UHV wafer bonding is performed in an ultra high vacuum (UHV) system. Inside this vacuum system the pressure is reduced down to 10-8 Pa, about 0.000 000 000 000 1 times the ordinary air pressure. This allows to handle surfaces with dangling bonds without degradation by chemical reactions for several hours. The surfaces of two wafers are converted into surfaces with a high density of dangling bonds by e.g. heating in the vacuum chamber. After cooling to room temperature the wafers are brought into contact. A bond wave propagates along the interface and a new solid is formed starting from two pieces. The bonds are covalent, giving the highest possible bonding strength. This method is called wafer bonding, because silicon and other semiconductor wafers are available with perfectly flat and clean surfaces. In principle all solid materials with mating surfaces can be bonded by this approach. The basic requirements are a roughness below 0.3 nm and a process to produce a high density of dangling bonds inside the vacuum chamber.