Abstract
Cr/Sc multilayer X-ray mirrors intended for normal incidence reflection in the water window wavelength range, λ =[2.4–4.4 nm] , have been grown by ion-assisted sputter deposition and characterized using soft and hard X-ray reflectivity. By extracting low-energy ions, with energies, E ion, ranging from 9 to 113 eV and with ion-to-metal flux ratios, Φ , between 0.76 and 23.1, from the sputtering plasma to the growing film, the nano-structure of the multilayer interfaces could be modified. A significantly increased soft X-ray reflectivity, using λ=3.374 nm, for Cr/Sc multilayers with layer thicknesses in the range 0.4–2.8 nm, was obtained when high ion-to-metal flux ratios, Φ Cr=7.1 and Φ Sc=23.1, and low energy ions, E ion=9 eV, were used. An experimental reflectivity of 5.5% was obtained at 76° for a multilayer with 400 bi-layers. Simulations of the reflectivity data showed that the interface widths are <0.425 nm. It could be concluded that roughness of low spatial frequency is reduced at lower ion energies than the high spatial frequency which was eliminated at the expense of intermixing at the interfaces at higher ion energies. The predicted performance of normal incidence multilayer mirrors grown at optimum conditions and designed for λ=3.374 and 3.115 nm indicates possible reflectivities of 6.5% and 14%, respectively.
Published Version
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