Coatings for FEL optics: preparation and characterization of B₄C and Pt

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 B 4 C 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.

Room-Temperature Formation of Hard BCx Films by Low Power Magnetron Sputtering

“…As seen from Figure 3, the atomic ratio of boron to carbon is lower than the stoichiometry of B 4 C, which is consistent with the results of other investigations [49][50][51]. It should be noted that most authors of works where boron carbide films were deposited using a B 4 C target do not provide the data on the total elemental composition of these films or the ratio of B/C atoms, and do not mention the content of oxygen and nitrogen [48,52,53]. Table 3 presents water or oxygen adsorbed in the reaction chamber during the deposition process and may be due to the film surface contamination after deposition during storage of the samples.…”

The beam transport system for the Small Quantum Systems instrument at the European XFEL: optical layout and first commissioning results

“…The coating density of the mirrors was measured by X-ray reflectometry using a laboratory source with a sub-nm accuracy on smaller superpolished silicon substrates coated together with each mirror (Sto ¨rmer et al, 2018). The geometric acceptance of the beamline was obtained by analytically calculating the propagation of a beam with source size and divergence as given in Table 1 through the optical layout schematically represented in Fig.…”

Metrology and characterization of two optical delay line mirrors before and after B4C coating at European XFEL

“…The homogeneity, reproducibility and run-to-run stability of the coating process was determined in previous investigations, described in the references. [5][6][7] For the 2 ODL mirrors considered in this study the coating thickness of B 4 C is 50 nm. After the deposition, the mirrors are measured again with at European XFEL with the same interferometer, method and lab set-up.…”