Unidirectional cross-interface grain growth in Cu–Cu direct bonding

Cu–Cu direct bonding plays a vital role in advanced chip stacking technology because it significantly shortens the signal transmission route, reduces the joint pitch, and improves the thermal resistance. Elimination of the bonding interface by cross-interface grain growth is crucial to achieve a reliable and robust Cu–Cu joint. However, sharp or partially zig-zag bonding interfaces are extensively observed as two texturally similar Cu surfaces are used for direct bonding. In this study, we demonstrate a bonding strategy by using two texturally different Cu surfaces especially with different grain sizes for Cu–Cu direct bonding. One Cu surface with a fine grain size (178 nm) has a high grain boundary density, significantly facilitating the shrinkage of interfacial voids and the formation of interfacial grain boundary. The large grain size difference between the two Cu surfaces (178 nm versus 1.8 μm) produces a driving force to move the bonding interface towards the smaller high-energy grains. Significant unidirectional cross-interface grain growth is achieved, effectively eliminating the original sharp bonding interface. Our results indicate that the grain engineering is a useful and reliable strategy for Cu–Cu direct bonding.