Nitrogen passivation formation on Cu surface by Ar–N₂ plasma for Cu-to-Cu wafer stacking application

Wafer stacking technology provides reduced interconnect delay, improved bandwidth, reduced form factor, and decreased cost. Solder-based metallic die bonding is presently utilized in high-volume manufacturing, but Cu-based metallic wafer bonding is quickly becoming a key bonding technique for next generation 3D IC and heterogeneous stacking applications. In this study, Ar–N₂ plasma treatment on Cu surface was investigated to passivate Cu surface with nitrogen and to enhance the bonding quality of Cu-to-Cu wafer bonding. The Ar–N₂ plasma treatment was performed by conventional DC sputtering under 5 mTorr working pressure with different Ar–N₂ partial pressures. Then, the effect of Ar–N₂ plasma treatment on Cu surface was evaluated structurally and electrically. It was observed that the Ar–N₂ plasma treatment with high nitrogen partial pressure over a sufficient plasma treatment time provided activated Cu surface, reduction of copper oxide and chemisorbed nitrogen, and copper nitride passivation. The Ar–N₂ plasma treatment of Cu surface was found to be a potential pretreatment method for Cu-to-Cu bonding.