Effects of Wafer Precleaning and Plasma Irradiation to Wafer Surfaces on Plasma-Assisted Surface-Activated Direct Bonding

The plasma-assisted surface-activated bonding technique enables one to directly bond heterogeneous materials. The surface roughness of wafers is an important factor for achieving bonding. The effects of precleaning and plasma surface activation processes on the surface roughness are investigated for silicon-on-insulator (SOI), Ce-substituted yttrium iron garnet (Ce:YIG), InP and LiNbO3. We found that an appropriate precleaning process reduces the surface roughness. Also, the oxygen plasma irradiation to the wafer surface for 10 or 30 s smoothens the surfaces. We achieved the bonding of SOI–Ce:YIG with a strength greater than 1.8 MPa, which is sufficient for application to silicon waveguide optical isolators and circulators.