Growth and Nano-structural Studies of Metallic Multilayers for X-ray Mirrors

A part of the Ph.D. project focused on growth and characterization of metal multilayers is presented in this licentiate thesis. The main interest in carrying out this research is to develop highly reflective normal-incidence condenser mirrors for soft X-ray microscopy studies in the water window (λ = 2.4− 4.2 nm) wavelength regime. Transition metals like Sc, Ti V, etc. have been considered because of the presence of their 2p-absorption edges within the water window. An anomalous dispersion at absorption edges has been utilized to get enhanced reflectance of soft X-rays. Since a single surface exhibits a very poor X-ray reflectivity, Cr/Sc, Cr/Ti, and Ni/V multilayers were grown in order to coherently add many reflections from several interfaces. The selection of Cr and Ni, as spacer layer, was made on the basis of their X-ray optical contrasts with the above-mentioned transition metals. The multilayer design, i.e., the individual layer thicknesses and the total number of bilayers, directly influences the resultant reflectance and careful determination was therefore made with the aid of computer simulations. All multilayers were grown on chemically cleaned Si substrates by ion-assisted dual target magnetron sputtering under high vacuum (v10−7 Torr) conditions. The effect of low and high ion-flux bombardment of low energy (<50 eV) Ar ions, on growing surfaces was studied for all material systems. Furthermore, a two-stage deposition of each individual layer with modulated ion-energies was applied in order to obtain smooth and abrupt interfaces with as small intermixing as possible. Ion-surface interactions were also theoretically considered for estimating an appropriate ion-flux and ion-energy range desired for sufficient ad-atommobilities. X-ray reflectivity and transmission electron microscopy have been the main probes for multilayer characterization in this work. For the Cr/Ti multilayer designed for normal incidence and grown with optimized two-stage ion-energy modulation, a peak reflectance of 2.1% was achieved at the Ti-2p absorption edge (λ = 2.74 nm). For a multilayer mirror designed for the Brewster angle a maximum reflectance of 4.3% was accomplished. These measurements were made at the synchrotron radiation source BESSY in Berlin. Specular reflectivity and diffuse scattering scans were utilized for quantitative and qualitative analysis of the vertical and lateral structure of the multilayers. At-wavelength measurements of a series of Cr/Ti multilayers revealed the accumulation of roughness with increasing number of bilayers (N > 100) for this material system. Hard X-ray reflectivity and diffractometry were used for quality checks of the multilayers for rapid feed-back to the deposition. In-situ annealing using hard X-ray reflectivity was also performed to assess the thermal stability of Cr/Ti multilayers. It was found that probably due to a strong thermal diffusion the degradation of multilayers (with bilayer period of 1.37 nm) in this material system occurs just above the growth temperature (v40◦C). The accumulation of a low spatial frequency “waviness” with increasing number of layers in Cr/Ti multilayers was investigated by transmission electron microscopy. The influence of process conditions on multilayer structure with different periodicities was investigated by TEM analyses of a series of three samples for each of the above-mentioned material system. The Cr/Sc multilayers have shown the most flat and abrupt interface structure without any significant roughness evolution when grown with optimum process parameters.