Characterization and reliability study of low temperature hermetic wafer level bonding using In/Sn interlayer and Cu/Ni/Au metallization

Low temperature hermetic wafer bonding using In/Sn solder interlayer and Cu/Ni/Au metallization was investigated for microelectromechanical system (MEMS) packaging application. The thin Ni layer was used as a buffer layer to control the diffusion process between solder interlayer and Cu. Bonding was performed in a vacuum wafer bonder at 180 and 150 °C for 20 min with a pressure of 5.5 MPa. It was found that bonding at 180 °C, voids free joints composed of high temperature intermetallic compounds (IMCs) were obtained with good hermeticity. However, bonding at 150 °C, voids were generated along the seal joint which caused poor hermeticity comparing with that bonded at 180 °C. Four types of reliability tests, i.e., pressure cooker test, high humidity storage, high temperature storage and temperature cycling test were performed to evaluate the reliability of the seal joints bonded at 180 °C. In the failed dies, the propagation of crack along bonding line was found. The possible failure mechanism was discussed, and feasible improvement methods were proposed.