29 August 2006 Understanding DC-bias sputtered thorium oxide thin films useful in EUV optics
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We used spectroscopic ellipsometry to determine the optical constants of seven thin-film ThO2 samples deposited by radio-frequency sputtering, thickness ranging between 24 and 578 nm, for the spectral range of 1.2 to 6.5. We used a hollow-cathode light source and vacuum monochromator to measure constants at 10.2 eV. None of the deposition parameters studied including DC-bias voltages successfully increase the n of (that is, densify) thoria films. The value of n at 3.0 eV is 1.86 ± 0.04. We find compelling evidence to conclude that the direct band gap is at ~5.9 eV, clarifying the results of others, some of whom observed the absorption edge below 4 eV. The edge in the two thickest films is of a narrow feature (FWHM=0.4 eV) with modest absorption (α~6μm-1, k~0.1). Absorption may go down briefly with increasing energy (from 6.2 to 6.5 eV). But at 10.2 eV absorption is very high and index low as measured by variable-angle reflectometry, α = 47.3 ± 5.5 μm-1 and k = 0.48 ± 0.05, and n=0.87 ± 0.12.
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William R. Evans, Sarah C. Barton, Michael Clemens, David D. Allred, "Understanding DC-bias sputtered thorium oxide thin films useful in EUV optics", Proc. SPIE 6317, Advances in X-Ray/EUV Optics, Components, and Applications, 631711 (29 August 2006); doi: 10.1117/12.687499; https://doi.org/10.1117/12.687499

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