Understanding DC-bias sputtered thorium oxide thin films useful in EUV optics

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. 1, 2 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 variableangle reflectometry, α= 47.3 ± 5.5 µm -1 and k= 0.48 ±0.05, and n=0.87 ±0.12.