X-Ray Photoelectron Spectroscopy to Examine

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X-Ray Photoelectron Spectroscopy to Examine
X-Ray Photoelectron
Spectroscopy to Examine
Molecular Composition
Amy Baker
R. Steven Turley
Brigham Young University
1
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.
2
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)

3
4
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
5
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
6
X-Ray Photoelectron Spectroscopy
7
How XPS works
K max  hv  
8
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)
9
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
10
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)
13
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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