Thorium Based Thin Films as
EUV Reflectors
Jed Johnson
Honors Defense

Reflectors in EUV range
EUV range is about
1001000Å
General Challenges:
- hydrocarbon buildup
- absorption
- high vacuum needed
Complex index of refraction: ñ=n+ik

Applications of EUV Radiation
EUV Lithography Thin Film or Multilayer Mirrors
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).

0.7
0.6
0.5
0.4
0.3
1
0.9
0.8
0.2
0.1
0
0
Calculated Reflectance vs. energy (eV) at 5 deg
100 200 300
Photon Energy
400 500
Gold
Ir
Ni
U
Thorium

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 UO
2
. (10 nm in 5 min)
Further oxidation to U
2
O nm in six to 12 months)
5 is less rapid. (5 – 10
Can even proceed to UO
3
!
Lower density = lower reflectance

A Possible Alternative: Thorium
Only one oxidation state:
ThO
2
. We know what we have!
The densities of UO
2
11 g/cm 3 ) and ThO
2 g/cm 3 ) are similar.
(about
(9.85
Rock stable: Highest melting point (3300 deg C) of any known oxide.

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
Calculated Reflectance vs. energy (eV) at 10 deg
Reflectance, S polarization at 10 degrees of various materials
100 200
Au
300
Energy in eV
Ni ThO2
400
UO2
500 600

0.2
0.1
0
0
0.6
0.5
0.4
0.3
0.7
0.8
First Thorium Reflectance Data
(Nov. ‘03 ALS)
M e asure d Re fle ctance of Th02 at 10 de gre e s
5
2.16-2.8 nm
12.4-18.8 nm
10
2.7-4.8 nm
17.2-25.0
15 20
Wavelength (nm)
4.4-6.8 nm
22.5-32.5
6.6-8.8 nm
25
8.4-11.6 nm
30
11.0-14.0 nm
35

Th vs. EUV Other Reflectors
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
Th001
UO2
UN
NiO on Ni
Ir
Au
4 6
Wavelength (nm)
8 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
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
Wavelength (nm)
25
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
6.6-8.8 nm
17.2-25.0
35 40

1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
70 80
“Zoomed in”
(and nm  eV) calc ThO2 42nm
Th001 measured
Th001 measured
Th001 measured
Th001 measured
90
Energy (eV)
100 110 120

1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
100
Higher Energies calc ThO2 42nm
Th001 measured
Th001 measured
Th001 measured
Th001 measured
Th001 measured
Th001 measured
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
SiO
2
(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: SiO
2 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
0.5
0.4
0.3
0.2
0.1
0
0 5
2.16-2.8 nm
12.4-18.8 nm
10
2.7-4.8 nm
17.2-25.0
15 20
Wavelength (nm)
4.4-6.8 nm
22.5-32.5
6.6-8.8 nm
25
8.4-11.6 nm
30
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
0.5
0.4
0.3
0.2
0.1
0
12 13 14
Reflection at 10 deg
Transmission at near-normal incidence
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)
ThO
2 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
4
3.5
3
2.5
2
1.5
1
0.5
0
66
Th004(Si)
ThO2(Si)
UO14g(Si)
UO12g(Si)
71 76 81
Energy (eV)
86 91 96

More XANES Data
Relative Intensity
3
2.5
2
1.5
1
0.5
0
270
ThO2
UO12g
Th004
275 280 285 290
Energy (eV)
295 300 305 310

XANES and Beta
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
12 13 beta
XANES Th004
XANES ThO2
14 15 wavelength (nm)
16 17 18