Effects of Current Stress for Low Temperature Cu/Sn/Cu Solid-State-Diffusion Bonding

In this work, current stress was applied to investigate electromigration (EM) effects for Cu/Sn/Cu solid-state-diffusion (SSD) bonding in 3D integration. Full-array bumps with different pitches were fabricated on wafers. Asymmetric structure of Cu/Sn bump and Cu bump was deposited by low-cost electroplating process. With optimized surface pretreatment, wafer-level Cu/Sn/Cu SSD bonding was performed at a low temperature of 200 °C for 30 min under a vacuum of 10-5 mbar. As-bonded wafers were annealed at 200 °C for 60 min under N2 atmosphere and then diced into dies. The bonding interface consisted of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu, without pure Sn remained and Sn overflow. After further annealing process for bonded dies, the middle layer of Cu6Sn5 has been completely exhausted, and the interface transformed to Cu/Cu3Sn/Cu. The bonding strength has reached above 70 MPa. Subsequently, the three-layer structure was subjected to EM test at 150 °C for 500 hours with current density of 2.0 × 104 A/cm2. No void or crack caused by atom migration appeared at the cathode and the anode. Bonding strength also kept at above 70 MPa, and no electrical degeneration occurred according to resistance measurement. It is concluded that high-quality Cu/Sn/Cu bonding was realized using SSD technology and exhibited an expected anti-electromigration capability.