Electromigration-induced drift in damascene and plasma-etched Al(Cu). I. Kinetics of Cu depletion in polycrystalline interconnects

Abstract

In this work, a direct and quantitative comparison is presented on the rate of electromigration-induced Cu depletion for polycrystalline damascene and reactive ion etched (RIE) Al(Cu). Kinetic data are derived from the incubation time, obtained from drift characteristics of both unpassivated and passivated Blech-type test structures between 155 and 230 °C. Since the incubation time represents the time necessary for the electron wind to deplete the critical length free of Cu, both electromigration (EM)-threshold and the rate of Cu depletion was systematically investigated. For the latter specific microstructural features, related to Al2Cu precipitate morphology and distribution, are discussed. EM-threshold on the other hand is a more intrinsic characteristic of the interconnect, depending on geometrical and mechanical properties (like aspect ratio and encapsulation). For geometrically equivalent, unpassivated structures, it was found that the improvement in incubation time for the damascene implementation is predominantly controlled by a higher critical length caused by its encapsulation inside the dielectric. For passivated structures, the impact of encapsulation on enlarging the maximum elastic stress buildup becomes similar. As a result, the difference in threshold between passivated damascene and RIE was observed to decrease. The incubation time in passivated structures becomes more importantly controlled by the kinetics of Cu depletion. It is demonstrated that it is most effective to maximize the degree of intergranular θ precipitation to retard the Cu depletion rate in polycrystalline structures. Implications on the relative EM performance under operating conditions for damascene and RIE Al(Cu) are also discussed.