| abstract |
The invention is directed to improved coated metal fluoride single crystal optical elements suitable for use in below 250 nm optical lithography, and particularly below 200 nm lithography. The coated elements of the invention can be lenses, windows, prisms and other elements used in lithographic methods, including the laser sources used therein. The invention is also directed to a method of removing the quasi-Bielby layer formed when a shaped optical element is polished. Removal of the quasi-Bielby layer prior to coating results in improved durability and optical transmission characteristics of the coated lenses. The coating material can be any material that does not impede the transmission of below 250 nm electromagnetic radiation. Fluorine doped silicon dioxide is the preferred coating material. |