Interfacial transformation of preoxidized Cu microparticles in a formic-acid atmosphere for pressureless Cu–Cu bonding

Wide bandgap (WBG) power devices have been attracting increasing attention for next-generation electronic applications. Cu–Cu bonding has been developed to accommodate the high temperature and current characteristics encountered in WBG semiconductors service conditions. In this study, we demonstrate and optimize a pressureless Cu–Cu bonding method by reductively sintering preoxidized Cu microparticles in a formic-acid atmosphere, and provide information on the bonding mechanism and relationship between behaviors of Cu microparticles and bonding quality. The transformation of Cu microparticles was examined at different preoxidation intervals, which were followed by a reduction reaction. The Cu oxide produced by preoxidation is critical for reliable Cu–Cu bonding because it forms linkages between Cu microparticles during the process; additionally, it fills the gaps between the individual particles to increase bonding density. In both thermal aging and cycling test, it was found that the Cu–Cu bonding exhibited similar microstructures and oxidation behavior, but followed different degradation and failure modes.