In-situ scanning electron microscope observation of electromigration-induced void growth in 30 nm ½ pitch Cu interconnect structures

In-situ electromigration tests have been performed inside a scanning electron microscope on 30 nm wide single damascene interconnects without vias, where a good resolution was obtained and drift velocities during void growth could be measured at 300 °C. These tests showed direct evidence that the cathode end of the line, where a polycrystalline grain cluster encounters a bigger grain, can act as a flux divergent point of Cu diffusion. Moreover, it was found that a thicker barrier suppresses barrier/interface diffusivity of Cu atoms, thereby slowing down electromigration-induced void growth. It was also demonstrated that Cobalt based metal caps are beneficial to electromigration for advanced interconnects where thinner barriers are required.