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Thorium Oxide Thin Films as EUV Reflectors

Thorium Oxide Thin Films as EUV Reflectors. Jed Johnson Dr. David D. Allred Brigham Young University. Reflectors in EUV range. EUV range is about 50-1500Å General Challenges: - multilayers required - absorption - high vacuum needed Applications - XUV lithography

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Thorium Oxide Thin Films as EUV Reflectors

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  1. Thorium Oxide Thin Films as EUV Reflectors Jed Johnson Dr. David D. Allred Brigham Young University

  2. Reflectors in EUV range • EUV range is about 50-1500Å • General Challenges: - multilayers required - absorption - high vacuum needed • Applications - XUV lithography - Soft x-ray microscopy - Astronomy • Complex index of refraction: ñ=n+ik

  3. Delta =1-n vs. Beta (absorption) • 30.4 nm (41 eV)

  4. Periodic table

  5. Thorium vs. Uranium • Thorium (11.7 g/cm^3) is less dense than uranium (19.1 g/cm^3). • In the EUV and Soft x-ray, uranium is in theory more reflective than thorium because of its density.

  6. Reflectivity vs. energy (eV)

  7. But the mirror’s surface will be oxidized. • Both uranium and thorium oxidize easily at room temperature. • The image mirror used UO2 as its top layer. • 4 Å in 5 min, tens of nm in week. • EUV at low angle only see top few nm.

  8. The 2 Dioxides are Similar! • Thin films usually start oxidizing when exposed to “air.” • The density, lattice structure, and therefore reflectivity of an element are altered by oxidation.

  9. Which oxide is better? • Oxidation occurs at room temperature for both uranium and thorium. • Uranium, however, has many oxidation states (UO2, U2O5, UO3 etc.). As long as there is unoxidized U nearby, most oxidation stops at UO2. But eventually, they could oxidize towards UO3. • This has lower density!

  10. The reflectance of UO3 is lower than UO2

  11. Advantages of Thorium Oxide • Rock stable: highest known melting point of any oxide. • Only one oxidation state, ThO2. We know what we have!

  12. Project Challenges • Thorium metal acquisition • Thorium Oxide sputtering target production • Long distances to analysis equipment

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