The effect of passivation thickness on the electromigration lifetime of Al/Cu thin film conductors

Abstract

Although it has become common practice to cover thin film conductors with tough passivation layers, the reliability of the underlying conductor has not been well established. Most investigators have observed an increase in lifetime for stripes covered with glass as compared to uncovered stripes, but have disagreed as to the extent and the cause of this effect. A research program was undertaken in which the electromigration lifetime of Al/Cu conductors was studied as a function of passivation thickness. Stripes covered with 1.5 μm of glass lasted longer than those which were not covered. A further increase in lifetime was found for samples with thicker glass, but a change in failure mode was also observed. All uncovered stripes and all stripes covered with thin glass failed by open circuit, whereas those with thick glass failed by short circuit, as well. Electromigration-induced compressive stresses are believed to be responsible for these observations.