Gas development at the interface of directly bonded silicon wafers: Investigation on silicon-based pressure sensors

Hydrophobic and hydrophilic silicon direct bonding (SDB) under vacuum conditions are compared with respect to the capability to form hermetically sealed cavities with minimal enclosed residual gas pressure, The quality of the enclosed vacuum inside the cavities of pressure-sensor test structures is monitored for different cavity layouts and annealing processes. The observed pressure increase during annealing is found to be strongly dependent on annealing temperature, subsequent storage time and on the bonded area surrounding each cavity. Trapped gaseous reaction products originating at the bonding interface are responsible for this effect. There is a significant difference in annealing-related gas evolution for hydrophobic and hydrophilic bonding surfaces, which can be explained by the existing theories of the bonding mechanism, Hydrophobic bonding leads to about 50% lower residual gas pressures compared with hydrophilic bonding. For hydrophilically bonded wafers a slow saturation of the gas pressure after annealing is observed.