Wafer Bonding

Wafer bonding is an integral part of the fabrication of MEMS, optoelectronics, and heterogeneous wafer stacks, including silicon-on-insulator. Wafer bonding can be divided into two technological groups: direct bonding and intermediate layer bonding. Direct bonding relies on the cohesive bond that is formed when the surfaces of two wafers are brought together under specific temperature and pressure conditions. Direct bonding relies on critical parameters such as surface energy, surface roughness, and surface morphology. Intermediate layer bonding relies on the cohesive bond that is formed when the surfaces of two wafers are mated with an intermediate layer. The intermediate layer can be an adhesive, polymer, solder, glass frit, or metal. Surface roughness and topography are less critical for bonding with an intermediate layer. Wafer bonding has found application in MEMS to fabricate MEMS devices, to encapsulate the MEMS device in an hermetic environment, and to transfer bond a complete MEMS device to a wafer with integrated circuits. Wafer bonding has found its earliest applications for pressure sensors and accelerometers, but the applications remain boundless.