Diffraction gratings with high efficiency and high groove density are required for EUV and soft x-ray spectroscopy techniques (such as Resonant Inelastic X-ray Scattering, RIXS) designed for state-of-the-art spectral resolution and throughput. We report on recent progress achieved at the Advanced Light Source (ALS) in development of ultra-dense multilayer coated blazed (MCB) gratings. In order to fabricate a high quality MCB grating, one should address two main challenges. The first one is to fabricate nano-period saw-tooth substrates with perfect groove profile and atomically smooth surface of the blazed facets. The second challenge relates to uniformed deposition of a multilayer on a highly corrugated surface of the substrates. We show that the required saw-tooth substrates with groove density up to 10,000 lines/mm can be fabricated using high resolution interference and e-beam lithography techniques followed by wet anisotropic etching of silicon. Peculiarities of growth on the saw-tooth substrates of a variety of multilayers, optimized to provide high diffraction efficiency in EUV wavelength range, are also under throughout investigation. With cross-sectional TEM we reveal a transformation of the structure of the multilayer stack, consisting in smoothing of the groove profile of a coated grating. The multilayer profiles measured with the TEM are used for diffraction efficiency simulations and investigation of the impact of the smoothing on grating performance. Thus, we show that a strong smoothing of the grating grooves results in deterioration of the blazing ability of the gratings.

Ultra-high frequency multilayer coated gratings which combine high dispersion and high diffraction efficiency are of eminent importance for high resolution EUV spectroscopy. Here we report on recent progress achieved in efficiency modelling of a high quality blazed grating which has been fabricated using electron beam lithography followed by wet anisotropic etching of Si. The 100-nm-period grating coated with an Al/Zr multilayer composed of 20 bi-layers demonstrated experimentally and theoretically an absolute efficiency of 13.3percent in the 1st diffraction order at 19.2 nm wavelength and 11 deg incidence angle. The groove profile of the grating was thoroughly characterized by AFM before and after the multilayer deposition. Investigation of the grating by TEM revealed a complex evolution of the groove profile from a triangular shape (bottom) to a sinusoidal one (top). The redundant metrology data were used for simulation of the grating efficiency with commercial PCGrate(R)-SX v.6.1 and v.6.4 codes. The grating boundary profile shapes and their horizontal shifts are the most difficult to determine parameters of the multilayer grating. The smoothing of boundary profiles is a crucial characteristic that dramatically affects on the grating efficiency. Yet, a good general agreement between theory and experiment was obtained for different wavelength and incident angles.

Multilayer coated blazed gratings with high groove density are the most promising candidate for high resolution EUV and soft x-ray spectroscopy. They combine the ability of blazed gratings to concentrate almost all diffraction energy in a desired high diffraction order with high reflectance of EUV and soft x-ray multilayers. The gratings however have to be of superior quality in order to realize their potential. Thus, the grating fabrication process should provide a perfect triangle groove profile and extremely smooth surface of the blazed facets. Here we report on recent progress in fabrication and characterization of the gratings designed for EUV applications. The gratings having groove density from 5000 to 10000 lines/mm were fabricated with scanning beam interference lithography or electron beam lithography followed by wet anisotropic etch of silicon wafers and multilayer deposition. The groove surface morphology was characterized with AFM and SEM, and evolution of the structure of the multilayer stack in the course of multilayer deposition onto the corrugated surface of the blazed grating was studied with TEM (Fig. 1). The grating diffraction efficiency measurements were performed at the ALS beam line 6.3.2. The grating coated with the Mo/Si multilayer composed of 30 bi-layers demonstrated diffraction efficiency more than 37percent in the 3rd diffraction order (Fig. 2). An impact of groove profile imperfections and perturbations of the multilayer stack was also investigated via efficiency simulations with PCGrate 6.1 code. The results of the study show good prospects to extend the technology towards soft x-rays.