11

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

22

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.

33

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)

44

55

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

66

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

77

X-Ray Photoelectron X-Ray Photoelectron SpectroscopySpectroscopy

88

How XPS worksHow XPS works

hvKmax

99

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

1010

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

1111

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

1212

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

1313

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 %

1414

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.

1515

ThOThO22 Results Results

1616

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

1717

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

1818

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

1919

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.

2020

* indicates where the sample stood for more than 4 or 5 minutes in between scans

2121

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.

2222

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

2323

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.