Combined surface activated bonding (SAB) approach for SiO2 direct wafer bonding in vacuum

A combined surface-activated bonding (SAB) technique has been developed for low-temperature direct wafer bonding of oxide-covered silicon wafers. This technique involves a combination of ion beam bombardment, silicon deposition, and water vapor exposure for surface activation before bonding in vacuum. Wafer bonding in vacuum minimizes interface-trapped species, such as water molecules, which is the main origin of hydrogen-induced and water-trapped voids. Trapped water can be eliminated from the bonding interface prior to the bonding step in vacuum, resulting in a void-free bonding interface. An increased bonding energy was achieved by 200°C post-bonding annealing. The increased bonding energy can be attributed to the greater number of Si-OH formed through hydroxylation of the silicon deposited on the SiO₂ surfaces. The present vacuum bonding approach is promising for low-temperature direct bonding of silicon-based wafers without either hydrogen-induced or water-trapped voids at the bonding interface.