Optical Properties and Application of Uranium-based Thin-Films for the

Extreme Ultraviolet and Soft X-ray Region

Richard L. Sandberg, David D. Allred, Marie K. Urry, Shannon Lunt, R. Steven Turley

Thanks toFellow EUV Members: Jed E. Johnson, Winston Larson, Kristi R. Adamson, Nikki Farnsworth, William R. Evans, and others from EUV Group, Andy Aguila & Eric Gullickson at ALS/CXRO

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

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August 4, 2004

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?• Pros: high density and many electrons (92) for absorption, high theoretical

reflectivity: low absorption and high index of refraction

• Con: chemically reactive (oxidizes in air to most abundant natural oxide UO2 at STP)

• We study different compounds of uranium, such as uranium dioxide (UO2) and uranium mononitride (UN), in search of compounds with the highest reflectance and most chemical stability.

• Previous Success: IMAGE Satellite Mirror Project—BYU uranium based mirrors (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

iiknn

,1

1~r

4.48nm

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August 4, 2004

Computed Reflectance at 10 degrees of various materials

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0.7

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0.9

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

Ref

lect

ance

Au Ni UO2 U Ir

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

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Schematic of DC magnetron sputtering system at BYU.

Sample PreparationThe UO, UN, Ni, and Au 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/RF SputteringThe uranium sputter targets used here were of depleted uranium metal (less than 0.2% U-235). UO2 deposited in two ways. Reactively sputtered (DC) in oxygen partial pressure (Lunt at oxygen partial pressure of 3x10-4 torr) or as pure uranium (RF) and then allowed to oxidize in ambient air. Uranium nitride was reactively sputtered (RF) in nitrogen partial pressure of about 10-5 torr.

•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.

•Ir Sample Prepared at Goddard Space Flight Center on Glass Slides

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•XRD Sample Thickness -UO2 30.0 nm (ρ=10.97 g/cm3) -UN 38.0 nm (ρ=10. g/cm3) -NiO on Ni 49.7 nm (ρ=6.67 g/cm3) -Au 29.5 nm (ρ=19.3 g/cm3) -Ir ?? (ρ=22.42 g/cm3)

Thickness Determined by XRD

m λ = 2d sin θ

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Oxidation of a UN Thin Film

95

100

105

110

115

1 10 100 1000Time (hrS.)

Per

cen

t ch

ange

in t

hic

kn

ess

IMD data

Fit

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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/Transmission Electron

Microscopes (SEM/TEM)

Atomic Force Microscopy (AFM)

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

Taking Reflectance Measurements at the ALS (Advance Light Source)

00.10.20.30.40.50.60.70.80.9

2.5 4.5 6.5 8.5 10.5 12.5

Wavelength (nm)

Ref

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

• Small Discrepancies arise from one region to another with the use of different filters.•XANES Capability• Normalization given by R=(Idetector-Idark)/(Ibeam-Idark)

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ALS Measured Reflectance Comparison at 5 deg

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2 3 4 5 6 7 8 9 10 11 12Wavelength (nm)

Ref

lect

ance

UO2UNNiO on NiIrAu

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ALS Measured Reflectance Comparison at 10 deg

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0.7

2 4 6 8 10 12Wavelength (nm)

Ref

lect

ance

UO2 UNNiO on NiIrAu

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August 4, 2004

ALS Measured Reflectance Comparison @ 15 deg

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0.05

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0.15

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0.25

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0.35

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0.45

0.5

4 5 6 7 8 9 10 11 12

Wavelength (nm)

Ref

lect

ance

UO2 UNNiO on NiIrAu

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Calculating Optical Properties of Uranium Oxide and Uranium Nitride from Reflectance

Lunt and Urry both used Kohn and Parratt’s equations to calculate reflectance from previously published value of δ and β. The values of δ and β were then adjusted until the difference between the measured reflectance and calculated reflectance were minimized. The measured reflectance scans were angle scans from the ALS. Urry studied uranium nitride and Lunt studied uranium oxide.

where

122

1

122

1,

mmmm

mmmmmp qNqN

qNqNf

1

1,

mm

mmms qq

qqf

imm Nq 22 cos

V.G. Kohn. Phys. Stat. Sol. 185(61), 61-70 (1995), L.G. Parratt. Physical Review 95 (2), 359-369 (1954).

1,,

1,,4, 1

mpmp

mpmpmmp rf

rfCr

1,,

1,,4, 1

msms

msmsmms rf

rfCr

/mmDiqm eC

Optical Properties of Uranium Oxide and Uranium Nitride

δ and β of UOx Top layer obtained from Lunt

  ALS Measured

λ (nm) δ β

4.6 0.0065 0.0011

5.6 0.0103 0.0016

6.8 0.0161 0.0031

8.5 0.0295 0.0134

10 0.0398 0.0269

12.5 0.0206 0.0091

14 0.0360 0.0151

15.5 0.0495 0.0216

17.5 0.0639 0.0338

 δ and β of UN from M. Urry 

 λ (nm)

13 0.01152 0.0595

14 0.0138 0.0416

δ and β of UO2 obtained from S. Lunt’s Thesis

  ALS MeasuredCXRO

Calculated

λ (nm) δ β δ β

4.6 0.0065 8.09E-04 0.0116 0.0011

5.6 0.0103 0.0012 0.0187 0.0025

6.8 0.0173 0.004 0.0302 0.0065

8.5 0.0298 0.0151 0.0491 0.0271

10 0.0344 0.0458 0.0674 0.0693

12.5 -0.0038 0.0129 0.0057 0.0399

14 0.0229 0.0103 0.0509 0.017

15.5 0.0362 0.0158 0.0782 0.0281

17.5 0.0547 0.0246 0.1058 0.0464

Lunt found that her samples were mostly UO2 with a top layer of an slightly higher oxidation state.

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2.5 4.5 6.5 8.5 10.5 12.5Wavelength (nm)

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lect

ance

Measured UO2

Computed UO2 (d=30 nm)

Computed UO2 with 0.5 nm C on top

Computed UO2 with C(density=1.5g/cc) 3 nm

00.10.20.30.40.50.60.70.80.9

1

2.5 3 3.5 4 4.5 5Wavelength (nm)

Ref

lect

ance

UOxComp UO2Comp UO2 with C cap

Measured Data compared with CXRO Previously Published Constants

Measured reflectance features do not agree with CXRO published constants. More work need to be done on measuring uranium’s optical constants.

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XANES (X-Ray Absorption Near Edge Spectroscopy)

XANES at ALS show additional absorption resonances not accounted for in Published Data at CXRO.

Relative XANES Scans of UO2 and ThO2

0.4

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0.8

1

1.2

1.4

1.6

1.8

280 285 290 295 300 305

Energy (eV)

Rel

ativ

e In

tens

ity

ThO2

UO2

U NVIOIV @ 286.3 eV *

*D. R. Lide (ed.), CRC Handbook of Chemistry and Physics, 71st edition, CRC Press, Boca Raton, 1990-91, p.10-256.

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Conclusions

• UO2 and UN reflect significantly more than Ni, Ir, and Au, the current materials with highest reflectance, between 4 and 9 nm.

• U reflectance differs from the reflectance predicted by the previously published uranium optical properties.

• Current preparation of UN is not stable in ambient air (oxidizes to UO2). Need to test oxidation of heated UN sample

Goals• Determine the optical properties of

UO2 below Shannon’s data (4.5 nm) and fill out UN optical properties data.

• Work with CXRO to amend the current uranium optical properties.

Questions? EUV Group Contact

Dr. David Allredallred@byu.edu(801) 422-3489

THANK YOU!! BYU EUV Optics

August 4, 2004