Low temperature solid–liquid interdiffusion wafer and die bonding based on PVD thin Sn/Cu films

Cu–Sn intermetallic diffusion has been studied with thin Sn and Cu films deposited by physical vapor deposition. It was found that annealing time is critical to the interdiffusion of copper and tin, leading to the formation of Cu₆Sn₅ and Cu₃Sn phases at temperatures of interest. Based on this observation, test wafer bonding have been implemented at 320 and 250 °C with sealing pressures of 0.6, 2, 4, 6 and 10 MPa. The experiments emphasized the importance of the sealing pressure and the existence of a sealing pressure threshold for the success of the bonding. When the sealing pressure was superior to that threshold, at 250 °C a thin (<4 µm) bonded Cu/Cu₃Sn/Cu joint was obtained, with none of the voiding defects previously reported by using electroplating.