Reducing Cu/Sn bonding wafer bow for fabrication of MEMS devices

This paper will focus on solid-liquid interdiffusion wafer level bonding process. The wafer fabrication process includes Cu/Sn bonding two thin down wafers. Such solid-liquid interdiffusion bonding process tends to induce stress on the bonded wafer. Therefore reducing wafer bow height after Cu/Sn bonding has been one of the main priorities in most MEMS integration processes. Several changes were introduced into the design and integration flow to help to reduce the wafer bow after Cu/Sn bonding. This includes adding dummy seal ring structures to the edge of the wafer, changing the process integration to bond a non-thinned down device wafer first and optimizing the Cu/Sn bonding recipe. Wafer bow height will be used as the main metrology data this bonding evaluation. FIB cross section will be used to verify the Cu/Sn bonding interface quality.