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X-Ray Photoelectron X-Ray Photoelectron Spectroscopy to Examine Spectroscopy to Examine
Molecular CompositionMolecular Composition
Amy BakerAmy Baker
R. Steven TurleyR. Steven Turley
Brigham Young UniversityBrigham Young University
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Why Extreme Ultraviolet?Why Extreme Ultraviolet?
Thin Film or Multilayer MirrorsThin Film or Multilayer MirrorsEUV LithographyEUV Lithography
Soft X-Ray MicroscopeSoft X-Ray Microscope Earth’s Magnetosphere in the EUVEarth’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?Why Thorium?
Only one oxidation state: ThOOnly one oxidation state: ThO22
Rock stable: Highest melting Rock stable: Highest melting point (3300 deg C) of any known point (3300 deg C) of any known oxide.oxide.
High Reflectance in the EUV (10-High Reflectance in the EUV (10-100nm)100nm)
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Will Thorium Work?Will Thorium Work? The mirror’s The mirror’s
surface will be surface will be oxidizedoxidized..
At optical At optical wavelengths, this wavelengths, this oxidation is oxidation is negligible. It is a negligible. It is a major issue for our major issue for our thin films, however. thin films, however.
We expect minimal We expect minimal oxidationoxidation
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Learn oxidation state of our thorium Learn oxidation state of our thorium samplessamples
Understand how composition Understand how composition changes with depthchanges with depth
Obtain an expression for oxidation as Obtain an expression for oxidation as a function of deptha function of depth
Purposes of X-Ray Purposes of X-Ray Photoelectron SpectroscopyPhotoelectron Spectroscopy
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X-Ray Photoelectron X-Ray Photoelectron SpectroscopySpectroscopy
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How XPS worksHow XPS works
hvKmax
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Electron Binding EnergyElectron Binding Energy
0
500
1000
1500
2000
2500
3000
3500
4000
4500
02004006008001000
Binding Energy (eV)
Co
un
ts
OTh
Th
C
4d3/2 4d5/2 1s
4f7/2
5d5/21s
Th5d3/2
4f5/2
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Peak ShiftsPeak Shifts
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
3.6K3.4K3.2K
3K2.8K
2.6K2.4K2.2K
2K1.8K1.6K1.4K1.2K
1K800600400200
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
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
Thorium peaks Thorium peaks on surfaceon surface
Thorium peaks Thorium peaks after oxygen is after oxygen is gonegone
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Depth ProfilingDepth Profiling Rastering:Rastering:
Argon ions knock off individual atomsArgon ions knock off individual atoms Variable angle scans:Variable angle scans:
More depth is obtained as x-ray gun More depth is obtained as x-ray gun and detector are moved towards and detector are moved towards incidenceincidence
Analyzer
X-ray Source
e-e-
e-
Sample
θ
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Variable Angle ResultsVariable Angle Results
Only penetrates about 150 Only penetrates about 150 Angstroms into the sampleAngstroms into the sample
This allows us to see surface This allows us to see surface contamination, but not contamination, but not composition with depthcomposition with depth
Results are averaged: cannot Results are averaged: cannot obtain resolved composition with obtain resolved composition with depthdepth
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Rastering ResultsRastering ResultsThorium Composition Sample 040207
-20
0
20
40
60
80
100
120
0 500 1000 1500 2000 2500 3000
Sputtering Time (s)
%
O %
Th %
Si %
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Too Much OxidationToo Much Oxidation
These samples were only a These samples were only a few hours old.few hours old.
We need more uniformity.We need more uniformity. Solution: Make ThOSolution: Make ThO22 mirrors. mirrors.
Reflection is similar to Th and Reflection is similar to Th and it should be more uniform.it should be more uniform.
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ThOThO22 Results Results
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ResultsResults Fully oxidized thorium is much Fully oxidized thorium is much
more uniform.more uniform. ThOThO22 shows definite promise as a shows definite promise as a
durable reflector in the EUV.durable reflector in the EUV. Rastering is an effective depth Rastering is an effective depth
profiling techniqueprofiling technique Variable angle can be used as a Variable angle can be used as a
surface techniquesurface technique
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Continued ResearchContinued Research
Include modeled interface in Include modeled interface in calculating optical constants from calculating optical constants from reflectance datareflectance data
Shape of sputtered area may affect Shape of sputtered area may affect rastering rate: use multilayer thin rastering rate: use multilayer thin film stack to explore shape of film stack to explore shape of sputtered regionsputtered region
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AcknowledgementsAcknowledgementsA special thanks toA special thanks to
R. Steven TurleyR. Steven Turley
David AllredDavid Allred
Matt LinfordMatt Linford
Yi LangYi Lang
BYU Thin Films GroupBYU Thin Films Group
Physics & Astronomy Department FundingPhysics & Astronomy Department Funding
ORCA Mentoring GrantORCA Mentoring Grant
NASA Space GrantNASA Space Grant
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Other Results of InterestOther Results of Interest
There was an increase in oxygen There was an increase in oxygen when the sample sat for more when the sample sat for more than 4 or 5 minutes in between than 4 or 5 minutes in between sputtering/scans.sputtering/scans.
This was observed for 5 out of 5 This was observed for 5 out of 5 samples that sat still between samples that sat still between scans.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?What Could This Be? Hypothesis: This is likely due to Hypothesis: This is likely due to
preferential sputtering.preferential sputtering. The argon ions will knock off The argon ions will knock off
oxygen atoms more readily than oxygen atoms more readily than thorium.thorium.
While sputtering, scans would While sputtering, scans would show less O than actually exists.show less O than actually exists.
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Future ResearchFuture Research
Test preferential sputtering Test preferential sputtering hypothesis.hypothesis.
Investigate other peak Investigate other peak anomalies: N, Aranomalies: N, Ar
Obtain accurate sputtering Obtain accurate sputtering ratesrates
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Future ResearchFuture Research
Shape of Shape of sputtered area sputtered area may affect the may affect the sputtering rate.sputtering rate.
Finally: Make and measure opticalFinally: Make and measure optical
constants for thin films of otherconstants for thin films of other elements.elements.