We report on recent progress in developing diffraction gratings which can potentially provide extremely high spectral resolution of 105-106 in the EUV and soft x-ray photon energy ranges. Such a grating was fabricated by deposition of a multilayer on a substrate which consists of a 6-degree blazed grating with a high groove density. The fabrication of the substrate gratings was based on scanning interference lithography and anisotropic wet etch of silicon single crystals. The optimized fabrication process provided precise control of the grating periodicity, and the grating groove profile, together with very short anti-blazed facets, and near atomically smooth surface blazed facets. The blazed grating coated with 20 Mo/Si bilayers demonstrated a diffraction efficiency in the third order as high as 33percent at an incidence angle of 11? and wavelength of 14.18 nm.

State of the art soft x-ray spectroscopy techniques like Resonant Inelastic X-ray Scattering (RIXS) require diffraction gratings which can provide extremely high spectral resolution of 105-106. This problem may be addressed with a sliced multilayer grating with an ultra-high groove density (up to 50,000 mm-1) proposed in the recent publication [Voronov, D. L., Cambie, R., Feshchenko, R. M., Gullikson, E., Padmore, H. A., Vinogradov, A. V., Yashchuk, V. V., Proc. SPIE 6705, 67050E (2007)]. It has been suggested to fabricate such a grating by deposition of a soft x-ray multilayer on a substrate which is a blazed saw-tooth grating (echellette) with low groove density. Subsequent polishing applied to the coated grating removes part of the coating and forms an oblique-cut multiline structure that is a sliced multilayer grating. The resulting grating has a short-scale periodicity of lines (bilayers), which is defined by the multilayer period and the oblique-cut angle. We fabricated and tested a Sc/Si multilayer sliced grating suitable for EUV applications, which is a first prototype based on the suggested technique. In order to fabricate an echellette substrate, we used anisotropic KOH etching of a Si wafer. The etching regime was optimized to obtain smooth and flat echellette facets. A Sc/Si multilayer was deposited by dc-magnetron sputtering, and after that it was mechanically polished using a number of diamond pastes. The resulting sliced grating prototype with ~;;270 nm line period has demonstrated a dispersive ability in the 41-49 nm photon wavelength range with a diffraction efficiency of ~;;7percent for the optimized 38th order assigned to the echellette grating of 10 mu m period.

Resonant Inelastic X-ray Scattering (RIXS) is the one of the most powerful methods for investigation of the electronic structure of materials, specifically of excitations in correlated electron systems. However the potential of the RIXS technique has not been fully exploited because conventional grating spectrometers have not been capable of achieving the extreme resolving powers that RIXS can utilize. State of the art spectrometers in the soft x-ray energy range achieve ~;0.25 eV resolution, compared to the energy scales of soft excitations and superconducting gap openings down to a few meV. Development of diffraction gratings with super high resolving power is necessary to solve this problem. In this paper we study the possibilities of fabrication of gratings of resolving power of up to 106 for the 0.5 1.5 KeV energy range. This energy range corresponds to all or most of the useful dipole transitions for elements of interest in most correlated electronic systems, i.e., oxygen K-edge of relevance to all oxides, the transition metal L2,3 edges, and the M4,5 edges of the rare earths. Various approaches based on different kinds of diffraction gratings such as deep-etched multilayer gratings, and multilayer coated echelettes are discussed. We also present simulations of diffraction efficiency for such gratings, and investigate the necessary fabrication tolerances.