Two-Step Plasma Treatment on Copper Surface for Low-Temperature Cu Thermo-Compression Bonding

The success of wafer-to-wafer or chip-to-chip bonding in 3-D packaging relies on the development of planarized, activated, and oxygen-free Cu surface. Preventing Cu oxidation is becoming a crucial issue in chip-to-chip bonding processes. In this study, a two-step plasma pretreatment process was demonstrated to prevent copper oxidation by virtue of forming a nitride passivation layer on the Cu surface. The two-step plasma pretreatment consisted of Ar plasma to activate the Cu surface, followed by N₂ plasma to passivate the Cu surface. It was found that the pressure in the Ar plasma step had the greatest effect on the formation of nitrate passivation on the Cu surface, and the lower the pressure in the Ar plasma step, the better the formation of copper nitrate in the N₂ plasma step. The two-step plasma pretreatment provided a nitride-passivated Cu surface and significantly improved the Cu-to-Cu bonding quality. This process may enable Cu-to-Cu bonding in an atmospheric environment.