Application of Au-Sn eutectic bonding in hermetic RF MEMS wafer level packaging

Recently the strong demands in wireless communication requires expanding development for the application of RF MEMS (Radio Frequency micro electro mechanical systems) sensing devices such as micro-switches, tunable capacitors because it offers lower power consumption, lower losses, higher linearity and higher Q factors compared with conventional communications components. To accelerate commercialization of RF MEMS products, development for packaging technologies is one of the most critical issues should be solved beforehand. Packaging for RF MEMS is more challenging compared with conventional IC (integrated Circuit) Packaging technologies because it has both electrical and mechanical component, a low temperature, and hermetic wafer level packaging technology is needed for RF MEMS device. Au-Sn metallization system has been successfully utilize for flip chip bonding in many applications such as optoelectronic packaging and microwave device because of their high strength, good wetting behaviors, and resistance for thermal fatigue compared with conventional Pb/Sn solder system. Au-Sn eutectic bonding is considered to be a promising low temperature, wafer level bonding technology. In this paper, Au-Sn eutectic bonding for RF MEMS application is presented, a closed square loop was designed for the bonding structure, test vehicle was prepared for DOE (Design of experiment) process for the optimization of bonding parameters, and bonding temperature and applied load are found to be the most critical parameters for the bonding result, bonding can be done at a relative low temperature below 300°C. For bonded samples, shear strength, warpage, insertion loss and hermetic tests etc. are performed for the evaluation of bonding quality, AES (Auger Electron Spectrum) and SEM (Scanning Electron Microscopy) was also made to investigate the microstructure of bonded interface, and reliability test such as thermal shock and high temperature, high humidity storage test was performed for the evaluation of bonding quality.