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 to
Fellow 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?
Thin Film or Multilayer Mirrors
EUV Lithography
(making really small computer chips)
EUV Astronomy
Soft X-Ray Microscopes
BYU EUV Optics
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.
August 4, 2004
Why Uranium?
•
•
•
•
BYU EUV Optics
August 4, 2004
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)
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.
BYU EUV Optics
~  n  ik  1    i
n
r
  1  n,
August 4, 2004
 k
Computed Reflectance at 10 degrees of various materials
0.9
0.8
Reflectance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
4
6
8
Au
10
12
Wavelength (nm)
Ni
UO2
14
U
16
Ir
BYU EUV Optics
Reflectance computed using the CXRO Website: http://www-cxro.lbl.gov/optical_constants/mirror2.html
August 4, 2004
18
20
Sample Preparation
The 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 Sputtering
The 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.
Schematic of DC magnetron
sputtering system at BYU.
BYU EUV Optics
August 4, 2004
•Ni/Au Resistive Thermal Evaporation
Evaporated 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
Thickness Determined by XRD
Film thickness (nm)
50
y = 40.398x2 - 116.4x + 109.12
R2 = 0.9995
45
40
35
UN (Dens.=14.3)
30
Poly. (UN
(Dens.=14.3))
25
0.6
BYU EUV Optics
August 4, 2004
m λ = 2d sin θ
0.8
1
1.2
Theta (for 4 peaks)
1.4
•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)
Oxidation of a UN Thin Film
Percent change in thickness
115
110
105
IMD data
Fit
100
95
BYU EUV Optics
1
August 4, 2004
10
Time (hrS.)
100
1000
Studying Our Samples
Ellipsometry
Scanning/Transmission Electron
Microscopes (SEM/TEM)
X-ray Photoelectron Spectroscope (XPS)
Atomic Force Microscopy (AFM)
BYU EUV Optics
August 4, 2004
Images courtesy of www.weizmann.ac.il/surflab/peter/afmworks, www.mos.org/sln/SEM/works/
5
Taking Reflectance Measurements at the ALS
(Advance Light Source)
BYU EUV Optics
August 4, 2004
• Small Discrepancies arise from
one region to another with the use
of different filters.
•XANES Capability
• Normalization given by
R=(Idetector-Idark)/(Ibeam-Idark)
Reflectance
Beamline 6.3.2 Reflectometer
• Bright synchrotron radiation
• 1-24.8 nm range
• High spectral purity
• Energy/wavelength or θ-2θ
scan capability
Sample of Data from the ALS
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2.5
Inage courtesy of http://www.lbl.gov/
4.5
6.5
8.5
Wavelength (nm)
10.5
12.5
ALS Measured Reflectance Comparison at 5 deg
0.9
0.8
0.7
Reflectance
0.6
0.5
0.4
UO2
UN
NiO on Ni
Ir
Au
0.3
0.2
0.1
0
2
BYU EUV Optics
August 4, 2004
3
4
5
6
7
Wavelength (nm)
8
9
10
11
12
ALS Measured Reflectance Comparison at 10 deg
0.7
0.6
UO2
UN
NiO on Ni
Ir
Au
Reflectance
0.5
0.4
0.3
0.2
0.1
0
2
BYU EUV Optics
August 4, 2004
4
6
8
Wavelength (nm)
10
12
ALS Measured Reflectance Comparison @ 15 deg
0.5
UO2
UN
NiO on Ni
Ir
Au
0.45
0.4
Reflectance
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
BYU EUV Optics
4
5
6
7
8
Wavelength (nm)
August 4, 2004
9
10
11
12
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.
rs ,m  C
4
m
f s ,m  rs ,m 1
1  f s ,m rs ,m 1
rp ,m  C
4
m
f p ,m  rp ,m 1
1  f p ,m rp ,m 1
where
f s ,m
qm  qm1

qm  qm1
qm  N m2  cos 2 i
f p ,m
N m2 1qm  N m2 qm1
 2
N m1qm  N m2 qm1
C m  e iqm Dm / 
V.G. Kohn. Phys. Stat. Sol. 185(61), 61-70 (1995), L.G. Parratt. Physical Review 95 (2), 359-369 (1954).
Optical Properties of Uranium Oxide and Uranium Nitride
δ and β of UN from M. Urry
λ (nm)


13
0.01152
0.0595
14
0.0138
0.0416
Lunt found that her samples were mostly UO2
with a top layer of an slightly higher oxidation
state.
δ and β of UO2 obtained from S. Lunt’s Thesis
CXRO
Calculated
ALS Measured
λ (nm)
β
δ
δ and β of UOx Top layer obtained
from Lunt
β
δ
ALS Measured
λ (nm)
β
δ
4.6
0.0065
8.09E-04
0.0116
0.0011
4.6
0.0065
0.0011
5.6
0.0103
0.0012
0.0187
0.0025
5.6
0.0103
0.0016
6.8
0.0173
0.004
0.0302
0.0065
6.8
0.0161
0.0031
8.5
0.0298
0.0151
0.0491
0.0271
8.5
0.0295
0.0134
10
0.0344
0.0458
0.0674
0.0693
10
0.0398
0.0269
12.5
-0.0038
0.0129
0.0057
0.0399
12.5
0.0206
0.0091
14
0.0229
0.0103
0.0509
0.017
14
0.0360
0.0151
15.5
0.0362
0.0158
0.0782
0.0281
15.5
0.0495
0.0216
17.5
0.0547
0.0246
0.1058
0.0464
17.5
0.0639
0.0338
Reflectance
Measured Data compared with CXRO Previously Published Constants
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
UOx
Comp UO2
Comp UO2 with C cap
2.5
3
3.5
Wavelength (nm)
4
4.5
5
1
0.9
Measured reflectance features
do not agree with CXRO
published constants. More
work need to be done on
measuring uranium’s
optical constants.
Reflectance
0.8
0.7
0.6
0.5
0.4
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
0.3
0.2
0.1
0
BYU EUV Optics
August 4, 2004
2.5
4.5
6.5
8.5
Wavelength (nm)
10.5
12.5
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
Relative Intensity
1.8
U NVIOIV @ 286.3 eV *
1.6
ThO2
1.4
UO2
1.2
1
0.8
0.6
0.4
280
285
295
290
300
Energy (eV)
BYU EUV Optics
August 4, 2004
*D. R. Lide (ed.), CRC Handbook of Chemistry and Physics,
71st edition, CRC Press, Boca Raton, 1990-91, p.10-256.
305
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
BYU EUV Optics
August 4, 2004
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 Allred
allred@byu.edu
(801) 422-3489
THANK YOU!!