Void Evolution via Coupled Creep and Electromigration in Confined Small Scale Interconnects

This paper proposes a three dimensional electromigration model for void evolution in small scale interconnects. Concurrent kinetics of creep flow and surface diffusion as well as the effect of surrounding material are considered to provide better understanding of the evolution process. The multiple kinetics and energetics are incorporated into a diffusive interface model. A semi-implicit Fourier spectral method and the preconditioned biconjugate-gradient method are proposed in the computations to achieve high efficiency and numerical stability. We systematically studied kinetic processes from diffusion dominated to creep dominated. Which process dominates, as revealed by the analysis, is determined by a combination of viscosity, mobility, interconnect thickness, and void radius. Previous studies on electromigration suggest that the void shape evolution is determined by the competition between the electron wind force and the surface energy. There exists a critical initial void shape, which determines whether a void evolves into a slit or not. However, our simulations show that in the same situation a creep dominated process can lead to a quite different morphology. A spherical void can evolve into a bowl shape, and further split into two smaller voids. It is also shown that the interconnect geometry has an important effect.