Low blaze angle gratings for x-ray and EUV applications

Abstract

Low groove density gratings with blaze angles as low as 0.1‡ are required for plane grating monochromators for x-ray synchrotron and Free Electron Laser applications. To achieve so small a blaze angles we developed a process of reduction of the blaze angle of a coarse Si grating fabricated by anisotropic wet etching. The coarse grating with a blaze angle of 4° is planarized by a polymer layer and then plasma etching is applied to remove the polymer and underlying silicon material. The appropriate ratio of etch rates of Si and the polymer material provides reduction of the groove depth and the blaze angle. We developed a set of reduction recipes which provide blaze angle reduction down to 0.04° with high accuracy and which preserves the perfect triangular shape of the grooves. The ultra-low blaze angle grating coated with a Mo/Si multilayer exhibits a record diffraction efficiency of 58% due to the perfect match of the groove depth with the multilayer d-spacing. This opens up wide possibilities for making highly accurate and efficient diffraction gratings for tender x-ray, free electron laser, and EUV lithography applications. The low blaze angle gratings have a perfect triangular groove profile and highly smooth surfaces of the blazed facets which ensures high diffraction efficiency of the x-ray gratings.