Grain structure effect on EM of Cu interconnects with CoWP capping - A statistical model

In this paper, EM statistics for Cu interconnects with CoWP capping was analyzed using a microstructure-based statistical model. The model was applied to a specific case where the interfacial diffusivity was effectively suppressed, leading to the grain boundary diffusion dominating the mass transport under EM. A direct correlation between the cluster length L and the EM lifetime t was established. Based on this relation and the weakest-link model, the EM lifetime and statistics were modeled for Cu interconnects with the measured cluster length distribution. The simulation results agree well with the experimental observations, confirming that the longer EM lifetime and the larger statistical deviation for the large grain structure stem from its shorter average cluster length and wider cluster length distribution. The weakest-link statistics show that only a few clusters close to the cathode end of the line control the EM statistics, which is consistent with the experimental observation that voids usually form adjacent to the cathode end of the line. This study suggests that for the CoWP capped Cu interconnects, the grain structure plays a key role in controlling the EM lifetime and statistics and will become increasingly important as the Cu interconnect continues to be scaled.