Advances in silica-based glasses for UV and vacuum-UV laser optics

The origins of pre-existing and laser-induced ultraviolet (UV) and vacuum ultraviolet (VUV) optical absorption in state-of-the-art glassy silicon dioxide and the ways to improve it are reviewed. The main causes of pre-existing absorption in UV/VUV are oxygen vacancies, hydroxyl (silanol) groups, and strained bonds/localized states due to glassy disorder. The main absorption bands induced by UV/VUV excimer lasers are due to oxygen vacancies and due to silicon and oxygen dangling bonds (E'-centers and non-bridging oxygen hole centers, respectively). The optimized glasses are achieved via an intricate balance between a good stoichiometry, use of network modifiers (F or OH) to reduce the number of strained bonds, minimized number of Si-OH-related absorbers and using of interstitial hydrogen for annealing of photoinduced defects. The optimization is different for KrF, ArF or F₂ excimer laser energies. The most significant advance to increase VUV transparency and laser toughness is fluorine doping. F-doped ("modified") silica glasses show superior transparency and radiation resistance in VUV region and are suitable for photomask substrates in F₂-laser based microlithography or for deep-UV optical fibers.