Electromigration in passivated Cu interconnects studied by transmission x-ray microscopy

Abstract

Time-resolved x-ray microscopy studies of the electromigration in inlaid Cu line/via structures were performed on focused-ion-beam-prepared cross sections of an advanced interconnect layer system. Multiple x-ray images were recorded at 1.8 keV photon energy while stressing the passivated Cu structures with an applied current. The image sequences show that void formation is a dynamic process, with voids being observed to nucleate and grow within the Cu via and migrate towards the via sidewall. Correlation of the real time x-ray microscopy images with postmortem high voltage transmission electron and scanning electron micrographs indicates that the void nucleation occurs at grain boundaries in the copper, and that the voids migrate along these grain boundaries during electromigration. By taking multiple images at different viewing angles, the three-dimensional arrangement of an interconnect stack with Cu line / via structures was reconstructed. In future studies time-resolved tomography will be used to visualize void dynamics within the volume, thereby identifying pathways for Cu diffusion.