A SIMULATION OF ELECTROMIGRATION-INDUCED TRANSGRANULAR SLITS

An on-chip aluminum interconnect carries an intense electric current a t an elevated temperature, motivating atoms to diffuse in the solid st ate, and inducing voids that may cause an open failure. Recent observa tions have shown that a void sometimes collapses to a slit running nea rly perpendicular to the electric current direction. Such a slit often lies inside a grain rather than along a grain boundary. An earlier ca lculation showed that diffusion on the void surface, driven by the ele ctric current, can cause a circular void to translate in an infinite, isotropic interconnect. It was suggested recently that this solution m ay be unstable, and that two forces compete in determining the void st ability: surface tension favors a rounded void, and the electric curre nt favors a slit. A linear perturbation analysis, surprisingly, reveal ed that the translating circular void is stable against infinitesimal shape perturbation. Consequently, the slit instability must have resul ted from finite imperfections. This article reviews the experimental a nd theoretical findings, and describes a numerical simulation of finit e void shape change. We determine the electric field by a conformal ma pping of complex variables, and update the void shape for a time step by a variational method. The simulation shows that a finite void shape imperfection or surface tension anisotropy can cause a void to collap se to a slit. (C) 1996 American Institute of Physics.