Void nucleation and growth during electromigration in 30 nm wide Cu lines: Impact of different interfaces on failure mode

We investigate void nucleation and growth during electromigration in 30 nm half pitch Cu lines. Diffusion interfaces are varied a) by using SiCN dielectric cap or a CoWP metal cap and b) by tuning the thickness of TaN/Ta barrier metal. The developed local sense EM test method and in-situ EM observations allow understanding void nucleation and growth stages. For the SiCN cap, independent of barrier thickness, there are two void growth modes sensitive to grain structure. In contrast, for the CoWP cap, a single mode independent of the grain structure is observed, where a nucleated void is pinned in the test line. We also show that Co diffuses into the interface between the barrier metal and Cu, and suppresses Cu diffusivity at that interface. As both Cu diffusivities at the cap and barrier interfaces are suppressed by the presence of Co, a CoWP cap is beneficial to electromigration for advanced interconnects where thinner barrier metals are required.