Evolution of interfacial voids in Cu-to-Cu joints

In this study, the Cu joints were fabricated by Cu pads with the highly (111)-oriented nano-twinned structure at 250 °C. We reported a new characterization approach by plan-view images of focused ion beam (FIB) to observe the evolution of interfacial voids in the Cu joints under annealing. The distribution function of interfacial voids and the kinetics of void evolution were then studied and analyzed. The evolution of interfacial voids was proposed to occur at different stages, which were dominant by plastic deformation, creep deformation, and void ripening caused by grain boundary and lattice diffusion. Significant void ripening was observed at early stage of bonding attributed to fast grain boundary diffusion. However, after the bonding interface was eliminated, the void sizes did not change due to slow lattice diffusion.