Abstract
X-ray mirrors are needed for beam shaping and monochromatization at advanced research light sources, for instance, free-electron lasers and synchrotron sources. Such mirrors consist of a substrate and a coating. The shape accuracy of the substrate and the layer precision of the coating are the crucial parameters that determine the beam properties required for various applications. In principal, the selection of the layer materials determines the mirror reflectivity. A single layer mirror offers high reflectivity in the range of total external reflection, whereas the reflectivity is reduced considerably above the critical angle. A periodic multilayer can enhance the reflectivity at higher angles due to Bragg reflection. Here, the selection of a suitable combination of layer materials is essential to achieve a high flux at distinct photon energies, which is often required for applications such as microtomography, diffraction, or protein crystallography. This contribution presents the current development of a Ru/C multilayer mirror prepared by magnetron sputtering with a sputtering facility that was designed in-house at the Helmholtz-Zentrum Geesthacht. The deposition conditions were optimized in order to achieve ultra-high precision and high flux in future mirrors. Input for the improved deposition parameters came from investigations by transmission electron microscopy. The X-ray optical properties were investigated by means of X-ray reflectometry using Cu- and Mo-radiation. The change of the multilayer d-spacing over the mirror dimensions and the variation of the Bragg angles were determined. The results demonstrate the ability to precisely control the variation in thickness over the whole mirror length of 500 mm thus achieving picometer-precision in the meter-range.
Highlights
Multilayer mirrors are widely used in a variety of laboratory instruments such as X-ray diffractometers and Xray fluorescence spectrometers, and increasingly at synchrotron storage rings
The Ru/C multilayer films were deposited in the Helmholtz-Zentrum Geesthacht (HZG) magnetron sputtering facility measuring 4.5 m in length, which is equipped with a load lock, and which is evacuated by a turbo
It is expected that an X-ray optical system of two Ru/C multilayer mirrors will achieve a high flux and a good energy resolution in a synchrotron beam, which is required for the investigation of very small crystals, especially proteins
Summary
Multilayer mirrors are widely used in a variety of laboratory instruments such as X-ray diffractometers and Xray fluorescence spectrometers, and increasingly at synchrotron storage rings. The perfection of the multiple stack can be improved by the presence of an amorphous layer; the selected sputtering conditions of the materials are important to accomplish the best possible reflectivity.35–37 This has been demonstrated at some synchrotron sources, where multilayers of Ru/B4C are used in monochromators for this mid-energy range.. It is important to mention that the sputtering facility of the Helmholtz-Zentrum Geesthacht (HZG) is designed to develop and optimise the preparation of mirrors having a length of 1500 mm with ultra-high precision in thickness uniformity over the entire deposition area. These can be single layer mirrors for total-reflection or high-flux multilayer mirrors.. The final optical system requires two identical multilayer mirrors to provide a high flux and a tuneable energy resolution
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