Precise knowledge of the wavelength-dependent refractive index of materials is required to accurately design, build and calibrate the in-band and out-of-band performance of EUV/x-ray instruments. Such instruments include exposure and patterning tools, imagers, microscopes and spectrometers for photolithography, plasma physics, synchrotron and laser science, solar physics and astrophysics. Yet, the available refractive index values in the EUV/x-ray are often unreliable. This is due to the extreme sensitivity of materials to contamination and oxidation, to the difficulty in fabricating appropriate thin film samples, to the presence of near-edge absorption fine structure, and to multiple reflections present at the longer EUV wavelengths, which are complicating the measurements. We are presenting a new methodology to measure the EUV refractive index and new sets of measurements for several important EUV materials. We use combinations of transmittance and reflectance data in the spectral range 826.5 eV (1.5 nm) to 15 eV (82.5 nm) and reveal for the first time highly resolved fine structure in the regions of L, M, N and O absorption edges, in both the absorptive and dispersive portions of the refractive index, resulting in improvements of up to a factor of 3 compared to earlier values. The improved refractive index accuracy is validated by sum rule tests and by simulating experimental data of multilayer coatings containing these materials.
Aperiodic multilayer interference coatings are of particular interest for a variety of hard x-ray applications, including target diagnostics, astrophysics, high energy physics and free-electron lasers. Such applications require large field of view along with the highest achievable photon efficiency for their optical components, pushing reflective multilayer coatings to their limits. This work investigates the design, experimental performance, modeling and optimization of high-reflectance aperiodic multilayers. Multilayer design starts with the implementation of an analytical method developed in the literature, which calculates the most efficient coating, featuring the highest achievable reflectivity with the least number of layers. A numerical optimization step is added for smoothing of high-frequency "ripples" or to comply with any specific requirement in terms of spectral or angular response. The design process also includes material-dependent specificities (e.g. typical roughness, interlayer formation) which are often crucial for accurate prediction of actual coating performance. We applied this method to develop novel high-reflectance broadband multilayers at 17.4 keV (Mo Kα emission line), working at angles of grazing incidence up to 0.6 degrees. The design methods employed in this work are presented, as well as the results obtained for a few multilayer systems, including Mo/Si, W/Si and W/SiC.
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