OPTICAL ELEMENT WITH AN ANTIREFLECTION COATING, PROJECTION OBJECTIVE, AND EXPOSURE APPARATUS COMPRISING SUCH AN ELEMENT

摘要

An optical element (14) transparent for radiation with a wavelength ? in the ultraviolet wavelength range below 250 nm, in particular at 193 nm, comprises a substrate (17) with a refractive index ns larger than 1.6, and an antireflection coating (16) formed on at least part of the surface of the substrate (17) between the substrate (17) and an ambient medium with a refractive index n_A, preferably with n_A=1.0. The antireflection coating (16) consists of a single layer of a material with a refractive index n_L of about n_L = vn_A n_S , in particular n_L > 1.3, and the optical thickness d_L of the single layer is about ?/4. The optical element (14) is preferably part of a projection objective (5) in a microlithography projection exposure apparatus (1) and located adjacent to a light-sensitive substrate (10).