Fabrication of ultralarge single order diffraction grating with high surface flatness

In this work, we present the fabrication process of a large (100mmx40mm) and thick (30mm) single order diffraction grating with a very flat reflecting surface. This grating is designed for a soft X-ray monochromator. Three single order diffraction gratings with different line densities were integrated on one silicon plate. The gratings are patterned on a 6.35mm thick substrate using direct e-beam writing and etched using high density plasma. Then the grating is glued on to a 23.7mm thick bulk silicon. The flatness of the surface of the gratings was well controlled and tested with an interferometer, the test results show that the peak-to-valley value of the surface is less than 60nm, which meet the requirement for a soft X-ray monochromator.