Voiding in passivated pure Al lines was observed  in situ  by using high-voltage scanning electron microscopy. Two different types of line were investigated. One type was deposited under high-purity conditions, and had a near-bamboo microstructure. The other type was deposited under conventional conditions, and had a polycrystalline microstructure with stress voids. The samples were observed while being tested under accelerated electromigration conditions. They were then thinned, and transmission electron microscopy was used to investigate the relationship between line microstructure and void nucleation. Electromigration voids and stress voids were found to nucleate at very specific and unique microstructural sites that required the intersection of a grain boundary with the line side-wall. Void movement after initiation was also dictated by the microstructure; with voids growing into, and causing failure, only in grains that were oriented such that a {111} plane was nearly perpendicular to the line. The conditions for void nucleation suggested that heterogeneities at the line side-wall were necessary for void nucleation.

T.Marieb, P.Flinn, J.C.Bravman, D.Gardner, M.Madden: Journal of Applied Physics, 1995, 78[2], 1026-32