Electromigration-induced transgranular failure mechanisms in single-crystal aluminum interconnects

Abstract

Passivated and unpassivated Al single-crystal lines with (110), (133), and (111) planes parallel to the substrate have been fabricated and electromigration tests have been performed to study transgranular failure mechanisms. Both erosion voids and slitlike voids with {111} facets were observed in single-crystal lines. The slitlike voids lie along the in-plane direction, which leads to minimum-surface-area voids among the crystallographically possible directions for the {111}-faceted voids. Voids that nucleate in the lines appear to be mobile and can move toward the cathode end and sometimes accumulate in the cathode pad. (110)-textured lines fail due to erosion voids, slitlike voids, and pad voids, with roughly equal probability. However, (111)- and (133)-textured lines failed predominantly due to pad voids. In both passivated and unpassivated single-crystal lines, the median time to failure, t50, is texture dependent, with t50(111)>t50(133)>t50(110), and with t50(111)⩾10×t50(110). The activation energy for failure for (110) single-crystal lines is about 1 eV, suggesting that Al/oxide interface diffusion may be the dominant mechanism in these lines, and by inference, in bamboo grains in lines with near-bamboo or fully bamboo microstructures. The current density exponent for failure times of single-crystal lines is about 2.5, which is close to that of polycrystalline lines. Based on the results of this study, coupled with results from an earlier model for calculation of lifetimes due to polygranular failure mechanisms, an improved scaling methodology for projection of lifetimes from test conditions to service conditions for near-bamboo interconnects is proposed.