The effect of a width transition on the electromigration reliability of Cu interconnects

We have conducted electromigration studies of Cu interconnects with wide-to-narrow transitions and observed bimodal failure distributions. The early failure mode corresponds to a fatal void at the width transition site while the late failure mode corresponds to a fatal void at the cathode. Also, we observed that as the distance between the cathode and transition decreases, the frequency of the early mode increases while the median time to failure of this mode decreases. Interestingly, we do not assert that the width transition site is as a site of flux divergence. Since a common test current is applied to the entire structure, the number of atoms entering and leaving the transition site should be constant, which was confirmed by simulation. Instead, we explain the experimental observations by different rates of void growth and motion, which are determined by current density. Because there is a much faster growth rate and motion in the narrow region than the wide region, voids that form in the narrow region coalesce at the transition thereby leading to early fails, and voids that form in the wide region lead to late (standard) cathode fails. Width transitions of various types and geometries are relatively common in product designs, therefore this mechanism should be included as part of the chip-level electromigration risk assessment.