Kinetics of Void Drift in Copper Interconnects

Abstract

We have observed the real-time behavior of electomigration-induced voids in both passivated and unpassivated copper interconnects in a Scanning Electron Microscope (SEM), and correlated void nucleation, growth, drift and stagnation with post-electromigration crystallographic microanalyses carried out using Electron Back-Scattered Diffraction (EBSD) analysis. Voids that nucleate at various locations along the interconnects often drift toward the cathode, where they grow, coalesce, and eventually cause electrical failure. In-situ SEM observations allowed for the tracking of void shapes and drift rates over long (multi-grain) distances. Changes in the size and the velocity of the voids were observed when the voids passed through different grains. These changes are attributed to the difference in diffusivity for different grain orientations. In passivated lines, voids were often trapped at individual grain boundaries, where they grew to cause failure, or de-trapped to continue to drift toward the cathode. In unpassivated lines, voids did not drift, but instead always nucleated and grew and grain boundaries. Locations at which voids grew in unpassivated lines, or at which voids were trapped and grew in passivated lines, were correlated with the crystallographic orientations of “upwind” and “downwind” grains. From these analyses, we find that the average electromigration interface diffusivities (z*D) as a function of grain orientation are ordered according to {100} > {111} > {110}. Quantitative analysis of void dynamics, correlated with crystallographic microanalyses, provides important data for modeling of electromigration-induced failure, and for process-optimization for improved reliability.