Wafer bonding is a phenomenon wherein at room temperature; mirror-polished, flat, and clean wafers of any material are brought in contact get attracted to each other by forces and adhere or bond. Wafer bonding is also known as direct bonding or fusion bonding. In maximum situations, the wafers that are included in actual applications are compound semiconductor wafers that consist of single-crystal materials such as silicon or gallium arsenide that are used in microelectronics or optoelectronics. When compared with that of covalently or ionically bonded solids the bonding at room temperature is usually weak. So for many applications, to strengthen the bonds across the interface the room-temperature-bonded wafers have to undergo heat treatment. Then one of the two wafers is thinned down to a thickness that may be in the range of many microns down to a couple of nanometers but depending on the specific application.
Today you will find that the most prominent applications of compound semiconductor wafer bonding are in the areas of silicon-on-insulator (SOI) devices and silicon-based sensors and actuators. SOI structures generally consist of a thin, top layer of single-crystal silicon, a layer of silicon dioxide (SiO2), and a silicon handle substrate that provides mechanical support. When the fabrication of SOI substrates is performed by wafer bonding, the silicon wafer that forms the top layer needs to be oxidized before bonding, and after bonding it needs to be thinned down to between 0.1 and 10 μm. When compared to devices on conventional silicon substrates, SOI devices that give hard radiation can operate at high temperatures and also have potentially higher packing density and lower power consumption.
Even after the dominance of silicon-related applications, wafer-bonding technology is by no means restricted to silicon wafers. Because of proper polishing and control of the chemistry of the surfaces, it has become possible to bond a variety of solids independently of their lattice parameter, structure (amorphous, poly crystalline, single-crystal), their crystallographic orientation, and or the thickness of the wafers. Hence, compound semiconductor wafer China bonding allows the fabrication of material combinations that were previously ruled out by most materials scientists, solid-state physicists, and electrical engineers, because by the conventional approach of epithelial growth these material combinations were not possible.
