Ion-beam polishing of fused silica substrates for imaging soft x-ray and extreme ultraviolet optics

We have studied the surface treatment of polished fused silica by neutralized Ar ions with energy of 500-1500 eV and incidence angles of 0-90 degrees. We found the following regularities: for samples that passed the standard procedure of deep polishing (initial effective roughness sigma(eff) similar to 0.5 nm), the effective roughness decreases to the ultrasmooth level (i.e., sigma(eff) similar to 0.25 nm in the range of spatial frequencies q is an element of[4.9 x 10(-2) - 63] mu m(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 mu m. For supersmooth samples (sigma(eff) < 0.3 nm), the effective roughness keeps the initial level at material removal down to tens of micrometers. The optimal ion energy range is 800-1300 eV (maximum smoothing effect); at higher energy some surface roughness degradation is observed. All the smoothing effects are observed at the incidence angle range theta(in) = 0-35 degrees. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 mu m per hour (J(ion) = 0.8 mA/cm(2)), which allows for deep aspherization of sized substrates. The technique allows for manufacturing the optical elements for extreme ultraviolet and soft x-ray wavelength ranges with a numerical aperture of up to 0.6. (C) 2016 Optical Society of America