Roughness characterization of large EUV mirror optics by laser light scattering

Optical components for extreme ultraviolet (EUV) lithography at a wavelength of 13.5 nm face tremendous requirements on the surface finish, because large amounts of the EUV light are lost as a result of roughness-induced scattering. In this paper we present a novel approach for the roughness characterization of large EUV mirror optics based on light scattering measurements at a wavelength of 442 nm. The high sensitivity to roughness and the robustness of this method are exemplified for a 660 mm diameter collector mirror substrate. Area covering images of the high-spatial frequency roughness are retrieved which enable a detailed prediction of the EUV reflectance prior to coating. The results are compared to EUV reflectance measurements after coating.