EARLY-STAGE ELECTROMIGRATION IN GOLD THIN-FILMS

The early stage of electromigration in thin gold films on polyimide ha s been investigated at room temperature using the resistometric techni que. While the resistance increase is initially linear, a saturation t endency is observed for longer stressing times at all stress current d ensities. A simple model is described which relates the saturation beh avior in the resistance change to the buildup of mechanical stress gra dients, which produce a counterflux of metal ions proportional to the stress gradient. The stress gradients arise due to nonuniformity in th e grain size in the polycrystalline thin metal films which produces cl uster regions of small grain size alternating with regions of large or near-bamboo grain size, which have larger and smaller metal ion diffu sivities, respectively. The dependence of the maximum level of the res istance's change and the rate of resistance increase on the stress cur rent density are experimentally characterized and compared with the mo del's predictions with good agreement. (C) 1998 American Institute of Physics. [S0021-8979(98)07320-4].