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X-Ray Photoelectron X-Ray Photoelectron MolecularMolecular

By Amy By Amy BakerBaker

Special Thanks to R. Steven Turley, David Allred, Matt Linford, Yi Lang, R. Steven Turley, David Allred, Matt Linford, Yi Lang, BYU ThinBYU Thin

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Spectroscopy to ExamineSpectroscopy to ExamineCompositionComposition

and Liz Streinand Liz Strein

Films Group,Films Group, Physics & Astronomy Department Funding , ORCA Mentoring GrantPhysics & Astronomy Department Funding , ORCA Mentoring Grant

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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

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Why Thorium?Why Thorium?

High Reflectance in the EUV (10-100nm)High Reflectance in the EUV (10-100nm) Only one oxidation state: ThOOnly one oxidation state: ThO22

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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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Einstein’s Photoelectric Einstein’s Photoelectric EffectEffect

When light shines on a metallic When light shines on a metallic surface, atoms in the metal surface, atoms in the metal absorb quantized packets of light absorb quantized packets of light or or photonsphotons and then eject and then eject electrons.electrons.

hvKmax

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How XPS worksHow XPS works

hvKmax

1010

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

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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 on Thorium peaks on surfacesurface

Thorium peaks Thorium peaks after oxygen is after oxygen is gonegone

Change in peak Change in peak shape due to shape due to oxygen bonding on oxygen bonding on the surfacethe surface

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Depth ProfilingDepth Profiling

Two methodsTwo methods Rastering:Rastering:

Argon ions knock off individual Argon ions knock off individual atomsatoms

Variable angle scans:Variable angle scans: More depth is obtained as x-ray More depth is obtained as x-ray

gun and detector are moved gun and detector are moved towards incidencetowards incidence

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Variable Angle ResultsVariable Angle Results

Only penetrates about 100 Only penetrates about 100 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

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Rastering ResultsRastering ResultsSample Composition

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

Depth (Angstroms)

Ele

men

t % Thorium

Oxygen

Silicon

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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

Fully oxidized thorium is uniform.Fully oxidized thorium is uniform. ThOThO22 shows definite promise as a shows definite promise as a

durable reflector in the EUV.durable reflector in the EUV.

Sample Composition

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100

Depth (Angstroms)

Ele

men

t %

Thorium

Oxygen

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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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Sample Composition

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300

Depth (Angstroms)

Ele

men

t % Thorium

Oxygen

Carbon

* **

*

* 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

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.

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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 Physics & Astronomy Department FundingFunding

ORCA Mentoring GrantORCA Mentoring Grant