Spin-on-glass smoothing of diamond turned optics for use in the extreme ultraviolet regime

Abstract

ABSTRACT Mirror substrates intended for use in the extreme ultraviolet (EUV) regime have exceedingly stringent requirements on surface finish. Rendering a substrate reflec tive at EUV wavelength and near normal angles of incidence requires these mirrors to be coated with high-quality multilayer coatings forming a Bragg reflector. These coatings typically have period requirements on the order of 7 nm. It is evident that any appreciable roughness on this scale would destroy the resonant behavior the Bragg structures significantly degrading the reflectivity of the mirror. A rule of thumb is that approximately 2% reflectivity is lost per A of surface roughness on the substrate. These requirements significantly increase the cost and fabrication time for EUV optics. This is especially true when dealing with non-conventional shapes such as toroids and fly-eye mirrors. Here we present a spin-on-glass process capable of generating super-polished parts from inexpensive substrates. The method is used to render complex diamond-turned optics compatible for use in the EUV regime. The method is described in detail and experimental results are presented demonstrating the effectiveness of the process. Issues of figure control are also discussed. Keywords: extreme ultraviolet, multilayer, roughness, diamond turning, spin-on-glass resist