Microstructure of AuSn Wafer Bonding for RF-MEMS Packaging

RF-MEMS is one of the most potential applications for MEMS products. Eutectic solder wafer bonding is one of the attractive methods for RF-MEMS wafer level packaging. A process of gold-tin hermetical wafer bonding was developed in SAIT, Korean. Different UBM systems and thin films of gold-tin were deposited on cap wafer, RF-MEMS device wafer and substrate wafer (if needed). The bonding was performed in N₂ ambience with pressure. The cross section of bonding layer had been studied using SEM/EDAX. The thickness of bonding layer is uniform, ranging from 5mum to 7mum. Pretreatment is important to obtain good adhesion and successful microstructure. Voids could be detected without ashing. Optimal process, such as plasma cleaning, would eliminate these voids. There existed different regions in the bonding layer due to inter-diffusion between Au-Sn and other elements. Different intermetallic formed at the bonding layer. The compositions of the intermetallic was identified by EDAX and analyzed according to the Au-Sn phase diagram. The microstructures of the bonding layer are similar for different bonding temperatures in experiments, which indicates lower bonding temperature can get the same hermetical sealing. Typically, there are Au rich layer, AuSn IMC layer and Sn/Au-Ni-Ti layer in the bonding layer between cap wafer and substrate wafer, while there are Au layer, Au-Sn-Ni compound layer in the bonding layer between device wafer and substrate wafer. From the EDAX analysis, different intermetallic compound (IMC) can be identified as AuSn₂, AuSn and other composition. Hermetical and shear strength test were performed for as-bonded dice. The test results indicated there is little difference among different bonding process. Fracture surfaces after shear test were investigated as well. The fracture was inside Au-Sn IMC. It indicates the UBM selected is suitable for application. Reliability test was also performed