Uranium Oxide as a Highly Reflective Coating from 2.7 to 11.6 Nanometers Richard L. Sandberg Thanks to Advisors: David D. Allred, R. Steven Turley Fellow.

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

Richard L. Sandberg

Thanks toAdvisors: David D. Allred, R. Steven Turley

Fellow EUV Members: Jed E. Johnson, Luke J. Bissel, William R. Evans,and others from EUV Group

Funding: BYU Physics Dept. Funding, ORCA Scholarship, SPIE Scholarship

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October 25, 2003

Why Uranium and Thorium?

• In the EUV, uranium and thorium have many electrons to interact with photons (light) and is more dense than many materials, causing them to interact with high energy EUV photons.

• We study different compounds of uranium and thorium, such as uranium-oxide (UO2), uranium-nitride (UN), and thorium-oxide (ThO2) in search of compounds with the best optical constants and that do not react with air.

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October 10, 2003

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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October 25, 2003

Computed Reflectance at 10 degrees of various materials

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2 4 6 8 10 12 14 16 18 20Wavelength (nm)

Ref

lect

ance

Au Ni UO2 U

Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html

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October 10, 2003

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

•XRD Sample Thickness -UO2 30.0 nm (ρ=10.97 g/cm3) -NiO 49.7 nm (ρ=6.67 g/cm3) -Au 28.5 nm (ρ=19.3 g/cm3) BYU EUV Optics

October 25, 2003

5

Studying Our Samples

Images courtesy of www.weizmann.ac.il/surflab/peter/afmworks, www.mos.org/sln/SEM/works/http://volta.byu.edu/adamson03.pdf, and http://www.swt.edu/~wg06/manuals/Gaertner117/ellipsometerHome.htm

Ellipsometry

X-ray Photoelectron Spectroscope (XPS)

Scanning/Tunneling Electron Microscopes (SEM/TEM)

Atomic Force Microscopy (AFM)

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October 10, 2003

Inage courtesy of http://www.lbl.gov/

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October 10, 2003

Taking Reflectance Measurements at the ALS (Advance Light Source)

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2.5 4.5 6.5 8.5 10.5 12.5

Wavelength (nm)

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lect

ance

Sample of Data from the ALS

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

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October 25, 2003

• Small Discrepancies arise from one region to another with the use of different filters.

• Normalization given by R=(Idetector-Idark)/(Ibeam-Idark)

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October 10, 2003

ALS Measured Reflectance Comparison at 5 deg

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2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5

Wavelength (nm)

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lect

ance

UO2 NiO Au

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October 10, 2003

ALS Measured Reflectance Comparison at 10 deg

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2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5

Wavelength (nm)

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UO2 NiO Au

BYU EUV Optics

October 10, 2003

ALS MeasuredReflectance Comparison @ 15 deg

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Wavelength (nm)

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UO2 NiO Au

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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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Wavelength (nm)

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UOx Comp UO2 with 3nm C cap

Measured Data compared with CXRO Atomic Scattering Factor Model

• Assumption: condensed matter may be modeled as a collection of non-interacting atoms. In the higher energy EUV, chemical bonds shouldn’t contribute.

**This is a bad assumption for the EUV where most electron resonances reside.

• Photons are scattered principally off electrons. More electrons = higher reflection.

Reflectance of Naturally Oxidized and Reactively Sputtered UO2

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Wavelength (nm)

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UO18-Naturally oxidized UO2 [i]

Lunt UO2-Reactively Sputtered [ii]

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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October 10, 2003

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Conclusions

• UO2 reflects significantly better than Ni and Au, the current materials with highest reflectance, between 4 and 11 nm.

• UO2 reflectance differs from the reflectance predicted by the atomic scattering factor model (ASF).

• 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.

Goals• Determine why UO2 measured

reflectance does not agree with ASF• Determine the optical properties of

UO2 below Shannon’s data (4.5 nm).• Publish results, finish thesis, and

graduate!

Successes• Presented at 2004 SPIE Annual

Meeting in San Diego, CA and published research in Proceedings of the SPIE

• Presented at 2004 SRI Meeting in San Fransisco, CA and published research in Proceedings of the SRI

THANK YOU!!