principles of electron beam microanalysis
TRANSCRIPT
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Principles of Electron Beam Microanalysis
EDS User School
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Outline
1.) Beam-specimen interactions
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
3.) Moseley‘s law
4.) Characteristic peaks: K-,L-, and M series
5.) Spatial resolution and excitation range in EDS analysis
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1.) Beam-specimen interactions
Backscattered Electrons (BSE)
Cathodoluminescence
Auger Electrons
Characteristic X-rays
Secondary Electrons (SE)
Transmitted Electrons
Absorbed Electrons
Electron Beam
Heat
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SEM - scanning electron microscopy EDS - energy dispersive spectrometry WDS - wavelength dispersive spectrometry AES - Auger electron spectroscopy XPS - X-ray photoelectron spectroscopy UPS - UV-light photoelectron spectroscopy SIMS - secondary ion mass spectrometry XRF - X-ray fluorescence spectroscopy PIXE - proton-induced X-ray emission CL - cathodoluminescence
Electrons Ions X-rays Light
Electrons SEM, AES XPS UPS
Ions SIMS
X-rays EDS, WDS PIXE XRF
Light CL
Accelerated from a source
Emitted from the specimen
1.) Beam-specimen interactions
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Basis: Atom model (N.Bohr)
1.) Beam-specimen interactions
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λ (nm) = 1.24 / E (keV)
1.) Beam-specimen interactions
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Outline
1.) Beam-specimen interactions
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
3.) Moseley‘s law
4.) Characteristic peaks: K-,L-, and M series
5.) Spatial resolution and excitation range in EDS analysis
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Advantage of electron beam microanalysis
Chemical analysis of a very small volume of material can be done - ideal method for characterisation of a microstructure in a sample !
Linear Analysed Assumed Amount dimension volume density of material
1µm à 10-12 cm3 à 7 g/cm3 à 7 x 10-12 g
Detection limit: 0,1%
Mass detection limit: 10-14 g
(For reference: Fe - atom weight is about 10-22 g)
Example:
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2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
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continuum or Bremsstrahlung (breaking radiation)
• results from deceleration of beam electrons in the electromagnetic field of the atom core
• combined with energy loss and creation of an X-ray with the same energy
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
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- Characteristic X-rays are formed by excitation of inner shell electrons
- Inner shell electron is ejected and an outer shell electron replaces it
- Energy difference is released as an X-ray
2 4 6 8 10 12 14keV
0
1
2
3
4
5
cps/eV
C Si Cr Cr Mn Mn
Fe
Fe
Ni Ni
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
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If beam energy E > EK then a K-electron may be excited
Energy of emitted photon can be calculated:
EPhot = E1 – E2
e.g.: Fe L → K
E1 = EK = 7.11 keV E2 = EL = 0.71 keV EKa = 6.40 keV
X-ray energy is the difference between two energy levels !
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
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X-ray and AUGER generation process
Emission of Auger electron Emission of X-ray
Auger and X-ray yield are competing processes
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C
Ge
Fluorescence yield (ω)
n ω= fraction of ionisation events producing characteristic X-rays (rest produce Auger electrons)
ω + A = 1 - ω increases with Z - ω for each shell: ωK ωL ωM - Auger process is favoured for low Z, - fluorescence dominates for high Z
ω ≈ 0.005 for C K ω ≈ 0.5 for Ge K
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Outline
1.) Beam-specimen interactions
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
3.) Moseley‘s law
4.) Characteristic peaks: K-,L-, and M series
5.) Spatial resolution and excitation range in EDS analysis
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3.) Moseley’s law
E = c1 (Z - c2)²
X-rays are characteristic because their specific energies are characteristic of the particular element which is excited.
Moseley’s law defines the relationship between the x-ray lines and the atomic number of the emitted atom.
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Detection limit of EDS
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Outline
1.) Beam-specimen interactions
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
3.) Moseley‘s law
4.) Characteristic peaks: K-,L-, and M-series
5.) Spatial resolution and excitation range in EDS analysis
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4.) Characteristic peaks: K, L, M series
L-family
• Energy of characteristic peaks is defined by element • The higher the atomic number Z the higher the peak energy
Kα
Kβ
Fe K-family
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The K-family of lines (1)
K-lines: vacancy in K-level is filled α- lines are L -K transitions β- lines are M-K transitions
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The K-family of lines (2)
- K lines in ED spectra are either a combination of Kα + Kβ peaks or a separated pair (Kβ weight then about 1/8 ... 1/10)
- Below element S (Kα = 2308 eV) it is not possible to resolve the two peaks with EDS à a Kβ shoulder may be visible on the high-energy side of the Kα
- line energy difference (Ka-Kß) is increasing with atomic number
- for SEM (30 kV Umax) K lines up to atomic number 42 (Mo) can be excited
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The K-family of lines (3)
S (16)Kα1,2 2308 eVKβ 2464 eVΔ (Kβ - Kα1,2) 156 eV
S (16) Ca (20)Kα1,2 2308 eV 3692 eVKβ 2464 eV 4013 eVΔ (Kβ - Kα1,2) 156 eV 319 eV
S (16) Ca (20) Mn (26)Kα1,2 2308 eV 3692 eV 5900 eVKβ 2464 eV 4013 eV 6492 eVΔ (Kβ - Kα1,2) 156 eV 319 eV 592 eV
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The L-family of lines (1)
• L-lines occur: vacancy in L-level is filled
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The L-family of lines (2)
Mo (42) Ce (58)Lα1 2292 eV 4839 eVLβ 2394 eV 5262 eVLl 2014 eV 4287 eV
Mo (42)Lα1 2292 eVLβ 2394 eVLl 2014 eV
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Intensity and energy of characteristic lines
- Energy of line is defined by - Element
- Type of transition
- Intensity of line is defined by - probability of producing a hole (vacancy)
- probability of electron transition
- probability of x-ray emission
- concentration
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Probability of producing a hole: Ionization cross-section for electrons
• Ionization cross section: probability of excitation
• maximum ionization cross section: 2,5 x Ebind
Ionisation cross section for electrons
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Ionization cross section for electrons
Fe
Cr
Ni
Cr: 33%
Fe: 33%
Ni: 33%
U = 10 keV
:bind
exc
EE Cr Fe Ni
1,847 1,561 1,337
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Atomic energy levels and line transition
Transitions have different probabilities
Lines have different intensities
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Line intensity relations
K- series: Z < 12 α1 : α2 : β1 = 100 : 50 : 15 13 > Z < 50 α1 : α2 : β1 = 100 : 53 : 18 L- series: 15 > Z < 90 α1 : β1 : γ1 = 100 : 52 : 10
Intensity of an x-ray line is determined by the transition probability of electrons from the outer to inner shell. These values are fixed for the lines of one series.
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Line intensity relations (2)
Lα1
Lβ1
Lβ2
Lγ1 Ll Lγ2/3
Lα2
Spectrum Barium L series, 15 kV α1 : β1 : γ1 = 100 : 52 : 10
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Line intensity relations (3)
Spectrum Barium L-series, 15 kV α1 : β1 : γ1 = 100 : 52 : 10
Ti-Kα1
Ti-Kβ1
Barium
BaTiO3
Spectrum BaTiO3, 15 kV Overlapped Ba L-series and Ti K-series
Lα1 (100)
Lβ1 (31)
Lβ2
Lγ1 (5) Ll Lγ2/3
Lα2
Lα1
Lβ1
Lβ2
Lγ1 Ll Lγ2/3
Lα2
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Outline
1.) Beam-specimen interactions
2.) EDS spectra: Origin of Bremsstrahlung and characteristic peaks
3.) Moseley‘s law
4.) Characteristic peaks: K-,L-, and M series
5.) Spatial resolution and excitation range in EDS analysis
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5.) X-ray range
Different excitation ranges for: - characteristic x-ray radiation and Bremsstrahlung,
- secondary electrons (SE) - back-scattered electrons (BSE)
sample surface
electron beam (E0)
secondary electrons
ca. 0.5 ... 5 µm ca. 10 µm3
back-scattered electrons
bremsstrahlung
X-rays
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5.) X-ray range
Dependence HV - x-ray range: Anderson and Hasler (1966) give the depth of X-ray production range (µm) as: RAH=0.064(E0
1.68 - Ec1.68) / ρ
E0: primary energy (keV), Ec: critical energy (keV), ρ: mean density (g/cm³)
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5.) X-ray range
• Monte Carlo electron-trajectory simulations of interaction volume in iron as function of primary beam energy
à With higher primary electron energy penetration depth is increasing
Rd ≈ 2,5 µm Rd ≈ 1,3 µm Rd ≈ 0,4 µm
EHT = 10 kV EHT = 20 kV EHT = 30 kV
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5.) X-ray range
n Monte Carlo electron-trajectory simulations of interaction volume as function of atomic number (EHT = 15 kV)
à With higher density penetration depth is decreasing
Carbon Rd ≈ 2 µm Iron Rd ≈ 0,6 µm Gold Rd ≈ 0,2 µm
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The kV compromise
Ichar increases with increasing E0/Ec
à X-ray signal improves
Rx increases with increasing E0/Ec
à X-ray spatial resolution degrades
0
0C
EU 2 ... 2,5E
= >
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