Electron Microprobe Electron Microprobe AnalysisAnalysis

XX--ray spectrometry:ray spectrometry:1.1. XX--ray generation and emissionray generation and emission2.2. XX--ray detection and measurementray detection and measurement

XX--ray energy and wavelengthray energy and wavelengthE=hE=hνν

h h : Planck's constant: Planck's constant((6.626x106.626x10--3434 Joule.secJoule.secor, 6.626x10or, 6.626x10--3434/1.6021x10/1.6021x10--1616 keV.seckeV.sec))

ν ν :: frequencyfrequency (= (= c/c/λλ))((c c : speed of light in vacuum: speed of light in vacuum== 2.99793x102.99793x101717 nm/secnm/secλ λ : wavelength): wavelength)

λλ ((nm)nm) = = cc//νν = = hc/Ehc/E = 1.2398/= 1.2398/E E (keV)(keV)

The electromagnetic spectrumThe electromagnetic spectrum

λ (nm) = 1.2398/E (keV)

SiKα1.74 keV0.71 nm

UMα3.17 keV0.39 nm

Ti KTi Kαα

Fe KFe Kαα

Ti KTi Kββ Fe KFe Kββ

K-shell

L-shell

Characteristic XCharacteristic X--ray generationray generation

Flowchart forX-ray generation

InnerInner--shell ionizationshell ionizationXX--ray and electron transitionray and electron transitionKKαα : : L to KL to K--shellshell KKβ: β: M to KM to K--shellshellLLαα : : M to LM to L--shellshell LLβ: β: N to LN to L--shellshellMMα: α: N to MN to M--shellshell

OvervoltageOvervoltage

Condition forCondition forinner shell ionizationinner shell ionization ::U U > 1> 1

U = E/Ecwhere, E is the beam energy (usually 10-25 keV)

Ec : critical excitation energyor absorption edge of the shell

KαLαMα

keV

InnerInner--shell ionization shell ionization crosscross--sectionsection

Q Q = 6.51x10= 6.51x10--2020 [([(nnssbbss)/()/(UEUEcc22)] ln()] ln(ccssU U ))

Q Q :: Cross sectionCross sectionnnss : # : # electrons in the shellelectrons in the shellbbss,,ccss : : constantsconstants

InnerInner--shell ionization shell ionization crosscross--sectionsection

AlK-shell

Ec= 1.84 keV

Best analytical condition, U≈5

XX--ray production rangeray production range

An analogous expression to the electron An analogous expression to the electron range ( R = KErange ( R = KE00

nn//ρ ρ ))

XX--ray Rangeray Range:: RR = = K(EK(E00nn -- EEcc

nn)/)/ρρwhere,where, KK = 0.064, = 0.064, nn = 1.68= 1.68

Always smaller than electron rangeAlways smaller than electron range

XX--ray depthray depth--distribution: distribution: φφ((ρρzz))

ρ = densityz = depthφ(Δρz) = X-ray intensity generated in a freestanding layerφ(ρz) at depth z = Intensity from depth ‘z’ divided by φ(Δρz)

crystalcrystal

detectordetector

WDSWDS WDSWDS

Wavelength Dispersive Spectrometer Wavelength Dispersive Spectrometer (WDS)(WDS)

BraggBragg’’s Laws Law

nnλλ = 2= 2dd sinsin θθ= path length ABC= path length ABC

θ = angle ofdiffraction

d = lattice spacing

n = order of diffraction(any integer)

WDS: Focusing geometryWDS: Focusing geometry

LL = = nnλ λ .R/d.R/d

take-offangle

angle ofdiffraction

Name 2d (Å) Type Elements usually analyzed LDEC 98 Ni/C Multi-layer B-O (Kα), optimized for C analysisSTE 100.4 Pb stearate B-O (Kα), optimized for C analysisLDE1 59.8 W/Si Multi-layer C-F (Kα), optimized for O analysisTAP 25.8 Thallium acid phthalate Na-P (Kα); Cu-Zr (Lα); Sm-Au (Mα)PET 8.742 Pentaerythritol S-Mn (Kα); Nb-Pm (Lα); Hg-U (Mα)LIF 4.028 Lithium fluoride Ti-Rb (Kα); Ba-U (Lα)

WDS: Analyzing crystalsWDS: Analyzing crystals

Curved diffracting crystalsCurved diffracting crystals

Johansson typebending curvature: 2Rpolished and ground to R

Johan typeonly bent to 2R,not ground

R

FWHM of fully focusing Johansson-type crystal ~10 eV

Some defocusing in Johan-type, but resolution is not compromised

WDS: Analyzing crystalsWDS: Analyzing crystalsVertical, horizontal and tilted spectrometersVertical, horizontal and tilted spectrometers

WDS: XWDS: X--ray detectorray detector(proportional counter)(proportional counter)

Flow proportional counter

Tungsten collection wire set at 1Tungsten collection wire set at 1--3 kV bias3 kV bias

Flow counter: 90% Flow counter: 90% ArAr +10% CH+10% CH44 (P(P--10); 10); polypoly--propylene windowpropylene window

Sealed counter: Sealed counter: XeXe or Kr; Be windowor Kr; Be window

WDS detector: Proportional counterWDS detector: Proportional counter

Collection wire bias range Collection wire bias range (applied potential): 1(applied potential): 1--3 kV3 kV

Bias is set so that Bias is set so that amplification is in the amplification is in the proportional regionproportional region

Amplification in proportional Amplification in proportional countercounter

(Bias)

Gas used for long wavelengthsGas used for long wavelengths: : 90% 90% ArAr +10% CH+10% CH44 (P(P--10)10)

Gas used for short wavelengthsGas used for short wavelengths: : XeXe or Kror Kr

Counting efficiency of gas in Counting efficiency of gas in proportional counterproportional counter

Lightelements

Heavyelements

WDS: changing the angle WDS: changing the angle of diffractionof diffraction

Theoretical and actual limits of Theoretical and actual limits of spectrometer movementspectrometer movement

WDS signal processingWDS signal processing

Single channel analyzer (SCA) Single channel analyzer (SCA) and and

pulse height analysis (PHA)pulse height analysis (PHA)

windowbaseline

Only pulses in this voltage interval are

counted

WDS: PHA setupWDS: PHA setup

Single Channel Analyzer (SCA) scanSingle Channel Analyzer (SCA) scan

BeamBeam--rasteredrastered image: image: electron beam electron beam rastersrasters over the area over the area to be imagedto be imagedStageStage--rasteredrastered image:image: electron electron beam is stationary, stage movesbeam is stationary, stage moves

Imaging with XImaging with X--rays: rays: compositional mappingcompositional mappingMg Ca

Na Ti

ZnZn--Sn compositeSn composite

XX--ray image artifact: backgroundray image artifact: background

Continuum XContinuum X--rays (background)rays (background)

XX--ray backgroundray background(maximum energy = electron beam (maximum energy = electron beam

energy, Eenergy, E00))

Characteristic XCharacteristic X--raysrays

ContinuumContinuumXX--raysrays

Generated by deceleration of beam Generated by deceleration of beam electrons in the electrons in the CoulombicCoulombic field field of outer shells of target atoms.of outer shells of target atoms.

Continuum XContinuum X--rays: background artifactrays: background artifact

Neither material contains CrNeither material contains Cr

But background counts for Cr :But background counts for Cr :

in in 11 in in 22Therefore, if a Cr XTherefore, if a Cr X--ray map is acquired, material ray map is acquired, material 22 will show higher Cr will show higher Cr than material than material 11

A composite made of 2 materials is being mapped:A composite made of 2 materials is being mapped:

22

11

Defocusing in beamDefocusing in beam--rasteredrasteredWDS XWDS X--ray mapsray maps

Resolution : # pixels (step size)Resolution : # pixels (step size)Signal: beam current and dwell time/pointSignal: beam current and dwell time/point

StageStage--rasteredrastered XX--ray mapping ray mapping parametersparameters

Acquisition time increasesAcquisition time increasesas resolution increasesas resolution increasesas dwell time/point increasesas dwell time/point increases

Large area stageLarge area stage--rasteredrastered imagesimages

Scale bar: 4 mmScale bar: 4 mm

BSEBSE OO MgMg CaCa

Resolution: 20 μm/step Image size: 30mm x 15mm#steps (pixels): 1500x750 Dwell time: 30 msec/pointBeam current: 30 nA Voltage: 15 kVAvg stage speed: 0.71 mm/secAcquisition time: 14 hrs

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

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