Soft X-ray Multilayer Mirrors
Abstract: This thesis is focused on the design, growth and characterization of soft x-ray multilayer mirrors with the emphasis on the wavelength region called the water window (λ=2.4-4.4 nm). The main application in mind is condenser mirrors for a high resolution compact soft x-ray microscope using a droplet-target laser-produced plasma (LPP) source operating at λ=3.374 nm. However, other applications, utilizing other wavelengths, have also been considered.The design of the multilayers which involves understanding of the details of the optical properties of the elements and the interaction between multilayer and electromagnetic radiation is reviewed. To design the multilayers, e.g., to determine the material system, the individual layer thicknesses, and the total number of layers, the simulation software package IMD was used. From the simulations it was also found that in order to realize high-reflectivity multilayer mirrors, interface imperfections, like roughness and intermixing need to be minimized.In particular the material systems W/B4C, Cr/Sc, and Ni/V grown by ion-assisted dualtarget magnetron sputter deposition, have been studied. The effect of various process parameters, especially the energy and flux of low-energy ion bombardment have been investigated in order to increase the understanding of the ion-surface interactions and the materials science of the multilayers.To characterize the multilayers mainly different x-rays techniques have been used; hard x-ray reflectivity, LPP soft x-ray reflectivity and also soft x-ray synchrotron reflectivity measurements at BESSY in Berlin and at the Advanced Light Source (ALS) in Berkeley. Both specular reflectivity and diffuse scattered intensity has been measured to assess the lateral and vertical structure of the multilayers.With an increased knowledge and understanding of the effects of the ion-surface interaction on the structure of the multilayers, advances were made in the development, with a following improvement in performance. The first multilayer material system investigated was W /B,C. However, because of a naturally high absorption of x-rays in W, the theoretically achievable reflectivity is very low, only about 12%.Cr/Sc, on the other hand, has potentially a much higher reflectivity of about 60%. Here different energies, but also fluxes, of ion assistance was investigated. It was found that, for a low ion flux, the ion energy, providing the optimum compromise between reduced roughness and induced intermixing, was rather high. By increasing the ion flux,the energy could be lowered, which in turn decreased the intermixing with a subsistent surface smoothening. This resulted in an improved reflectivity, where maximum nearnormal incidence reflectivities of R=5.5% and R=14.5% were achieved at λ=3.374 nm and at the Sc absorption edge (λ=3.115 nm), respectively.However, although fairly high reflectivities were obtained, the trade-off between roughness reduction and interface mixing persisted. Analytical calculations, based on a binary collision approximation, revealed that an interface mixing of ±1 atomic distance is unavoidable when a continuous flux of assisting ions is used. To overcome this limitation, a sophisticated interface engineering technique was employed, where the first part of each layer was grown using a high-flux of low energy ions, and the remaining part with a slightly higher ion energy. This method was demonstrated to largely eliminate the intermixing while maintaining the smoothening effect of ion-assistance.Using this novel modulated low-energy and high-flux ion-assistance technique extremely flat and abrupt interfaces were obtained. Soft x-ray Cr/Sc and Ni/V multilayer mirrors with near-normal incidence reflectivities of 20.7% and 2.7% at the Sc and V(λ=2.43 nm) absorption edges, respectively, were realized. Multilayer optimized for the Brewster angle showed a reflectance of R=26.7% and a polarizing power of Rs/Rp=5450 for Cr/Sc and R=l0% and Rs/Rp=4190 for Ni/V, when realized with engineered interfaces.Finally, also the long-term goal of producing a large-area, spherically-shaped, normalincidence soft x-ray multilayer condenser mirror for a compact soft x-ray microscope was accomplished.
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