Electromigration recovery and short lead effect under bipolar- and unipolar-pulse current

Electromigration (EM) of Copper interconnects is studied under unipolar pulse current (UPC) at 1.25 kHz ~ 1.25 MHz and bipolar pulse current (BPC) at 100 kHz ~ 5.0 MHz. For symmetric BPC, where a time-averaged current density (J_avg) is 0, we found interconnect failures by EM-induced void formation. This means EM is not fully recovered during the opposite-polarity pulse current. The degree of EM recovery is found to be 0.7 ~ 0.9, where 1 is for full-recovery. Also, the short lead EM effect is investigated under UPC. We found short lead EM lifetimes are modeled by peak current density (J_peak), instead of J_avg. Also, we observed, during the off-time between low frequency pulse currents, mechanical back-stress may reduce the stress gradient built by UPC and increase the EM lifetimes of the short leads.