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Development of electron microscope

1897 : Thompson describes the existence of negativelycharged particles (electrons)

1925: De Broglie theorised that electrons have wave-likecharacteristics, addressing the wave/particle duality

1927: Thompson and reid demonstrated the wave natureof electrons by diffraction expeiments

1931: Ruska et. Al. build the first electron microscope(Nobel prize in 1986)

1949: Philips commercialises the EM100….

~ 80 years

Near-field scanning optical microscopy

Scanning Probe Microscopy

Atomic force microscopy

scanning tunnelingmicroscope

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OPTICAL OPTICAL

MICRSOCOPEMICRSOCOPE

SEMSEM

TEMTEM

OMOM

TEMTEM

SEMSEM Diffraction pattern

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SCANNING ELECTRON MICROSCOPE (SEM)SCANNING ELECTRON MICROSCOPE (SEM)

The basic steps involved in all Electron Microscopy are:

�� AA streamstream ofof electronselectrons isis formedformed byby thethe electronelectron sourcesource ((electricelectric gungun))

andand acceleratedaccelerated towardtoward thethe specimenspecimen usingusing aa positivepositive potentialpotential..

�� ThisThis streamstream ofof electronelectron isis focusedfocused usingusing aperturesapertures andand magneticmagnetic

lenseslenses intointo aa thin,thin, focused,focused, monochromaticmonochromatic beambeam

�� ThisThis beambeam isis focusedfocused ontoonto thethe specimenspecimen usingusing aa magneticmagnetic lenslens

�� InteractionsInteractions occuroccur insideinside thethe specimenspecimen,, affectingaffecting thethe electronelectron beambeam..

TheseThese interactionsinteractions areare detecteddetected andand transformedtransformed intointo anan imageimage

SEM componentsSEM components

1.1. Microscope column:Microscope column:

�� Electron gun (source of electrons)Electron gun (source of electrons)

�� Electromagnetic lensesElectromagnetic lenses

�� AperturesApertures

�� Scan coils (used to adjust beam position)Scan coils (used to adjust beam position)

2. Specimen stage (chamber)Specimen stage (chamber)

3.3. Vacuum pumping systemVacuum pumping system (keep clean, filament oxidation, air will scatter e(keep clean, filament oxidation, air will scatter e--s)s)

4.4. Secondary electronSecondary electron and and backscattered electronbackscattered electron detectorsdetectors (receive signals used (receive signals used in imaging)in imaging)

5.5. XX--ray detectorray detector (EDX or EDS) (EDX or EDS) –– used to detect xused to detect x--rays for chemical analysisrays for chemical analysis

6.6. Electronic control and imaging systemElectronic control and imaging system: to control the SEM, including focusing, : to control the SEM, including focusing, magnification and imagingmagnification and imaging

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Material Analysis

Clay Nylon Gold wire

Fiber Copper interconnect WoodDiamond powder

SEM PRINCIPLESSEM PRINCIPLES

•• The SEM The SEM uses electronsuses electrons instead of light to form an image.instead of light to form an image.

•• A beam of electrons is produced by the A beam of electrons is produced by the electron gunelectron gun

•• An anode attracts the electrons and directs them towards the sampleAn anode attracts the electrons and directs them towards the sample

•• A series of electromagnetic lenses focuses the beam A series of electromagnetic lenses focuses the beam

�� One type of lens is the One type of lens is the condenser lenscondenser lens, which demagnifies the , which demagnifies the

beam to decrease the divergence angle.beam to decrease the divergence angle.

�� A second type of lens is the A second type of lens is the objective lensobjective lens, which further narrows , which further narrows

the beam to focus it on the sample.the beam to focus it on the sample.

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• Once the beam hits the sample, several electrons are ejected from the

sample. These include:

�� Secondary Electrons (SE)Secondary Electrons (SE)

�� BackBack--Scattered Electrons (BSE)Scattered Electrons (BSE)

�� XX--raysrays

�� Auger ElectronsAuger Electrons

�� Cathodoluminescence (CL)Cathodoluminescence (CL)

• Detectors collect these secondary and back-scattered electrons and

convert them into a signal that is sent to a viewing screen similar to a

TV, thus producing an image

1.1. TYPES OF ELECTRON GUNS (electron source)TYPES OF ELECTRON GUNS (electron source)

�� Tungsten hairpin Filament (most commonly used)Tungsten hairpin Filament (most commonly used)

�� Lanthanum Hexaboride (LaBLanthanum Hexaboride (LaB66))

�� Field Emission (FE) (W or C cathode)Field Emission (FE) (W or C cathode)

•• ElectronsElectrons areare producedproduced fromfrom thethe filamentfilament byby thermionicthermionic emissionemission..

•• CurrentCurrent isis passedpassed throughthrough thethe filamentfilament inin orderorder toto heatheat itit sufficientlysufficiently toto aa

TT ==27002700 KK..

•• HeatingHeating isis accomplishedaccomplished byby runningrunning aa 33--44 ampamp currentcurrent throughthrough thethe

filamentfilament..

•• ThereThere isis aa largelarge spreadspread inin electronselectrons fromfrom thethe filamentfilament tiptip..

•• TheThe filamentfilament isis aa atat largelarge negativenegative potentialpotential

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Tungsten hair pin gun

•• TheThe electronselectrons areare initiallyinitially focusedfocused byby thethe WehneltWehnelt gridgrid cylindercylinder “cap”“cap”

whichwhich isis biasedbiased negativenegative atat --200200 toto --300300 VV withwith respectrespect toto thethe filamentfilament..

••

•• TheThe WehneltWehnelt reducesreduces thethe spacespace chargecharge (suppresses(suppresses emissionemission fromfrom thethe

filamentfilament exceptexcept atat itsits tips)tips)..

•• TheThe electronselectrons areare acceleratedaccelerated towardstowards thethe anodeanode ((1010--100100 µµmm inin

diameter)diameter)

•• TheThe electronselectrons convergeconverge atat aa pointpoint betweenbetween thethe WehneltWehnelt capcap andand anodeanode..

ThisThis pointpoint isis calledcalled thethe “cross“cross--over”over” andand isis thethe locationlocation ofof thethe effectiveeffective

electronelectron sourcesource

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•• The anode has a hole in its centre and is biased from 1 to 50 keV with The anode has a hole in its centre and is biased from 1 to 50 keV with

respect to the filamentrespect to the filament--WehneltWehnelt

•• This bias accelerates the electrons and forms the beam. This bias accelerates the electrons and forms the beam.

•• The electron flux is minimal until the temperature of the filament is The electron flux is minimal until the temperature of the filament is

approximately 2500 K.approximately 2500 K.

•• Above this temperature it is predicted that the electron flux will increase Above this temperature it is predicted that the electron flux will increase

with increasing temperature, until the filament melts.with increasing temperature, until the filament melts.

•• In practice, however, the electron emission reaches a plateau termed In practice, however, the electron emission reaches a plateau termed

“saturation”, due to the self“saturation”, due to the self--biasing effects of the Wehnelt cap. biasing effects of the Wehnelt cap.

Oxidation

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ELECTRON BEAM LOCATIONELECTRON BEAM LOCATION

•• Filament position is very importantFilament position is very important

•• Correct position optimises the electron flow Correct position optimises the electron flow

�� Less heat requiredLess heat required

�� Extended filament lifeExtended filament life

FILAMENT LIFEFILAMENT LIFE

•• Maintenance of high vacuumMaintenance of high vacuum

•• Cleanliness of the electron gunCleanliness of the electron gun

•• Filament currentFilament current

ACCELERATING VOLTAGEACCELERATING VOLTAGE

•• Voltage varies between 2 Voltage varies between 2 -- 30 KV30 KV

•• Electrons have energy of the Electrons have energy of the

accelerating voltageaccelerating voltage

•• at 25 KV, the energy of the at 25 KV, the energy of the

electrons = 25 eV. electrons = 25 eV.

2.2. ELECTROMAGNETIC LENSESELECTROMAGNETIC LENSES

• Electrons are negatively charged particles

� Path can be changed by a magnetic field

• Electron microscope lenses are essentially a coil of wire around a metal

cylinder, called solenoid.

• The electron path diversion depends on:

� magnetic field

� velocity

� direction

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2.1.2.1. CONDENSER LENSESCONDENSER LENSES

•• TheThe electronelectron beambeam isis divergentdivergent afterafter

passingpassing throughthrough thethe anodeanode andand mustmust bebe

collimatedcollimated byby thethe condensercondenser lens(es)lens(es) andand

aperturesapertures intointo aa relativelyrelatively parallelparallel beambeam..

•• TheThe electronelectron beambeam convergesconverges andand passespasses

throughthrough aa focalfocal pointpoint (which(which movesmoves upup andand

downdown withwith thethe strengthstrength ofof thethe magneticmagnetic

field)field)

•• TheThe condensercondenser lenslens controlscontrols thethe amountamount ofof

currentcurrent thatthat passespasses downdown thethe restrest ofof thethe

microscopemicroscope columncolumn (controls(controls thethe intensityintensity

oror brightnessbrightness ofof thethe electronelectron beambeamElectromagnetic lensElectromagnetic lens

2.2. OBJECTIVE LENSES2.2. OBJECTIVE LENSES

•• TheThe objectiveobjective lenslens isis locatedlocated atat thethe basebase ofof thethe columncolumn justjust aboveabove thethe

samplesample..

•• ThisThis lenslens focusesfocuses thethe electronelectron beambeam ontoonto thethe samplesample andand controlscontrols itsits

finalfinal sizesize andand positionposition..

33.. SCANNINGSCANNING COILSCOILS

•• TheThe scanningscanning coilscoils areare housedhoused insideinside thethe objectiveobjective lenslens..

•• TheyThey causecause thethe beambeam toto scanscan overover thethe specimenspecimen surfacesurface inin anan

adjustableadjustable wayway..

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4. ASTIGMATISM4. ASTIGMATISM

•• Astigmatism enlarges the effective beam size (spot size)Astigmatism enlarges the effective beam size (spot size)

•• The spot size may not be clear due to:The spot size may not be clear due to:

�� aberrationsaberrations

�� aperture contaminationaperture contamination

�� column contaminationcolumn contamination

•• StigmatorStigmator coilscoils housedhoused withinwithin thethe scanningscanning coilscoils consistconsist ofof aa setset ofofelectromagneticelectromagnetic lenseslenses thatthat controlcontrol thethe finalfinal beambeam shapeshape byby applyingapplying aamagneticmagnetic fieldfield toto makemake thethe beambeam circularcircular..

•• TheyThey alsoalso correctcorrect forfor anyany dirtydirty columncolumn oror aperturesapertures

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44.. APERTURESAPERTURES

•• AperturesApertures areare usedused toto

�� reducereduce oror excludeexclude extraneousextraneous

electronselectrons

�� reducereduce aberrationsaberrations

SPECIMENSPECIMEN CHAMBERCHAMBER

•• HousesHouses thethe specimenspecimen andand thethe variousvarious

detectorsdetectors (SE,(SE, BSE,BSE, EDX,EDX, ……))

SPECIMENSPECIMEN STAGESTAGE

•• ThereThere areare fivefive typestypes ofof motionmotion (TXYZR)(TXYZR)

�� Tilt,Tilt, X,X, Y,Y, Z,Z, && RotationRotation

Large SEM specimen chamber

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MAGNIFICATIONMAGNIFICATION

•• The mechanism by which the image is magnified is very simple and The mechanism by which the image is magnified is very simple and does not involve any lensesdoes not involve any lenses

•• The CRT area is fixed, and only the area scanned varies.The CRT area is fixed, and only the area scanned varies.

•• The magnification depends on the The magnification depends on the

�� Spot (beam) sizeSpot (beam) size

�� Area scannedArea scanned

•• The magnification The magnification M = the side length of the CRT / side length of the M = the side length of the CRT / side length of the scanned area. scanned area.

•• Magnification is proportional to the applied voltage (M Magnification is proportional to the applied voltage (M ↑↑ with with ↑↑ V)V)

•• Magnification is inversely proportional to the working distance (WD) (M Magnification is inversely proportional to the working distance (WD) (M ∝∝ 1/ WD)1/ WD)

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Electron Beam Electron Beam –– Specimen InteractionSpecimen Interaction

•• As the beam strikes each point on the specimen, various responses are As the beam strikes each point on the specimen, various responses are

produced due to produced due to specimen specimen -- electron interactionselectron interactions. .

•• In almost all types of electron microscopes primary electrons enter the In almost all types of electron microscopes primary electrons enter the

specimen and the same or different electrons leave it again to form the specimen and the same or different electrons leave it again to form the

image.image.

•• Several of these responses can be collected by suitable detectors, Several of these responses can be collected by suitable detectors,

amplified and displayed in the CRT to form an image (or for chemical amplified and displayed in the CRT to form an image (or for chemical

analysis)analysis)

Incident Beam

SpecimenSpecimen

X-rays

• Through thickness

• Composition info

Auger electrons• Surface sensitive

• Composition information

Primary backscattered

electrons (BSE)

• Atomic number

• Topographical/ Imaging

Cathodoluminescence

Electrical information

Secondary electrons (SE)

Topographical/Imaging

Specimen current• Electrical current

The types of signals producedThe types of signals produced

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Interaction VolumeInteraction Volume

•• Electron penetration volume and depth Electron penetration volume and depth

depend on:depend on:

–– Electron beam energyElectron beam energy

–– More energy, deeper penetration and More energy, deeper penetration and

larger interaction volumelarger interaction volume

–– Atomic number (Z) of specimenAtomic number (Z) of specimen

–– Higher Z, lower penetration and Higher Z, lower penetration and

volumevolume

Effect of accelerating voltage on size Effect of accelerating voltage on size

of interaction volume specimenof interaction volume specimen

Voltage decreases

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Effect of atomic number on size of Effect of atomic number on size of

interaction volume specimeninteraction volume specimen

Z increases

High voltage

Low voltage

LOW ATOMIC NUMBER HIGH ATOMIC NUMBER

e-

e-e-

e-

e-

X-ray X-ray

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Types of Electron ScatteringTypes of Electron Scattering

•• There are two types of electron scattering when the electron beam There are two types of electron scattering when the electron beam

strikes the specimen:strikes the specimen:

1.1. Elastic ScatteringElastic Scattering

�� Electrons Electrons retain retain all of their energy all of their energy -- backscattered electrons (BSE)backscattered electrons (BSE)

2.2. Inelastic ScatteringInelastic Scattering

�� Electrons Electrons loselose energy by interacting with the specimen energy by interacting with the specimen –– Secondary Secondary

electrons (SE) and Xelectrons (SE) and X--raysrays

SECONDARY ELECTRONS (SE)SECONDARY ELECTRONS (SE)

�� SESE havehave veryvery lowlow energyenergy (<(< 5050 eV)eV)comparedcompared toto 3030 KVKV ofof thethe electronelectronbeambeam

�� TheyThey areare traveltravel veryvery slowlyslowly asas theytheyleaveleave thethe specimenspecimen

�� TheThe SESE areare createdcreated throughoutthroughout thetheprimaryprimary excitationexcitation volume,volume, butbut ifif theytheyareare nearnear thethe surface,surface, theythey cancanescapeescape

�� SinceSince theythey havehave lowlow energy,energy, deeperdeeperonesones areare easilyeasily absorbedabsorbed (other(otherinteractions)interactions)

SE DetectorSE Detector

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SECONDARY ELECTRONS DETECTIONSECONDARY ELECTRONS DETECTION

•• TheThe SESE areare detecteddetected byby aa detectordetector

knownknown asas thethe EverhartEverhart--ThornleyThornley

detectordetector ((19601960))..

•• ScintillatorScintillator

�� WhenWhen thethe ee-- strikestrike thethe scintillator,scintillator, itit

producesproduces lightlight

•• LightLight guideguide

�� TransportTransport lightlight toto PMTPMT

•• PhotomultiplierPhotomultiplier (PMT)(PMT)

�� AmplifyAmplify lightlight andand

�� ConvertConvert itit toto electricalelectrical signalsignal

Number of SE that escape from the sample Number of SE that escape from the sample

depends on the sample surfacedepends on the sample surface

Sample surface

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BACKSCATTERED ELECTRON (BSE) BACKSCATTERED ELECTRON (BSE)

DETECTIONDETECTION

�� The BSE have energies > 50 eV The BSE have energies > 50 eV

�� They travel in straight linesThey travel in straight lines

�� They have weaker signal than SEThey have weaker signal than SE

�� They are detected by using purpose They are detected by using purpose

built backscattered electron built backscattered electron

detectors. detectors.

BSE DetectorBSE Detector

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EFFECT OF MICROSCOPE VARIABLESEFFECT OF MICROSCOPE VARIABLES

Polymer Metal

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EFFECT OF BEAM (SPOT) SIZE ON EFFECT OF BEAM (SPOT) SIZE ON IMAGEIMAGE QUALITYQUALITY

Better ResolutionBetter ResolutionPoorer ResolutionPoorer Resolution

Bigger spot (Beam) size Bigger spot (Beam) size

Smaller spot (Beam) size Smaller spot (Beam) size

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Longer WD: poor resolution Longer WD: poor resolution

Shorter WD: better resolutionShorter WD: better resolution

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�� Higher voltageHigher voltage will provide will provide better better

resolutionresolution (since high voltage generates (since high voltage generates

shorter wavelength of electrons)shorter wavelength of electrons)

�� However, higher voltage also causes an However, higher voltage also causes an

increase in the interaction volumeincrease in the interaction volume: this : this

will will reduce resolutionreduce resolution

Voltage and Resolution

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�� Smaller electron beam angle Smaller electron beam angle increases increases the the

interaction volumeinteraction volume: thus : thus reducing reducing

resolutionresolution

Electron Beam Angle and Resolution

DEPTH OF FIELD (DOF)DEPTH OF FIELD (DOF)

�� The DOF is increased:The DOF is increased:

1.1. By reducing the final lens apertureBy reducing the final lens aperture

2.2. By increasing the working distance (lower the specimen: increase By increasing the working distance (lower the specimen: increase

the distance between the final lens and the specimen)the distance between the final lens and the specimen)

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Increasing the DOF by reducing the Increasing the DOF by reducing the final lens aperturefinal lens aperture

Increasing the DOF by increasing the WDIncreasing the DOF by increasing the WD

Aperture size vs. Resolution and DOFAperture size vs. Resolution and DOF

WD vs. Resolution and DOFWD vs. Resolution and DOF

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Filament Filament -- DOFDOF Filament Filament -- ResolutionResolution

VARIABLE PRESSURE DETECTOR VARIABLE PRESSURE DETECTOR (VPSE)(VPSE)

�� VariableVariable pressurepressure technologytechnologyenablesenables investigatinginvestigating nonnon--conductingconductingspecimensspecimens withoutwithout priorprior preparationpreparation

VPSE DetectorVPSE Detector

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NonNon--conducting material: conducting material:

charging effect, difficult to imagecharging effect, difficult to image

VariableVariable PressurePressure TechnologyTechnology::

TheThe VPVP systemsystem operatesoperates toto

neutraliseneutralise thethe surfacesurface byby emittingemitting

aa gasgas intointo thethe samplesample chamberchamber

SEM with VPSE detectorSEM with VPSE detector

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SPECIMEN PREPARATION FOR SEMSPECIMEN PREPARATION FOR SEM

•• Image quality can be limited by specimen preparationImage quality can be limited by specimen preparation

•• There are 3 requirements for preparing specimens for SEMThere are 3 requirements for preparing specimens for SEM

1.1. Remove all water, solvents, or other materials that could evaporate Remove all water, solvents, or other materials that could evaporate while in the vacuumwhile in the vacuum

2.2. Firmly mount the specimenFirmly mount the specimen

3.3. NonNon--metallic samples (nonmetallic samples (non--conductive) should be coated so they conductive) should be coated so they are electrically conductive. Metallic samples can be directly placed are electrically conductive. Metallic samples can be directly placed into the SEM. (noninto the SEM. (non--conducting materials will lead to charging: conducting materials will lead to charging: deflection of SE, beam deflection and SE burstsdeflection of SE, beam deflection and SE bursts))

Note: conditions 1 & 3 are valid for conventional SEM’s onlyNote: conditions 1 & 3 are valid for conventional SEM’s only

Gold Sputtering MachineGold Sputtering Machine