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 Astronomy
Brigham Young University
*allred@byu.edu; phone 1 801 422-3489; fax 1 801 422-0553; xuv.byu.edu
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October 25, 2003
Overview
• OUTLINE
– Why EUV and soft XRays?
– Why uranium?
– What do the current
models predict?
– Methods.
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October 25, 2003
• 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
• Sample preparation
model (ASF) needs revision for
• Reflectance measurement
compounds. Oxidized uranium
– Results.
reflectance differs from that
predicted by ASF.
– Conclusions.
3. Reflectance of air-oxidized
uranium (UO2) matches
reactively sputtered UO2. 2
Why Extreme Ultraviolet (EUV) and Soft X-Rays?
Thin Film or Multilayer Mirrors
EUV Lithography
(making really small computer chips)
EUV Astronomy
Soft X-Ray Microscopes
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The Earth’s magnetosphere in the EUV
Images from www.schott.com/magazine/english/info99/ and www.lbl.gov/Science-Articles/Archive/xray-inside-cells.html.
October 25, 2003
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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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October 25, 2003
Delta vs. beta plot for several elements at 4.48 nm
4.48nm
Note: Nickel and its neighboring 3d elements are the
nearest to uranium in this area.
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ñ r  n  ik  1    i
  1  n,
October 25, 2003
 k
5
Computed Reflectance, ASF model, CXRO
0.9
0.8
0.7
Reflectance
0.6
0.5
0.4
0.3
0.2
0.1
0
2
3
4
5
6
7
8
Wavelength (nm)
Ni
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October 25, 2003
NiO
U
9
10
11
12
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).
6
Sample Preparation
The 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 Sputtering
The 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 Evaporation
Evaporated Ni wire from a resistively heated
tungsten boat (RD Mathis Co.) in a large,
cryopumped, stainless steel “bell jar” coater.
Schematic of DC magnetron
sputtering system at BYU.
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Taking Reflectance Measurements at the
Advanced Light Source (ALS) Beamline 6.3.2
Beamline 6.3.2 Reflectometer
• Bright synchrotron radiation
• 1-24.8 nm range
• High spectral purity
• Energy/wavelength or θ-2θ
scan capability
Schematic of ALS beamline 6.3.2 courtesy of http://www-cxro.lbl.gov/ALS6.3.2/.
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
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* Discrepancies in data appear between wavelength scans because samples were
removed and from chamber and then re-measured.
October 25, 2003
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0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Reflectance
Reflectance
Measured reflectances of UOx, NiO on Ni, and Ni on quartz at 5 degrees from 2.7-11.6 nm
2.5
3
NiO on Ni
4.5
5
4.4
Ni on Quartz
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Reflectance
Reflectance
UOx
3.5
4
Wavelength (nm)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
6.6
7.1
UOx
7.6
8.1
Wavelength (nm)
NiO on Ni
Ni on Quartz
8.6
4.9
5.4
5.9
6.4
Wavelength (nm)
UOx NiO on Ni Ni on Quartz
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
8.4
8.9
UOx
9.4
9.9
10.4
Wavelength (nm)
NiO on Ni
10.9
Ni on Quartz
11.4
9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Reflectance
Reflectance
Measured reflectance at 10 degrees of UO2, NiO on Ni, Ni on Quartz from 2.7-11.6 nm
2.7
3.2
3.7
4.2
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
4.4
4.7
4.9
Ni on Quartz
UOx
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Reflectance
Reflectance
NiO on Ni
6.6
7.1
7.6
8.1
NiO on Ni
Ni on Quartz
6.4
8.6
NiO on Ni
Ni on Quartz
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
8.4
9.4
Wavelength (nm)
UOx
5.9
Wavelength (nm)
Wavelength (nm)
UOx
5.4
10.4
11.4
Wavelength (nm)
UOx
NiO on Ni
Ni on Quartz
10
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Reflectance
Reflectance
Measured reflectance at 15 degrees of UO2, NiO on Ni, and Ni on Quartz from 2.7-11.6 nm.
2.5
3
3.5
4
4.5
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
4.4
4.9
Wavelength (nm)
UOx
NiO on Ni
UOx
Ni on Quartz
0.6
Reflectance
Reflectance
0.7
0.5
0.4
0.3
0.2
0.1
0
7.1
7.6
8.1
Wavelength (nm)
UOx
NiO on Ni
Ni on Quartz
5.9
6.4
Wavelength (nm)
0.8
6.6
5.4
8.6
NiO on Ni
Ni on Quartz
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
8.4
8.9
UOx
9.4
9.9 10.4
Wavelength (nm)
NiO on Ni
10.9
Ni on Quartz
11.4
11
Reflectance
Reflectance of Naturally Oxidized and
Reactively Sputtered UO2
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3
5
7
9
11
Wavelength (nm)
13
15
17
UO18-Naturally oxidized UO2 [i]
Lunt UOx on UO2-Reactively Sputtered [ii]
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[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
12
46 to 584 Angstroms, Masters Thesis, Dept. of Physics and Astronomy, BYU, Provo, UT 2002.
Reflectance of Measured UO2 and Computed Models
1
0.9
0.8
Reflectance
Reflectance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
22
3
44
5
66
7
8
9
10
11
12
Wavelength (nm)
Wavelength
(nm)
Measured UOx
Computed UOx (d=30 nm)
Computed UOx with 0.5 Measured
nm C on top
Computed
UOx with
UOx Measured
Computed
UOx UOx (d=30
nm) C(density=1.5g/cc) 3 nm
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Acknowledgements
Hollilyn 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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Conclusions
1. 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.
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October 25, 2003