Void Formation and Growth During Electromigration in Thin Films

One of the modes of electromigration damage is shown to be related to irregularities in the grain-boundary structure of films. Based on the restriction of electromigration to grain boundaries, calculations are performed to determine the magnitude and distribution of excess vacancy concentrations at sites where electromigration flux divergencies may be present. The attainment of an equilibrium vacancy supersaturation (Δc/c0) is found to be practically instantaneous. The maximum supersaturations are less than 1 under all conditions used. This is found to be inadequate for void formation by vacancy condensation unless heterogeneous nucleating sites are present (e. g., oxide particles, boundary-surface oxide intersection). The rate of observable hole formation is accounted for by either void growth at a grain-boundary triple-point junction or by grain-boundary grooving, each being related to the electromigration flux. Grooving seems most likely in pure films with all boundaries having similar character (e. g., preferred orientation).