This process was studied at about 350C, in a sub-micron width Al-Cu/TiN line, by means of depth-resolved  in situ  transmission electron microscopy. It was found that voiding at the cathode was characterized by the uniform thinning of a grain, and by mass transport from no-current flow ledges with a (111) faceted plane. The adjacent grain began to void when the grain upstream in the electron flow was almost depleted. The voiding proceeded in a manner that was similar to that in the upstream grain. Voiding between the cathode and the anode began mainly at the intersection of the Al top surface a grain boundary. It occurred mainly downwards along the anode side of the grain boundary until it reached the Al bottom surface; causing a gap in the Al. It then grew laterally towards the anode. The cathode side surface of the void remained stationary during void evolution.

H.Okabayashi, M.Komatsu, H.Mori: Japanese Journal of Applied Physics, 1996, 35[1-2B], 1102-6