Abstract: A silicon wafer direct bonding mechanism is proposed as follows. Silicon dangling bonds on the thermal oxidized silicon surface adsorb OH groups to form chemical bonds. A pair of these mirror-polisned silicon wafers in contact is attracted by van der Waal's force at room temperature. At 200-400℃, OH groups located on both surfaces interact, and interface silanol bonds are formed between two surfaces. The polymerization of silanol bonds takes place at a temperature above 800℃ to form siloxane (Si-O-Si) and water. Water molecules diffuse through the interface oxide to react with the bulk silicon. It results in an enhancement of bonding strength and in a local vacuum at uncontacted voids and contributes to the elimination of voids. Water molecular diffusion coefficient increases exponentially with temperature and there is a turning point around 1050℃ after which the increasing rate slows down significantly. Based on the above bonding mechanism, an optimized bonding process has been designed and 3" wafers have been directly bonded successfully 1-3μm CMOS devices using the SOI/SDB material have been fabricated and good performance has been shown. Typical values of electron and hole channel mobility at low drain voltage are 680 cm²/V. sec. and 320 cm²/V. sec., respectively.