Silicon Echelles for Soft X-Ray and Extreme UV Radiation: Influence of the Camber of the Reflective Facet on the Diffraction Efficiency

— The influence of the reflective facet parameters, primarily its camber, on the diffraction efficiency of perfect silicon gratings with blaze angles of 3°–4° and a groove density of 500 and 250 mm –1 , which are intended for operation in soft X-ray and extreme UV radiation, is studied. The investigated diffraction gratings are fabricated by the wet etching of Si (111) vicinal wafers and characterized by atomic-force microscopy to determine the relief of the grooves. For one of the considered gratings with a Mo/Si multilayer coating, a record-breaking absolute efficiency of ~40% at a wavelength of 13.5 nm in the –8th order of unpolarized radiation was obtained recently. The possibility of achieving high efficiencies of high diffraction orders, with a number of ~10 and more is considered; this means the possibility of unique X-ray echelle fabrication. An estimate of the influence of the introduced camber parameter of the reflected facet on the maximum achievable order number of highly efficient diffraction is obtained. The absolute and relative (grating) efficiencies are determined by simulation using the PCGrate™ code based on the method of boundary integral equations and groove profiles measured using atomic-force microscopy.