At wavelength metrology of high-reflectance multilayer mirror in the water window

Multilayer mirrors for the water window spectral range (photon energies from 284 to 543 eV, which correspond, respectively, to the C–K and O–K absorption edges) have become key components for microscopy, synchrotron and free electron laser sources, semiconductor microanalysis, and x-ray spectroscopy. The emergence of new x-ray sources and new applications in this spectral domain (e.g., soft x-ray photolithography and attosecond science) requires more efficient optical components with precise metrology of their performance. We have recently reported high experimental peak reflectance at near-normal incidence near the Sc L2,3 absorption edge (E ≈ 400 eV) by the experimental optimization of Cr/Sc-based multilayers. The short wavelength and the large number of multilayer periods raise some specific issues for the at-wavelength reflectance measurements. We report here on a systematic study of potential bias in the measurements of high-reflectance multilayer mirror in the water window based on a comparison between experimental data from the three different synchrotron beamlines as well as theoretical calculations and simulations. In particular, we discuss the effects of spectral resolution and angular resolution on the peak reflectance as well as the influence of the sample lateral uniformity and of the light scattered by the multilayer sample (diffuse scattering). As a result of this study, we determined the peak reflectance of this multilayer mirror to be 35% at E = 396 eV at near-normal incidence, which is the highest reflectance reported in the water window spectral range.