Size-dependent kinetics of reactive diffusion in nano-grained Ag-Sn thin films

The kinetics of lateral Ag₃Sn intermetallic phase propagation during Sn diffusion into nano-grained Ag films has been studied by optical microscopy, SEM and electron microprobe in a temperature range 140 – –200 °C. The Sn and Ag films of various thicknesses were deposited onto glass substrate with small overlap (5–10 μm). It was detected that the rates of Ag₃Sn propagation in thin film couples exceed more than two orders of magnitude the propagation rate in a bulk of massive Ag-Sn samples. Kinetics of the phase propagation depends on the grain size in Ag-film: the smaller grain size, the faster is the propagation rate. The mechanism of phase propagation is proposed in assumption that the kinetics is controlled by grain boundary (GB) diffusion through the intermetallic layer with subsequent chemical reaction between Ag and Sn atoms at the Ag₃Sn -Ag interface. By comparison of experimental data on Ag₃Sn propagation with the proposed theory the Arrhenius equation D_iδ=8.1×10^-14exp-55.4kJ/molRTm³/s was obtained for products D_i δ (D_i is the GB diffusion coefficient of Sn in the Ag₃Sn phase, δ is the GB width).