A direct quantitative comparison was made of the rate of electromigration-induced Cu depletion in polycrystalline damascene and reactive ion-etched Al-Cu. Kinetic data were derived from incubation times; as deduced from the drift characteristics of unpassivated and passivated Blech-type test structures between 155 and 230C. The incubation time represented the period which was necessary for the electron wind to deplete a critical length of Cu. Both the electromigration threshold and the rate of Cu depletion were systematically investigated. The latter was related to the Al2Cu precipitate morphology and distribution. The electromigration threshold was a more intrinsic characteristic of the interconnect, and depended upon geometrical and mechanical properties such as the aspect ratio and the encapsulation. In the case of geometrically equivalent unpassivated structures, it was found that the improvement in incubation time for damascene samples was controlled mainly by the higher critical length which was caused by its encapsulation within a dielectric. In the case of passivated structures, the effect of encapsulation in increasing the maximum elastic-stress build-up became similar. Therefore, the difference in thresholds between passivated damascene and reactive ion-etched material decreased. The incubation time for passivated structures became more closely controlled by the kinetics of Cu depletion. It was demonstrated that it was most effective to maximize the degree of intergranular precipitation in order to retard the Cu depletion rate in polycrystalline structures.

Electromigration-Induced Drift in Damascene and Plasma-Etched Al(Cu) - Kinetics of Cu Depletion in Polycrystalline Interconnects. J.Proost, A.Witvrouw, K.Maex, J.D’Haen, P.Cosemans: Journal of Applied Physics, 2000, 87[1], 86-98