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 (B4C), 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 B4C 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 B4C 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 B4C. The thermal stability of the B4C layers on the various substrate materials was investigated.

Keywords: FEL; X-ray optics; coatings; mirrors.

Figure 1

Specular X-ray reflectivity as a function of the incidence angle for Au, B₄C, Mo and Rh coatings. The scans were performed using Cu radiation (8048 eV). The results from IMD simulations are also indicated. The linear plot shows that the critical angle is higher when the film density is increased. The logarithmic scale of the insert depicts thickness oscillations (Kiessig oscillations). The distance between the maxima is smaller for thicker films.

Figure 2

B₄C coating layer thickness in the tangential direction (x, which is parallel to the long axis) of the mirror.

Figure 3

B₄C coating layer thickness in the sagittal direction (y, which is perpendicular to the long axis) of the mirror.

Figure 4

Uniformity of the B₄C film layer thickness. The coating was deposited on 70 sapphire substrates, which are evenly disributed over the available area of 1500 mm (tangential) by 120 mm (sagittal). The coloured z-scale is the normalized layer thickness difference given by (t−t _m)/t _m, where t _m is the mean layer thickness along the tangential direction (16 positions, y = 60 mm) and t is the measured thickness at 70 positions within the mirror area (x, y). Contour lines have been interpolated in order to demonstrate the changes. The values of normalized difference decrease towards the outer regions of the area.

Figure 5

Ratio (m) of the difference in thickness to difference in y position as a function of the inclination angle (insert: geometrical dimensions between the sputtering source and substrate).

Figure 6

Variation in layer thickness of annealed boron carbide and carbon coatings.

Figure 7

Surface images obtained using optical interferometry with a magnification of ×20: (a) uncoated silicon substrate and (b) magnetron-sputtered B₄C coating on a Si substrate.

Figure 8

One-dimensional PSD function of a B₄C coating on silicon.