Room-temperature direct bonding of silicon and quartz glass wafers

We demonstrate a facile bonding method for combining Si/Si, Si/quartz, and quartz/quartz wafers at room temperature (∼25 °C) using a one-step O₂/CF₄/H₂O plasma treatment. The bonding strengths were significantly improved by adding a small amount of CF₄ into the oxygen plasma, such that reliable and tight bonding was obtained after storage in ambient air for 24 h, even without employing heat. Moreover, by introducing water vapor during O₂/CF₄ plasma treatment, uniform wafer bonding was spontaneously achieved without applying an external force. The fluorinated surface asperities appear to be softened more easily by the interfacial water stress corrosion, enabling reliable bonding at room temperature. Additionally, adding an optimized amount of water vapor to the O₂/CF₄ plasma increases sufficiently the amount of hydroxyl groups without eliminating the CF₄ effect. The additional water adsorbed on the surface may help to close the gap between the bonded wafers, resulting in better bonding efficiency.