction analysis and transmission electron microscopy analysis were u sed to identify phases present in as prepared and reacted diffusion couples . Complete reactions to form a single phase or mixtures of the three low te mperature equilibrium silicides (Cu3Si, Cu15Si4, and Cu5Si) were observed. Upon initial heating of samples from room temperature, heat flow signals we re observed with differential scanning calorimetry corresponding to the gro wth of Cu3Si. At higher temperatures (> 525 K) and in the presence of exces s Cu, the more Cu-rich silicides, Cu15Si, and Cu5Si formed. Based on differ ential scanning calorimetry results for samples with average stoichiometry of the phases Cu3Si and Cu5Si, enthalpies of formation of these compounds w ere measured. Considering the reaction of these phases forming from Cu and a-Si, the enthalpies were found to be -13.6 +/- 0.3 kJ/mol for Cu3Si and -1 0.5 +/- 0.6 kJ/mol for Cu5Si. The growth of Cu3Si was found to obey a parab olic growth law: x(2) = k(2)t, where x is the thickness of the growing sili cide, k(2) is the temperature dependent reaction constant, and t is the rea ction time. Also, the form of the reaction constant, k(2), was Arrhenius: k (2) = k(0) exp(-E-a/k(b)T) with k(b) being Boltzmann's constant and the pre factor, k(0) = 1.5 x 10(-3) cm(2)/s, and activation energy, E-a = 0.98 eV. These results indicate a much slower reaction to form Cu3Si in thin film Cu /a-Si diffusion couples than indicated by previous researchers using mostly bulk samples of Cu and crystalline Si (x-Si). (C) 1999 American Institute of Physics. [S0021-8979(99)03320-4].