4. Electron-Sample Interaction, Scattering Process - Electron Microscopy and Diffraction

8/9/2019 4. Electron-Sample Interaction, Scattering Process - Electron Microscopy and Diffraction

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Electron MicroscopyElectron Microscopy

and Diffractionand Diffraction

4.4. ElectronElectron-- specimen interaction,specimen interaction,

Scattering process and applicationScattering process and application

Do MinhDo Minh NghiepNghiepMaterials Science CenterMaterials Science Center


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ContentContent

Interaction of primary beam and specimenInteraction of primary beam and specimen

Interaction volume and signals obtainedInteraction volume and signals obtained Scattering processScattering process

Secondary electron image (SEI) and detectorSecondary electron image (SEI) and detector

Backscattered electron image (BSEI) and detectorBackscattered electron image (BSEI) and detector

XX--ray spectraray spectra

Factors influencing resolution: voltage and ZFactors influencing resolution: voltage and Zsamplesample

01.01.2009 2Materials Science Center, HUT


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InteractionInteraction

modemodeand volumeand volume



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The size and shape of theThe size and shape of the

region of primaryregion of primary

excitation can beexcitation can beestimated by carrying outestimated by carrying out

simulationssimulations that usethat use

Monte Carlo calculationsMonte Carlo calculations

and take into account theand take into account the

composition (composition (ZZ), thickness), thickness((dd) of the specimen and) of the specimen and

accelerated voltage (accelerated voltage (VV))

MonteMonte--Carlo simulationCarlo simulation



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An interactionAn interaction

volume can also bevolume can also be

used to predictused to predict thethe

types of signals thattypes of signals that

will be produced andwill be produced and

the depth from whichthe depth from which

they canthey can escape.escape.

MonteMonte CarloCarlo simulationssimulations ofof electronelectron trajectoriestrajectories areare basedbased onon 11)) thethe

energyenergy ofof thethe primaryprimary beambeam electron,electron, 22)) thethe likelihoodlikelihood ofof anan interaction,interaction, 33))

thethe changechange inin directiondirection andand energyenergy ofof thethe electron,electron, 44)) thethe meanmean freefree pathpath

ofof thethe electronelectron andand 55)) aa randomrandom factorfactor forfor anyany givengiven interactioninteraction..




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http://www.small-world.net/efs.htm




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Interaction volumeInteraction volume

Interaction

volume

Incident beamIncident beam

ElectronElectron--samplesample interactioninteraction volumevolume hashas aa pearpear shapeshape (left)(left)..

ActualActual imageimage of of interactioninteraction volumevolume betweenbetween incidentincident beambeam andand

samplesample surfacesurface (right)(right) showingshowing shapeshape andand sizesize ofof primaryprimary excitationexcitation regionregion



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Interaction depth of signalsInteraction depth of signals

1. Auger electron

(0,10,1--2 nm2 nm)

2. Secondaryelectron (10 nm10 nm)

3. Backscattered

electron ( 5 mm)

4. Charact. X-ray

5. Continiium X-ray

6. Fluorescence

Xray ( 1010 mmmm)

Primary Signals:

in t th cpin t tn x ng-cTia rngen

Prim.

beam

Spec.

surface

1. Auger electron

2. Sec. electron

3. Back-scat.

electron

4. Charac.

Xray

5. Xray continium

6.Xray

f

luorescence



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Determination of interaction depthDetermination of interaction depth


Ec = 7 eV; Eo = 20 keV; r = 7 g.cm-3; d = 0.8 mm


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Use of signalsUse of signalsWhen the electron beam strikes a sample, bothWhen the electron beam strikes a sample, both photonphoton andand

electron signals are emitted:signals are emitted:

XrayXray:: samplesample

compositioncomposition(SEM+EDS/WDS)(SEM+EDS/WDS)

Incident electron

Specimen

Auger electronAuger electron::

surfacesurface compositioncomposition

Backscattered electronBackscattered electron::

topographic info and phasetopographic info and phase

(Z) contrast(Z) contrast (SEM)(SEM)

Xray fluorescenceXray fluorescence::

elemental compositionelemental composition

(EPMA)(EPMA)Secondary electronSecondary electron::

surface topographysurface topography

(SEM)(SEM)

Electric currentElectric current::

electrical propertyelectrical property

Transmitted electronTransmitted electron:: microstructure, crystal structure, compositionmicrostructure, crystal structure, composition (TEM+EELS)(TEM+EELS)



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Informations from interactionInformations from interaction

Electron and photon signals give infos aboutElectron and photon signals give infos about- Topography of surface

- Microstructure and /or morphology

- Crystal and/or defect structure

- Chemical composition- Electrical current

- Local magnetic field



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Scattering processScattering process



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ElectronElectronscatteringscattering

ScatteringScattering: change in the: change in the

primary motion directionprimary motion direction

Names:Names:

- without energy loss:

elastic scattering

- with energy loss:

inelastic scattering



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Electron scatteringElectron scattering Elastic (coherent) scattering: electron changes its

trajectory but energy unchanged

- Scattered transmitted electron (diffraction)

- Back-scattered electron Inelastic (incoherent) scattering: electron changes its

trojectory and loses a part of energy for secondary

processes

- Secondary electron

- Auger electron

- Continium and characteristic X-ray radiation

- Cathodoluminiscence

01.01.2009 Materials Science Center, HUT 14


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TheThe probabilityprobability ofof anan

elasticelastic vsvs.. ananinelasticinelastic collisioncollision isis

basedbased primarilyprimarily onon

thethe atomicatomic weightweight ofof

thethe specimenspecimen (Z).

Electron scatteringElectron scattering


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Electron scatteringElectron scattering

A beam electron interacts with the electrical fieldA beam electron interacts with the electrical field

of theof the nucleusnucleus of a specimen atomof a specimen atom

Resulting in:Resulting in:-- a change in the direction of the beam electrona change in the direction of the beam electron

without a significant change in the energywithout a significant change in the energy of theof the

beam electronbeam electron

-- backscattered electrons (>50 eV) and continuousbackscattered electrons (>50 eV) and continuous

xx--rays are formed, electron diffractionrays are formed, electron diffraction

Elastic scattering

A beam electron interacts with theA beam electron interacts with the

electrical field ofelectrical field of electronselectrons of a specimenof a specimen

atomatom Resulting in:Resulting in:

-- a transfer of energya transfer of energy to the specimen atomto the specimen atom

-- secondary electrons (


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How signals are formedHow signals are formed

DeDe--excitation occurs byexcitation occurs by

release of Xrelease of X--ray photonray photonShellelectronreleased

Shellelectronreleased

asanAugerele

ctron

asanAugerele

ctron

HighHigh--energyenergyprimary electronprimary electron

Primary electron inPrimary electron in--elastically scatteredelastically scattered

Secondary electronSecondary electron

A shell electron fallsA shell electron falls

to the vacant shellto the vacant shell

Characteristic XCharacteristic X--rayrayAuger electronAuger electron

A hole is createdA hole is created

in the shellin the shellKL MM



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Secondary electron:< 50 eV

Back-scattered

electron: > 80 % of

primary electron

energy

X-ray: 0.520 keV

SignalSignalenergyenergy

Backscattered

Electron Signal

Inelastically

scattered e-



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Auger electronAuger electron



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Low energy electrons emitted from the upper 2Low energy electrons emitted from the upper 2--3 nm of the surface and3 nm of the surface and

Contains information about the element that produced it based on its energyContains information about the element that produced it based on its energy

Auger electronAuger electron



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An Auger spectrum for Aluminum showing peaksAn Auger spectrum for Aluminum showing peaks

for different electron replacement eventsfor different electron replacement events

Auger electron spectrumAuger electron spectrum



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Secondary electronSecondary electron(SE) image and detector(SE) image and detector



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Secondary electron (SE)Secondary electron (SE) Secondary electrons areSecondary electrons are

usually the result of anusually the result of an

inelastic collision in whichinelastic collision in which

the energy of the primarythe energy of the primarybeam is partly transferredbeam is partly transferred

to an electron that is thento an electron that is then

emitted from the atom asemitted from the atom as

SE.SE.

Secondary electronsSecondary electronstypically have an energytypically have an energy

of 50 eV or less.of 50 eV or less.



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Although secondaryAlthough secondary

electrons are producedelectrons are produced

throughout the interactionthroughout the interaction

region, they can only escaperegion, they can only escapefrom the uppermost portionfrom the uppermost portion

due to their low energy.due to their low energy.

SE gives info of topographySE gives info of topography

SecondarySecondary

electronelectron



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TheThe angleangle atat whichwhich thethe beambeamstrikesstrikes thethe specimenspecimen andand thethe

distancedistance fromfrom thethe surfacesurface areare

importantimportant factorsfactors inin howhow muchmuch

ofof signalsignal escapesescapes fromfrom thethe

specimenspecimen..

Rough area: more signals,

Flat area: fewer signals

Thick sample thin sample

Primary electron Edge effect and

sample thickness

MoreSE

escape

on edges

Primary electron

Fewer SE

escape on

flat areas

Primary electron

Thick sample

Thin sample

SE escape from both sides

SurfaceSurface

topographytopography

contrast by SEcontrast by SE


Fewer SE

escape


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SurfaceSurfacetopographytopography

contrast by SEcontrast by SE

Sometimes one can takeSometimes one can take

advantage of the this effectadvantage of the this effect

and increase useableand increase useable

signal by tilting thesignal by tilting the

specimen towards thespecimen towards thedetector and at an angledetector and at an angle

relative to the primaryrelative to the primary

beam.beam.

Sample

surface



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DetectorsDetectors

SE detector:SE detector:

-- Lateral: side mountedLateral: side mounted

-- Annular: inAnnular: in--lenslens

BSE detector:BSE detector:

-- Solid state detectorSolid state detector


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The position of theThe position of the

secondary electronsecondary electron

detector also affects signaldetector also affects signal

collection and shadow.collection and shadow.

An inAn in--lens detector withinlens detector within

the column is morethe column is more

efficient at collectingefficient at collecting

secondary electrons thatsecondary electrons thatare generated close to theare generated close to the

final lens (i.e. shortfinal lens (i.e. short

working distance).working distance).

SE detectorSE detector



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AnAn inin--lenslens detectordetectordoesdoes notnot

useuse aa faradayfaraday collectorcollector asas

thisthis wouldwould affectaffect thethe primaryprimary

beambeam electrons,electrons, butbut insteadinsteaddependsdepends onon thethe naturalnatural

trajectorytrajectory ofof thethe secondarysecondary

electronselectrons toto strikestrike itit..

ItIt takestakes advantageadvantage ofof thethe

focusingfocusing actionaction ofof thethe lenslens totobringbring thesethese SESE toto crosscross overover

andand thenthen spreadspread outout toto strikestrike

thethe annularannular detectordetector..



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Side mounted: deep image InSide mounted: deep image In--lens: distinckt surfacelens: distinckt surface




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Amplifier

CRTCRT

~ 300V

Photocathode

Dinodes

Photoelectrons SE

4. Photomultiplier tube

3. Light guide pipe

Photons

+10kV

SE

2. ScintillatorPhosphorus crystal

Electrode1. Faraday

cage/grid


SE detector system usually consists of 4 parts: (1) a Faraday cage,

(2) a scintillator, (3) a light guide pipe, (4) a photomultiplier.



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Photomultiplier


Everhart -Thornley detector setup withFaraday cage biased to +300 V to collect SE


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A conventional secondary electron detector is positioned off to the side of theA conventional secondary electron detector is positioned off to the side of the

specimen. A faraday cage (kept at a positive bias) draws in the low energyspecimen. A faraday cage (kept at a positive bias) draws in the low energy

secondary electrons. The electrons are then accelerated towards a scintillatorsecondary electrons. The electrons are then accelerated towards a scintillator

which is kept at a very high bias in order to accelerate them into the phosphorus.which is kept at a very high bias in order to accelerate them into the phosphorus.

SE detector: Faraday cageSE detector: Faraday cage



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TheThe EverhartEverhart--ThornleyThornley detectordetector hashas anan aluminumaluminum coatingcoating (+(+1010--1212

keV)keV) thatthat alsoalso servesserves toto reflectreflect thethe photonsphotons backback downdown thethe lightlight pipepipe..

SE detector: scintillatorSE detector: scintillator

Scintillator



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The scintillator is aThe scintillator is a

phosphor crystal thatphosphor crystal that

absorbs an electronabsorbs an electron

and generates aand generates a

photon.photon.



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The photonsThe photons

produced in theproduced in the

scintillator arescintillator are

carried down acarried down a

fiber optic lightfiber optic light

pipe out of thepipe out of themicroscope.microscope.




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Most of the secondary electron detector lies outside of the SEMMost of the secondary electron detector lies outside of the SEM

chamber and is based on a photomultiplier tube (PMT)chamber and is based on a photomultiplier tube (PMT)

SE detector: photomultiplierSE detector: photomultiplier



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The PMT is converting the incoming photons into electrons which are then drawn toThe PMT is converting the incoming photons into electrons which are then drawn to

dynodes kept at a positive bias. The dynodes are made of material with a low workdynodes kept at a positive bias. The dynodes are made of material with a low work

function and thus give up excess electrons for every electron that strikes them. Thefunction and thus give up excess electrons for every electron that strikes them. The

result multiplies the signal contained in each photon produced by the scintillator.result multiplies the signal contained in each photon produced by the scintillator.




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1.1. The electronic signal fromThe electronic signal from

the PMT is further increasedthe PMT is further increased

by a signal amplifier.by a signal amplifier.

22. Thus an increase in gain. Thus an increase in gain

is accomplished by voltageis accomplished by voltage

applied to the dynodes of theapplied to the dynodes of thePMT and alters the contrast ofPMT and alters the contrast of

the image.the image.

3.3. An increase in the blackAn increase in the black

level is made by increasinglevel is made by increasing

the current in the amplifier andthe current in the amplifier and

alters the brightness of thealters the brightness of theimage.image.

4.4. Signal is thus increased atSignal is thus increased at

the scintillator, PMT, andthe scintillator, PMT, and

amplifier.amplifier.




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BackBack--scatteredscattered

electron (BSE) imageelectron (BSE) imageand detectionand detection



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Backscattered electronsBackscattered electrons

are the result of elasticare the result of elastic

collisions with atoms of thecollisions with atoms of thespecimen.specimen.

They result in emittedThey result in emitted

electrons that have anelectrons that have an

energy of 80 % or more ofenergy of 80 % or more of

the original energy of thethe original energy of the

primary beam electron.primary beam electron.

BackBack--scattered electron (BSE)scattered electron (BSE)



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BackscatteredBackscattered

electrons are alsoelectrons are also

produced throughoutproduced throughout

the interaction regionthe interaction regionbut because of theirbut because of their

greater energy cangreater energy can

escape from deeper inescape from deeper in

the specimen.the specimen.

Backward scattering

about 180 o.

BackBack--scattered electron (BSE)scattered electron (BSE)



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ZZ--contrastcontrastby BSEby BSE Production of backscattered

electrons varies with atomic

number (Z). Higher atomic number

elements appear brighter (or

scatter more effectively) than

lower atomic number

elements.

Resulting image shows

elemental contrast.

SEI

BSEI



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Backscatter electronsBackscatter electrons

have a greater energyhave a greater energy

and can escape fromand can escape from

deeper within thedeeper within the

specimen than canspecimen than can

secondary electrons, butsecondary electrons, but

because they are morebecause they are more

readily produced by highreadily produced by high

atomic weight elementsatomic weight elementsthey can be used tothey can be used to

visualize differences invisualize differences in

elemental composition.elemental composition.

BSE detectorBSE detector



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Repulsed SEs

BSEs

Primary e-

-50 V

Primary e-

BSEs

Backward

SEs

+300V

Convert

target

Objective

SEs fromsample

Primary e-

Grid



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The topography of the specimen will also affect the

amount of backscatter signal and so backscatter

imaging is often carried out on flat polished samples




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Since backscattered electrons have a high energy theySince backscattered electrons have a high energy they

cannot be collected by way of a Faraday cage or other devicecannot be collected by way of a Faraday cage or other device



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The most common design is a four quadrant solid stateThe most common design is a four quadrant solid state

detector that is positioned directly above the specimendetector that is positioned directly above the specimen



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Two image modes:

- COMPO (A+B)

composition image

- TOPO (A-B)topography image


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Blood cells withBlood cells with

nuclei stained withnuclei stained with

a silver compounda silver compoundare visible inare visible in

backscatter mode,backscatter mode,

even though theyeven though they

are beneath theare beneath the

surface of the cellsurface of the cellmembrane.membrane.

SEI

BSEI

Example 1Example 1



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Gold particles onGold particles on E. coliE. coliappear as bright white dots due toappear as bright white dots due to

the higher percentage of backscattered electrons comparedthe higher percentage of backscattered electrons compared

to the low atomic weight elements in the specimen.to the low atomic weight elements in the specimen.

Example 2Example 2



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Backscatter image of nickel in a leafBackscatter image of nickel in a leaf

Example 3Example 3

NiNi


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Backscatter image of a composite (polished cementBackscatter image of a composite (polished cement

fragment) in which low atomic weight particles appearfragment) in which low atomic weight particles appear

dark and high atomic weight particles are white.dark and high atomic weight particles are white.

Example 4Example 4



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BSE Detector: Large-area scintillator detector

is more suitable than E-T detector. Because ofelectric discharge of the last, BSEs lose a bit

of energy in the gas medium and have enough

energy to activate scintillator.

SE Detector: Gas-Amplification Detector (GasPhase Detector)

Electron detectors inElectron detectors in

Environmental SEMEnvironmental SEM



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Environmental SEMEnvironmental SEM



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Environmental

Environmental

SEM

SEM



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SE detector in Environmental SEMSE detector in Environmental SEM



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Environmental electrons are a form of secondary electrons thatEnvironmental electrons are a form of secondary electrons thatare produced via interactions of secondary electrons producedare produced via interactions of secondary electrons produced

by the specimen that strike gas molecules in the chamber, thusby the specimen that strike gas molecules in the chamber, thus

amplifying the signal.amplifying the signal.




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An environmental SEM or ESEM actually requires gas of

some sort (usually water vapor) to create the signal and

can operate at elevated pressures as high as 1 x 10 Torr

Movie of melting sample in ESEM




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10 min.

15 min.0 min.

20 min.

Watching paint dry in ESEMWatching paint dry in ESEM

ExampleExample01.01.2009 60Materials Science Center, HUT


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The ESEM uses a special detector



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XX--ray spectrumray spectrum



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XX--raysrays areare indirectlyindirectly producedproduced

whenwhen anan electronelectron isis displaceddisplaced

throughthrough aa collisioncollision withwith aa

primaryprimary beambeam electronelectron andand isisreplacedreplaced byby anotheranother electronelectron..

TheThe resultantresultant lossloss ofof energyenergy isis

givengiven offoff inin thethe formform ofof anan XX--rayray..

TheThe energyenergy willwill alwaysalways bebe lesslessthanthan thethe energyenergy ofof thethe primaryprimary

beambeam electronelectron..

XX--raysrays



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Because of their highBecause of their high

energyenergy (5-20 keV) XX--raysrays

can escape from verycan escape from very

deep in the specimen.deep in the specimen. X-rays are used for

elemental analysis (EDS,

WDS)

XX--ray spectrumray spectrumEDS


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Resolution and itsResolution and itsinfluencing factorsinfluencing factors



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Resolution in an SEM isResolution in an SEM is

ultimately determined byultimately determined by

the size of the regionthe size of the region

from which signal isfrom which signal isproduced.produced.

Thus for the sameThus for the same

region of excitation theregion of excitation the

resolution from the threeresolution from the three

signals differs andsignals differs and

decreases fromdecreases from

secondary to backscattersecondary to backscatter

and Xand X--rays.rays.

Resolution decreases asResolution decreases as

interaction volume increasesinteraction volume increases



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If the region of excitation remains small then signal will beIf the region of excitation remains small then signal will beproduced from a small region and there will be no overlappingproduced from a small region and there will be no overlapping

from adjacent regions. In this case each individual spot isfrom adjacent regions. In this case each individual spot is

resolved from its neighbors.resolved from its neighbors.

Resolution and excitation regionResolution and excitation region



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Even a slight increase in size of the region of signalEven a slight increase in size of the region of signal

production can result in decreased resolution.production can result in decreased resolution.




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If the beam is scanned in exactly the same positions butIf the beam is scanned in exactly the same positions butthe region of excitation is larger then the regions of signalthe region of excitation is larger then the regions of signal

production will also be larger and overlap with adjacentproduction will also be larger and overlap with adjacent

ones. Such an image would thereforeones. Such an image would therefore not be resolvednot be resolved..




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Factors affecting size ofFactors affecting size of

the interaction region are:the interaction region are:

Diameter of the primaryDiameter of the primary

beambeam ddBB Energy of the primaryEnergy of the primary

beambeam EEBB

Atomic weight of theAtomic weight of thespecimenspecimen ZZsampsamp Coating of specimenCoating of specimen

Interaction volume are affected byInteraction volume are affected by

ddBB, E, EBB and Zand Zsampsamp



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How dHow dBB, E, EBB and Zand Zsampsamp affectaffect

the resolutionthe resolution


Resolution

Resolution

Power,RP

Interaction

volume,Vint

Beam

diameter,dB

Beam

energy,EB

Atomic

weight,Zsamp

Coating

thin, heavy thick, light


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Affect of beam crossoverAffect of beam crossover

FinalFinal primaryprimary beambeam probeprobe sizesize

ddBB fromfrom aa fieldfield emitteremitter isis ((1010--

100100)x)x smallersmaller thanthan thatthat of of aa

conventionalconventional tungstentungsten filamentfilamentoror LaBLaB66 emitteremitter.. ThisThis isis oneone

reasonreason whywhy FESEMsFESEMs havehave thethe

bestbest imageimage resolutionresolution..

FESEMs also tend to remain stable at very low acceleratingFESEMs also tend to remain stable at very low acceleratingvoltages (0.5voltages (0.5 5 keV) resulting in shallow (5 keV) resulting in shallow (nngnng) regions of) regions of

excitation and thus higher image resolution.excitation and thus higher image resolution.



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Overlapping of signal production is also the primary reason why itOverlapping of signal production is also the primary reason why it

is so critical to have the beam of an SEM properly stigmated.is so critical to have the beam of an SEM properly stigmated.

Even if the size of the region is kept small, it is only those regionsEven if the size of the region is kept small, it is only those regions

which are perfectly circular that will produce the best resolution.which are perfectly circular that will produce the best resolution.

Resolution and stigmatismResolution and stigmatism



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Astigmatic regions may not reduce imageAstigmatic regions may not reduce image

resolution in one dimension.resolution in one dimension.




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But can still reduce resolution by overlappingBut can still reduce resolution by overlapping

with adjacent regions.with adjacent regions.




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Example 1:Example 1: affect of coatingaffect of coating


GoldGold

ChromiumChromium

Mycoplasma pneumonia sputtered by

Au gives better image than by Cr


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Affect ofAffect of

acceleratedaccelerated

voltagevoltageand atomicand atomic

weightweight

77

Z RP

E RP

Increasing Z

In

creasing

E

01.01.2009 Materials Science Center, HUT


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The relationship of accelerating voltage (EThe relationship of accelerating voltage (Eoo) to atomic weight) to atomic weight

(Z) of the specimen and its affect on the depth of penetration(Z) of the specimen and its affect on the depth of penetration

is in reverse order.is in reverse order.

Affect of accelerated voltageAffect of accelerated voltage

and atomic weightand atomic weight



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At higher voltage image is brighterdue to more signal

Example 2:Example 2:

affect of accelerating voltageaffect of accelerating voltage


3,0 keV3,0 keV 20,0 keV20,0 keV


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ButBut reducedreduced resolutionresolution because ofbecause of

larger interaction volume caused.larger interaction volume caused.

Example 3:Example 3:

affect of accelerating voltageaffect of accelerating voltage

3,0 keV3,0 keV 20,0 keV20,0 keV


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ResolutionResolution -- cathode materialcathode material --

voltage relationshipvoltage relationship


4. Electron-Sample Interaction, Scattering Process - Electron Microscopy and Diffraction

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Transcript of 4. Electron-Sample Interaction, Scattering Process - Electron Microscopy and Diffraction