Abstract
Large X-ray mirrors are required for beam transport at both present-day and future free-electron lasers (FELs) and synchrotron sources worldwide. The demand for large mirrors with lengths up to 1 m single layers consisting of light or heavy elements has increased during the last few decades. Accordingly, surface finishing technology is now able to produce large substrate lengths with micro-roughness on the sub-nanometer scale. At the Helmholtz-Zentrum Geesthacht (HZG), a 4.5 m-long sputtering facility enables us to deposit a desired single-layer material some tens of nanometers thick. For the European XFEL project, the shape error should be less than 2 nm over the whole 1 m X-ray mirror length to ensure the safe and efficient delivery of X-ray beams to the scientific instruments. The challenge is to achieve thin-film deposition on silicon substrates, benders and gratings without any change in mirror shape. Thin films of boron carbide and platinum with a thickness in the range 30-100 nm were manufactured using the HZG sputtering facility. This setup is able to cover areas of up to 1500 mm × 120 mm in one step using rectangular sputtering sources. The coatings produced were characterized using various thin-film methods. It was possible to improve the coating process to achieve a very high uniformity of the layer thickness. The movement of the substrate in front of the sputtering source has been optimized. A variation in B4C layer thickness below 1 nm (peak-to-valley) was achieved at a mean thickness of 51.8 nm over a deposition length of 1.5 m. In the case of Pt, reflectometry and micro-roughness measurements were performed. The uniformity in layer thickness was about 1 nm (peak-to-valley). The micro-roughness of the Pt layers showed no significant change in the coated state for layer thicknesses of 32 nm and 102 nm compared with the uncoated substrate state. The experimental results achieved will be discussed with regard to current restrictions and future developments.
Highlights
Long X-ray mirrors are required for photon beam transport and shaping in the beamlines at free-electron lasers (FELs) and synchrotron sources
The experimental results of FEL coatings are divided into three parts and the results are discussed according to the technical requirements for X-ray mirrors
The micro-roughness of the Pt layers on the silicon substrate were investigated in the mid- and high-spatial-frequency range by means of white-light interferometry
Summary
Long X-ray mirrors are required for photon beam transport and shaping in the beamlines at free-electron lasers (FELs) and synchrotron sources. The required technical specifications lead to a shape error of less than 2 nm peak-to-valley (PV) for the mirrors. These extraordinarily high specifications are needed to preserve the wavefront and maintain the coherence of the FEL beam. A particular scientific instrument at the European XFEL, the Materials Imaging and Dynamics (MID) beamline, requires mirrors with a second stripe of coating materials, which possess a higher critical angle than the standard coating of B4C (Madsen et al, 2013). The layer thickness values in the tangential direction of the mirrors along 1500 mm are shown for the most important FEL coatings
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