Surface characterization of optics for EUV lithography

Abstract

The surface topography of optics fabricated for Extreme Ultraviolet Lithography has been measured using a combination of phase-measuring interferometery and atomic force microscopy. Power Spectral Densities were computed over spatial frequencies extending from 2.0×10-8 nm-1 to 7.7×10-2 nm-1. Roughness values for frequencies greater than 1.0×10-6 nm-1 were 0.64 nm rms for a spherical optic and 0.95 nm rms for an aspheric optic. These values are significantly larger than 0.088 nm rms, which as obtained using a spherical optic representative of current limits in surface polishing technology.