Low vacuum wafer bonding

Compared to bonding wafers in air, bonding of hydrophilic silicon wafers performed in low vacuum (∼700 Pa or a few torr) leads to much stronger bonds at the bonding interface after annealing at temperatures as low as 150°C. The bond energy reached is close to that of thermal silicon oxide itself. For hydrophilic wafer pairs bonded in air, a high bond energy at the bonding interface can also be realized by a low vacuum storage prior to the annealing, or a low vacuum annealing at 150°C after bonding. These low vacuum effects appear to be associated with a significant reduction of trapped nitrogen at the bonding interface. Trapped nitrogen prevents an intimate contact of the bonding surfaces during annealing and thus prevents formation of covalent bonds. Because a difference in thermal expansion coefficients is usually present between different wafers, in order to avoid excess thermal stresses the low vacuum bonding approach is crucial for bonding of dissimilar materials in applications such as microelectromechanical systems and has been applied to bonding silicon to materials other than silicon which have hydrophilic surfaces.