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Oxford Instruments The Business of Science ® © Oxford Instruments 2010 ED and WD X-ray Analysis Elemental analysis in the SEM The ‘what’, ‘where’ and ‘how much’?
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The Business of Science®© Oxford Instruments 2010
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
Ti Ni Cr HfTi Ni W WHf TaHfTa
Ti Ta
HfW
Ta
0 1 2 3 4 5 6 7 8 9 10
keVFull Scale 20347 cts Cursor: -0.016 (2615 cts)
Spectrum 1
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X-ray Generation
K lines L lines
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X-ray line series
SnSn SnSn
Sn
Sn
Sn
2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
keVFull Scale 32058 cts Cursor: 2.614 (1014 cts)
Sn
Ca
Ca
2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
keVFull Scale 69667 cts Cursor: 2.611 (653 cts)
Ca
Ca K series 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
Cr Fe
CrNi
Fe
-Mn
+Mn
-Co
Mn+Co
Co
5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2
keVFull Scale ED 78582 cts Full Scale WD 30 (100xcts/s) Cursor: 5.085
Spectrum 1
PrLaCe
PrCeNd Pr Pr
Nd
La
NdLa
La
CePr Ce
La
Nd
LaNd
Ce
Ce
4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2
keVFull Scale ED 1552 cts Full Scale WD 18814 (100xcts/s) Cursor: 4.146
rare earth
resolution
sensitivity
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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
Ca
CrC CrCa
Cr
Mg
Al
CrO
Si
1 2 3 4 5 6 7 8
keVFull Scale 1042 cts Cursor: 0.211 (21 cts)
Cr FeMg
Cr CrFe
Fe
1 2 3 4 5 6 7 8
keVFull Scale 94 cts Cursor: 0.241 (4 cts)
Fe
Fe
Cr
FeAl
Mg
Cr
Cr
O
1 2 3 4 5 6 7 8 9
keVFull Scale 1421 cts Cursor: 0.221 (26 cts)
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Interaction volume
1µm
Ni 5kV
Si 5kV
0.1 µm
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
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
Ti
Ti Ti NiCr
Mo
Al Fe
MoCr
Fe
Ni Cr Ni
Fe
1 2 3 4 5 6 7 8 9
keVFull Scale 4591 cts Cursor: 0.077 (161 cts)
Grid Spectrum(3,3)
• 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
• Typically 0.1% to 0.5% for ED• Order of magnitude better for WD
• Exact detection limit depends on operating conditions and composition
Cr Fe
Cr Ni
Fe
Ti
-Mn
+Mn
Co
+Co
Co
-Co
Mn
5 5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8
keVFull Scale ED 72759 cts Full Scale WD 32 (100xcts/s) Cursor: 4.908
Spectrum 5
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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
