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Apri l, 2005

Microscopic Composition Measurement at NanoscaleMicroscopic Composition Measurement at Nanoscale

DSSC Seminar

Lin Wang

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OutlineOutline

General Introduction

Energy Dispersive Spectroscopy (EDS)

Wavelength Dispersive Spectroscopy (WDS) Electron Energy Loss Spectroscopy (EELS)

Auger Electron Spectroscopy (AES)

X-Ray Photoelectron Spectroscopy (XPS) Summary

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General IntroductionGeneral Introduction

When High Energy Electron Beam Meets MaterialWhen High Energy Electron Beam Meets Material

By Analyzing Emitted X-RayEnergy Dispersive Spectroscopy (EDS) + SEM/ TEMWavelength Dispersive Spectroscopy (WDS) + SEM

By Analyzing Inelastically Scattered ElectronsElectron Energy Loss Spectroscopy (EELS) +TEM

By Analyzing Emitted Auger ElectronsAuger Electron Spectroscopy (AES)

When XWhen X--Ray Meets MaterialRay Meets Material By Analyzing Emitted Photoelectrons

X-Ray Photoelectron Spectroscopy (XPS)

AnalyticalElectronMicroscope(AEM)

SurfaceCharacterization

Take a thin TransmittingSpecimen as an example

{AEM with EDS}

(AEM with EELS)

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Principles of EDSPrinciples of EDS

EnergyEnergy DispersiveDispersive (X(X--Ray) Spectroscopy (EDS)Ray) Spectroscopy (EDS)

X-Ray counting is done by measuring the x-ray photon energies with aSi(Ni)solid state detector

Different characteristic X-Ray lines of elements represent the typesand relative amounts of elements in the sample

The number of counts of each peak may be converted to weightconcentration using standard (more accurate) or standardlesscalculations

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Energy Resolution (Peak Broadening) measured by FWHM (Full Width

at Half Maximum of the peak)

Peak overlap

Principles of EDS (cont.)Principles of EDS (cont.)

With TEMWith SEM

Gaussian Distribution

Y=AAexp{-1/2*[(EA-E)/ ]2}

FWHM=2.355

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Application Examples of EDSApplication Examples of EDS

EDS Line Profile Software automationallows simplified composition profiling atnanometer resolution using EDS.

Sample: Si80Ge20 islands grown on Si at800C followed by Si capping.

EDS Mapping - EDS Mapping allowsvisualization of the phase separationprocess, which can be coupled with point-by-point quantitative analysis.

Sample: Cu50Ag50 ball milled at 230 C

PointPoint

AnalysisAnalysis11--D ProfilingD Profiling 22--D MappingD Mapping

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Principles of WDSPrinciples of WDS

WavelengthWavelength DispersiveDispersive (X(X--Ray) Spectroscopy (WDS)Ray) Spectroscopy (WDS)

With SEM

X-Ray counting is done by using a Bragg

reflector to wavelength-filter the x-rays on their

way to the detector

Braggs Law

N

=2dsin

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Capacities of WDSCapacities of WDS

Advantages

Superior spectral resolution (X10 times better than EDS)

Can detect light element Z 4

Can detects 0.1%-several ppm More accurate

Disadvantages

The equipment is more expensive

More Time consuming

More difficult to use

Usage Identification of spectrally overlapped elements such as W or Ta in Si,

or N in Ti Detect of low concentration species (down to 100 or even 10ppm)

such as P or S in metals Analysis of Low atomic number elements such as oxidation in metals

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Comparison between EDS&WDSComparison between EDS&WDS

EDS: Normally detects Z 10(below

Na) thus typically cant be

detected O, N, C. Sometimes ifusing windowless or thin

window it can detect Z 4

Parallel technique

Can quickly scan for a widerang of possible elements

Good for use both with SEM orTEM specimen

Time of a typical run take a few

minutes

WDS: Detects Z 4

Serial technique

slow but can provide excellentresolution

Requires very high x-raygeneration rates thus TEM

samples cant provide highcount (x-ray generation) rate

Time of a typical run take hours

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Principles of EELSPrinciples of EELS

Electron Energy Loss Spectroscopy (EELS)Electron Energy Loss Spectroscopy (EELS)

With TEM

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Principles of EELS (continued)Principles of EELS (continued)

Electron Energy Loss Spectroscopy (EELS)Electron Energy Loss Spectroscopy (EELS)

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Capacities of EELSCapacities of EELS

Usage

Light element spectroscopy for concentration, electronic and chemicalstructure analysis at ultrahigh lateral resolution

Sample Requirements

Solids and specimens must be transparent to electrons with about 10-200nm thickness;

sample size: 3mm diameter thin foil

Not destructive to sample

Limitation

Range of elements: Z=3-92

Lateral resolution: 1nm-10m, depending on the diameter of the incidentelectron probe size and the thickness of the specimen

Sampling depth: with thickness of specimen 10-200nm

Detection limits:10-21g

Accuracy: with standards 1-2 at.%, without standards 10-20 at.%

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Application Examples of EELSApplication Examples of EELS

EELS Mapping - The breakdown of the Li/Ni ordering at the surfacecan be seen as changes in the EELS fine structure and Ni-to-O ratio.

Sample: LiNi0.80Co0.20O2 from Li-Ion battery after aging

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Principles of AESPrinciples of AES

Auger Electron Spectroscopy (AES)Auger Electron Spectroscopy (AES)

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Capacities of AESCapacities of AES

Sample Requirements

vacuum compatible materials

no destruction except to electron beam sensitive materials and duringdepth profiling

Vacuum Requirements: 10-10 torr

Limitation

Range of elements: All except H and He

Lateral resolution: 10-30nm for Auger analysis and even less for imaging

Sampling depth: 0.5-10nm

Detection limits: 0.1-1at.% Accuracy: 30% if using published elemental sensitivity

10% if using standards that closely resemble the sample

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Applications of AESApplications of AES

Composition analysis of the 0-3nm region near the surface for allelements except H and He

Scanning Auger Microscopy

Depth-composition profiling and thin film analysis High lateral resolution surface chemical analysis and

inhomogeneity studies to determine compositional variations

Surface diffusion and segregation

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Principles of XPSPrinciples of XPS

XX--Ray Photoelectron Spectroscopy (XPS)Ray Photoelectron Spectroscopy (XPS)

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Capacities of XPSCapacities of XPS

Sample Requirements

vacuum compatible materials, flat samples preferred

no destruction except to X-ray sensitive materials and during

depth profiling

Limitation

Range of elements: All except H and He Lateral resolution: 5mm-75m

Sampling depth:0.5-5nm

Detection limits: 0.01-0.3at.%

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Applications of XPSApplications of XPS

Routinely used in industry and research whenever elemental orchemical state analysis is needed at surfaces and interfaces as well asspatial resolution requirements are not demanding (typically greaterthan 150m).

Eg.s:

examination for and identification of surface contaminations

Evaluation of materials processing steps (cleaning, plasma etching, thermal

oxidation, silicide thin-film formation etc.) Evaluation of thin-film coating or lubricants

Failure analysis for adhesion between components(air oxidation , corrosionetc.)

Tribological (or wear) activity

Effectiveness of surface treatment of polymers or plastics

Surface composition differences for alloys

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SummarySummary

All of the five methods can give both microscopic imaging functionand qualitative plus quantitative composition analysis function

Quantitative composition measurement will be more accurate withstandards esp. standards in similar composition range

For EDS, the major limitation is the peak broadening and light elementincapability

For WDS, comparing with EDS, it has high spectral resolution but ittakes longer for each run

EELS can detect light element and has a high lateral resolution

AES and XPS are surface composition detection techniques but AEShas a nm-scale lateral resolution which is much better than XPS

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ReferenceReference

J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L,Sawyer, J. Michael, Scanning Electron Microscopy and X-RayMicroanalysis, Kluwer Academic / Plenum Publishers (2003)

C. Brundle, C. Evans, S. Wilson, L. Fitzpatrick, Encyclopedia ofMaterials Characterization, Butterworth-Heinemann (1992)

D. Williams, C. Carter, Transmission Electron Microscopy, Pledum(1996)

D. J. OConnor, B. A. Sexton, R.St. C. Smart, Surface Analysis Methods

in Materials Science, Springer (2003) http://cmm.mrl.uiuc.edu/Gallery/STEM/STEMGallery.htm