Evaluation of Ion Milled Soft X-ray Multilayer Mirrors for Reflection Wavefront Correction

Abstract

Surface roughness and soft x-ray reflectance of soft x-ray Mo/Si multilayer mirrors ion-milled for physical optical reflection wavefront correction with 0.1 nm accuracy have been measured. The multilayer mirror was milled by Ar ion beam accelerated at 500 V using area-selected mask template with 10×10 mm2 opening. Milled surface at peripheral was rougher than that at center of milling area. Measured maximum roughness of 0.47 nm RMS would cause only under 0.01% reduction of reflectance by theoretical estimation. 44% peak reflectance of the ion-milled multilayer at λ=14.3 nm is practical enough for composing soft x-ray objectives such as Schwarzschild objective. These results prove that ion milling method is effective and gentle milling for precise figure error correction of soft x-ray multilayer mirrors.