Lawrence Berkeley National Laboratory

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.