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Proceedings Paper

Understanding DC-bias sputtered thorium oxide thin films useful in EUV optics
Author(s): William R. Evans; Sarah C. Barton; Michael Clemens; David D. Allred
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Paper Abstract

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.

Paper Details

Date Published: 29 August 2006
PDF: 12 pages
Proc. SPIE 6317, Advances in X-Ray/EUV Optics, Components, and Applications, 631711 (29 August 2006); doi: 10.1117/12.687499
Show Author Affiliations
William R. Evans, Brigham Young Univ. (United States)
Sarah C. Barton, Brigham Young Univ. (United States)
Michael Clemens, Brigham Young Univ. (United States)
David D. Allred, Brigham Young Univ. (United States)

Published in SPIE Proceedings Vol. 6317:
Advances in X-Ray/EUV Optics, Components, and Applications
Ali M. Khounsary; Christian Morawe, Editor(s)

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