1. 1 X-Ray Photoelectron Spectroscopy to Examine Molecular Composition Amy Baker R. Steven Turley Brigham Young University

2. 2 Why Extreme Ultraviolet? Thin Film or Multilayer Mirrors EUV Lithography Soft X-Ray Microscope Earth’s Magnetosphere in the EUV Images from www.schott.com/magazine/english/info99/ and www.lbl.gov/Science-Articles/Archive/xray-inside-cells.html.

3. 3 Why Thorium? Only one oxidation state: ThO 2 Only one oxidation state: ThO 2 Rock stable: Highest melting point (3300 deg C) of any known oxide. Rock stable: Highest melting point (3300 deg C) of any known oxide. High Reflectance in the EUV (10- 100nm) High Reflectance in the EUV (10- 100nm)

4. 4

5. 5 Will Thorium Work? The mirror’s surface will be oxidized. The mirror’s surface will be oxidized. At optical wavelengths, this oxidation is negligible. It is a major issue for our thin films, however. At optical wavelengths, this oxidation is negligible. It is a major issue for our thin films, however. We expect minimal oxidation We expect minimal oxidation

6. 6 Learn oxidation state of our thorium samples Learn oxidation state of our thorium samples Understand how composition changes with depth Understand how composition changes with depth Obtain an expression for oxidation as a function of depth Obtain an expression for oxidation as a function of depth Purposes of X-Ray Photoelectron Spectroscopy

7. 7 X-Ray Photoelectron Spectroscopy

8. 8 How XPS works

9. 9 Electron Binding Energy O Th C 4d 3/2 4d 5/2 1s 4f 7/2 5d 5/2 1s Th 5d 3/2 4f 5/2

10. 10 Peak Shifts Thorium peaks on surface Thorium peaks on surface Thorium peaks after oxygen is gone Thorium peaks after oxygen is gone

11. 11 Depth Profiling Rastering: Rastering: Argon ions knock off individual atoms Argon ions knock off individual atoms Variable angle scans: Variable angle scans: More depth is obtained as x-ray gun and detector are moved towards incidence More depth is obtained as x-ray gun and detector are moved towards incidence Analyzer X-ray Source e- Sample θ

12. 12 Variable Angle Results Only penetrates about 150 Angstroms into the sample Only penetrates about 150 Angstroms into the sample This allows us to see surface contamination, but not composition with depth This allows us to see surface contamination, but not composition with depth Results are averaged: cannot obtain resolved composition with depth Results are averaged: cannot obtain resolved composition with depth

13. 13 Rastering Results

14. 14 Too Much Oxidation These samples were only a few hours old. These samples were only a few hours old. We need more uniformity. We need more uniformity. Solution: Make ThO 2 mirrors. Reflection is similar to Th and it should be more uniform. Solution: Make ThO 2 mirrors. Reflection is similar to Th and it should be more uniform.

15. 15 ThO 2 Results

16. 16 Results Fully oxidized thorium is much more uniform. Fully oxidized thorium is much more uniform. ThO 2 shows definite promise as a durable reflector in the EUV. ThO 2 shows definite promise as a durable reflector in the EUV. Rastering is an effective depth profiling technique Rastering is an effective depth profiling technique Variable angle can be used as a surface technique Variable angle can be used as a surface technique

17. 17 Continued Research Include modeled interface in calculating optical constants from reflectance data Include modeled interface in calculating optical constants from reflectance data Shape of sputtered area may affect rastering rate: use multilayer thin film stack to explore shape of sputtered region Shape of sputtered area may affect rastering rate: use multilayer thin film stack to explore shape of sputtered region

18. 18 Acknowledgements A special thanks to R. Steven Turley David Allred Matt Linford Yi Lang BYU Thin Films Group Physics & Astronomy Department Funding ORCA Mentoring Grant NASA Space Grant

19. 19 Other Results of Interest There was an increase in oxygen when the sample sat for more than 4 or 5 minutes in between sputtering/scans. There was an increase in oxygen when the sample sat for more than 4 or 5 minutes in between sputtering/scans. This was observed for 5 out of 5 samples that sat still between scans. This was observed for 5 out of 5 samples that sat still between scans.

20. 20 * indicates where the sample stood for more than 4 or 5 minutes in between scans

21. 21 What Could This Be? Hypothesis: This is likely due to preferential sputtering. Hypothesis: This is likely due to preferential sputtering. The argon ions will knock off oxygen atoms more readily than thorium. The argon ions will knock off oxygen atoms more readily than thorium. While sputtering, scans would show less O than actually exists. While sputtering, scans would show less O than actually exists.

22. 22 Future Research Test preferential sputtering hypothesis. Test preferential sputtering hypothesis. Investigate other peak anomalies: N, Ar Investigate other peak anomalies: N, Ar Obtain accurate sputtering rates Obtain accurate sputtering rates

23. 23 Future Research Shape of sputtered area may affect the sputtering rate. Shape of sputtered area may affect the sputtering rate. Finally: Make and measure optical Finally: Make and measure optical constants for thin films of other elements.