Finishing of Ge nanomachined surfaces for X-ray crystal optics

Surface nanomachining based on single-point diamond turning is a viable technology for the preparation of high-quality surfaces. It found its first extensive use in infrared optics, but it is also attractive for X-ray optics. The challenge is that the X-rays used in laboratories have wavelengths 4 orders of magnitude shorter than infrared radiation. Though recent studies have shown promising results, periodic surface ripples as a footprint of the regular cutting tool scanning proved to be an unavoidable feature, producing unwanted effects in reciprocal space. Hence, the presented paper addresses the problem of finishing Ge monocrystalline X-ray surfaces while keeping all benefits of nanomachining. To do so, chemomechanical polishing, Ar+ ion beam bombardment, and UV laser irradiation were tested separately. Chemomechanical polishing proved to be rather time consuming, whereas ion beam bombardment was not sufficient to suppress the ripples even at the highest available energies. In contrast, UV laser irradiation turned out to be a suitable and time-efficient finishing tool to fully suppress the ripples while keeping the local rms surface roughness around 1 nm.