Hydrophilic Co-bonding Strategy of Cu-Cu and Al₂O₃-Al₂O₃ for 3D Integration

Cu/SiO₂ hybrid bonding has been considered one of the most promising advanced packaging technologies. However, due to the inferior thermal conductivity and potentially lower bonding strength of SiO₂ compared with atomic layer deposited Al₂O₃, Cu/Al₂O₃ hybrid bonding has become an attractive technology to be developed. In this work, activation and bonding conditions for Cu-Cu and Al₂O₃-Al₂O₃ hydrophilic bonding are comprehensively investigated, with a combined formic acid and inductively coupled plasma (ICP) activation strategy being proposed. Plasma activation conditions for cobonding of Cu-Cu and Al₂O₃-Al₂O₃ interfaces, such as plasma type and activation time, are optimized. The influence of formic acid treatment on Cu surface resistivity and its compatibility with Al₂O₃ bonding is investigated. Additionally, the influence of annealing processes on bonding quality is analyzed. It is observed that Ar ICP-treated Cu and Al₂O₃ surfaces exhibit minimum root-mean-square roughness values of 1.53 and 0.125 nm, respectively, at an optimized ICP activation duration of 100 s. The combined formic acid and ICP activation strategy achieves the lowest sheet resistance of 0.31 Ω/sq on Cu surfaces. After annealing, the shear force of Cu-Cu and Al₂O₃-Al₂O₃ bonding is enhanced by 51% and 163%, respectively. Through synergistic formic acid-ICP activation followed by annealing at 300 °C, tightly bonded Cu-Cu and Al₂O₃-Al₂O₃ interfaces are realized with shear strengths of 4.05 and 1.65 MPa. This work provides experimental references and guidance for Cu/Al₂O₃ hybrid bonding technology.