Electromigration: Void Dynamics

Abstract

The detailed knowledge of the void evolution is helpful to understand and interpret traditional electromigration (EM) lifetime experiments. As void formation and subsequent evolution is highly dynamic, an in situ observation experiment was employed using an SEM allowing for void detection down to a few nanometers in diameter. For void nucleation, it has been suggested that due to the large anisotropy in modulus in Cu, incompatible grain boundaries can act as stress concentrators for both stress-induced and EM-induced voiding. In addition, during growth, the shape change and migration have been associated with the microstructure. To better understand void formation and evolution, we examined the Cu microstructure using EBSD during the experiment without interruption. We tested interconnects consisting of Cu-based damascene (M1) drift type structures, capped with SiCN. Void formation could be confirmed to be associated with features such as incompatible grain boundaries in some instances. In other cases, triple points and grain boundary/sidewall intersections were found as void nucleation sites. Void growth and migration is found strongly correlated with the driving current density. We show conditions under which void migration takes place or is suppressed. In particular, a critical current density of approximately 10 $\mbox{mA}/\mu\mbox{m}^{2}$ is observed, below which no void migration occurs. At current densities of 15 $\mbox{mA}/\mu\mbox{m}^{2}$ and above, all voids migrated at a net drift velocity proportional to the current density. Independent of migration, all voids are observed to grow at rates consistent with published data for a mixed near bamboo microstructure and interface diffusion.