Thermodynamic and kinetic study of solid state reactions in the Cu–Si system Available to Purchase

It has been shown that significant changes in the course of solid state reactions can be realized by decreasing length scale, temperature, or by varying parent microstructures. In the case of the formation of $Cu3Si$ by interdiffusion of Cu and Si, previous research has shown that over a large temperature range reaction rates are determined by the rate of grain boundary diffusion 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 reactions, as well as the effect of decreasing the temperatures and length scales of the reactions. Multilayered thin film diffusion couples of Cu and $a-Si$ were prepared by sputter deposition, with most average composite stoichiometries close to that of the equilibrium phase $Cu3Si.$ Layer thicknesses of the two materials were changed such that the modulation (sum of the thickness of one layer of Cu and $a-Si),$ λ, varied between 5 and 160 nm. X-ray diffraction analysis and transmission electron microscopy analysis were used to identify phases present in as prepared and reacted diffusion couples. Complete reactions to form a single phase or mixtures of the three low temperature equilibrium silicides $(Cu3Si,$ $Cu15Si4,$ and $Cu5Si)$ were observed. Upon initial heating of samples from room temperature, heat flow signals were observed with differential scanning calorimetry corresponding to the growth of $Cu3Si.$ At higher temperatures (>525 K) and in the presence of excess Cu, the more Cu-rich silicides, $Cu15Si,$ and $Cu5Si$ formed. Based on differential scanning calorimetry results for samples with average stoichiometry of the phases $Cu3Si$ and $Cu5Si,$ enthalpies of formation of these compounds were 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 −10.5±0.6 kJ/mol for $Cu5Si.$ The growth of $Cu3Si$ was found to obey a parabolic growth law: $x2=k2t,$ where x is the thickness of the growing silicide, $k2$ is the temperature dependent reaction constant, and t is the reaction time. Also, the form of the reaction constant, $k2,$ was Arrhenius: $k2=k0 exp(−Ea/kbT)$ with $kb$ being Boltzmann’s constant and the prefactor, $k0=1.5×10−3 cm2/s,$ and activation energy, $Ea=0.98 eV.$ 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 $(x-Si).$