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

ABSTRACT

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°. We found the following regularities for samples that passed the standard procedure of deep polishing (initial effective roughness σ(eff)∼0.5 nm), the effective roughness decreases to the ultrasmooth level (i.e., σ(eff)∼0.25 nm in the range of spatial frequencies q∈[4.9×10(-2)-63] µm(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 µm. For supersmooth samples (σ(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 θ(in)=0-35°. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 µm per hour (J(ion)=0.8 mA/cm2), 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.