A reflectometer for at-wavelength characterisation of gratings

The design for an UHV-reflectometer for XUV-radiation is presented, which is dedicated to at-wavelength characterisation on high precision gratings. At-Wavelength Metrology is a powerful and necessary characterisation tool for the development and characterisation of optical elements. Since the optical constants of the coating materials involved are dependent on wavelength, information on e.g. reflectivity can only be obtained at-wavelength and cannot be provided by ex-situ methods.In our institute a technology centre for production and characterisation of highly efficient precision gratings is established. Within this project a reflectometer for at-wavelength characterisation of the fabricated blazed gratings is developed and manufactured. This reflectometer complements the SXR-metrology instrumentation at BESSY-II: the existing reflectometer and the polarimeter/ellipsometer chamber for polarisation studies on magneto-optical samples or non-magnetic multilayers.The main feature of the reflectometer is the possibility to incorporate real gratings with a length up to 600 mm, adjustable in six degrees of freedom by a custom designed tripod system. The reflectivity is measured between −180° and +180° incidence angle for both s- and p-polarisation geometry. A variety of detectors with a high dynamic range is accessible.The reflectometer is coupled permanently to the new optics beamline on a BESSY-II bending magnet operating in the UV, EUV and soft x-ray range with the polarisation adjustable to either linear or elliptical. The station will be available by the end of 2013.