Electromigration in VLSI metallization (Invited Paper)

Abstract

There is increasing concern regarding reliability of electromigration-induced and thermal stress-induced failures in submicron interconnects carrying the projected high current density and in multilevel interconnection with W studs. 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 half that of Al- Cu flat lines. The discontinuity of Cu supply at Al-Cu/W interfaces account 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 threshold frequency for drastic change in lifetime. However, lifetime increases with duty cycle as t50 varies as 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.