Fabrication of a Bragg beam splitter for hard x-ray free-electron lasers

We report a fabrication method of an ultrathin silicon crystal as a beam splitter for the hard x-ray regime based on the Bragg diffraction operated in the symmetric Bragg geometry, and evaluation results of crystalline perfection at SPring-8. A sub-10-mu m thick Si( 511) crystal was fabricated with a reactive dry etching method using atmospheric-pressure plasma. Following the evaluation of topography and diffractometry, the crystal was found to be strain-free, and capable of splitting a monochromatic x-ray beam into two branches with almost 1:1 splitting ratio.