Silicon Direct Bonding

This chapter provides an explanation to Silicon Direct Bonding. Direct bonding generally means joining of two pieces together without an intermediate layer or external force. If the surfaces are flat, clean and smooth, they can stick together when brought into contact and form a weak bonding based on physical forces at room temperature. Hydrophilic bonding is used commercially for example to produce SOI-wafers and microelectromechanical structures. In hydrophobic Si bonding, silicon is directly joined to another silicon wafer without an intermediate oxide layer, that is, not even native oxide. It has been suggested that this bonding method could replace epitaxial growth in some applications. Direct wafer bonding of silicon wafers is a robust and simple method to fuse two wafers together, but it is not suitable for every application. The bond-strengthening influence of short plasma activation is clear and has been reported in numerous articles. UHV bonding has also been reported as a method for making room-temperature hydrophobic bonding. Thermally grown oxides on prime polished silicon wafer surface can be bonded without further treatments. It is possible to use CVD oxides to replace the thermal oxide. CVD polysilicon layers are commonly used in MEMS applications. If polysilicon forms the surface layer on the silicon wafer, the encapsulation (bonding to glass) is usually carried out by anodic bonding, which is not as sensitive to surface roughness as direct bonding. The crystallization does not cause voids to the bonded interface