Observations of electromigration induced void nucleation and growth in polycrystalline and near-bamboo passivated Al lines

Electromigration voiding in passivated pure Al lines was observed in situ using high voltage scanning electron microscopsy. Two different types of lines were investigated; one deposited under high purity conditions with a near-bamboo microstructure, and one deposited under conventional conditions with a polycrystalline microstructure with stress voids present. The samples were observed while being tested under accelerated electromigration conditions. Samples were then thinned and analyzed with transmission electron microscopy techniques to investigate the relationship between line microstructure and void nucleation. Electromigration voids and stress voids were seen to nucleate at very specific, unique, microstructural sites that require the intersection of a grain boundary with the line sidewall. Void movement after initiation is also dictated by microstructure, with voids only growing into and causing failure in grains oriented with a {111} plane near perpendicular to the line. The conditions for void nucleation suggest that heterogeneities at the line sidewall are necessary for void nucleation.