10 October 2005 Determining Optical Constants for ThO 2 Thin Films Sputtered Under Different Bias Voltages from 1.2 to 6.5 eV by Spectroscopic Ellipsometry William R. Evans Brigham Young University Honors Thesis Defense

10 October Our Goal – EUV Applications Extreme Ultraviolet Optics has many applications. These Include: –EUV Lithography –EUV Astronomy –Soft X-ray Microscopes A Better Understanding of materials for EUV applications is needed. EUV Lithography EUV Astronomy The Earth’s magnetosphere in the EUV Soft X-ray Microscopes

10 October ThO 2 A number of studies by our group have shown that thorium and thorium oxide (ThO 2 ) have great potential as highly reflective coatings in the EUV. In certain regions, ThO 2 may be the best monolayer reflector that has yet been studied.

10 October Sputtering Sputtering is the process by which we prepare our thin film samples. Argon ions from a highly energetic argon plasma “sputter” thorium atoms off of a target. When done in the presence of oxygen, a ThO 2 thin film is deposited on the substrate.

10 October Biased Sputtering Image Source: Sample holder set to a negative bias voltage. The negative bias attracts argon ions from the plasma which pound the thin film, like cold-working metal. Hypothetically, this should produce a smoother, denser film. Higher density films have higher n.

10 October Problems with The EUV Problems with making measurements in the EUV: –Non-local – ALS in Berkley, CA –Cost –Time Commitment –Cleanliness –Roughness Biased Sputtering: Large number of Samples

10 October Spectroscopic Ellipsometry Advantages with Spectroscopic Ellipsometry: –Local –Less cost –Simultaneously fits thickness, n, and k –Quick and simple for large numbers of samples –Cleanliness and roughness are less of an issue (larger wavelengths)

10 October Data Fitting The data were modeled using the J. A. Woollam ellipsometry software. –n is modeled parametrically using a Sellmeier model which fits ε 1 using “poles” (mathematically: discontinuities in the complex plane). –The Sellmeier model by itself doesn’t account for absorption or k. (i.e. All of the poles are on the real axis.) –k can be added in separately, either by fitting k point by point, or by modeling ε 2 with different “oscillators” (parameterized functional distributions).

10 October Data Fitting (Cont.) Once we obtained the experimental data, we fit n to the ellipsometry reflectance data on silicon wafers using the Sellmeier model, assuming no k. Next, we fit k point by point to the transmission data on quartz slides. Using the first fit of k as a reference, we fit an oscillator curve to the point by point fit of k, and then to the experimental data. Finally, we re-fit n to the silicon data, (Sellmeier) taking into account the absorption that we found from the quartz samples.

10 October Preview of Results We found that n is dispersive over the whole range (1.0 – 6.5 eV), with values of –1.82 ± 0.06 at 1.2 eV –1.85 ± 0.06 at 2.5 eV –1.93 ± 0.06 at 4.0 eV –2.24 ± 0.07 at 6.0 eV. We found no dependence of n on bias voltage, thickness, sputter pressure, deposition rate, or any other parameter we looked at. An absorption feature at about 6.5 eV is most likely a narrow band with FWHM of about 0.4 eV. There is evidence that ThO 2 has both direct and indirect band gaps at about 6.10 ± 0.15 eV and 2.8 ± 0.45 eV, respectively.

10 October n

10 October n (Cont.)

10 October n not related to Bias Voltage

10 October Absorption Feature

10 October Acknowledgements Dr. Allred Dr. Turley The BYU EUV Thin Film Optics Group, past and present BYU Department of Physics and Astronomy, BYU Office of Research and Creative Activities, and Rocky Mountain NASA Space Grant Consortium for support and funding Kristin Evans