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Choosing the right
EDS detector Keith Thompson
April 28 2014
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EDS and SEM go hand-in-hand
Electron Microscopy provides the imaging
EDS provides the “chemistry”
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EDS provides a look at material composition
Point & Shoot, Line Scan and Mapping
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Electron Microscopes
• Material Science
• Electronics
• Petrochemical
• Mining
• Metals
• Semiconductor
• Life Science
Many options for
electron microscopes
Many options for EDS
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EDS detector: many features to consider
Many active areas Various tube size… even oval tubes
Ever Faster Acquisitions
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6 Proprietary & Confidential
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• There are 3 main drivers in specifying an EDS
detector
• Energy resolution @ Mn k-alpha
• Sensitive to: “Xx”
• Solid angle
• How relevant are these specifications in determining
the performance of an EDS detector?
• How to choose the right EDS detector
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Energy Resolution
Which detector is right for my application?
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Energy resolution
Measured:
Width of the Mn k-alpha peak @
half the peak height
- Why does this spread occur?
- What is good enough?
- Is this a valuable metric?
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Energy resolution
System noise drives uncertainty
in the measured energy of the
incident X-ray.
This uncertainty creates a
natural variation in the energy
histogram
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System noise drives energy resolution
• In a SiLi-based EDS, the capacitance is directly related to the active
area of the device.
• Capacitance starts as BIG
• As active area increases, capacitance increases
Needs LN
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SDD crystals change the equation on resolution
Get much smaller
And have potential to get smaller still.
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Longer integration times result in superior energy resolution
Shorter integration times are required for high count rates
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What does high count rate resolution look like?
123 eV @ 6.4 msec
183 eV @ 0.2 msec
EDS detectors are routinely specified @ 2,000 – 3,000 cps
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
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What does high count rate resolution look like?
Often-times 185 eV is just fine
2000 eV
peak-to-peak
2000 eV
peak-to-peak
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Point & Shoot analysis of a geological sample
Qualitative Identification
Often-times higher energy
resolution is just fine
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Point & Shoot analysis of a geological sample
Mapping
Fast mapping higher resolution
is just fine on this sample.
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What does high count rate resolution look like?
Sometimes no resolution
is ever good enough.
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
• Sometimes poor resolution is just fine
• Other times even really good resolution will never be good enough
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What does high count rate look like?
128 eV @ 6.4 msec
160 eV @ 0.4 msec
How about down here?
(input cps)
(standard EDS)
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What does high count rate look like?
Difficult to even define resolution
@ 1,000,000 cps
(input cps)
(standard EDS)
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Can we improve on this?
It is possible to design an SDD with excellent low energy performance
76 eV
78 eV
114 eV
66 eV
67 eV
90 eV
55 eV
61 eV
114 eV
(input cps)
(Extreme EDS)
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25 Proprietary & Confidential
Can we improve on this?
It is possible to design an SDD with excellent low energy performance
57 eV
67 eV
50 eV
62 eV
50 eV
62 eV
(input cps)
(Extreme EDS)
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A look at the numbers
Group 1 = Extreme EDS
Group 2 = standard EDS Study involved over 300 detectors
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A look at the numbers
All “129 eV” detectors ~ 10 eV difference in light element
Group 1 = Standard EDS
Group 2 = Extreme EDS
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How much of an impact?
Extreme detector, 10 kcps
Standard detector, 10 kcps
Standard detector, 1 Mcps
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
• Sometimes poor resolution is just fine
• Other times the resolution will never be good enough
• Low energy part of the spectrum is affected more dramatically than the
moderate to higher part of the spectrum
• Some detectors hold it together
• Other detectors falls apart
• Specifications at Mn-ka (5.9 keV) are not reflective of low energy
performance.
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Light element sensitivity
“Sensitive to:” Which detector is right for my application?
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Light element sensitivity: “Sensitive to”
•EDS detectors often carry a light element
sensitivity specification termed as:
• “Sensitive to”
•Why this specification?
•What does it really indicate?
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Light element sensitivity: “Sensitive to”
• The detector system absorbs x-rays
• Window between SEM chamber and crystal
• Thin metal layer on detector crystal to avoid cathodoluminescence
• Some detectors use N2 backfill
window
detectorcrystal
pre-amp
cold finger
insulator
X-raysliquid Nitrogen
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X-ray absorption in windows
Na
O B
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X-ray absorption in windows
O
B Li Be
Li detection is not possible with a window and has challenges well beyond
window technology
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Light element sensitivity: “Sensitive to Be”
Extreme EDS
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Light element sensitivity: “Sensitive to BN”
Extreme EDS
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Light element sensitivity: “Sensitive to B”
Compact EDS
Pure B metal
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Compact EDS detector
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Light element sensitivity: “Sensitive to B”
8x sensitivity Extreme EDS
Compact EDS
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Light element sensitivity: “Sensitive to B”
0
1000
2000
3000
4000
5000
6000
0
200
400
600
800
1000
1200
1400
1600
1800
2000
80 180 280 380 480
EDS
cou
nts
WD
S co
un
ts
Energy eV)
B - WDS
B - EDS
Trace B (2% B in Fe-Cr) is harder
B
C
0
2000
4000
6000
8000
10000
0
20000
40000
60000
80000
100000
120000
80 180 280 380 480
EDS
cou
nts
WD
S co
un
ts
Energy eV)
B - WDS
B - EDS
B metal is easy for EDS/WDS
B
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
• Sometimes poor resolution is just fine
• Other times the resolution will never be good enough
• Low energy part of the spectrum is affected more dramatically than the
moderate to higher part of the spectrum
• Some detectors hold it together
• Other detectors falls apart
• Specifications at Mn-ka (5.9 keV) are not reflective of low energy
performance.
• Sensitivity
• Detection to B isn’t always detection to B
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•Is light element sensitivity just
about my detector and window
technology?
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Light element sensitivity: Variable pressure mode
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Light element sensitivity: Variable pressure mode
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Light element sensitivity: Variable pressure mode
C Cu-L
Pure B metal
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Light element sensitivity: Variable pressure mode
No VP
50 Pa
200 Pa
No VP
Extreme detector
Compact EDS detector
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
• Sometimes poor resolution is just fine
• Other times the resolution will never be good enough
• Low energy part of the spectrum is affected more dramatically than the moderate to higher part of the spectrum
• Some detectors hold it together
• Other detectors falls apart
• Specifications at Mn-ka (5.9 keV) are not reflective of low energy performance.
• Sensitivity
• Detection to B isn’t always detection to B
• Variable pressure mode has a major impact on light element detection
• As do many, many other factors.
• The only good light element Quant requires full standards
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Light Element Detection – Li mapping
Windowless Extreme EDS detector
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
• Sometimes poor resolution is just fine
• Other times the resolution will never be good enough
• Low energy part of the spectrum is affected more dramatically than the moderate to higher
part of the spectrum
• Some detectors hold it together
• Other detectors falls apart
• Specifications at Mn-ka (5.9 keV) are not reflective of low energy performance.
• Sensitivity
• Detection to B isn’t always detection to B
• Variable pressure mode has a major impact on light element detection
• As do many, many other factors.
• The only good light element Quant requires full standards
• The technology for light element detection exists today. You need to specifically plan for it.
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Light element sensitivity: “Sensitive to”
•Determine what you need
• Is it important to your application? • Light element detection? Or mapping?
• Transition metals?
• Do you work in VP mode?
• How critical is quant?
•Be specific & avoid ambiguity.
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What detector is best for my
application?
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Transition metal: Qualitative analysis
10 mm2 active area
133 eV @ Mn k-a
Sensitive to B
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Point & Shoot analysis of a geological sample
Qualitative Identification
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Point & Shoot analysis of a geological sample
Mapping
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High resolution Spectral Imaging @ low kV
3 kV, 0.5 nA
100 mm2 active area
129 eV @ Mn k-a
Sensitive to Be
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Complex structures in a TEM
Quantitative element map of an advanced
semiconductor gate structure
100 mm2 active area
129 eV @ Mn k-a
Sensitive to Be
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Geological mapping at 5 kV
5 kV, 2 nA
100 mm2 active area
129 eV @ Mn k-a
Sensitive to Be
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Example – Multiphase sample with peak overlaps
• Detector: UltraDry 10mm2 SDD
• Resolution: 129eV MnKa FWHM
• Accelerating Voltage: 7kV
• Magnification: 500x
• Map resolution: 256x192
• Storage Rate: 107,000cps
• Acquisition Time: 5 minutes
10 mm2 detector @ high beam current (> 25 nA)
60 mm2 detector @ low beam current (< 2 nA)
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Example – Multiphase Sample – Raw Count Maps
Si_K
Ta_M
W_M
Ni_L
Cu_L
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Multiphase Sample – Net Count Maps
Si_K
Ta_M
W_M
Si, Ta, W: No detector can separate these peaks
Peak deconvolution algorithms cleanly separate the peaks
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Example – Mo, S, Ba Multiphase Sample
• Detector: UltraDry 10mm2 SDD
• Resolution: 129eV MnKa FWHM
• Accelerating Voltage: 7kV
• Magnification: 500x
• Map resolution: 256x192
• Acquisition Time: 3 minutes
10 mm2 detector @ high beam current (> 25 nA)
60 mm2 detector @ low beam current (< 2 nA)
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Example – Mo, S, Ba – Raw Count Element Maps
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Example – Mo, S, Ba – Net Count Maps
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Example – Mo, S, Ba – Phase Maps
Distinguishing the three main phases is not
possible without robust phase identification
involving Principle Component Analysis 2100 2150 2200 2250 2300 2350 2400 2450 2500
eV
MoLa SKa
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Take-away points
• Energy resolution
• Speed matters: faster acquisition = worse resolution
• Detectors are specified at slow rates
• Sometimes poor resolution is just fine
• Other times the resolution will never be good enough
• Low energy part of the spectrum is affected more dramatically than the moderate to higher part of the spectrum
• Some detectors hold it together
• Other detectors falls apart
• Specifications at Mn-ka (5.9 keV) are not reflective of low energy performance.
• Sensitivity
• Detection to B isn’t always detection to B
• Variable pressure mode has a major impact on light element detection
• As do many, many other factors.
• The only good light element Quant requires full standards
• The technology for light element detection exists today. You need to specifically plan for it.
• Post-processing algorithms
• Peak deconvolution, background subtraction and matrix correction algorithms are critical to high quality mapping
• Phase mapping is even more powerful than these element mapping algorithms
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Summary
• EDS detectors have made many advances over the last
several years
• Know your application
• Discern what’s really important.
• Energy resolution
• Light element / Low energy
• Algorithms and post-processing techniques
• Specify carefully.