The thickness dependence of the flow stress of capped and uncapped polycrystalline Ag thin films

The flow stress of capped and uncapped polycrystalline silver films on oxidized silicon wafers is reported as a function of Ag film thickness (in the range of 0.2-1.2 mu m) and temperature (in the range of -50-500 degrees C). While the flow stress in capped films increases with decreasing film thickness, the flow stress in uncapped films increases when film thickness is decreased from 1.2 to 0.5 mu m, but then decreases with further decreases in film thickness. A high temperature regime of purely plastic strain accommodation extends to lower temperatures in thinner uncapped films than in capped films, with a transition temperature to elastoplastic behavior characterized by an apparent activation energy consistent with diffusive mechanisms of strain relief. These results suggest that surface diffusion to, and down, grain boundaries (whose spacing scale with film thickness) mediates strain accommodation in uncapped films. (C) 1998 American Institute of Physics. [S0003-6951(98)00943-7].