The Effect of Cu Concentration and Distribution on the Lifetimes of Submicron, Bamboo Al(Cu) Runners

Abstract

ABSTRACTWe have studied the effect of Cu concentration and initial distribution on the electromigration lifetimes of 0.3 – 0.8 μm wide, ≥ 99% bamboo Al interconnects. The mechanism by which Cu reduces electromigration in polygranular interconnects may not be important in bamboo interconnects, so it is not possible to predict whether Cu will have a positive or negative effect in this case. Samples containing 0.0, 0.5, and 0.9 wt. % Cu were tested at 225° and 2×106 A/cm2. For the Cu-containing samples, two different initial Cu distributions were tested: one in which the Cu was quenched into supersaturated solution within the Al grains (no precipitates) and another with many small, closely-spaced Al2Cu precipitates. We find that Cu still improves the mean time to fail of bamboo lines, although not as dramatically as in polygranular lines. More importantly, it significantly decreases the standard deviation in failure time, thus increasing the time to the first failure. Unlike in polygranular lines, increasing the Cu concentration from 0.5 to 0.9 wt. % has little effect on lifetime, and deliberately forming Cu precipitates prior to electromigration stressing reduces lifetimes. The best performance for submicron, highly bamboo runners was obtained from Al(0.5 wt. % Cu) with no precipitates prior to electromigration stressing.