Theoretical analysis of current-induced void dynamics in metallic thin films
This thesis addresses electromigration-induced void dynamics in metallic thin films. This has been a problem of major interest both in terms of fundamental understanding of driven mass transport and microstructural evolution in solids and for addressing important interconnect reliability concerns in integrated circuits. Recent theoretical work in this area has revealed extremely rich nonlinear dynamical phenomena induced by void surface electromigration and emphasized the role of surface diffusional anisotropy and current crowding effects in void surface morphological evolution and film failure.
Our theoretical non-linear analysis is based on self-consistent numerical simulations of current-induced migration and morphological evolution of voids in metallic thin films; this analysis accounts rigorously for current crowding effects that become particularly important in narrow films, as well as surface curvature effects that can be particularly strong due to the strong anisotropy of adatom diffusion on void surfaces. The morphological evolution of the void surface is determined coupled self-consistently with the electric field distribution in the conducting film that contains the void; a boundary-element method is used for the computation of the electrostatic potential in conjunction with a front-tracking method to monitor void surface propagation. A two-dimensional (2D) implementation is followed in the plane of a long metallic film of finite width; this 2D representation is based on the assumption that the void extends throughout the film thickness, which is consistent with experimental observations.
This thesis consists of a comprehensive study of current-driven morphological evolution of single and multiple voids, including the interactions between voids in metallic thin films and the resulting film electrical properties. Specifically, we have examined in detail void migration, surface wave propagation on voids, and void-void interactions in metallic thin films under surface electromigration conditions. The predictions for the evolution of interconnect line electrical resistance based on our simulations of void morphological evolution are in good agreement with experimental data from accelerated electromigration testing in the literature.