Because photolithographic optical apparati project very detailed circuit patterns onto wafers, they need to use very short wavelength light sources. Current devices use 248 nm KrF (krypton fluoride) or 193 nm ArF (argon fluoride) excimer lasers as light sources, which are capable of creating circuit patterns of a width of about 100 nanometers. Various other light sources are currently being studied to get this size down to the 50 nanometers that will be required in the future. In addition to fluoride dimer excimer lasers with wavelengths of 157 nm and X-ray lasers, EUV (Extreme Ultra-Violet) light sources are attracting keen attention. EUV sources have wavelengths of just 13 nanometers, and so should be well able to create patterns of under 50 nanometers. However, because almost all materials absorb EUV, refractive optical elements cannot be used to control the EUV path, and special mirrors, known as multilayer coating mirrors because their surfaces are coated with multilayer thin-film coatings, are being developed. The coatings consist of a large number of alternating layers of materials having slightly different refractive indices to provide a resonant reflectivity to EUV. The manufacture of multilayer reflectors, in turn, requires very high precision coating technologies to deposit coatings with atomic-level thicknesses.