Optical constants of SrF2 thin films in the 25–780-eV spectral range

Abstract

The transmittance and the optical constants of SrF 2 thin films, a candidate material for multilayer coatings operating in the extreme ultraviolet and soft x-rays, have been determined in the spectral range of 25-780 eV, in most of which no experimental data were previously available. SrF 2 films of various thicknesses were deposited by evaporation onto room-temperature, thin Al support films, and their transmittance was measured with synchrotron radiation. The transmittance as a function of film thickness was used to calculate the extinction coefficient k at each photon energy. A decrease in density with increasing SrF 2 film thickness was observed. In the calculation of k, this effect was circumvented by fitting the transmittance versus the product of thickness and density. The real part of the refractive index of SrF 2 films was calculated from k with Kramers-Kr€ onig analysis, for which the measured spectral range was extended both to lower and to higher photon energies with data in the literature combined with interpolations and extrapolations. With the application of f-and inertial sum rules, the consistency of the compiled data was found to be excellent. V

Tuning sum rules with window functions for optical constant evaluation

“…We start using the new sum rules with a self-consistent set of data reported for thin films of SrF 2 [13]. Data sources for the characterization of SrF 2 included experimental data in the ranges of 0.01-0.1 eV [14, 15], 9.8-35 eV [16], 25-780 eV [13].…”

“…Data sources for the characterization of SrF 2 included experimental data in the ranges of 0.01-0.1 eV [14, 15], 9.8-35 eV [16], 25-780 eV [13]. At photon energies up to 3×10 4 eV, where no experimental data were available, an independent-atom approximation was used in [13] that combined the optical constants of Sr [17] with the semi-empirical data of Henke et al [18] for F; the latter was downloaded from the web of the Center for x-ray Optics [19]. For energies in the 3×10 4 -4×10 5 eV range, the calculations of Chantler et al [20] were used.…”

“…All curves were normalized to the largest value over the spectrum to simplify the comparison. The small-energy range data taken from Rodríguez-de Marcos et al [13], which come from Bosomworth [15], did not reach a small enough energy as to calculate the sum rules with more weight on the small-energy range. To solve this, we extrapolated Bosomworth data down to 10 −5 eV with a power function.…”

EUV refractive index measurements with improved accuracy and their impact in component modeling

“…In the last 25 years, we have successfully employed the reflectance and transmittance methods to determine accurately the refractive index of a large number of materials in the EUV/soft x-ray range, including Si, Mo, Be, Ir, Y, B, B4C, SiO, SrF2 [20][21][22][23][24]. It is worthwhile noting that our experimental data for the refractive index of Si and Mo [16,17], which have been included in the CXRO database, enabled for the first time the accurate modeling of Mo/Si multilayer coatings for EUV lithography near 13.5 nm wavelengths.…”

Y2O3 optical constants between 5 nm and 50 nm

“…There has been an increased interest in the optical constants for compounds in the extreme ultraviolet. For example, the optical constants of silicon carbide [15][16][17][18], boron carbide [19][20][21], aluminum nitride [22], silicon monoxide [23] and dioxide [24], zirconium dioxide [25], and various fluorides [26] have been reported in the EUV and VUV. This interest is partly for the materials' use in optical reflectors, either as single-surface or multilayer reflectors.…”