Surface Protection for Semiconductor Direct Bonding

Semiconductor direct wafer bonding is a widely used process for fabricating 3-dimensional structures, especially engineered substrates such as SOI wafers or cavity wafers with and without insulating layers at the bonding interface. The investigations described here concern cavity SOI wafers without insulating layers, as used for discrete and integrated pressure sensors. Semiconductor direct bonding is the initial bonding by mechanical contacting of high-quality surfaces activated by wet chemical and/or plasma treatments, followed by thermal annealing at temperatures and times related to the chosen activation. This ultimately results in a very strong mechanical bond, which can survive all of the subsequent process steps, such as those needed to process MEMS pressure sensors. The requirement for the direct bonding is that the wafers to be bonded have a very high quality; this means very low roughness, low waviness and no surface residues or contamination. In this paper, the focus is on roughness in relation to the bonding process and process integration.