Uranium Oxide and Uranium Nitride as Highly Reflective Coatings from 2.7 to 11.6
Nanometers
Richard L. Sandberg, Marie K. Urry, Shannon Lunt David D. Allred, R. Steven Turley
Thanks toFellow EUV Members: Jed E. Johnson, Luke J. Bissel, 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
BYU EUV Optics
April 19, 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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April 19, 2004
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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April 19, 2004
Computed Reflectance at 10 degrees of various materials
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
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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April 19, 2004
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). After sputtering, the uranium oxide was allowed to oxidize naturally in laboratory air. Uranium nitride was reactively sputtered (RF) in nitrogen partial pressure of about 10-5 torr, bellow 10-4 torr as suggested by L. Black et al., Journal of Alloys and Compounds 315, 36-41 (2001).
•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
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April 19, 2004
m λ = 2d sin θ
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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April 19, 2004
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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April 19, 2004
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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April 19, 2004
ALS Measured Reflectance Comparison at 5 deg
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
2 3 4 5 6 7 8 9 10 11 12Wavelength (nm)
Ref
lect
ance
UO2UNNiO on NiIrAu
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April 19, 2004
ALS Measured Reflectance Comparison at 10 deg
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2 4 6 8 10 12Wavelength (nm)
Ref
lect
ance
UO2 UNNiO on NiIrAu
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April 19, 2004
ALS Measured Reflectance Comparison @ 15 deg
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
4 5 6 7 8 9 10 11 12
Wavelength (nm)
Ref
lect
ance
UO2 UNNiO on NiIrAu
Optical Properties of Uranium Oxide and Uranium Nitride
kn
iiknn
,1
1~r
δ and β of UO2 obtained from S. Lunt’s Thesis
ALS Measured CXRO 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
δ and β of UN from M. Urry
λ (nm)
13 0.01152 0.0595
14 0.0138 0.0416
1,,
1,,4, 1
mpmp
mpmpmmp rf
rfCr
1,,
1,,4, 1
msms
msmsmms rf
rfCr
122
1
122
1,
mmmm
mmmmmp qNqN
qNqNf
1
1,
mm
mmms qq
qqf
/mmDiqm eC
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).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
2.5 4.5 6.5 8.5 10.5 12.5Wavelength (nm)
Ref
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 Atomic Scattering Factor Model
Measured reflectance features do not agree with CXRO atomic scattering factors. More work need to be done on measuring uranium’s optical constants.
• Photons are scattered principally off electrons. More electrons = higher reflection.
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XANES (X-Ray Absorption Near Edge Spectroscopy)
XANES at ALS show additional absorption resonances not accounted for in ASF Data at CXRO.
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April 19, 2004
Relative XANES Scans of UO2 and ThO2
0.4
0.6
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.
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 atomic scattering factor model (ASF).
• 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 existing uranium atomic scattering factor data.
Questions? EUV Group Contact
Dr. David [email protected](801) 422-3489
THANK YOU!! BYU EUV Optics
April 19, 2004