Transcript Presentation - Physics and Astronomy

Thorium Based Thin Films as
EUV Reflectors
Jed Johnson
Honors Defense
Reflectors in EUV range
EUV range is about
100-1000Å
General Challenges:
- hydrocarbon buildup
- absorption
- high vacuum needed
Complex index of
refraction: ñ=n+ik
Applications of EUV Radiation
Thin Film or Multilayer Mirrors
EUV Lithography
EUV Astronomy
Soft X-ray Microscopes
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.
Creating Thin Films
• Ions from an induced argon plasma bombard a target. Atoms are
then ejected from the target and accumulate as a coating on the
substrate.
Measuring Reflectance
Data is taken primarily at the ALS (Advanced Light Source) at
LBNL in Berkeley, CA. Accelerating electrons produce high
intensity synchrotron radiation.
Why Actinides?
Beta vs. delta scatter plot at 4.48 nm
Note: Nickel and its neighboring 3d elements are the
nearest to uranium in this area.
ñ r  n  ik  1    i
  1  n,
 k
Periodic table
δ vs. (δ + β)
30.4 nm (41 eV)
Thorium vs. Uranium
Why such a large difference in optical properties?
Thorium (11.7 g/cm^3) is less dense than uranium
(19.1 g/cm^3).
Calculated Reflectance
vs. energy (eV) at 5 deg
1
0.9
0.8
Reflectance
0.7
Gold
Ir
Ni
U
Thorium
0.6
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
Photon Energy
400
500
However….
The mirror’s surface will be oxidized.
At optical wavelengths, this oxidation is
negligible. It is a major issue for our thin films
though.
Problems with Uranium
Immediate oxidation to UO2. (10 nm in 5 min)
Further oxidation to U2O5 is less rapid. (5 – 10
nm in six to 12 months)
Can even proceed to UO3!
Lower density = lower reflectance
A Possible Alternative: Thorium
Only one oxidation state:
ThO2. We know what we
have!
The densities of UO2 (about
11 g/cm3) and ThO2 (9.85
g/cm3) are similar.
Rock stable: Highest melting
point (3300 deg C) of any
known oxide.
Calculated Reflectance
vs. energy (eV) at 10 deg
Reflectance, S polarization 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
0
100
200
300
400
Energy in eV
Au
Ni
ThO2
UO2
500
600
First Thorium Reflectance Data
(Nov. ‘03 ALS)
M easured Reflectance of Th02 at 10 degrees
0.8
0.7
0.6
Reflectance
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Wavelength (nm)
2.16-2.8 nm
2.7-4.8 nm
4.4-6.8 nm
12.4-18.8 nm
17.2-25.0
22.5-32.5
6.6-8.8 nm
8.4-11.6 nm
11.0-14.0 nm
35
Th vs. EUV Other Reflectors
0.9
0.8
Reflectance
0.7
0.6
0.5
Th001
UO2
UN
NiO on Ni
Ir
Au
0.4
0.3
0.2
0.1
0
2
4
6
8
Wavelength (nm)
10
12
Between 6.5 and 9.4 nm, Th is the best
reflector we have measured.
Measured and Calculated
Reflectance at 10 deg
0.9
0.8
Reflectance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
5
2.16-2.8 nm
8.4-11.6 nm
22.5-32.5
10
15
20
25
Wavelength (nm)
2.7-4.8 nm
11.0-14.0 nm
calc. AFM CXRO S polarized
4.4-6.8 nm
12.4-18.8 nm
30
35
6.6-8.8 nm
17.2-25.0
40
“Zoomed in”
(and nm  eV)
1.00
0.90
calc ThO2 42nm
Th001 measured
Th001 measured
Th001 measured
Th001 measured
Reflectance
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
70
80
90
100
Energy (eV)
110
120
Higher Energies
1.00
0.90
calc ThO2 42nm
Th001 measured
Th001 measured
Th001 measured
Th001 measured
Th001 measured
Th001 measured
Reflectance
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
100
150
200
Energy eV
250
300
Einstein’s Atomic Scattering Factor
Model
Photons are scattered
principally off electrons.
More electrons = higher
reflection.
Assumption: condensed
matter may be modeled
as a collection of noninteracting atoms. In the
higher energy EUV,
chemical bonds shouldn’t
contribute. (except near
threshold regions)
Can the ASF model be applied in
the visible light range?
Silicon (opaque)
and oxygen
(colorless gas)
combine to form
SiO2 (quartz).
Clearly the
chemical bonds
have a dramatic
effect on the
compound’s
properties.
Where then is the ASF model
valid?
At some point,
ASF model and
measured data
should converge.
Unpublished
BYU study: SiO2
plots never
converged up to
300 eV.
Possibility #1
Bad experiment! Data has never been so clean though
and the features are clear. Curve was reproduced March
2004.
Beamline 6.3.2 coordinator at ALS has no explanation.
M e asure d Re fle ctance of Th02 at 10 de gre e s
0.8
0.7
0.6
Reflectance
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Wavelength (nm)
2.16-2.8 nm
2.7-4.8 nm
4.4-6.8 nm
12.4-18.8 nm
17.2-25.0
22.5-32.5
6.6-8.8 nm
8.4-11.6 nm
11.0-14.0 nm
35
Possibility #2
The sputtered
film wasn’t
pure thorium.
Possibly an
alloy?
EDX w/ SEM
indicates 
Carbon and Oxygen
Cutting fluid residue left on target?
Thorium carbide?
Hydrocarbon contaminant?
Carbon Impurities in silicon? (EDX “sees through”)
Surface XPS only sees Th.
Bottom Line: none of
these small
contributions could
have caused a drop
from ~70% to ~10%
reflectance.
Possibility #3
Maybe chemistry IS playing a larger role in this
region than previously expected.
Could the atomic scattering model need
modification in this range?
Transmission Measurements
Below 14 nm, there is a feature common to
reflection and transmission measurements.
1
0.9
0.8
0.7
0.6
Reflection at 10
deg
0.5
Transmission at
near-normal
incidence
0.4
0.3
0.2
0.1
0
12
13
14
15
wavelength (nm)
16
17
18
Optical Constant Fitting
Th003 is the only
transmission sample
to be made and
analyzed to date.
Least squares
procedure fits for
optical constants and
film thickness.
Characterization Issues
Film thickness
(more XRD and
ellipsometry)
Film composition
(XPS)
Roughness (AFM)
Optical Constant Data
17.0 nm
13.9 nm
Comparison: Calculated beta from transmission.
17.0 nm: 0.0330
13.9 nm: 0.1078
thickness: 197 Ǻ (XRD)
Good Agreement!
Conclusions
1. Th exhibits the highest reflectance of any
measured compound from 6.5 to 9.4 Ǻ.
2. The Atomic Scattering Factor model may
need revision in the EUV.
3. Constants obtained from the fitting program
are reasonable.
Future Research
Film oxidation (rate)
Film composition (modeling grain boundaries,
interfaces)
ThO2 constant determination
Roughness effects on reflectance / modeling
Theoretical ASF research
Acknowledgments
BYU XUV Research Group colleagues
Dr. David D. Allred
Dr. R. Steven Turley
BYU Physics Department Research
Funding
X-Ray Absorption Near Edge
Structure (XANES)
Induced current is measured in wire
connected to sample as EUV photons
strike it.
Absorption information.
Theoretical multiple scattering calculations
are compared with experimental XANES
spectra in order to determine the
geometrical arrangement of the atoms
surrounding the absorbing atom.
XANES Data
5
4.5
Relative Intensity
4
Th004(Si)
3.5
ThO2(Si)
3
UO14g(Si)
2.5
UO12g(Si)
2
1.5
1
0.5
0
66
71
76
81
86
Energy (eV)
91
96
More XANES Data
Relative Intensity
3
ThO2
Relative Intensity
2.5
UO12g
2
Th004
1.5
1
0.5
0
270
275
280
285
290
Energy (eV)
295
300
305
310
XANES and Beta
0.14
0.12
beta
0.1
beta
XANES Th004
0.08
XANES ThO2
0.06
0.04
0.02
0
12
13
14
15
wavelength (nm)
16
17
18