1. 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

2. 10 October 20052 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

3. 10 October 20053 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.

4. 10 October 20054 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.

5. 10 October 20055 Biased Sputtering Image Source: http://www.passforge.com/gallery.html 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.

6. 10 October 20056 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

7. 10 October 20057 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)

8. 10 October 20058 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).

9. 10 October 20059 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. 10 October 200510 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.

11. 10 October 200511 n

12. 10 October 200512 n (Cont.)

13. 10 October 200513 n not related to Bias Voltage

14. 10 October 200514 Absorption Feature

15. 10 October 200515 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