Surface Characterization for and by Semiconductor Wafer Direct Bonding

Semiconductor direct wafer bonding is a very attractive process without any additional bonding layer to stack and join wafers, in order to fabricate special substrates, such as SOI or wafers having cavities sealed with thin membranes for special MEMS processes. With plasma activation, low temperature direct wafer bonding becomes also usable for MEMS applications, in which cap wafer bonding is required at the end of the process. While the surface activation processes required for direct bonding are well investigated and understood, the surface roughness of the layers to be bonded seems to be the limiting factor for this bonding technology, especially if chemical mechanical polishing (CMP) processes cannot be used. In this paper a method is introduced to investigate the bondability of the wafer surface in order to improve the surface roughness and the pre-bonding process.