Fabrication of atomically precise, super smooth, and damage-free x-ray optics

An atomically precise, super smooth, and damage-free surface is highly demanded for x-ray mirrors, multilayer optics, channel-cut crystal monochromators (CCM), and gratings. An ultra-precision optic with a figure error of several nm is crucial for single-nanometer spatial resolution, signal strength, and contrast. Moreover, sub-angstrom root-mean-square surface roughness is beneficial for high reflectivity and the lowest unwanted scattering. Additionally, a damage-free surface with no alter layer is greatly desired for CCM and grating substrates because it is essential for the high reflectivity of the CCM and the uniform etching rate of the grating’s ruling. In the manufacturing of x-ray mirrors, to obtain the desired figure error, a non-contact figuring method, such as ion beam figuring (IBF), plasma chemical vaporization machining (PCVM), or Elastic Emission Machining (EEM), is usually employed thanks to its high controllability and stability. EEM has proved the best performance in terms of achieving a low figure error and maintaining a good surface roughness. After shaping/figuring, a finishing method is usually applied to improve its surface roughness without distorting its figure error. Recently, Catalyst-Referred Etching (CARE) has realized its potential and applicability to x-ray mirror manufacturing as a finishing method. Thanks to its removal mechanism, a highly ordered surface with a root-mean-square of 0.03 nm RMS is attained. In the polishing of a CCM, because a mechanical method is usually used to polish its surface at the narrow gap, the residual mechanical damage induced a low reflectivity and low spatial resolution. PCVM with a wire electrode has recently been proposed and demonstrated its excellent performance. The damage-free surface of a CCM with a gap of less than 100µm has been successfully realized by PCVM. In this paper, recent achievements in figuring (EEM), surface finishing (CARE), and damage removal (PCVM) are presented and discussed.