Dislocation pile-ups as sites for formation of electromigration-induced transgranular slit-like voids in Al interconnects

Abstract

Electromigration-induced voiding in metal interconnects in Si integrated circuits is a serious reliability concern. The microstructure of narrow interconnects subject to post-pattern anneal is expected to be bamboo-like in character. These structures are best described as chains of single crystals, with grain boundaries perpendicular to the interconnect axis. In these microstructures, two distinct types of void morphologies have been reported in Al-alloy interconnects: large, wedge shaped erosion voids (E-voids), and narrow slit-like voids (S-voids). A summarized below, electromigration experiments conducted on single-crystal Al interconnects have clearly shown that the transition of erosion voids to slit-like voids is very strongly dependent on the crystallography of the interconnect, and also that there is some inhomogeneously distributed feature which triggers S-void formation, even in single-crystal interconnects. In summary, the authors feel that the strong crystallographic dependence of the S-voids, the possible effects of the enormous mechanical stresses (in excess of 1 GPa in some cases) which can exist in such interconnects, and the stochastic nature of the development of slit-like features, have not been adequately captured in the existing models. In what follows, the authors present a model for a role that dislocation pile-ups may play in reducing the energy of transition ofmore » E-voids to S-voids, and for controlling the location of this transition.« less