Direct Observation of the Effects of Cu Distribution on Electromigration Phenomena in Submicron Al Interconnects

Abstract

AbstractWe have performed time-resolved studies of the electromigration (EM) – induced formation, growth, migration, and spatial distribution of Al2Cu precipitates in submicron Al(0.5 wt. % Cu) runners. Of particular interest is the formation of the precipitates in relationship to the local microstructure of the runners, and the formation of voids and hillocks with respect to the location of the Al2Cu precipitates. Runners of widths 0.3, 0.5, 0.8, and 1.0 ýtm were tested at a current density of 2×106 A/cm2 and temperatures between 200 and 300°C. The evolution of the microstructure of the runners was evaluated using plan-view TEM, SEM, and EDX. The formation of voids and hillocks, and the growth of precipitates proceeds more rapidly as the width of the runners increases. However, a far greater number of precipitates nucleate in the 0.3 μm-wide line than in any of the others. As expected, voids are most likely to form near the cathode and hillocks near the anode. Large Al2Cu precipitates form near the anode long before failure occurs. Interestingly, the cathode-side bonding pad is not observed to serve as a good reservoir of Cu for the runners. Although the formation of voids near the cathode seems to be associated with the depletion of Cu in this region, hillocks and voids elsewhere are more likely to form at precipitates than in other parts of the line. Thus Al2Cu precipitates can act as sites for preferential EM-damage, reducing interconnect reliability.