The Effects of Al Doping and Metallic-Cap Layers on Electromigration Transport Mechanisms in Copper Nanowires

We investigate electromigration transport mechanisms in Cu and Cu alloy damascene conductors. We show that the drift velocity exhibits a dependence on microstructure. We find that Cu-Al alloys exhibit a small increase in grain boundary diffusion activation energy compared to pure Cu and a reduction in the diffusion prefactor for Cu/cap interfacial transport. Cu-silicide- and CoWP-cap layers are both effective in reducing the interfacial component of electromigration primarily through increases in interface diffusion activation energy. The Cu silicide cap also impacts grain boundary electromigration as a result of silicon doping of grain boundaries during processing, while the CoWP cap has no measurable impact on grain boundary transport. The positive impact of Al doping and metallic-cap layers on electromigration is additive, suggesting the potential for impurity doping and metallic caps to be combined to optimize for reliability across the geometry ranges encountered in circuits.