Transcript X-Ray Photoelectron Spectroscopy to Examine Molecular

X-Ray Photoelectron
Spectroscopy to Examine
Molecular Composition
Amy Baker
R. Steven Turley
Brigham Young University
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Why Extreme Ultraviolet?
Thin Film or Multilayer Mirrors
Soft X-Ray Microscope
EUV Lithography
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.
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Why Thorium?
Only one oxidation state: ThO2
 Rock stable: Highest melting point
(3300 deg C) of any known oxide.
 High Reflectance in the EUV (10100nm)

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Will Thorium Work?



The mirror’s surface
will be oxidized.
At optical
wavelengths, this
oxidation is
negligible. It is a
major issue for our
thin films, however.
We expect minimal
oxidation
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Purposes of X-Ray Photoelectron
Spectroscopy



Learn oxidation state of our thorium
samples
Understand how composition changes
with depth
Obtain an expression for oxidation as a
function of depth
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X-Ray Photoelectron Spectroscopy
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How XPS works
Kmax  hv  
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Electron Binding Energy
4500
Th
4000
4f5/2
3500
4f7/2
Counts
3000
2500
2000
Th
4d3/2 4d5/2
O
1s
1500
C
1000
1s
500
0
1000
800
600
400
Th
5d3/2 5d5/2
200
0
Binding Energy (eV)
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Peak Shifts

3.6K
3.4K
3.2K
3K
2.8K
2.6K
2.4K
2.2K
2K
1.8K
1.6K
1.4K
1.2K
1K
800
600
400
200
354.9
352.9
350.9
348.9
346.9
344.9
342.9
340.9
338.9
336.9
334.9
332.9
330.9
328.9
Thorium peaks
on surface
326.9

3.2K
3K
2.8K
2.6K
2.4K
2.2K
2K
1.8K
Thorium peaks
after oxygen is
gone
1.6K
1.4K
1.2K
1K
800
600
400
200
354.9
352.9
350.9
348.9
346.9
344.9
342.9
340.9
338.9
336.9
334.9
332.9
330.9
328.9
326.9
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Depth Profiling

Rastering:
Argon ions knock off individual atoms

Variable angle scans:
More depth is obtained as x-ray gun and
detector are moved towards incidence
θ
ee-
e-
Sample
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Variable Angle Results
Only penetrates about 150 Angstroms
into the sample
 This allows us to see surface
contamination, but not composition
with depth
 Results are averaged: cannot obtain
resolved composition with depth

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Rastering Results
Thorium Composition Sample 040207
120
100
%
80
O%
60
Th %
40
Si %
20
0
-20 0
500
1000
1500
2000
2500
3000
Sputtering Time (s)
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Too Much Oxidation
 These
samples were only a few
hours old.
 We need more uniformity.
 Solution: Make ThO2 mirrors.
Reflection is similar to Th and it
should be more uniform.
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ThO2 Results
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Results
Fully oxidized thorium is much more
uniform.
 ThO2 shows definite promise as a
durable reflector in the EUV.
 Rastering is an effective depth
profiling technique
 Variable angle can be used as a
surface technique

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Continued Research


Include modeled interface in calculating
optical constants from reflectance data
Shape of sputtered area may affect
rastering rate: use multilayer thin film
stack to explore shape of sputtered
region
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Acknowledgements
A special thanks to
R. Steven Turley
David Allred
Matt Linford
Yi Lang
BYU Thin Films Group
Physics & Astronomy Department Funding
ORCA Mentoring Grant
NASA Space Grant
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Other Results of Interest
There was an increase in oxygen
when the sample sat for more than 4
or 5 minutes in between
sputtering/scans.
 This was observed for 5 out of 5
samples that sat still between scans.
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* indicates where the sample stood for more than
4 or 5 minutes in between scans
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What Could This Be?
 Hypothesis:
This is likely due to
preferential sputtering.
 The argon ions will knock off
oxygen atoms more readily than
thorium.
 While sputtering, scans would
show less O than actually exists.
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Future Research
 Test
preferential sputtering
hypothesis.
 Investigate other peak anomalies:
N, Ar
 Obtain accurate sputtering rates
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Future Research

Shape of
sputtered area
may affect the
sputtering rate.
Finally:
Make and measure optical
constants for thin films of other elements.
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