Preparation and characterization of B₄C coatings for advanced research light sources

Abstract

X-ray optical elements are required for beam transport at the current and upcoming free-electron lasers and synchrotron sources. An X-ray mirror is a combination of a substrate and a coating. The demand for large mirrors with single layers consisting of light or heavy elements has increased during the last few decades; surface finishing technology is currently able to process mirror lengths up to 1 m with microroughness at the sub-nanometre level. Additionally, thin-film fabrication is able to deposit a suitable single-layer material, such as boron carbide (B 4 C), some tens of nanometres thick. After deposition, the mirror should provide excellent X-ray optical properties with respect to coating thickness errors, microroughness values and slope errors; thereby enabling the mirror to transport the X-ray beam with high reflectivity, high beam flux and an undistorted wavefront to an experimental station. At the European XFEL, the technical specifications of the future mirrors are extraordinarily challenging. The acceptable shape error of the mirrors is below 2 nm along the whole length of 1 m. At the Helmholtz-Zentrum Geesthacht (HZG), amorphous layers of boron carbide with thicknesses in the range 30-60 nm were fabricated using the HZG sputtering facility, which is able to cover areas up to 1500 mm long by 120 mm wide in one step using rectangular B 4 C sputtering targets. The available deposition area is suitable for the specified X-ray mirror dimensions of upcoming advanced research light sources such as the European XFEL. The coatings produced were investigated by means of X-ray reflectometry and interference microscopy. The experimental results for the B 4 C layers are discussed according to thickness uniformity, density, microroughness and thermal stability. The variation of layer thickness in the tangential and sagittal directions was investigated in order to estimate the achieved level of uniformity over the whole deposition area, which is considerably larger than the optical area of a mirror. A waisted mask was positioned during deposition between the sputtering source and substrate to improve the thickness uniformity; particularly to prevent the formation a convex film shape in the sagittal direction. Additionally the inclination of the substrate was varied to change the layer uniformity in order to optimize the position of the mirror quality deposited area during deposition. The level of mirror microroughness was investigated for different substrates before and after deposition of a single layer of B 4 C. The thermal stability of the B 4 C layers on the various substrate materials was investigated.

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

“…3.8 mrad, which means that the film density of the magnetronsputtered B 4 C layer is 2.37 g cm À3 and this is 94% from the bulk value of 2.52 g cm À3 (Thé venot, 1990). Previous results are in good agreement (Stö rmer et al, 2010(Stö rmer et al, , 2011(Stö rmer et al, , 2016aKozhevnikov et al, 2015).…”

“…The generator power values were 800 W MF (mid-frequency) for B 4 C at a rectangular source (304.8 mm  88.9 mm) and 80 W DC for Pt at a circular source (diameter of 76.2 mm). Small silicon substrates (20 mm  60 mm  0.7 mm) for test coatings were fixed on a large mirror dummy as described previously (Stö rmer et al, 2016a). In the case of B 4 C we used the maximum deposition length of 1500 mm; for Pt we reduced the process to 500 mm in order to save cost and material.…”

“…The challenge is to deposit stable and uniform coatings over a long deposition length. The first part is primarily dedicated to B 4 C coatings and their newest experimental results due to thickness uniformity along the whole deposition length of 1500 mm, which were improved distinctly compared with former results (Stö rmer et al, 2016a). In the second part the X-ray optical properties of Pt coatings are introduced according to layer thickness, density and micro-roughness.…”

“…The specifications on polishing errors are about a factor of ten more stringent compared with well established mirrors for synchrotron radiation (European XFEL, 2016). The first FEL coatings with amorphous boron carbide (B 4 C) on singlecrystalline silicon substrates have been deposited and investigated (Stö rmer et al, 2016a). The Helmholtz-Zentrum Geesthacht (HZG) sputtering facility was used to coat the worldwide unique 1 m-long silicon substrates to extraordinarily high specifications.…”

Topological Constraint Theory Analysis of Rigidity Transition in Highly Coordinate Amorphous Hydrogenated Boron Carbide

“…Here, we apply rigidity theory to a unique and unusually coordinated material, boron carbide. This material has garnered interest for a variety of applications, including nuclear reactor coatings (Greuner et al, 2004;Buzhinskij et al, 2009), neutron detection (Robertson et al, 2002;Caruso, 2010;Gervino et al, 2013), low-k dielectrics and related layers for integrated circuits (Han et al, 2002;Nordell et al, 2016bNordell et al, , 2017, and various specialized coatings (Keski-Kuha et al, 1998;Chen et al, 2006;Hu and Kong, 2014;Azizov et al, 2015;Störmer et al, 2016). The particular amorphous hydrogenated boron carbide (a-BC:H) variant described here was produced in the form of thin films by plasma-enhanced chemical vapor deposition (PECVD) from a single-source molecular ortho-carborane (o-C 2 B 10 H 12 ) precursor to form a disordered polymeric carboranebased network.…”