Electromigration damage due to copper depletion in Al/Cu alloy conductors

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Abstract
The problem of damage nucleation due to electromigration-induced copper depletion in an Al/Cu alloy is treated. The diffusion of aluminum in thin-film lines is sharply reduced by alloying with small amounts of copper. In this model, copper is depleted in a region at the cathode end of an Al/Cu alloy line. Assuming that the aluminum diffusion in this copper-deficient region is much faster than the diffusion of the copper, a quasisteady-state stress profile is established between the blocking boundary at the cathode and the copper front moving at the drift velocity. The time to reach a critical stress to initiate failure is shown to depend on the inverse of the square of the current density as observed experimentally. The model is appropriate for failure at contacts and vias, and at the end of long polycrystalline clusters in near-bamboo lines.

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