C₂H₄ reductive plasma treatment on copper surface for copper bonding

This study investigated the effects of C₂H₄ plasma treatment on a Cu surface and its influence on the quality of Cu-to-Cu bonding. C₂H₄ plasma is applied to enhance Cu bonding by producing reactive hydrogen for effective oxide removal and activation, while forming a protective carbon layer at lower temperatures with simpler processing than forming gas. An 8-inch Si wafer with a 700 nm thick SiO₂ layer was used as the substrate, onto which 50 nm Ti and 1 μm Cu layers were deposited by DC magnetron sputtering. The Cu surface was treated with C₂H₄ plasma for varying durations (10 ∼ 90 s) to modify its surface characteristics. XPS (x-ray photoelectron spectroscopy) analysis showed that plasma treatment reduced the surface oxides and contaminants, with a noticeable increase in Cu₂O. AFM (atomic force microscopy) measurements indicated that the plasma treatment decreased the surface roughness by over 40%, with the roughness decreasing as the plasma-treatment time increased. Cu-to-Cu bonding was performed at 260 °C, followed by annealing at 200 °C. A bonding quality was assessed using SAT (scanning acoustic tomography), FE-SEM (field emission scanning electron microscopy), and TEM (transmission electron microscopy), revealing that the plasma-treated sample exhibited fewer voids and a diffused Cu interface, suggesting improved bonding compared to the non-plasma-treated sample. The results indicate that C₂H₄ plasma treatment enhances the Cu surface, leading to improved Cu-to-Cu bonding quality.