Fundamentals Of Scanning Electron Microscopy And Energy .

Fundamentals of Scanning ElectronMicroscopy and Energy DispersiveX-ray Analysis in SEM and TEMTamara RadetiÉ,University of BelgradeFaculty of Technology and Metallurgy,Beograd, SerbiaNFMC Spring School on Electron Microscopy, April 2011Outline SEM– Microscope features– BSE– SE X-ray EDS– X-rays - origin & characteristics– EDS system– Qualitative & quantitative analysisNFMC Spring School on Electron Microscopy, April 2011

SEM: Basic PrinciplesThe scanned image is formedpoint by point in a “serial fashion”.“microscope”control console(electronics) Electron source - gunElectron column- lenses- apertures- deflection coilsSpecimen chamber withdetectorsNFMC Spring School on Electron Microscopy, April 2011SEM: Electron Gun Generates & accelerates electrons to an energyrange 0.1-30 keV. Characteristics:– Emission current, ie– Brightness,E current/area.solid angle 4ip/S2dp2Dp2– Source size– Energy spread– Beam stability– Lifetime & costNFMC Spring School on Electron Microscopy, April 2011

SEM: Electron EmittersW- hairpinLaB6E 105E 106 A/cm2srds 5-50 PmA/cm2srds 30-100 PmField emission- Cold- Thermal- SchottkyE 108 A/cm2srds 5nmNFMC Spring School on Electron Microscopy, April 2011SEM: Electron Lenses1/f 1/p 1/qMagnification M q/pDemagnification m p/qElectromagnetic lens defects: Spherical aberration: ds 1/2CsD3 Chromatic aberration:dc CcD('E/E0) Diffraction: dd 0.61O/D AstigmatismNFMC Spring School on Electron Microscopy, April 2011

SEM : working distancePolepiecedetectorWorkingdistanceSample at idealworkingdistance for XraymicroanalysisSample at incorrectworking distance forX-ray microanalysis The working distance is defined as the distancebetween the lower pole piece of the objective lensand the plane at which the probe is focused.NFMC Spring School on Electron Microscopy, April 2011SEM: Deflector (scan) Coils Magnification Lspecimen/LCRTNFMC Spring School on Electron Microscopy, April 2011

Operator control of SEM LensesEffect of aperture sizeEffect of condenser lens strengthdp (dG2 dS2 dd2 dC2)1/2dmin KCs1/4l3/4(ip/EO2 1)3/8Effect of working distanceNFMC Spring School on Electron Microscopy, April 2011SEM: Interaction Volume Monte Carlo simulation Penetration depth: 1-5 PmNFMC Spring School on Electron Microscopy, April 2011

SEM: Backscattered electronsBSEAl6(Fe,Mn)Mg2Si K nbse/nb ibse/ib K(I) KncosINFMC Spring School on Electron Microscopy, April 2011SEM: BSE DetectorsElectron beamScintillatorlight pipeto photomultiplierSolid angle ofBSE detectorspecimen Everhart-Thornley detector (with negativebias) Scintillator detector The solid state detectorNFMC Spring School on Electron Microscopy, April 2011

SEM: Secondary electrons SE I - surface SE II - surface volume SE III & IV - noiseNFMC Spring School on Electron Microscopy, April 2011SEM: SE detector Everhart-Thornley detector (withpositive bias)NFMC Spring School on Electron Microscopy, April 2011

SEM: Other signalsNFMC Spring School on Electron Microscopy, April 2011Generation of X-raysIncident e- beamMEjected e(SE)LK Characteristic X-raysphotons Auger electronsContinuum X-rays orbremsstrahlungradiationContinuum X-rayEmissionCharacteristic X-rayEmissionnucleiNFMC Spring School on Electron Microscopy, April 2011

Electron TransitionsNFMC Spring School on Electron Microscopy, April 2011Families of Characteristic linesNFMC Spring School on Electron Microscopy, April 2011

EDS Spectra for Families of X-ray linesNFMC Spring School on Electron Microscopy, April 2011EDS: SEM - vs - STEM (TEM)SEM e beam energy 0.1-20 keV min probe size 1nm interaction volume PmSTEM (TEM) e beam energy 100-400keV min probe size 1nm specimen thickness 100 nmNFMC Spring School on Electron Microscopy, April 2011

EDS: SEM - vs - STEM (TEM) The energy of the incident electron must be larger than ionizationenergy for a given shell.U is overvoltage, U E/EJOptimal value of Uป2NFMC Spring School on Electron Microscopy, April 2011EDS User InterfaceNFMC Spring School on Electron Microscopy, April 2011

EDS SystemX- ray Detector:Detects and convertsX-rays into electronicsignalsPulse processor:Measures theelectronic signals ofeach X-raymeasured.ElementW eight%SiCrMnFeNiCuMo0.3218.84 20.002 1.3970.25 200.15100.00Analyzer:Displays & analyses data.NFMC Spring School on Electron Microscopy, April 2011EDS: Detector componentsWindow: Be UltrathinFETCollimatorCryostatCrystal: Si(Li) Intrinsic GeElectron trapNFMC Spring School on Electron Microscopy, April 2011

EDS: How the detector works? creation of electron/hole pair 3.8 eV in average electronic noise & incoplete charge collectionreduce energy resolution.NFMC Spring School on Electron Microscopy, April 2011EDS: Pulse detection and analysis Time constant, W- shorter W: higher count rate, but peakbroadening (lower energy resolution)- longer W: better resolution, butincrease in dead time Dead time– Dead time , % (1-Rout/Rin)x100%NFMC Spring School on Electron Microscopy, April 2011

EDS: Energy resolutionFWHM2 kE FWHMnoise2 FWHM of Mn KD, about 130eVLow energy resolution: FWHM of F KDLow energy resolution: FWHM of C KDNFMC Spring School on Electron Microscopy, April 2011XEDS: ArtifactsSi “escape peak” 1.74keV bellow truecharacteristic peakpositionThe sum peak too high count rateThe internalfluorescencepeak A small Si KD peak at1.74keVNFMC Spring School on Electron Microscopy, April 2011

EDS: Qualitative Analysis Spectrum : peak identification– Families of peaks– Peak overlaps / deconvolution– Artifacts & spurious peaks– Background subtractionCount rate & collection timeLimits:– Probe size– Spatial resolution– Energy resolution– Detectability limitNFMC Spring School on Electron Microscopy, April 2011EDS: Qualitative AnalysisAlSi Profile (line spectrum)Ti Elemental mappingNFMC Spring School on Electron Microscopy, April 2011

EDS: Quantitative Analysis for SEM Acqusition under best conditions– Flat specimen, avoid coating, homogeneous specimen (at list in theanalysis/interaction volume)– High count rate, but optimum dead time– Optimum overvoltage Standards : ci/cstd Ii/Istd kiMatrix correction ZAF– Z : effect of R & S– A : absorption– F: secondary fluorosence Recalculate concentrations Other corrections: PROZA, PaP, XPPStandardless analysisNFMC Spring School on Electron Microscopy, April 2011EDS: Quantitative Analysis for TEMCA/CB KAB.IA / IBCA,B - concentration in wt%IA,B - peak intensityKAB - Cliff-Lorimer factorkAB kAR / kBRTypical k ASi curve for K lines for a Be window detector (after P. J. Sheridan [1989] )kAR AA wR QB aR eR / AR wA QA aA eAA atomic weightw fluorescent yieldQ ionisation cross sectiona the fraction of the total line, e.g. K / (K K ) for a Ka linee the absorption due to the detector window at that line energy.NFMC Spring School on Electron Microscopy, April 2011

SEM - EDS - TEMNFMC Spring School on Electron Microscopy, April 2011SEM - EDS - TEMSTATEGrain size, mAs-cast384 ehomogenizationAl6(Fe,Mn)Al-Mg-Si based Precipitates freezoneÕs width,area fraction, % particles, areafraction, %m3.41 0.91.78 0.6615.47 6.68549 501.95 0.640.33 0.2517.84 8.85566 503.44 1.240.93 0.4815.91 7.36

TEM Specimen Preparation inMaterials ScienceTamara RadeticDept. Metall. Eng., Faculty of Technology & Metallurgy,University of Belgrade, SerbiaNFMC Spring School on Electron Microscopy, April 2011Why is the specimen preparation thatimportant?No good specimen - No good TEM! Microscopic - vs - macroscopic No simple rules for various materials &purposes Artifacts Arts -vs- Science ?NFMC Spring School on Electron Microscopy, April 2011

Basic requirements Representative of the material under investigationArtifact freeClean, without contaminationMechanically rigid and stableResistant to the electron beam irradiationElectrically conductiveLarge area transparent to the electron beamNFMC Spring School on Electron Microscopy, April 2011Principals for a TEM specimenpreparation Safety Time-effective Cost-effective RepeatableNFMC Spring School on Electron Microscopy, April 2011

Size of a TEM specimen Diameter: 3mm, 2.3mm or even 1mm!– Reduce size of a bulk specimen.– Use grid support for small specimens. Thickness: 10-200nm– Material (chemical composition)– Kind of the experiment (HREM-vs-CTEM, EELS-vsEDS)NFMC Spring School on Electron Microscopy, April 2011Choice of a specimen preparationtechniqueSpecimen-self-supported (bulk)-supported by a grid (thin films, nano-particlesect)Material:-ductile (metals)-hard & brittle (semiconductors & ceramics)Geometry:-plan view-cross-sectionElectrical properties:-conductive-insulatingNFMC Spring School on Electron Microscopy, April 2011

Types of specimen preparationtechniques Mechanical Mechanical polishing to the electrontransparency (Tripod)CleavageUltramicrotomyCrushingMechanical ionic Grinding, dimpling ion millingIonic FIBChemical Electro-chemical polishingChemical polishing or etching ReplicaThin film depositionPhysicalNFMC Spring School on Electron Microscopy, April 2011Step 1: Slicing the specimenÇป1000PmDuctile materials (metals)¾Cut slice 500-1000 Pm thick:- chemical wire/string saw- wafering saw (not diamond!!!)- spark erosionBrittle materials(semiconductors, ceramics )¾Cut slice ป1000 Pm thick:- diamond wafering saw¾Cleave (NaCl, MgO )¾UltramicrotomeQuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.NFMC Spring School on Electron Microscopy, April 2011

Step 2: GrindingÇป1000Pmป100Pm Mechanically grind down the specimen to about 100Pm inthickness.Ductile materials: manuallyHard materials: grinding/polishing machinesQuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.NFMC Spring School on Electron Microscopy, April 2011Step 3: Cutting 3mm diskDuctile materials (metals)¾ Mechanical disc punchBrittle materials (semiconductors, ceramics )¾ Ultrasonic drill¾ Grinding (slurry) drill¾ Spark erosion (for conducting specimens)NFMC Spring School on Electron Microscopy, April 2011

Step 4: Prethinning the disk - dimpling Plan view & cross-section specimens 10Pm100 Pm Plan view: thin film on the substrate 10Pm100 PmQuickTime and adecompressorare needed to see this picture.NFMC Spring School on Electron Microscopy, April 2011Step 5: Final thinning of the diskÇ Electropolishing Ion MillingNFMC Spring School on Electron Microscopy, April 2011

Electropolishing Only for electrically conducting specimensNFMC Spring School on Electron Microscopy, April 2011Ion Milling Ion milling involves bombarding a thin TEM specimen withenergetic ions (Ar ) and sputtering material from thespecimen until it is thin enough to be electron transparent.NFMC Spring School on Electron Microscopy, April 2011

Plasma Cleaningbefore2min cleaningNFMC Spring School on Electron Microscopy, April 2011Preparation of a cross-sectionspecimenQuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.NFMC Spring School on Electron Microscopy, April 2011

General tools & equipmentQuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.QuickTime and adecompressorare needed to see this picture.NFMC Spring School on Electron Microscopy, April 2011Useful links .com/www.struers.comwww.buehler.com/NFMC Spring School on Electron Microscopy, April 2011

Fundamentals of Scanning Electron Microscopy and Energy Dispersive X-ray Analysis in SEM and TEM Tamara RadetiÉ, University of Belgrade Faculty of Technology and Metallurgy, Beograd, Serbia NFMC Spring School on Electron Microscopy, April 2011 Outline SEM – Microscope features – BSE –SE † X-ray EDS – X-rays - origin & characteristics