MS482 Materials Characterization - KAISTenergymatlab.kaist.ac.kr/layouts/jit_basic... · MS482...
Transcript of MS482 Materials Characterization - KAISTenergymatlab.kaist.ac.kr/layouts/jit_basic... · MS482...
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