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ED and WD X-ray Analysis
Elemental analysis in the SEM
The ‘what’, ‘where’ and ‘how much’?
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‘What’
-
Identifying the elements
• Locate the electron beam on the region of
the sample
• Start acquisition
• Spectrum shows peaks
• Elements are identified and labelled
• The higher the peak is above the
background, the higher the concentration of
that element
Spectrum 1
Ta
W
Hf
Ta
Hf
Ti
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Ti
Ni
0
1
2
3
Full Scale 20347 cts Cursor: -0.016 (2615 cts)
Ti
4
5
Ni
Cr
6
7
Hf
8
Ta
W
Hf Ta W
9
10
keV
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X-ray Generation
K lines
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L lines
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X-ray line series
Ca
Ca
Sn
Sn
Sn
Sn
Ca
2.8
3
3.2
3.4
3.6
Full Scale 69667 cts Cursor: 2.611 (653 cts)
3.8
4
Ca K series
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Sn
4.2
4.4
4.6
4.8
keV
Sn
Sn
2.8
3
3.2
3.4
3.6
Full Scale 32058 cts Cursor: 2.614 (1014 cts)
3.8
4
4.2
Sn
4.4
4.6
4.8
keV
Sn L series
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ED and/or WD?
• Energy dispersive
• Measures X-ray from its energy
• Wavelength dispersive
• Measures X-ray from its wavelength
• Energy in keV = 12.398/ wavelength in Angstroms
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ED detectors
• Old type Si(Li) detectors needed
to be kept at liquid nitrogen
temperature
• New type SDD detectors are
cooled to Peltier temperature
• X-ray energy converted to
charge pulse and then to a
voltage pulse
• Simultaneous acquisition of
elements Be to U
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WD spectrometer
• Diffraction according to Bragg’s law
• nλ=2dsinθ
• Much better peak resolution then ED
• Much better sensitivity for trace
elements
• Sequential analysis of elements Be to U
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ED and WD together
rare earth
Ce
resolution
Ce
Nd
La
Nd
La
Pr
Ce
Nd
Ce
La
Nd
La
Ce
Pr
La
Nd
Ce
La Pr
Pr
Pr
4.2
4.4
4.6
4.8
5
5.2
Full Scale ED 1552 cts Full Scale WD 18814 (100xcts/s) Cursor: 4.146
5.4
5.6
5.8
6
6.2
keV
Spectrum 1
Fe
Co
+Co
Mn
Cr
sensitivity
Ni
-Co
+Mn
-Mn
Fe
Cr
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5.2
5.4
5.6
5.8
6
6.2
6.4
6.6
Full Scale ED 78582 cts Full Scale WD 30 (100xcts/s) Cursor: 5.085
6.8
7
7.2
7.4
7.6
7.8
8
8.2
keV
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What can be analysed?
• Just about anything you can put in the SEM!
• Microanalysis – typical volume analysed about 1µm
• Depends on accelerating voltage and density
• Nanoanalysis – need to reduce electron beam penetration
• Reduce accelerating voltage
• For best results the sample should be flat and polished and
conducting
• But often you can achieve adequate results from ‘rough’
samples
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Typical applications
•
•
•
•
Comparing ‘good’ and ‘bad’ samples
Identifying compositions in fine grain structures
Identifying sources of contamination
Measuring variation in composition across an interface
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Industrial and research applications
•
•
•
•
•
•
•
•
•
Failure analysis - automotive, aerospace, semiconductors
Materials research
Quality Control
Photovoltaics
Light Emitting Diodes
Thin film analysis
Artefact conservation
Steel inclusions
Gun Shot Residue forensics
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‘Where’ - Mapping
• Scan the electron beam and
acquire X-ray information at
each pixel position
• Display the results as a series of
maps for each element or as a
single colour image
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Individual maps
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ED spectrum at each point
Fe
Fe
Cr
Cr
Mg
1
2
3
4
5
Full Scale 94 cts Cursor: 0.241 (4 cts)
Cr
Fe
6
7
8
keV
O
Cr
Fe
Cr
Si
Mg
Al
O
Cr
Cr
Fe
Cr
Fe
Al
Mg
Ca
C
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1
2
3
4
5
Full Scale 1421 cts Cursor: 0.221 (26 cts)
6
7
8
Ca
1
2
3
4
5
Full Scale 1042 cts Cursor: 0.211 (21 cts)
Cr
Cr
6
7
8
keV
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9
keV
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Interaction volume
Ni 5kV
0.1 µm
1µm
Si 5kV
0.4 µm
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Mapping large areas
• Field width 0.53mm
3x3 maps stitched
• WD geometry requirement
– will defocus at low mag.
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‘How much’ – Quantitative Analysis
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Element
Weight%
Al K
1.42
Ti K
1.29
Cr K
17.46
Fe K
33.65
Ni K
42.29
Mo L
3.88
Totals
100.00
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Processing for quant
Grid Spectrum(3,3)
Fe
Ni
Cr
Ni
Fe
Mo
Mo
Cr
Ti
Al
Cr
Ti
Fe
Ni
Ti
1
2
3
Full Scale 4591 cts Cursor: 0.077 (161 cts)
•
•
•
•
4
5
6
7
8
9
keV
Measure peak areas
Compare with standards
Apply inter-element corrections
For accurate results, the sample should be flat, polished,
homogeneous (on the micro-scale) and conducting
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Detection limits
Spectrum 5
Fe
Co
+Co
Mn
Cr
+Mn
Ni
-Co
-Mn
Fe
Cr
Ti
5
5.2
5.4
5.6
5.8
6
6.2
6.4
Full Scale ED 72759 cts Full Scale WD 32 (100xcts/s) Cursor: 4.908
Co
6.6
6.8
7
7.2
7.4
7.6
7.8
8
keV
• Typically 0.1% to 0.5% for ED
• Order of magnitude better for WD
• Exact detection limit depends on operating conditions and composition
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ED or WD?
• ED for major elements, fast analysis, rough samples
Ideal for a ‘quick look’
• WD for minor and trace elements and overlaps
Better sensitivity and resolution, but slow
• ED and WD are complementary techniques for analysis
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