Abstract

Recent efforts to push optical lithography deeper into the ultraviolet has run up against unexpected difficulties as a result of having to incorporate crystalline materials in the optical systems. 193 nm lithography needs a second UV material to complement fused silica for correcting chromatic aberrations, and 157 nm lithography needs entirely crystalline refractive optics, since fused silica is not transparent enough at this wavelength. This incorporation of crystalline optics was not initially considered a serious problem since these have been used routinely for UV optics, and a well-known symmetry argument seemed to provide assurance that cubic-symmetry crystals such as CaF₂ would be free of the intolerable natural birefringence of non-cubic crystals like MgF₂.

© 2002 Optical Society of America