Wds technology june 2015

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Steve Seddio

WDS spectrometer technologies:Not all WDS spectrometers are created equally

What is WDS?

nλ = 2d sinθ

• The result:

• Best energy resolution

• Best peak-to-background

Why WDS?

The Challenge of WDS

• Hypothetical sample in an SEM

• For microanalysis, sample is normal to the electron beam

• X-rays are emitted from the excitation volume in a

3-dimensional hemispherical wavefront

• 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.

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

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

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

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

• Diffractor rotation affects solid angle

Rowland Circle WDS: Solid Angle

102030405060

Solid angle as a function of θ

X-ray Energy (keV)

0.1 1 10

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

X-ray Energy (keV)

0.1 1 10

Mo/B4C or C/W

2d = 145

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

Ni/C or C/W

2d = ~100

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

Ni/C or Cr/Sc or C/W

2d = ~80

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

W/Si or C/W

2d = 60

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

2d = 45

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

2d = 25.757

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

2d = 8.742

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

LiF (200)

2d = 4.027

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

LiF (220)

2d = 2.848

Rowland Circle WDS: SEM solid angle

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

210 mm SEM

SAdiff = 661 mm2

LiF (200)2d = 4.027

PET2d = 8.742

TAP2d = 25.757

Mo/B4C2d = 200

Cr/Sc2d = ~80

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

X-ray Energy (keV)

0.1 1 10

Rowland Circle WDS: EPMA Solid Angle

160 mm EPMA

SAdiff = 704 mm2

210 mm SEM

SAdiff = 661 mm2

X-ray Energy (keV)

0.1 1 10

160 mm EPMA

SAdiff = 704 mm2

210 mm SEM

SAdiff = 661 mm2

160 mm EPMA

SAdiff = 1320 mm2

X-ray Energy (keV)

0.1 1 10

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

• 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

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

X-ray Energy (keV)

0.1 1 10

Polycapillary

X-ray Energy (keV)

0.1 1 10

Solid Angle ≠ IntensityGrazing incidence

Polycapillary

Hybrid

X-ray Energy (keV)

0.1 1 10

Effective S

Grazing incidence

Polycapillary

Hybrid

X-ray Energy (keV)

0.1 1 10

Effective S

Grazing incidence

Polycapillary

Hybrid

210 mm SEM

• 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

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

Seddio and Fournelle 2015

X-ray Energy (keV)

0.1 1 10

Effective S

LiF (200)2d = 4.027

LiF (220)

2d = 2.848

X-ray Energy (keV)

0.1 1 10

Effective S

LiF (200)2d = 4.027

X-ray Energy (keV)

0.1 1 10

Effective S

PET2d = 8.742

LiF (200)2d = 4.027

X-ray Energy (keV)

0.1 1 10

Effective S

TAP2d = 25.757

PET2d = 8.742

X-ray Energy (keV)

0.1 1 10

Effective S

TAP2d = 25.757

X-ray Energy (keV)

0.1 1 10

Effective S

Ni/C2d = ~100

W/Si2d = 60

X-ray Energy (keV)

0.1 1 10

Effective S

Ni/C2d = ~100

Mo/B4C2d = 200

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

Wds technology june 2015

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