Thorium Oxide Thin Films Thorium Oxide Thin Films as EUV Reflectorsas EUV Reflectors

Jed JohnsonJed Johnson

Dr. David D. AllredDr. David D. Allred

Brigham Young UniversityBrigham Young University

Reflectors in EUV rangeReflectors in EUV range

• EUV range is about 50-EUV range is about 50-1500Å1500Å

• General Challenges:General Challenges:- multilayers required- multilayers required- absorption- absorption- high vacuum needed- high vacuum needed

• ApplicationsApplications- XUV lithography- XUV lithography- Soft x-ray microscopy- Soft x-ray microscopy- Astronomy- Astronomy

• Complex index of Complex index of refraction: refraction: ñ=n+ik

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

Periodic tablePeriodic table

Thorium vs. UraniumThorium vs. Uranium

• Thorium (11.7 g/cm^3) is less dense than Thorium (11.7 g/cm^3) is less dense than uranium (19.1 g/cm^3).uranium (19.1 g/cm^3).

• In the EUV and Soft x-ray, uranium is in In the EUV and Soft x-ray, uranium is in theory more reflective than thorium theory more reflective than thorium because of its density.because of its density.

Reflectivity vs. energy (eV)Reflectivity vs. energy (eV)

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Photon Energy

Ref

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Ir

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Thorium

But the mirror’s surface will be But the mirror’s surface will be oxidized.oxidized.

• Both uranium and thorium oxidize Both uranium and thorium oxidize easily at room temperature.easily at room temperature.

• The image mirror used UO2 as its top The image mirror used UO2 as its top layer.layer.

• 4 Å in 5 min, tens of nm in week.4 Å in 5 min, tens of nm in week.

• EUV at low angle only see top few EUV at low angle only see top few nm. nm.

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

an element are altered by oxidation.an element are altered by oxidation.

Reflectance, S polarization at 10 degrees of various materials

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Energy in eV

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Au Ni ThO2 UO2

Which oxide is better?Which oxide is better?

• Oxidation occurs at room temperature Oxidation occurs at room temperature for both uranium and thorium. for both uranium and thorium.

• Uranium, however, has many oxidation Uranium, however, has many oxidation states (UOstates (UO22, U, U22OO55, UO, UO33 etc.). As long as etc.). As long as there is unoxidized U nearby, most there is unoxidized U nearby, most oxidation stops at UOoxidation stops at UO22. But eventually, . But eventually, they could oxidize towards UOthey could oxidize towards UO33. .

• This has lower density!This has lower density!

The reflectance of UOThe reflectance of UO33 is lower than is lower than UOUO22

Reflectance, S polarization at 10 degrees of various materials

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Energy in eV

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Au Ni ThO2 UO2 UO3

Advantages of Thorium Advantages of Thorium OxideOxide

• Rock stable: highest known melting Rock stable: highest known melting point of any oxide.point of any oxide.

• Only one oxidation state, ThO2. We Only one oxidation state, ThO2. We know what we have!know what we have!

Project ChallengesProject Challenges

• Thorium metal acquisitionThorium metal acquisition

• Thorium Oxide sputtering target Thorium Oxide sputtering target productionproduction

• Long distances to analysis equipment Long distances to analysis equipment