Date post: | 22-Jan-2018 |
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Steve Seddio
WDS spectrometer technologies:Not all WDS spectrometers are created equally
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What is WDS?
nλ = 2d sinθ
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• The result:
• Best energy resolution
• Best peak-to-background
Why WDS?
EDS
WDS
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The Challenge of WDS
• Hypothetical sample in an SEM
• For microanalysis, sample is normal to the electron beam
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The Challenge of WDS
• X-rays are emitted from the excitation volume in a
3-dimensional hemispherical wavefront
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The Challenge of WDS
• Get diverging X-rays to diffract off of a diffractor and then be
counted by a detector at a meaningful intensity
• Flat diffractors:
• Simple geometry
• X-rays interact with diffractor with different θ
• X-rays continue to diverge after diffraction → low count rates at detector.
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Meeting the Challenge
• All WDS spectrometers consist of
• Diffractor
• Proportional counter (a.k.a., detector)
• Flowing P10 gas (90% Ar, 10% CH4)
• Sealed Xe
• Some WDS spectrometers include an X-ray optic near sample
• There are two types of spectrometers that have been
developed to meet the WDS challenge
• Rowland circle WDS
• Parallel beam WDS
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Meeting the Challenge: Intensity
• For WDS, the count rate is a function of
• X-ray generation• Accelerating voltage
• Beam current
• Vacuum
• Sample composition
• Sample preparation
• Solid Angle and Optic • Diffractor size
• Diffractor distance from sample
• Size of optic
• Reflectance and/or transmittance of optic
• Other• Diffractor “reflectance”
• Type of gas in detector
• Pressure of gas in detector
• Transmittance of detector window
• Ambient temperature
• Presence / transmittance of spectrometer window
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Rowland Circle WDS: The Classic Solution (c. 1882)
• Curved diffractors focus the diffracted X-rays on to detector
• Requires complex motion so that the sample, diffractor, and
detector remain on a circle of fixed radius
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Rowland Circle WDS: The Classic Solution
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Rowland Circle WDS: The Classic Solution
• To maintain RC geometry, diffractor must move away from
sample for low-energy X-rays
• The result is a decrease in solid angle → low intensity (c/(s×nA))
for low energy X-rays
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Rowland Circle WDS: The Classic Solution
• Diffractor rotation affects solid angle
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Rowland Circle WDS: Solid Angle
102030405060
Solid
Angle
θ
Solid angle as a function of θ
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Rowland Circle WDS: SEM Solid Angle
Mo/B4C
2d = 200
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
Moon or Sun
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Mo/B4C or C/W
2d = 145
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Ni/C or C/W
2d = ~100
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Ni/C or Cr/Sc or C/W
2d = ~80
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
W/Si or C/W
2d = 60
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
W/Si
2d = 45
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
20
X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
TAP
2d = 25.757
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
PET
2d = 8.742
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
LiF (200)
2d = 4.027
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Ang
le (
sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
LiF (220)
2d = 2.848
Rowland Circle WDS: SEM solid angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
210 mm SEM
SAdiff = 661 mm2
24
X-ray Energy (keV)
0.1 1 10
Solid
Angle
(sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Rowland Circle WDS: SEM Solid Angle
Solid angle as a function of energy
210 mm SEM
SAdiff = 661 mm2
Be B O Al Si Ti Fe Cu Sr
LiF (200)2d = 4.027
PET2d = 8.742
TAP2d = 25.757
Mo/B4C2d = 200
Cr/Sc2d = ~80
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Rowland Circle WDS: The Electron Microprobe (EPMA)
• Electron microscope with typically 5 RC-WDS spectrometers
• Spectrometers can concurrently analyze the same or different elements
• 2 or 4 diffractors in each spectrometer
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X-ray Energy (keV)
0.1 1 10
Solid
Angle
(sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Rowland Circle WDS: EPMA Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
160 mm EPMA
SAdiff = 704 mm2
210 mm SEM
SAdiff = 661 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Angle
(sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Rowland Circle WDS: EPMA Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
160 mm EPMA
SAdiff = 704 mm2
210 mm SEM
SAdiff = 661 mm2
160 mm EPMA
SAdiff = 1320 mm2
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X-ray Energy (keV)
0.1 1 10
Solid
Angle
(sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Rowland Circle WDS: EPMA Solid Angle
Solid angle as a function of energy
Be B O Al Si Ti Fe Cu Sr
160 mm EPMA
SAdiff = 704 mm2
210 mm SEM
SAdiff = 661 mm2
160 mm EPMA
SAdiff = 1320 mm2
LiF (200)2d = 4.027
PET2d = 8.742
TAP2d = 25.757
Ni/C2d = ~100
W/Si2d = 60
W/Si2d = 45
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Rowland Circle WDS: The Classic Solution
• Pros
• Excellent energy resolution
• Excellent peak-to-background
• Cons
• Complicated spectrometer geometry
• Best results when the chamber is designed for the spectrometer.
• X-rays measured with large θ have low intensities
• Typically relies on an optical microscope to ensure sample is at proper
working distance
Not commonly available to the SEM user.
Requires a horizontal geometry
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Parallel Beam WDS: A Modern Approach
• Parallel beam WDS spectrometer
• Collimating optic is located near (~ 20 mm) from sample
• Grazing incidence
• Polycapillary
• Hybrid
• Parallel X-ray beam incident on diffractor
• No Rowland circle geometry needed
• Diffractor is flat
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Parallel Beam WDS: A Modern Approach
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X-ray Energy (keV)
0.1 1 10
Solid
Angle
(sr)
0.00
0.01
0.02
0.03
0.04
0.05
0.06
Parallel Beam WDS: A Modern Approach
Polycapillary
Be B O Al Si Ti Fe Cu Sr
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X-ray Energy (keV)
0.1 1 10
Solid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Parallel Beam WDS: A Modern Approach
Solid Angle ≠ IntensityGrazing incidence
Polycapillary
Hybrid
Be B O Al Si Ti Fe Cu Sr
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X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Parallel Beam WDS: A Modern Approach
Grazing incidence
Polycapillary
Hybrid
Be B O Al Si Ti Fe Cu Sr
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X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Parallel Beam WDS: A Modern Approach
Grazing incidence
Polycapillary
Hybrid
210 mm SEM
Be B O Al Si Ti Fe Cu Sr
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Parallel Beam WDS: A Modern Approach
• Pros
• Excellent energy resolution
• Excellent peak-to-background
• Excellent intensity
• Cons
• Cannot accommodate a slit to modestly improve energy resolution with
a large intensity cost
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Head-to-Head Comparison: SEM and EMP
• Identical accelerating voltage and beam current
• PB-WDS with hybrid optic
• 1 spectrometer
• Sealed Xe detector
• Flat diffractors
• 160 mm RC-WDS
• 5 spectrometers
• 3 detectors with 0.1 atm P10 flow
• 2 detectors with 1 atm P10 flow
• Curved diffractors (Johann and Johansson)
X-ray Energy (keV)
0.1 1 10E
ffective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Seddio and Fournelle 2015
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X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Head-to-Head Comparison: SEM and EMP
LiF (200)2d = 4.027
LiF (220)
2d = 2.848
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Head-to-Head Comparison: SEM and EMP
X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
LiF (200)2d = 4.027
40
X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Head-to-Head Comparison: SEM and EMP
PET2d = 8.742
LiF (200)2d = 4.027
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Head-to-Head Comparison: SEM and EMP
X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
TAP2d = 25.757
PET2d = 8.742
42
Head-to-Head Comparison: SEM and EMP
X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
TAP2d = 25.757
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Head-to-Head Comparison: SEM and EMP
X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Ni/C2d = ~100
W/Si2d = 60
44
Head-to-Head Comparison: SEM and EMP
X-ray Energy (keV)
0.1 1 10
Effective S
olid
Angle
(sr)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Ni/C2d = ~100
Mo/B4C2d = 200
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Conclusions
• RC-WDS
• Excellent solution when microscope is designed primarily for WDS
• Yields impractically low intensities on SEMs
• PB-WDS
• Unrivaled low energy X-ray intensity
• High energy X-ray intensities consistent with the best RC-WDS
intensities on an SEM