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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
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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..
MonteMonte--Carlo simulationCarlo simulation
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http://www.small-world.net/efs.htm
MonteMonte--Carlo simulationCarlo simulation
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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
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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
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01.01.2009 15Materials Science Center, HUT
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
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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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01.01.2009 27Materials Science Center, HUT
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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01.01.2009 29Materials Science Center, HUT
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..
SE detectorSE detector
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Side mounted: deep image InSide mounted: deep image In--lens: distinckt surfacelens: distinckt surface
SE detectorSE detector
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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 detectorSE detector
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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01.01.2009 32Materials Science Center, HUT
Photomultiplier
SE detectorSE detector
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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01.01.2009 35Materials Science Center, HUT
The scintillator is aThe scintillator is a
phosphor crystal thatphosphor crystal that
absorbs an electronabsorbs an electron
and generates aand generates a
photon.photon.
SE detector: scintillatorSE detector: scintillator
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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.
SE detector: scintillatorSE detector: scintillator
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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.
SE detector: photomultiplierSE detector: photomultiplier
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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.
SE detector: photomultiplierSE detector: photomultiplier
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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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BSE detectorBSE detector
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
BSE detectorBSE detector
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01.01.2009 47Materials Science Center, HUT
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
BSE detectorBSE detector
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01.01.2009 48Materials Science Center, HUT
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
BSE detectorBSE detector
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BSE detectorBSE detector
01.01.2009 Materials Science Center, HUT 49
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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01.01.2009 52Materials Science Center, HUT
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.
SE detector in Environmental SEMSE detector in Environmental SEM
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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
SE detector in Environmental SEMSE detector in Environmental SEM
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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.
Resolution and excitation regionResolution and excitation region
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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..
Resolution and excitation regionResolution and excitation region
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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
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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.
Resolution and stigmatismResolution and stigmatism
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But can still reduce resolution by overlappingBut can still reduce resolution by overlapping
with adjacent regions.with adjacent regions.
Resolution and stigmatismResolution and stigmatism
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Example 1:Example 1: affect of coatingaffect of coating
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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
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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
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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
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