SEM2 - Electron Microscopy Group
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THE UNIVERSITY OF BIRMINGHAM Centre for Electron Microscopy Chemical Analysis in the SEM Ian Jones 1. Basic processes: why x-rays 2. EDX 3. WDX 4. Quantification 5. Other detectors Chemical analysis in the SEM 1. Basic processes: why x-rays 2. EDX 3. WDX 4. Quantification 5. Other detectors Chemical analysis in the SEM Elastic and Inelastic scattering Elastic and Inelastic scattering Electron bounces off whole atom Electron interacts with one of the orbital electrons Secondary electrons, X-rays Elastic ≡ no loss of energy Inelastic ≡ energy is lost Sample-Electron Interaction For SEM imaging backscattered and secondary electrons are important. Sample-Electron Interaction Electron beam Electron beam Electron beam Electron beam Electron beam Electron beam Low atomic number Low atomic number Medium atomic number Medium atomic number High atomic number High atomic number Monte Carlo simulation of 100 Monte Carlo simulation of 100 electron trajectories electron trajectories (E=25keV) in different (E=25keV) in different materials. materials.
Transcript of SEM2 - Electron Microscopy Group
THE UNIVERSITYOF BIRMINGHAM
Centre for Electron Microscopy
Chemical Analysis
in the
SEM
Ian Jones
1. Basic processes: why x-rays
2. EDX
3. WDX
4. Quantification
5. Other detectors
Chemical analysis in the SEM
1. Basic processes: why x-rays
2. EDX
3. WDX
4. Quantification
5. Other detectors
Chemical analysis in the SEMElastic and Inelastic scatteringElastic and Inelastic scattering
Electronbounces offwhole atom
Electroninteracts withone of theorbitalelectrons
Secondaryelectrons, X-rays
Elastic ≡ no loss of energyInelastic ≡ energy is lost
Sample-Electron Interaction
For SEM imaging backscattered andsecondary electrons are important.
Sample-Electron Interaction
Electron beamElectron beam Electron beamElectron beam Electron beamElectron beam
Low atomic numberLow atomic number
Medium atomic numberMedium atomic number
High atomic numberHigh atomic number
Monte Carlo simulation of 100Monte Carlo simulation of 100electron trajectorieselectron trajectories(E=25keV) in different(E=25keV) in differentmaterials.materials.
Characteristic X-ray
Kα => transition from L to K ShellKβ => transition from M to K ShellKγ => transition from N to K Shell
Lα => transition from M to L ShellLβ => transition from N to L Shell
Ephoton ~ Z2
TransitionsTransitionsFluorescence yields
Fluorescence yields for K and L shells for 5 ≤ Z ≤ 110.
Probability of aspecific excited atomemitting a photon inpreference to anAuger electron.
YBa2Cu3O7E0 = 10 keVSimulated spectrum, as emitted
Ba LY LCu LO K
Photon energy (keV)
Inte
nsity Measurement challenges:
Natural peak widths ~ 1 eVComplex spectraComplex backgroundPoor P/B (relatively high continuum background)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
1. Basic processes: why x-rays
2. EDX
3. WDX
4. Quantification
5. Other detectors
Chemical analysis in the SEM
EDX detectorEDX detector
Liquid N2 Dewar
Si (Li) Detector(detectioncrystal)
Field Effect Transistor(FET) Window
Electron trapCollimator
To preamplifier
Monolithic Semiconductor Energy Dispersive X-ray Spectrometer
Woldseth, 1973
2005Resolution (LN2, ~ 80 K): 129 eV at MnKα (10 mm2); 140 eV (50 mm2)Limiting count rate: ~ 2 kHz (best resolution); 30 kHz (resolution ~ 180 eV)
3 mmGold electrode (rear)
“The last piece of authentic 1960ssemiconductor electronics to still make a buck!” (David Joy)
-500V
( 0 V)
Fitzgerald, R, Keil, K. and Heinrich, K. Science v 159 (1968) 528“Solid-State Energy-Dispersion Spectrometer for Electron-Microprobe X-ray Analysis”
Schematic of EDS detector (Reed)
A Typical EDX SpectrumA Typical EDX Spectrum
Schematic of sum peaks (Reed)
Schematic of EDS pulse processor (Reed)
A Typical EDX SpectrumA Typical EDX Spectrum
X-ray spectrum
X-ray energy
The first stage in quantification is to convert the EDX spectrum to
numbers of characteristic x-rays.
There are two approaches:
•Modelling
•Filtering
The EDX units in the EM Centre use filtering. The spectrum is passed
through a top hat filter, which effectively double differentiates it. This
results in narrower peaks and aids deconvolution. Each peak included
in your analysis is stored as a profile, along with its electronic
characteristics (peak width etc). The relevant profiles are then converted
to be compatible with your spectrum and fitted to it.
FilteringFiltering
Things under your control:
• Energy range of spectrum
• Discriminator setting
(time constant)
• Count rate
Energy Timeconstant
Countrate
Peakwidth
√ √ √
Sumpeaks
√ √
EDS has now been replaced by SDD
SDD Backsurface
Central anode,80 µm diameter
Resistor bridge
Ring electrodes
Silicon Drift Detector (SDD)
300 µmSDDs are thin!
SDDs have a complexback surface electrodestructure.
Area 5 mm2 to 100 mm2
X-rays
The anode of an SDDis ~ 0.005 mm2 for a 50 mm2 detector, about1/10,000 the area of EDS
E. Gatti and P. Rehak, 1984 Mapping in the SEM
Comment:
Alumix 231
(Al14Si2.5Cu0.5Mg)
3 min 18 min
SDD Count rate 130 kcps
Dr MQ Chu
1. Basic processes: why x-rays
2. EDX
3. WDX
4. Quantification
5. Other detectors
Chemical analysis in the SEM WDX
AnalysingCrystalSample
Detector
Schematic diagram of a WDX analysis system
X-ray
Electron beam
2dsinθ = λ
By continuously changing θ, different x-raywavelengths can be selected in turn, and byappropriately positioning a detector, the x-rayintensity is measured as a function of wavelength.
How WDX works
ExamplesExamples
The ED spectrum from an alloy containing0.15 wt% Si. The red line shows the
expected peak position for Si, but it isdifficult to be positive about reliable
identification
In the WD spectrum from the same sampleas in , the improvement in peak to
background ratio means there is no doubtthat Si is present.
ExamplesExamples
ED and WD spectra from a nickel-based superalloy. The WDspectra shows the lines from W, Ta and Re clearly separated,
whereas this is not the case in the ED spectrum
Analysing crystalsName Formula Range λ (A) Range z (Kα) Range z (Kβ)
Lithiumfluoride
LiF 0.25 – 2.70.35 – 3.8
22 – 6819 - 58
> 56> 49
Quartz α-SiO2 0.58 – 6.3 15 - 46 > 49
PET C(CH2OH)4 0.76 – 8.2 14 - 40 > 36
PbSt [CH3(CH2)16CO]2Pb
8.7 - 94 5 - 12 20 - 36
sinθ = λ/2d
Main characteristics
Energy resolution: 5 eV
Detection limit: Be
Signal to noise ratio: 103
Time for analysis: long (300-3000 sec)
Extracting the peak intensities is relatively straightforward, because the peaks are so narrow.
WDX
1. Basic processes: why x-rays
2. EDX
3. WDX
4. Quantification
5. Other detectors
Chemical analysis in the SEM
Quantification
For an element A, its concentration in wt%
is
!
cA
= A
specimen
NA
standard
N
where
!
A
specimen
N is the number of x-rays
from the specimen
and
!
A
standard
N is the number of x-rays from
the pure element standard under identical
conditions.
ZAF corrections
This rough first estimate is refined via five ‘ZAF’
corrections:
Z Backscattering
Z Stopping power
A Absorption
F Fluorescence by characteristic x-rays
F Fluorescence by Bremsstrahlung
which are iterated.
Notes
1. Specimens should be smooth.
2. Compound standards are OK.
3. The closer a standard’s composition is to that ofthe specimen, the better.
4. Standards should be single phase andhomogeneous.
5. Spatial resolution depends on beam voltage andbut is likely to be a substantial fraction of a µm.
!
Z
1. Basic processes: why x-rays
2. EDX
3. WDX
4. Quantification
5. Other detectors
Chemical analysis in the SEM
Microcalorimetry Approach to EDS
Need heat capacity C to be small:1. Low temperature of operation2. Small absorber volume3. Insulators and superconductors
Temperature
Time
CEν
CG
τ =
G ThermalConductance
Eν
CHeat
X-rayThermometer
Capacity
ΔEFWHM = 2.36√ kT2C
For area ~ mm2, thickness ~ few µmΔEFWHM ~ few eV at T = 100 mK
JEOL 6400 SEM
NIST MicrocalorimeterCryostat
ConventionalSi(Li) EDS
1. Liquid N2 to 77K2. Liquid He to 4 K3. Adiabaticdemagnetizationrefrigerator to100 mK
The end
