MS482MaterialsCharacterization(재료분석)

LectureNote9:SEM

Byungha ShinDept.ofMSE,KAIST

1

2016FallSemester

CourseInformationSyllabus1. Overviewofvariouscharacterizationtechniques (1lecture)2. Chemicalanalysistechniques (8lectures)

2.1. X-rayPhotoelectronSpectroscopy(XPS)2.2. UltravioletPhotoelectronSpectroscopy(UPS)2.3. AugerElectronSpectroscopy(AES)2.4. X-rayFluorescence(XRF)

3. Ionbeambasedtechniques (4lecture)3.1. RutherfordBackscatteringSpectrometry(RBS)3.2. SecondaryIonMassSpectrometry(SIMS)

4. Diffractionandimagingtechniques (7lectures)4.1. Basicdiffractiontheory4.2. X-rayDiffraction(XRD)&X-rayReflectometry(XRR)4.3. ScanningElectronMicroscopy(SEM)&

EnergyDispersiveX-raySpectroscopy(EDS)4.4. TransmissionElectronMicroscopy(TEM)

5. Scanningprobetechniques (1lecture)5.1. ScanningTunnelingMicroscopy(STM)5.2. AtomicForceMicroscopy(AFM)

6. Summary:Examplesofrealmaterialscharacterization (1lecture)*CharacterizationtechniquesinblueareavailableatKARA(KAISTanalysiscenterlocatedinW8-1)

ScanningElectronMicroscopy(SEM)

SEM is an excellent technique for obtaining high resolution images.

~

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TypicalInstrumentViewManyOptionsareBuiltAroundaCommonCore

ElectronGun

CondenserLens

ObjectiveLens

Sample

CathodeRayTube

SecondaryElectronDetector

Amplifier

• Whenresolutionbeyondanopticalmicroscopeisneeded

• Firstlookatanewproblemtoseeifitisadeposit,particles,pits,etc.

• Criticaldimensionmeasurementsofsmallfeatures

• AlternativetoTEMwhenconsideringtimeandmoney

KeyApplications

SourceofImagedElectrons

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Incident electron beamSourceofAugerelectronsignal

Sourceofsecondaryelectron(looselyboundoutershellelectrons)signal

SourceofBackscatteredelectrons

Sourceofelectron-excitedcharacteristicX-rays

SourceofBremsstrahlung

Sourceofsecondaryfluorescence

SourceofImagedElectrons

• Secondaryelectronmode:lowKEofSEà shallowescape(sampling)depth;intensitymainlydeterminedbysurfacetopography

• Backscatteredmode:surfacetopography+largecontributionfromZcontrast (brighterasZincreases);largersamplingdepth(~X100ofSE)

Secondaryelectron(1– 50eV)

Backscatteredelectrons

ElectronBeam-SpecimenInteractionMonteCarlosimulationoftrajectoriesof20keV electronbeamstrikingSi • Elasticscattering:

- electron-nucleusinteraction- cross-section,Q(cm-2)

𝑄(> 𝜙E) = 1.62×10LME𝒁𝟐

𝑬𝟐 cotM𝜙E2

• Inelasticscattering:- electron-electroninteraction𝑑𝐸𝑑𝑠

keVcm = −7.85×10Z

𝒁𝜌𝐴𝑬𝒊

ln1.166𝐸^

𝐽

𝐽(keV) = (9.76𝑍 + 58.5𝑍LE.cd)×10Le

ElectronBeam-SpecimenInteractionMonteCarlosimulationoftrajectoriesofenergeticelectronbeam

10keV 20keV 30keV

• Q ~1/E2 anddE/ds ~1/E• StraightertrajectoriesnearthesurfacewithhigherE(reducedQ)

• LargerinteractionvolumewithhigherE

• Q ~Z2 anddE/ds ~Z• IncreasingbackscatteringwithhigherZ (reducedQ)• SmallerinteractionvolumewithhigherZ

20keV,Carbon

20keV,Iron

BackscatteredElectrons

𝜂 =𝑛hij𝑛h

=𝑖hij𝑖h

#ofbeamelectronsincidentonthespecimen

Beamcurrentinjectedintothespecimen

Z dependenceofh Tiltangledependenceofh

• h increaseswithZ (strongerdependenceatlowZ’s)à Z(composition)contrast

• h increaseswithq (duetodominanttendencyofelasticscatteringintheforwarddirection)à surfacetopography

E0 =20keV

(anglebetweentheincidentbeamandthesurfacenormal)

SecondaryElectrons

𝛿 =𝑛ij𝑛h

=𝑖ij𝑖h

• d increaseswithdecreasingE (forE >E1)duetosmallerinteractionvolumeàmoresurfacesensitivewithlowerE

• Shallowsamplingdepth&strongdependenceofd withqà surfacetopography

• Thesecantbehaviordoesn’tapplyatE <E1 (afractionofkeV – afewkeV)

𝛿 = 𝛿E sec 𝜃

(<~1keV)(<~5keV)

R:primaryelectronpathwithinadistanceofR0 fromthesurface

Atq=0o (notilt),SEsignalfromR0Withatilt,SEsignalfromR=R0secq

BSEvsSEimagesBack Scattered Electron image

Secondary Electron image at 20 kV Secondary Electron image at 5 kV

Silica-coatedAunanoparticles

SEMinstrumentation

V0:accel.voltagedp:probesizeap:convergenceangleIp:probecurrentObjectiveLens

CondenserLens

ElectronGun:Thermalvs.FieldEmissionW Thermal LaB6 Thermal W Field Emitter

ß2mmà

ß500µmà

Diameterof First >

Crossover90µm-100µm 5µm-10µm 20Å-100Å

AdvantagesofFEsources• Smallerspotsize:small

virtualsourcesizeandhighnumericalaperture=smallspotsizeandhighdepthoffield

• Excellentlowvoltageperformance:lowenergyspreadleadstolessimagedistortionfromchromaticaberration

• Longlifetime

SEMlenses

ObjectiveApertureSize• Optimalapertureangleminimizingaberration• Convergenceangleà imagedepthoffocus• Probecurrent

MagneticLens• Strongestmagneticfieldnearthesideofthepolepieces (softmagneticmaterialssuchasiron)à strongerdeflectionfurtheroff-axis

aa:apertureangle

a2:convergenceangle

EffectofWorkingDistance

WorkingDistance

IncreasingW (whilekeepingthefocus):• Largerspotsize(degradedimageresolution)

• Probecurrentapproximatelythesame

• Improveddepthoffocus

EffectofCondenserLensStrength

CondenserLensStrength

Weakercondenserlens:• Largerspotsize(degradedimageresolution)

• Higherprobecurrent

LensAberrationsSphericalAberration

a

𝑑n =12𝐶n𝛼

e

sphericalaberrationcoefficient

ApertureDiffraction

𝑑q =0.61𝜆𝛼

• Limitinga reducessphericalaberrationbutreducestheprobecurrent

• Dependenceona oppositetosphericalaberration

LensAberrationsChromaticAberration

chromaticaberrationcoefficient

AstigmatismFromnotperfectlycylindricallens

𝑑s = 𝐶s𝛼Δ𝐸𝐸E • Fixedbyastigmator (deviceapplyinga

weaksupplementmagneticfield)• Repeat:X-stigmator controlà focusà

y-stigmator controlà focus• Worsechromaticaberrationfor

smalleraccelerationvoltage(E0)

ResolutionofSEM

Resolution, 𝑑M = 𝑑nM + 𝑑sM + 𝑑unM + 𝑑qM = 𝑓(𝛼)

At10keV,l ≈ 0.012nmwithCs ≈ 20nm,Cc ≈ 10nm,DE ≈ 2eVTofinda thatminimizesd,setd(d2)/da =0andsolveforaà aopt ≈ 3X10-3 rads,dR ≈5nm(ForbestSEM’s,dR≈1nm)

StrengthsandLimitations

• Strengths–Relativelysimpletooperate,canprovideimageresolutionontheorderof1-2nm.

–Canbecoupledtoseveralanalyticaltechniques–Relativelyfastimaging–Widerangeofsampletypes

• Limitations–Vacuumcompatibilitytypicallyrequired–Mayneedtoetchforcontrast– Imagingmayspoilsubsequentanalyses–Sizerestrictionsmayrequirecutting–Resolutionisastrongfunctionofsampleandpreparation.

SEM-EnergyDispersiveX-raySpectroscopy(EDS)

EDS is a fast method for elemental identification (all elements with Z > B). ©CopyrightEvansAnalyticalGroup®

Primary&SecondaryProcesses

Electron

ExcitedIon

Augerelectronemission

FluorescentX-rayRelaxationProcess1

RelaxationProcess2

Background

• EDSdetectorsareparalleldetectors:– Capabilities:

• Surveytechnique• Maps• Theexperimentscanbefast.

– Concerns:• Peakoverlap• Detectorartifactsincludingsumpeaks,pileup&escapepeaks.

EDScanbequantitativeforhomogenous,polished,conductivesamples,withsimilarreferencestandards.MostEDSsamplesdon’tmeetthesecriteria,soEDSisnormallynotconsideredtobequantitative.

ImageandEDSspectrumof0.3micronaluminaparticleonSi(3keV beam).

TypicallyEDSisbestforparticlesthatare>0.5microns.

SmallParticleAnalysis

C

O

Al

Si

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3keV 10keV

SEMofresidue

HigherBeamEnergySamplesDeeper

C

N

O

F

Al

Si

CNOF

Al

Si

ThinLayerAnalysis

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• Strengths– FastelementalID– CoupledwithanSEMorSTEMcanprovidemapping– <1umspotsizeforlowkV– Readilyavailable

• Limitations– Quantificationrequiresstandards– Insulatorscanbemoredifficult– Spectralinterferences– Relativelylowsensitivity(1%-0.1at%),worseforlowZ

StrengthsandLimitations