Wafer bonding for optical MEMS

A low temp. direct bonding method that is CMOS compatible is described. The method is applied in the manufg. of advanced spatial light modulators with a matrix of up to 512 * 2048 individually addressable monocryst. silicon mirrors. The key processing step is the layer transfer of a 260nm thick silicon membrane from an SOI wafer to the CMOS wafer, achieved by wafer bonding and removal of the SOI handle wafer. The CMOS wafers were planarized with various glasses such as undoped and boron doped silica glass (USG, BSG). It was shown that USG is an ideal material for planarization and bonding, while BSG tends to form interface defects (bubbles) during annealing. Polished surfaces of USG are characterized by a low surface roughness (RMS < 0.5nm), which makes it ideally suited for direct bonding. The layer system, however, causes large stresses, which are partially compensated by an intermediated SOG layer.