Soft x-ray optical constants of sputtered chromium thin films with improved accuracy in the L and M absorption edge regions

In this study, we determine with improved accuracy the complex index of refraction n =1 - delta + i beta of sputtered chromium thin films for photon energies ranging from 25 eV to 813 eV. These data include the first absolute measurements of the absorption fine structure near the Cr-L edge. First, we verified by combining Rutherford Backscattering Spectrometry and grazing-incidence x-ray reflec-tometry that the sputtered thin films were pure Cr with a density consistent with tabulated values. Then, we demonstrated that the Cr surface oxide layer remains stable when the samples are exposed to air for up to 4 years. The Cr absorption coefficient beta was determined from the transmittance of freestanding Cr thin films with various thicknesses, measured at the ALS synchrotron radiation source. A model is proposed to correct the transmittance data from the spectral contamination of the source. Finally, we used the new beta values, combined with theoretical and tabulated data from the literature, in order to calculate the delta values by the Kramers-Kronig relation. The improvement in the accuracy of beta values is demonstrated by the f-sum rule. An additional validation of the new Cr optical constants (delta, beta) is performed by comparing the simulated and experimental reflectance of a Cr/B4C multilayer mirror near the Cr-L-2,L-3 edge. Published by AIP Publishing.