The diffusion of Cu through electron-beam evaporated thin films was studied by monitoring electrical resistance changes, as a function of time, at 343 to 543K. The object was to determine to what extent the presence of Al affected Cu diffusion through Ag thin films. It was found that the activation energy and the pre-exponential factor, as determined from the Arrhenius plot, decreased with increasing Al content of the alloy film (table 17). However, as a whole the diffusion coefficient increased with increasing Al content. Transmission electron microscopy and thin-film X-ray diffraction studies of 9.2 and 18.2at%Al films indicated that the microstructures and the corresponding selected-area diffraction patterns varied as a function of the Al content. Upon adding Al, the Cu reacted to form Al2Cu (-phase) precipitates; even at room temperature. It was suggested that precipitation occurred along the grain boundaries of Cu, and along the alloy Cu interface. This reduced grain growth but, upon annealing such films, this retarding effect of precipitates upon grain growth was removed. The enhancement of the diffusion coefficient, with increasing Al content, was attributed to the fact that the concentration gradient and the associated stress gradient at the interface and grain boundaries produced dislocations.

R.Roy, S.K.Sen: Thin Solid Films, 1993, 223[1], 189-95

Table 16

Diffusivity of 110mAg along Mutually Perpendicular

Directions in (011) Ag/Cu Interphase Boundaries