Surface Finishing Method Using Plasma Chemical Vaporization Machining for Narrow Channel Walls of X-Ray Crystal Monochromators
Takashi Hirano*,, Yuki Morioka*, Shotaro Matsumura*, Yasuhisa Sano*, Taito Osaka**, Satoshi Matsuyama*, Makina Yabashi**, and Kazuto Yamauchi*
*Department of Precision Science and Technology, Graduate School of Engineering, Osaka University
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
**SPring-8 Center, RIKEN, Sayo, Japan
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.
-  P. Emma et al., “First lasing and operation of an å ngstrom-wavelength free-electron laser,” Nature Photon., Vol.4, pp. 641-647, 2010.
-  T. Ishikawa et al., “A compact X-ray free-electron laser emitting in the sub-å ngström region,” Nature Photon., Vol.6, pp. 540-544, 2012.
-  H. Fujimoto, A. Waseda, and X. Zhang, “Profile Measurement of Polished Surface with Respect to a Lattice Plane of a Silicon Crystal Using a Self-Referenced Lattice Comparator,” Int. J. Automation Technol., Vol.5, No.2, pp. 179-184, 2013.
-  M. Yabashi, K. Tamasaku, S. Kikuta, and T. Ishikawa, “X-ray monochromator with an energy resolution of 8×10-9 at 14.41 keV,” Rev. Sci. Instrum., Vol.72, pp. 4080-4083, 2001.
-  K. Tono et al., “Beamline, experimental stations and photon beam diagnostics for the hard x-ray free electron laser of SACLA,” New J. Phys., Vol.15, pp. 083035, 2013.
-  A. Diaz et al., “Coherence and wavefront characterization of Si-111 monochromators using double-grating interferometry,” J. Synchrotron Rad., Vol.17, pp. 299-307, 2010.
-  I. Sergueev, R. Döhrmann, J. Horbach, and J. Heuer, “Angular vibrations of cryogenically cooled double-crystal monochromators,” J. Synchrotron Rad., Vol.23, pp. 1097-1103, 2016.
-  U. Bonse and M. Hart, “Tailless x-ray single-crystal reflection curves obtained by multiple reflection,” Appl. Phys. Lett., Vol.7, pp. 238-240, 1965.
-  G. Faigel et al., “New Approach to the Study of Nuclear Bragg Scattering of Synchrotron Radiation,” Phys. Rev. Lett., Vol.58, pp. 2699-2701, 1987.
-  A. V. Zozulya et al., “Wavefront preserving channel-cut optics for coherent x-ray scattering experiments at the P10 beamline at PETRAIII,” J. Phys. Conf. Ser., Vol.499, pp. 012003, 2014.
-  P. K. Dhillon and S. Sarkar, “Non-monotonic roughening at early stages of isotropic silicon etching,” Appl. Surf. Sci., Vol.284, pp. 569-574, 2013.
-  Y. Mori, K. Yamauchi, K. Yamamura, and Y. Sano, “Development of plasma chemical vaporization machining,” Rev. Sci. Instrum., Vol.71, pp. 4627-4632, 2000.
-  T. Osaka et al., “Fabrication of ultrathin Bragg beam splitter by plasma chemical vaporization machining,” Key Eng. Mater., Vol.523-524, pp. 40-45, 2012.
-  T. Osaka et al., “A Bragg beam splitter for hard x-ray free-electron lasers,” Opt. Express, Vol.21, pp. 2823-2831, 2013.
-  T. Hirano et al., “Development of speckle-free channel-cut crystal optics using plasma chemical vaporization machining for coherent x-ray applications,” Rev. Sci. Instrum., Vol.87, p. 063118, 2016.
-  Y.-P. Zhao, J. T. Drotar, G.-C. Wang, and T.-M. Lu, “Roughening in Plasma Etch Fronts of Si(100),” Phys. Rev. Lett., Vol.82, pp. 4882-4885, 1999.
-  T. Osaka et al., “Wavelength-tunable split-and-delay optical system for hard X-ray free-electron lasers,” Opt. Express, Vol.24, pp. 9187-9201, 2016.
-  T. Osaka et al., “Characterization of temporal coherence of hard X-ray free-electron laser pulses with single-shot interferograms,” IUCrJ, Vol.4, pp. 728-733, 2017.
-  T. Hirano et al., “Performance of a hard X-ray split-and-delay optical system with a wavefront division,” J. Synchrotron Rad., Vol.25, pp. 20-25, 2018.
-  H. Seidel, L. Csepregi, A. Heuberger, and H. Bäumgartel, “Anisotropic Etching of Crystalline Silicon in Alkaline Solutions,” J. Electrochem. Soc., Vol.137, pp. 3612-3626, 1990.