On diffusion-controlled interface microstructure of vapor deposited bilayer thin film of Sn/Cu

Depth dependent concentration profiles for bilayer film of Sn (500 A)/Cu (500 A) are determined at different temperatures by X-ray photoelectron spectroscopy (XPS). Although the nature of the diffusion profile for the sample studied immediately after deposition could be explained by the existing laws, the profiles for others are found to be distinctly different and are not explained by the theories of nucleation and growth of the inter-metallic phases. Measured value of the inter-diffusion coefficient for Cu points out to the grain boundary as well as interstitial diffusion processes. It is also observed that the compositions across the bulk of the films become uniform on annealing at higher temperatures and the width of this region increases with annealing. However, the composition close to the surface is found to be entirely different from that of the bulk even on prolonged heating. The findings possibly demonstrate the importance of physical surface in influencing the solid-state reactions.