X-Ray Photoelectron Spectroscopy to Examine Molecular Composition

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

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. Why Thorium? • Only one oxidation state: ThO2 • Rock stable: Highest melting point (3300 deg C) of any known oxide. • High Reflectance in the EUV (10-100nm)

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

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

7. X-Ray Photoelectron Spectroscopy

8. How XPS works

9. Th 4f5/2 4f7/2 Th O 4d3/2 4d5/2 1s Th C 5d3/2 5d5/2 1s Electron Binding Energy

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

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

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

13. Rastering Results

14. Too Much Oxidation • These samples were only a few hours old. • We need more uniformity. • Solution: Make ThO2 mirrors. Reflection is similar to Th and it should be more uniform.

15. ThO2 Results

16. Results • Fully oxidized thorium is much more uniform. • ThO2 shows definite promise as a durable reflector in the EUV. • Rastering is an effective depth profiling technique • Variable angle can be used as a surface technique

17. Continued Research • 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

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. 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. • This was observed for 5 out of 5 samples that sat still between scans.

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

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

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

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