On the characterization of ultra-precise X-ray optical components: advances and challenges inex situmetrology

Abstract

To fully exploit the ultimate source properties of the next-generation light sources, such as free-electron lasers (FELs) and diffraction-limited storage rings (DLSRs), the quality requirements for gratings and reflective synchrotron optics, especially mirrors, have significantly increased. These coherencepreserving optical components for high-brightness sources will feature nanoscopic shape accuracies over macroscopic length scales up to 1000 mm. To enable high efficiency in terms of photon flux, such optics will be coated with application-tailored single or multilayer coatings. Advanced thin-film fabrication of today enables the synthesis of layers on the sub-nanometre precision level over a deposition length of up to 1500 mm. Specifically dedicated metrology instrumentation of comparable accuracy has been developed to characterize such optical elements. Second-generation slope-measuring profilers like the nanometre optical component measuring machine (NOM) at the BESSY-II Optics laboratory allow the inspection of up to 1500 mm-long reflective optical components with an accuracy better than 50 nrad r.m.s. Besides measuring the shape on top of the coated mirror, it is of particular interest to characterize the internal material properties of the mirror coating, which is the domain of X-rays. Layer thickness, density and interface roughness of single and multilayer coatings are investigated by means of X-ray reflectometry. In this publication recent achievements in the field of slope measuring metrology are shown and the characterization of different types of mirror coating demonstrated. Furthermore, upcoming challenges to the inspection of ultra-precise optical components designed to be used in future FEL and DLSR beamlines are discussed.

High precision tilt stage as a key element to a universal test mirror for characterization and calibration of slope measuring instruments

“…12,27,[37][38][39][40][41] This can be thought of as a repeatability ( Below we describe a custom-made tilt stage Model N-310K021 designed and manufactured at Physik…”

“…[5][6][7][8][9] The key to achieving the high quality of these x-ray optics is metrology, used for both optical fabrication at the vendor's facilities and for performance optimization of the optics and optical systems at the light sources. [10][11][12][13] With that, the accuracy of the metrology has to be a few times better than the required optical quality.…”

A new compact soft x-ray spectrometer for resonant inelastic x-ray scattering studies at PETRA III

“…The grating's residual slope error and its radius of curvature were measured with the BESSY-NOM slope measuring profiler. 34 The slope error in the spatial frequency range between 1mm -1 up to the aperture length cause aberrations in the optical system. The measured values are presented in table I.…”

“…The measured micro-roughness, approximately 2 nm rms, is a typical value for standard gratings manufactured by different vendors. 34 The measure blaze angle is listed in table I. The grating's residual slope error and its radius of curvature were measured with the BESSY-NOM slope measuring profiler.…”

Small angle deflectometer with submillimeter lateral resolution for flatness measurements of optics

“…Currently, the best possible mirror production technology can provide figure /form errors for 400-500 mm length in the range of 2 nm (PV) [1]. The form of these mirrors can be measured by small angle deflectometers [2,3]. Most of them apply the slope measurement technique using commercially available autocollimators like the autocollimator Elcomat 3000 from Moeller-Wedel Optical GmbH.…”