A study was made of damage nucleation due to electromigration-induced Cu depletion in Al-Cu alloys. It was noted that the diffusion of Al in thin-film lines was sharply reduced by alloying with small amounts of Cu. In the present model, Cu was assumed to be depleted in a region at the cathode end of an Al-Cu alloy line. By again assuming that the diffusion of Al in this Cu-deficient region was much faster than the diffusion of Cu, a quasi steady-state stress profile was established between the blocking boundary at the cathode, and the Cu front (which moved at the drift velocity). The time which was required to reach a critical stress, and initiate failure, was shown to depend upon the inverse of the square of the current density as measured experimentally. The model was applicable to failure at contacts and vias, and at the end of long polycrystalline clusters in near-bamboo lines.

J.R.Lloyd, J.J.Clement: Applied Physics Letters, 1996, 69[17], 2486-8