Nano-machining for advanced X-ray crystal optics

Abstract

We present our recent technological achievements in development of the nano-machining of active X-ray crystal optics surfaces. This technique uses a single crystal diamond tool with extremely precise and temperature stabilized positioning system. We can prepare various simple (flat, spherical, cylindrical) as well as more complex free-form surfaces (including aspherical ones). The objective is to prepare high-quality surfaces of the desired shape with sub-nanometer surface roughness and low sub-surface damage of the crystal lattice. The final surface roughness of a flat Ge surface below 0.4 nm (RMS) was achieved for a very slow regime of the processing using feed rates≤0.25 mm/min. At such a slow feed rate, a parasitic surface grating formed due to rastering the diamond tool along the surface was minimized. Surface roughness Ra was reached well below the level of 3 nm stated by the machining device manufacturer in specifications, which is a very good result. The sub-surface damage varied periodically and followed the surface morphology. Raman peak is shifting to higher phonon frequencies in commercially polished sample as in our sample produced using nano-machining which indicates lower compressive local stresses using our technology. The TEM analysis showed an amorphous layer with thickness of ≈30 nm which origin is the subject of our further research. We would like to emphasize that we do not observe any dislocations in the crystalline phase of germanium (110) laying under amorphous layer. We can conclude from our pilot experiments that the diamond tool nano-machining is a promising technology for high-quality surface treatment to replace less homogeneous chemical methods used in production of X-ray channel-cut crystal optics.