Note 1 to entry: Silicon fusion bonding is used to form impurity diffusion layers or insulation layers inside a wafer by bonding two silicon wafers, one or both of which may be oxidized. The technology is also used to bond wafers that contain impurities of different species or concentrations, as an alternative process to in-depth impurity diffusion or epitaxial growth where high temperatures and a long process time are required. The main problem with silicon fusion bonding is its high process temperature; all lower-temperature processes should take place after the bonding. Studies are ongoing to lower the process temperature by the application of plasma oxidation treatment before bonding, and to apply the technology to bond non-silicon materials. By bonding oxidized wafers, the silicon-on-insulator (SOI) structure can be obtained, in which an insulation layer is sandwiched by two silicon layers. The SOI structure is used to separate integrated element components by oxide and other dielectric materials to improve performance; for example, to manufacture photodiode arrays and so on. Another application of the technology is bonding wafers that have been bored or cut with grooves, to obtain precise structures made inside a wafer. This technique is used to make pressure sensors, and heat exchangers for laser diodes with an internal cooling structure, and so on.