Low Temperature, Wafer Level Au-In Bonding for ISM Packaging

A low process temperature, hermetic and reliable wafer level packaging (WLP) technology is required for image sensor module (ISM) packaging. Eutectic bonding is regarded as one of the most common used methods for WLP. Au-Sn metallization system has been applied as a wafer level bonding technology in many applications, but it still has process temperature around 300degC which is not applicable for temperature sensitive materials contained device wafer like ISM. In this paper, a fluxless Au-In solder system with Au, In multilayer metallizations has been developed and fabrication process is also presented, the metallization is achieved using e-beam evaporation, test vehicle was then prepared for bonding quality evaluation. Bonding process is performed at 180degC with static force for a relatively long dwelling time of 30minutes in N2 ambience, finally a void free joint is formed. Microstructure observation reveals a combination of different Au-In intermetallic compounds AuIn2 and AuIn at the interface. Shear strength around 20MPa could be obtained for as-bonded samples, and a remelting temperature over 300degC is confirmed using thermomechanical analysis (TMA) test. Real time helium leak rate test are performed to check hermeticity of the package, samples are also subjected to pressure cooker test (PCT) for evaluation of bonding performance after reliability test