Outgassing of Water Molecules During Silicon Fusion Bonding

Wafer bonding is a key technology for wafer level packaging in MEMS applications. By bonding a cap wafer to a MEMS device wafer, the mechanical structure can be sealed in a package and protected against the outside environment. Fusion bonding is a simple direct wafer bonding technique resulting in high bond strength compared to many other bonding methods. In this study, we present a method to bond a cap wafer to a MEMS device wafer which has released moving mass on it. Wafer surfaces are brought into a contact (pre-bonded) in vacuum conditions followed with high temperature annealing to strengthen the bond. Outgassing of water molecules remaining in the MEMS wafer before wafer bonding was found to be the main reason for bonding failures. Scanning acoustic microscopy was used to compare the bonding results between different pre-bonding vacuum levels. Good bonding results were achieved at higher vacuum level. This packaging method appears to be a suitable technique for encapsulating mechanical sensors with moving mass or other micro structures.