Direct bonding of glass substrates

We report on investigations of direct bonding of glass materials for application as optical devices or high precision mechanical stages under vacuum. SiO2 (fused silica) wafers of about 0.8 mm thickness and 200mm diameter were bonded to massive SiO2 substrates of up to 20 mm thickness. With suitable wet cleaning and a low pressure Nitrogen plasma surface activation, rather high quality (optically transparent) bonds were obtained. Successful bonding was achieved at relatively low temperatures of about 250°Celsius in vacuum under compressive forces of several tens of kN, using a commercial bond equipment. Remaining bonding defects were analyzed by transmitting polarized light and measuring stress birefringence. At particle related defects in the bonding area, stress levels of the order of 0.2 MPa were discerned. Small unbound regions typically emerged at locations where no significant pressure was applied during the bonding process. Crack-opening indicated a bonding strength of about 0.6 J/m².