Enhancement of diffraction efficiency and spectral flux of laminar-type diffraction gratings coated with Ni/La/C layers in soft X-ray region

A conventional nickel (Ni) coated laminar-type grating used at an incidence angle (α) of 87.07° attains a diffraction efficiency of ca. 0.14 for λ= 6.76 nm. We have been performed a feasibility study to enhance its diffraction efficiency dramatically. To this end we found that lanthanum (La) is one of the promising candidates for Ni coated gratings. However, if La was used as the top layer, the reflectivity would deteriorate due to hydroxides and cracks attributed to its vulnerability to moisture in the air. To circumvent these issues we introduced a thin cap layer of carbon (C). We prepared a laminar-type grating having grating period (σ) of 0.893 μm (or groove density of 1120.4 lines/mm); groove depth of 15 nm; duty ratio (= ridge width/σ) of 0.4. We successively coated the layers of Ni, La, and C layers having thicknesses of 30.0, 25.4, and 4.0 nm, respectively. For the new grating the observed maximum diffraction efficiency at λ = 6.76 nm was 0.357 at α = 83.52°, and it was over twice higher than that of 0.173 of the conventional Ni coated grating at α = 84.42°.13th International Conference on Synchrotron Radiation Instrumentation (SRI2018