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Uranium Oxide as a Highly Reflective Coating from 2.7 to 11.6 Nanometers

William R. Evans, Richard L. Sandberg, David D. Allred*, Jed E. Johnson, R. Steven Turley

Department of Physics and AstronomyBrigham Young University

*allred@byu.edu; phone 1 801 422-3489; fax 1 801 422-0553; xuv.byu.edu

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Overview• RESULTS

1. Uranium reflects more than nickel, the current metal of choice for soft X-Ray applications, between 4 and 11 nm.

2. The atomic scattering factor model (ASF) needs revision for compounds. Oxidized uranium reflectance differs from that predicted by ASF.

3. Reflectance of air-oxidized uranium (UO2) matches reactively sputtered UO2.

• OUTLINE– Why EUV and soft X-

Rays?– Why uranium?– What do the current

models predict?– Methods.

• Sample preparation• Reflectance measurement

– Results. – Conclusions.

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Why Extreme Ultraviolet (EUV) and Soft X-Rays?

Images from www.schott.com/magazine/english/info99/ and www.lbl.gov/Science-Articles/Archive/xray-inside-cells.html.

EUV Lithography(making really small computer chips)

Thin Film or Multilayer Mirrors

EUV Astronomy

The Earth’s magnetosphere in the EUV

Soft X-Ray Microscopes

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Why Uranium?• Uranium has many electrons to interact with photons (light) and is more dense

than many materials, causing them to interact with high energy EUV photons.

• High Theoretical Reflectivity: Low absorption and high index of refraction

• Previous Success: IMAGE Satellite Mirror Project (Launched March 25, 2000)

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Note: Nickel and its neighboring 3d elements are the nearest to uranium in this area.

Delta vs. beta plot for several elements at 4.48 nm

kn

iikn

,1

1ñ r

4.48nm

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Computed Reflectance, ASF model, CXRO

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Ni NiO U UO2

Reflectances for Ni, NiO, U, and UO2 predicted by the atomic scattering factor model from the Center for

X-Ray Optics (CXRO) website (www-cxro.lbl.gov).

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Schematic of DC magnetron sputtering system at BYU.

Sample PreparationThe uranium oxide and nickel samples were deposited on pieces polished silicon test wafers (100 orientation). Quartz crystal monitors were used to measure the sputtering and evaporation rates.

•U DC Magnetron SputteringThe uranium sputter targets used here were of depleted uranium metal (less than 0.2% U-235). After sputtering, the uranium was allowed to oxidize naturally in laboratory air.

•Ni Resistive Thermal EvaporationEvaporated Ni wire from a resistively heated tungsten boat (RD Mathis Co.) in a large, cryopumped, stainless steel “bell jar” coater.

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Taking Reflectance Measurements at the Advanced Light Source (ALS) Beamline 6.3.2

Schematic of ALS beamline 6.3.2 courtesy of http://www-cxro.lbl.gov/ALS6.3.2/.

Beamline 6.3.2 Reflectometer• Bright synchrotron radiation• 1-24.8 nm range• High spectral purity• Energy/wavelength or θ-2θ scan capability

ALS Reflectance Measurements• UOx means UO2, U most abundant natural oxide• NiO on Ni means Ni sample on Si oxidized by UV cleaning lamp• Ni on quartz means Ni on quartz slide* Discrepancies in data appear between wavelength scans because samples were removed and from chamber and then re-measured.

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Measured reflectances of UOx, NiO on Ni, and Ni on quartz at 5 degrees from 2.7-11.6 nm

00.10.20.30.40.50.60.70.8

4.4 4.9 5.4 5.9 6.4Wavelength (nm)

Ref

lect

ance

UOx NiO on Ni Ni on Quartz

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2.5 3 3.5 4 4.5 5Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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6.6 7.1 7.6 8.1 8.6Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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8.4 8.9 9.4 9.9 10.4 10.9 11.4Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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Measured reflectance at 10 degrees of UO2, NiO on Ni, Ni on Quartz from 2.7-11.6 nm

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4.4 4.9 5.4 5.9 6.4

Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

00.10.20.30.40.50.60.70.8

2.7 3.2 3.7 4.2 4.7

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UOx NiO on Ni Ni on Quartz

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6.6 7.1 7.6 8.1 8.6

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UOx NiO on Ni Ni on Quartz

00.10.20.30.40.50.60.70.8

8.4 9.4 10.4 11.4

Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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Measured reflectance at 15 degrees of UO2, NiO on Ni, and Ni on Quartz from 2.7-11.6 nm.

00.10.20.30.40.50.60.70.8

4.4 4.9 5.4 5.9 6.4

Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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2.5 3 3.5 4 4.5 5

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UOx NiO on Ni Ni on Quartz

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UOx NiO on Ni Ni on Quartz

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8.4 8.9 9.4 9.9 10.4 10.9 11.4Wavelength (nm)

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UOx NiO on Ni Ni on Quartz

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Reflectance of Naturally Oxidized and Reactively Sputtered UO2

00.10.20.30.40.50.60.70.80.9

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3 5 7 9 11 13 15 17Wavelength (nm)

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UO18-Naturally oxidized UO2 [i]Lunt UOx on UO2-Reactively Sputtered [ii]

[i] Sandberg et a., Advances in Mirror Technology for X-Ray, EUV Lithography, Laser, and Other Applications , Ali M. Khounsary, Udo Dinger, Kazuya Ota, Editors, Proc. SPIE 5193, SPIE, Bellingham, WA, 2003. [ii] Shannon Lunt, Determining the Indices of Refraction of Reactively Sputtered Uranium Dioxide Thing Films from 46 to 584 Angstroms, Masters Thesis, Dept. of Physics and Astronomy, BYU, Provo, UT 2002.

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Reflectance of Measured UO2 and Computed Models

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Measured UOx

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Reflectance of Measured UO2 and Computed Models

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Measured UOx Computed UOx (d=30 nm)

Reflectance of Measured UO2 and Computed Models

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Measured UOx Computed UOx (d=30 nm)

Computed UOx with 0.5 nm C on top Computed UOx with C(density=1.5g/cc) 3 nm

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AcknowledgementsHollilyn Drury and Megan Rowberry (Provo High

School) aided in sputtering the uranium films studied. An SPIE scholarship and department funding aided

Richard Sandberg in the research. We also acknowledge gratefully the financial

contributions of V. Dean and Alice J. Allred and Marathon Oil Company (US Steel) and Nan Ellen Ah You for gifts to Brigham Young University for thin film research.

We would also like to thank the ALS for the beamtime used to make the optical measurements.

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Conclusions1. Uranium oxide reflects significantly better than nickel, the current

material with highest reflectance, between 4 and 11 nm. 2. Uranium oxide reflectance differs from the reflectance predicted

by the atomic scattering factor model (ASF).

3. Reflectances of naturally oxidized uranium (UO2) matches reactively sputtered UO2 –Thus the material can be made in a number of different ways and is stable enough for practical use.