Room-temperature diffusion in Cu/Ag thin-film couples caused by anodic dissolution

Thin, 100-nm films of first silver and then copper were deposited consecutively onto inert substrates by magnetron sputter deposition. Constant anodic current densities were applied at room temperature to dissolve the outer copper film to varying depths. The 50Cu/50Ag interface, derived from the auger electron spectroscopic concentration-depth profile, initially moved into the copper toward the outer dissolving surface, indicating enhanced diffusion of copper into silver. After longer times at all anodic current densities, the interface reversed and moved back toward the underlying silver-rich layer, indicating that eventually diffusion of silver into copper predominated. The reversal time was inversely proportional to the anodic current density. These effects are explained by anodic formation of subsurface vacancies which migrate as divacancies to the copper/silver interface where they affect interface movements by the well-known Kirkendall mechanism. Calculated diffusivities up to 10−12 cm2/s at maximum anodic current densities of 900 μA/cm2 are dramatically above any that are normally observed at room temperature.