ELECTROMIGRATION AND RELIABILITY IN SUBMICRON METALLIZATION AND MULTILEVEL INTERCONNECTION

There are increasing reliability concerns of electromigration-induced and thermal stress-induced failures in submicron interconnects and in multilevel interconnection with W studs. The electromigration characteristics of Al and Al-Cu submicron lines, two-level Al-Cu lines with W studs, Al fine lines under pulsed current stressing at high frequencies, and Al and Al-Cu fine lines under temperature cycling have been systematically studied. Lifetime is affected by grain size, grain morphology and bend structure in submicron metal lines. The lifetime of W stud chains is less than a half of that of Al-Cu flat lines. The discontinuity of the supply of Cu at Al-Cu/W interfaces accounts for most of the reduction in the electromigration resistance of W stud chains. Under pulsed current stressing at frequencies 50-200 MHz, our data indicate no drastic change in lifetime within this frequency range. However, lifetime increases with duty cycle as t50 is-proportional-to r-2.7, which is a remarkable improvement over an average current density model. Lifetime also depends explicitly on both current-on time and current-off period. The extra thermal stress induced by temperature cycling shortens the lifetime of both Al and Al-Cu fine lines by more than an order of magnitude. Our results also show that the addition of Cu in Al fine lines improves the resistance to thermal stress-induced failures, probably by the suppression of grain boundary sliding and migration.