1. 16 Aug 2006 16 Aug 2006 Understanding DC-Biased Thoria Thin Films Useful in EUV Optics William R. Evans, Sarah Barton, David D. Allred Brigham Young University The International Society for Optical Engineering

2. 16 Aug 20062 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. 16 Aug 20063 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. 16 Aug 20064 Results: n

5. 16 Aug 20065 n not related to Bias Voltage or Thickness

6. 16 Aug 20066 Results: Absorption There is a narrow absorption feature at about 6.2 eV, with full width half max of about 0.4 eV.

7. 16 Aug 20067 Comparing to the Literature In reviewing the literature, there seems to be a couple of different band gaps that people detect: Graphic From: Rivas-Silva, et. al.

8. 16 Aug 20068 Comparing to the Literature Sviridova & Suikovskaya measured sample thickness and absorption for several different wavelengths near where thorium goes transparent, for thorium chloride and thorium nitrate. From this we determine a band gap of ~5.92 eV. We obtained a value in the same range.

9. 16 Aug 20069 Comparing to the Literature Mahmoud reports a very clear band gap of about 3.84 eV. However, his samples were deposited on glass of unspecified composition.

10. 16 Aug 200610 What we think might be going on... If the middle band were centered at about -9.8 eV in stead of -11.8 eV, the ~6 eV band gap reported in the majority of the thin film sources would be explained as a jump from the valence band to the middle band. Also, if the conduction band started at about -6 eV in stead of about -7 eV, the ~4 eV band gap reported by Mahmoud and others could be explained by a transition from the middle band, which had some electrons in it due to mild doping, transitioning into the conduction band.

11. 16 Aug 200611 Measurements at 10.2 eV We used a McPherson Vacuum monochromator at BYU to measure optical constants of our ThO 2 thin films at the 10.19 eV Kα line of Hydrogen.

12. 16 Aug 200612 Measurements at 10.2 eV

13. 16 Aug 200613 Measurements at 10.2 eV

14. 16 Aug 200614 Conclusions First of all, we have shown that DC Biased sputtering cannot be expected to significantly affect the optical constants of ThO 2 thin films. –This is not surprising considering the extremely high melting point of ThO 2. Secondly, exactly what is going on with the band gap of ThO 2 is still not really understood. –It appears that there are two fundamental band gaps in ThO 2, but more research is needed. –We are in the process of making additional measurements on ThO 2 between 6 and 9 eV.

15. 16 Aug 200615 Acknowledgements R.S. Turley and The BYU EUV Thin Film Optics Group, past and present. BYU Department of Physics and Astronomy BYU Office of Research and Creative Activities Rocky Mountain NASA Space Grant Consortium V. Dean and Alice J. Allred, Marathon Oil Company Kristin Evans