Electromigration-induced void growth in bamboo structures

Abstract

A novel resistometric technique enables the investigation of single void nucleation and growth induced by electromigration (EM) for aluminum (Al) lines having a perfect bamboo structure in comparison with single-crystal Al lines. Fine tungsten (W) voltage probes were fabricated at every 4 μm along the Al line with grain sizes of 10 μm or more. Local electrical resistance changes have confirmed that a void nucleated only at the grain boundary and no damage appeared within the grains. The measured values of the local electrical resistance changes were converted to EM-induced void growth rates. The vacancy flux was deduced from the void growth rate under the assumption that a vacancy volume is equal to the atomic volume. It has been clarified that the vacancy fluxes for a bamboo-structured Al line were about one order magnitude smaller than the ideal vacancy fluxes in the Al lattice derived from the Nernst–Einstein relation. The vacancy fluxes for single-crystal Al lines were also quantified under an accelerated test at a high electric current density with a temperature gradient. These results suggest that the bamboo grain boundaries have a blocking effect on vacancy flow. This blocking produces vacancy supersaturation near the grain boundary, and the reduction in vacancy flux is well explained by the back-diffusion due to the corresponding concentration gradient.