Novel hydrophilic SiO2 wafer bonding using combined surface-activated bonding technique

Low-temperature hydrophilic SiO₂-SiO₂ wafer bonding has been performed in vacuum by a new combined surface-activated bonding (SAB) technique. In this technique, wafers are irradiated by ion beam bombardment and simultaneously deposited with silicon by in situ silicon sputter deposition, and then terminated with Si-OH groups by water vapor exposure prior to bonding in vacuum. A surface energy of more than 1 J/m² was achieved by 200 °C postbonding annealing. A void-free oxide intermediate layer with a thickness of about 15 nm was observed at the bonding interface by transmission electron microscopy (TEM). The increased bonding energy can be attributed to the greater number of Si-OH formed through hydroxylation of the silicon deposited on the SiO₂ surfaces.