optical properties and application of uranium-based thin-films for the extreme ultraviolet and soft...
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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
BYU EUV Optics
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
BYU EUV Optics
August 4, 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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August 4, 2004
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
BYU EUV Optics
August 4, 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
BYU EUV Optics
August 4, 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). 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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August 4, 2004
•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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August 4, 2004
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
BYU EUV Optics
August 4, 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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August 4, 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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August 4, 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
BYU EUV Optics
August 4, 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
BYU EUV Optics
August 4, 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
BYU EUV Optics
August 4, 2004
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.
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 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.
BYU EUV Optics
August 4, 2004
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
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.
BYU EUV Optics
August 4, 2004
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
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