Influence of anisotropic surface diffusivity on electromigration induced void migration and evolution

Microelectronic circuits often fail because of voids and cracks that cause open circuits in their interconnects. The voids form due to severe thermal stresses and high current densities present in the lines during service. Typical interconnect lines contain large numbers of voids which gradually grow and change their shape during the life of the line. It has been observed that many open circuits occur when a round void collapses into a slit perpendicular to the direction of electric current flow. O. Kraft and E. Arzt [Acta Mater. 45, 1599 (1997)] have shown that these slits may be caused by electromigration induced surface diffusion. Anisotropy in the surface diffusivity plays a key role in the formation of slit voids. In this article, our objective is to present a detailed parametric study of electromigration induced void evolution in interconnects. We idealize an interconnect as a two-dimensional electrically conducting strip which contains an initially semi-circular void. We neglect the stresses for simplicity. A finite element method is used to predict the evolution of the void after an electric field is applied to the strip. It is shown that the void may: (i) Evolve to a stable shape as it migrates down the line; (ii) break up into several smaller voids or separate from the boundary; (iii) form a slit which severs the strip. The conditions necessary for each type of behavior are explored in detail. The implications on interconnect reliability are discussed.