Effect of Formic Acid Vapor In Situ Treatment Process on Cu Low-Temperature Bonding

Low-temperature Cu/Cu direct bonding technology using formic acid vapor in situ treatment was developed. Effect of formic acid vapor treatment conditions on Cu surface and bonding was studied. Cu surface oxide was reduced using formic acid vapor in situ treatment at 150 and 200°C, respectively. With higher temperature and longer treatment time, surface reduction is more effective. Grain boundary etching was found on chemical mechanical polished Cu film after initial treatment by formic acid. However, Cu surface roughness is minimally influenced by long-time formic acid vapor treatment. Cu film/Cu film direct bonding was realized in N₂ atmosphere with formic acid vapor in situ treatment under temperature below 200°C. For lower treatment temperature, longer treatment time is required to achieve good bonding. The bonding strength is about 9.0 MPa when Cu surface is treated at 200°C for 10 min.