Electromigration damage in aluminum film conductors

Abstract

A current induced "open" in an aluminum stripe is caused by an accumulation of vancancies at some point in the film, i.e., by a nonvanishing divergence in the ion flux. Work reported in the literature has dealt predominantly with electromigration damage resulting from a nonuniform temperature profile. This paper concerns damage sites related to structural irregularities at grain boundary intersections. The nature of these structural irregularities was studied in fine-grained films having preferred orientation. We show that in a film where a high degree of preferred orientation exists, the diffusivity is considerably less than in a film of mixed orientation. When boundary mobility is more or less uniform, i.e., in highly oriented films, the predominant damage sites are at points where abnormally large grains statistically are found along the stripe length. Damage here arises mainly from the difference in effective grain boundary area at the junction between the small equiaxed structure and the large grains. In films that have a mixed orientation, another mechanism becomes of considerable importance. Flux divergences can now result from the intersection of boundaries having different diffusivities associated with them. This may manifest itself as a local change in the activation energy for diffusion or as a change in the number of available diffusion sites per boundary. We have made steady state calculations of the local vacancy supersaturations to be expected at each of the divergent sites mentioned above, i.e., local changes in grain size or in boundary mobility, and have found that relatively small changes in grain size or boundary mobility are sufficient to produce a critical vacancy supersaturation for void nucleation and growth to occur. From the results of this work, we predict that the ideal material with respect to long lifetime and high reliability would have the following characteristics: large grains to reduce total mass flux through any sample cross section, equiaxed structure to eliminate gross grain size divergences, and preferred orientation to reduce the diffusivity per boundary.