Electrodeposition of nanocrystalline Cu for Cu-Cu direct bonding

Cu-Cu direct bonding is a promising technology to construct a temperature resistant bonding structure for emerging power devices and vehicle electronics. The main challenge is the ease oxidation of Cu and slow solid-state atomic diffusion at the bonding surface. In this study, nanocrystalline Cu is constructed on the bonding surfaces using electrodeposition and direct bonding is carried out in a formic acid atmosphere. Microstructural and strength analyses are performed to investigate the bonding performance of the Cu-Cu joints. The bonding strength analysis indicates that a well-bonded Cu-Cu joint with a high shear strength of 71 MPa can be achieved by direct bonding of two Cu surfaces with a nanocrystalline structure (average grain size = 78 nm). Microstructural analysis shows that significant grain growth accompanying formation of twins at the bonding interface contribute to high bonding strength, which is attributed to enhanced atomic diffusion on the nanocrystalline Cu surfaces.