Precise control of the blaze angle of x-ray diffraction gratings via planarization and plasma etching

High diffraction efficiency of a grating can be achieved by use of inclined facets. This type of grating, normally referred to as a blazed grating, has facets that are arranged so that there is an equal angle on the incident and diffracted sides, and thus can be thought of as reflecting light into a particular order. Conventionally blazed gratings are made by diamond mechanical ruling, or more recently by anisotropic etching of silicon. However it is difficult with these processes to achieve a very low blaze angle as well as to precisely control it. This has become particularly important for applications involving Free Electron Lasers, where a very grazing incidence angle has to be used to avoid damage, and for extension of the working range up to high energies on synchrotrons. In each case, the very small angular size of the source results in a low line density, which in turn results in a low blaze angle. In high groove density multilayer blazed gratings, high precision for the blaze angle is required to match the groove depth to the multilayer d-spacing. We have developed a process which gives the possibility for alteration of the groove profile of a fabricated grating and tune the blaze angle with high precision. The method is based on planarization of the grating grooves by deposition of a SiO2 sacrificial layer followed by smoothing the surface with Ar plasma etch. Finally, a reactive plasma etching is used to etch off the sacrificial layer together with the surface layer of the Si grating. The optimized plasma etching provides a certain ratio of etch rates of the sacrificial layer and Si and results in reduction of the blaze angle down to a desired value.