High Reflectance Nanoscale V/Sc Multilayer for Soft X-ray Water Window Region

Abstract

V/Sc multilayer is experimentally demonstrated for the first time as a high reflectance mirror for the soft X-ray water window region. It primarily works at above the Sc-L edge (λ = 3.11 nm) under near normal incidence while a second peak appears at above the V-L edge (λ = 2.42 nm) under grazing incidence. The V/Sc multilayer fabricated with a d-spacing of 1.59 nm and 30 bilayers has a smaller interface width (σ = 0.27 and 0.32 nm) than the conventional used Cr/Sc (σ = 0.28 and 0.47 nm). For V/Sc multilayer with 30 bilayers, the introduction of B₄C barrier layers has little improvement on the interface structure. As the number of bilayers increasing to 400, the growth morphology and microstructure of the V/Sc layers evolves with slightly increased crystallization. Nevertheless, the surface roughness remains to be 0.25 nm. A maximum soft X-ray reflectance of 18.4% is measured at λ = 3.129 nm at 9° off-normal incidence using the 400-bilayers V/Sc multilayer. According to the fitted model, an s-polarization reflectance of 5.2% can also be expected at λ = 2.425 nm under 40° incidence. Based on the promising experimental results, further improvement of the reflectance can be achieved by using a more stable deposition system, exploring different interface engineering methods and so on.

Figure 1

Theoretical s-polarized reflectance curves of V/Sc and Cr/Sc multilayers. No interface width is added in the structure. Both multilayers have a d-spacing of 1.59 nm and Γ = 0.5.

Figure 2

Grazing incidence x-ray reflectance measurements of 30 bilayers Cr/Sc multilayer (a), V/Sc multilayer (b), V/Sc with 0.1 nm (c) and 0.2 nm thickness B₄C (d) at both interfaces. The fitted results of pure Cr/Sc and V/Sc multilayers are also shown in (a) and (b).

Figure 3

Real part of the refractive index profile of the V/Sc (thick lines) and Cr/Sc (thin lines) multilayers with ideal layer structure (dashed lines) and experimentally fabricated structure (solid lines).

Figure 4

Grazing incidence x-ray reflectance measurement and the fitted curve of the 400-bilayers V/Sc multilayer.

Figure 5

AFM images of the surface morphology of V/Sc multilayers with 30 bilayers (a) and 400 bilayers (b).

Figure 6

TEM images of the whole 400-bilayers V/Sc multilayer stack (a), the bottom layers (b), middle layers (c), and the SAED pattern of the middle layers (d). Sc is depicted by the bright layers and V by the dark layers.

Figure 7

Measured SXR reflectivity curves and the fitted results of the 400-bilayers V/Sc multilayer at the incidence of 9° and 7°. The simulated reflectance curve at 40.05° incidence is also shown.