Wafer-Level Hermetic Bonding Using Sn/In and Cu/Ti/Au Metallization

Low-temperature hermetic wafer bonding using In/Sn interlayer and Cu/Ti/Au metallization was investigated for microelectromechanical systems packaging application. In this case, the thin Ti layer was used as a buffer layer to prevent the diffusion between solder interlayer and Cu after deposition and to save more solders for diffusion bonding process. Bonding was performed in a wafer bonder at 180 and 150 degrees C for 20 min with a pressure of 5.5 MPa. It was found that bonding at 180 degrees C voids free seal joints composed of high-temperature intermetallic compounds were obtained with good hermeticity. However, with bonding at 150 degrees C, voids were generated along the seal joint, which caused poor hermeticity compared with that bonded at 180 degrees C. After four types of reliability tests-pressure cooker test, high humidity storage, high-temperature storage, and temperature cycling test-dies bonded at 180 degrees C showed good reliability properties evidenced by hermeticity test and shear tests. Results presented here prove that high-yield and low-temperature hermetic bonding using Sn/In/Cu metallization with thin Ti buffer layer can be achieved.