Experimental evaluation of threshold current density for electromigration damage in Al interconnect line with reservoir and sink structure

Abstract

The reservoir is an expansion at the cathode-end in the electric line, which improves the lifetime for electromigration (EM). In contrast, the sink, an additional part at the anode-end, leads to rapid failure. The immortal threshold current density, the critical current density that does not cause EM damage, is an important evaluation factor in EM reliability. Previous numerical studies suggested that the threshold current density increases with the use of reservoir and decreases with the use of sink. In this study, a current loading test using simplified reservoir/sink Al lines was performed to experimentally verify the reservoir/sink effect on the threshold current density. The 2D modeled reservoir/sink structures were fabricated by Al line, Cr via, and SiO2 passivation layers. Typical EM phenomena such as atomic disappearance at the cathode-end due to the growth of voids and generation of hillocks at the anode-end were observed. The reservoir/sink condition of the line affected the rate of change of the electrical potential drop. The threshold current density increased when the reservoir structure was used and decreased when the sink structure was used. It was demonstrated that the reservoir is useful not only for expanding the EM lifetime but also for improving the threshold current density.