The early stages of electromigration in thin films on polyimide were investigated at room temperature by using resistometric techniques. Although the resistance increase was initially linear, a tendency to saturate was observed at longer stressing-times, for all stress current densities. A simple model was described which related the saturation of resistance change to a build-up of mechanical stress gradients. The latter produced a counter-flux of metal ions which was proportional to the stress gradient. The stress gradients arose due to a non-uniformity of the grain size in the thin polycrystalline films. This produced regions of small grain size which alternated with regions of large or near-bamboo grain size. These regions had higher and lower metal-ion diffusivities, respectively.

Early-Stage Electromigration in Gold Thin Films S.Bai, K.P.Roenker: Journal of Applied Physics, 1998, 84[8], 4248-54