Dr. Nilanjan Chatterjee

©Massachusetts Institute of Technology

Electron Probe MicroElectron Probe Micro--AnalysisAnalysis(EPMA)(EPMA)

Imaging and micrometerImaging and micrometer--scale scale chemical compositional chemical compositional

analysis of solidsanalysis of solids

Secondary electron (SE)

Back-scatteredelectron (BSE)Cathodoluminescence

(CL)

Characteristic X-ray

Electronbeam

Specimen

Signals produced inSignals produced inThe Electron MicroprobeThe Electron Microprobe

Qualitative analysis: phase identificationQualitative analysis: phase identificationBackBack--scattered electron (BSE) image: spatial variations in scattered electron (BSE) image: spatial variations in chemical compositionchemical composition

XX--ray spectra with energy dispersive spectrometer (EDS): ray spectra with energy dispersive spectrometer (EDS): element identificationelement identification

Elemental XElemental X--ray maps with EDS or wavelength dispersive ray maps with EDS or wavelength dispersive spectrometer (WDS)spectrometer (WDS)

SemiSemi--quantitative analysis: phase quantitative analysis: phase abundanceabundance

•• Polished surfacePolished surface•• BackBack--scattered electron scattered electron

imageimage•• Function of Function of

compositioncomposition

•• Thin sectionThin section•• Polarized transmitted Polarized transmitted

light imagelight image•• Function of optical Function of optical

propertiesproperties

Optical MicroscopeOptical Microscopeversusversus

Scanning Electron MicroscopeScanning Electron Microscope

ElectronElectron--specimen interactionsspecimen interactions

Elastic ScatteringElastic Scattering Inelastic ScatteringInelastic ScatteringEE11 = = EE00 , large , large φφee EE11<<EE0 0 , small , small φφii

-- BackBack--scatteredscattered -- Characteristic XCharacteristic X--raysrayselectronelectron -- Secondary electronSecondary electron

-- CathodoluminescenceCathodoluminescence

Beam electronSpecimen atom

Scattered beam electron

Elastic scattering crossElastic scattering cross--sectionsection

QQ(>(>φφee)) = = 1.62x101.62x10--2020 ((ZZ22//EE22)cot)cot22((φφee/2)/2)

QQ: cross section (events.cm: cross section (events.cm22/e/e--.atom).atom)

φφee: elastic scattering angle: elastic scattering angle

ZZ: : atomic numberatomic number

EE: beam energy: beam energyEE11 = = EE00 , large , large φφee

Electron BackElectron Back--scatteringscattering(High angle elastic scattering)(High angle elastic scattering)

η = nBSE/nB

where nB: # beam electrons, nBSE: # back-scattered electronsFor compounds, η = Σ cjηj

where cj : concentration of element ‘j ’

BSE image of rockBSE image of rock

BSE compositional contrastBSE compositional contrast

Electrongun

Condenserlens

OpticalMicroscope

WDS

LN2

EDSSE

Objective lens

BSE

Stage

JEOL JXAJEOL JXA--8200 8200 SuperprobeSuperprobe

Highvacuum

WDS

Liq N2for

EDS

Sampleexchange

Perpendicular geometry

Electron gun: W or LaB6 filament, thermionic emissionAccelerating voltage: 1-40 kV (typical: 10-25 kV)

Electron lenses (condenser and objective):Beam diameter: 1 nm-1 μm (typical: 0.1-1 μm)Beam current: 1 pA-1 μA (typical: 10-100 nA)

Scanning electron imaging:Back-scattered electron (BSE): compositional imaging, 40-4000xSecondary electron (SE): topographic imaging, 40-360,000x

X-ray spectrometry:Energy dispersive (EDS): element detection5 Wavelength dispersive (WDS): concentration measurement

Features:Features:

Capabilities:Capabilities:

BackBack--scattered electron detectorscattered electron detector

Top view of specimenAnnular, Annular, solidsolid--state state diode, split diode, split into two into two semisemi--circles, circles, A and B.A and B.

Compositional and topographic Compositional and topographic imaging with BSE detectorimaging with BSE detector

A+BA+BCompositionalCompositional

modemode

AA--BBTopographicTopographic

modemode

BSE image resolution:BSE image resolution:electron interaction volumeelectron interaction volume

Electron Range Electron Range ((KanayaKanaya--Okayama)Okayama)::

RR = = KKEE00nn//ρρ

where,where, KK = 0.0276 = 0.0276 AA//ZZ 0.8890.889

nn = 1.67;= 1.67; ρρ = density= density

Typical ranges (Typical ranges (15 kV15 kV, perpendicular beam):, perpendicular beam):CC 1.8 1.8 μμmmFeFe 1.1 1.1 μμmmUU 0.8 0.8 μμmm

Phase identification: EDS XPhase identification: EDS X--ray spectraray spectraIlmenite (ilm):

FeTiO3

hbl: hydrousCa-Fe-Mg-Al-silicate

plg:Na-Ca-Al-silicate

Ato# 1 8 11 12 13 14 19 20 22 26ilm: O Ti Fegrt: O Mg Al Si Ca Febt: H O Mg Al Si K Fehbl: H O Mg Al Si Ca Feplg: O Na Al Si Ca

Energy Dispersive XEnergy Dispersive X--ray ray Spectrometer (EDS)Spectrometer (EDS)

EDS detector: a EDS detector: a ‘‘pp--nn’’ layer of layer of intrinsic intrinsic Si(LiSi(Li)) semiconductor; semiconductor; Be windowBe window(Si is usually electron deficient because it (Si is usually electron deficient because it contains B, a contains B, a pp--typetype impurity. It is coated impurity. It is coated with Li, an with Li, an nn--typetype dopantdopant with excess with excess electrons to create a electrons to create a ‘‘pp--nn’’ intrinsic intrinsic semiconductor)semiconductor)

Multichannel analyzer (MCA) Multichannel analyzer (MCA) processes the Xprocesses the X--ray signalray signal

Phase abundance: BSE & XPhase abundance: BSE & X--ray mapsray maps

Scale bar: 4 mmScale bar: 4 mm

BSEBSE OO MgMg CaCa

•• Olivine (Mg,Fe)Olivine (Mg,Fe)22SiOSiO4 4 -- light in O and Mg maps light in O and Mg maps –– 19.9%19.9%•• AugiteAugite Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO6 6 -- light in Ca map light in Ca map –– 19.6%19.6%•• Crystallized fineCrystallized fine--grained matrix grained matrix –– 60.5%60.5%

Other imaging techniques: Other imaging techniques: secondary electron (SE)secondary electron (SE)

(Inelastic scattering)(Inelastic scattering)

Electrons from specimen surface are mobilized by beam electrons(useful in studying surface features)

Emitted at low energies (typical: <10 eV)EE11<<EE0 0 , small , small φφii

Secondary electron detectorSecondary electron detector

Side view of specimen

Imaging with secondary electron Imaging with secondary electron detectordetector

--veve Faraday cage biasFaraday cage biasless SEless SE

less topographic contrastless topographic contrast

++veve Faraday cage biasFaraday cage biasmore SEmore SE

better topographic contrastbetter topographic contrast

Other imaging techniques:Other imaging techniques:CathodoluminescenceCathodoluminescence (CL)(CL)

Band gap energy, Band gap energy, EEgapgap, is a property of the semiconductor, is a property of the semiconductorTrace impurities change Trace impurities change EEgapgap by adding additional energy by adding additional energy states in the band gapstates in the band gap

CathodoluminescenceCathodoluminescence imagingimagingLight from CL is intercepted at the optical

microscope and analyzed with a wavelength discriminating spectrometer

JXA-733

Auto focus

Zircon

JXA-8200

Optical gratingspectrometer

MIT OpenCourseWarehttp://ocw.mit.edu

12.141 Electron Microprobe Analysis by Wavelength Dispersive X-ray SpectrometryJanuary (IAP) 2010

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