Surface Finishing Method Using Plasma Chemical Vaporization Machining for Narrow Channel Walls of X-Ray Crystal Monochromators

Abstract

Channel-cut Si crystals are useful optical devices for providing monochromatic X-ray beams with extreme angular stability. Owing to difficulties in the high-precision surface finishing of narrow-channel inner walls of the crystals, typical channel-cut crystals have considerable residual subsurface crystal damage and/or roughness on their channel-wall reflection surfaces that decrease intensity and distort the wavefronts of the reflected X-rays. This paper proposes a high-precision surface finishing method for the narrow-channel inner walls based on plasma chemical vaporization machining, which is a local etching technique using atmospheric-pressure plasma. Cylinder- and nozzle-shaped electrodes were designed for channel widths of more than 5 and 3 mm, respectively. We optimized process conditions for each electrode using commercial Si wafers, and obtained a removal depth of 10 μm with a surface flatness and roughness of less than 1 μm and 1 nmRMS, respectively, which should allow the damaged layers to be fully removed while maintaining the wavefront of coherent X-rays.