choosing the right eds detector - thermo scientific

Choosing the right EDS detectorKeith Thompson

April 7 2014

2 Proprietary & Confidential

EDS and SEM go hand-in-hand

Electron Microscopy provides the imaging

EDS provides the “chemistry”


EDS provides a look at material composition

Point & Shoot, Line Scan and Mapping


Electron Microscopes

• Material Science• Electronics• Petrochemical• Mining• Metals• Semiconductor• Life Science

Many options for electron microscopes

Many options for EDS


EDS detector advances

Larger active areasVarious tube size… even oval tubes

Faster Acquisitions


• There are 3 main drivers in specifying an EDS detector

• Energy resolution @ Mn k-alpha• Sensitive to• Solid angle

• How relevant are these specifications in determining the performance of an EDS detector?

• How do I choose the right detector for my lab?

Energy Resolution

Which detector is right for my application?


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?


How x-ray detection works:

X-rays generate a small current of e- – hole pairs

Next stage: charge-sensitive pre-amplifier stage

Next stage: Pulse processor measures the amplitude


Signal

Time

Function of Silicon Drift Detector

Charge Collection:

Event 1 signal 1

Event 2 signal 2

Event 3 signal 3

t1

t2

t3


0

5

10

15

20

25

30

35

0 100 200 300 400

Volta

ge

Time

0

0.2

0.4

0.6

0.8

1

1.2

50 60 70 80 90 100 110 120 130 140 150

Energy (keV)

Coun

ts

Peaking time

X-ray induced voltage step

Every system has noise. This noise drives energy resolution


Sources of uncertainty: Noise

• Internal or “system”• Inherent in the system: SDD module, wire bonding, cabling, electronics and

other overall architecture design

• Measured in the factory in a “golden” environment.

• External or “Environmental” • Any external noise source: motors, old equipment, switching circuits, EM

interference, UPS, poor power, ground-loops and so forth


System noise

• 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


Get much smallerAnd have potential to get smaller still.


Move averaging, less uncertainty, better resolution

Less averaging, more uncertainty, worse resolution


Longer integration times result in superior energy resolutionShorter integration times are required for high count rates


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


Take-away points

• Energy resolution• Laboratory noise impacts performance• speed matters: faster acquisition = worse resolution

• Detectors are specified at slow rates



Often-times 185 eV is just fine

2000 eVpeak-to-peak

2000 eVpeak-to-peak



Sometimes we need 4 eV


Take-away points


• Detectors are specified at slow rates• Sometimes poor resolution is just fine• Other times the resolution will never be good enough


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


What does high count rate look like?

Difficult to even define resolution @ 1,000,000 cps

(input cps)Standard EDS


Can we improve on this?

It is possible to design an SDD with excellent low energy performance

76 eV78 eV114 eV

66 eV67 eV90 eV

55 eV61 eV114 eV

(input cps)Extreme EDS


Can we improve on this?

It is possible to design an SDD with excellent low energy performance

57 eV67 eV

50 eV62 eV

50 eV62 eV

(input cps)Extreme EDS


Take-away points


• 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


• High quality light element energy resolution

• Low quality light element energy resolution

• Can we quantify the difference between these two detector designs?


A look at the numbers

Group 1 = Standard EDSGroup 2 = Extreme EDS


A look at the numbers

All “129 eV” detectors~ 10 eV difference in light element

Group 1 = Standard EDSGroup 2 = Extreme EDS


How much of an impact?

Extreme detector, 10 kcps

Standard detector, 10 kcps

Standard detector, 1 Mcps


Take-away points

• Energy resolution• Laboratory noise impacts performance

• speed matters: faster acquisition = worse resolution


• Sometimes poor resolution is just fine

• Other times the resolution will never be good enough


• Energy resolution specifications up at Mn –ka (5.9 keV) are not reflective of the performance in the low energy spectrum.

Light element sensitivity“Sensitive to ”Which detector is right for my application?


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?



• 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


X-ray absorption in windows

NaOB


X-ray absorption in windows

O

BLi Be

Li detection is not possible with a window and has challenges well beyond window technology


Light element sensitivity: “Sensitive to Be”

Extreme EDS


Light element sensitivity: “Sensitive to BN”

Extreme EDS


Light element sensitivity: “Sensitive to B”

Compact EDS

Pure B metal


Compact EDS detector



8x sensitivity Extreme EDS

Compact EDS



0

1000

2000

3000

4000

5000

6000

0200400600800

100012001400160018002000

80 180 280 380 480

EDS

coun

ts

WD

S co

unts

Energy eV)

B - WDSB - 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

coun

ts

WDS

cou

nts

Energy eV)

B - WDS

B - EDS

B metal is easy for EDS/WDS

B


Take-away points


• speed matters: faster acquisition = worse resolution• Detectors are specified at slow rates





• Sensitivity• Detection to B isn’t always detection to B


•Is light element sensitivity just about my detector and window technology?


Light element sensitivity: Variable pressure mode



C Cu-L

Pure B metal



No VP

50 Pa

200 Pa

No VP

Extreme detector



Take-away points








• Variable pressure mode has a major impact on light element detection


Light Element Detection – Li mapping

Windowless Extreme EDS detector


Take-away points









• The technology for light element detection exists today. You need to specifically ask & plan for it.



•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.

What detector is best for my application?


Point & Shoot analysis of geological



Spectral Imaging Map: Geological


Example – Multiphase sample with peak overlaps

• Detector: UltraDry 10mm2 SDD• Resolution: 129eV MnK FWHM• Accelerating Voltage: 7kV• Magnification: 500x• Map resolution: 256x192• Storage Rate: 107,000cps• Acquisition Time: 5 minutes


Example – Multiphase Sample – Raw Count Maps

Si_KTa_MW_M

Ni_LCu_L


Multiphase Sample – Net Count Maps


Multiphase Sample – Net Count Maps

Si_KTa_MW_M

Si, Ta, W: No detector can separate these peaks

Peak deconvolution algorithms cleanly separate the peaks


Example – Mo, S, Ba Multiphase Sample

• Detector: UltraDry 10mm2 SDD• Resolution: 129eV MnK FWHM• Accelerating Voltage: 7kV• Magnification: 500x• Map resolution: 256x192• Acquisition Time: 3 minutes


Example – Mo, S, Ba – Raw Count Element Maps


Example – Mo, S, Ba – Net Count Maps


Example – Mo, S, Ba – Phase Maps

Distinguishing the three main phases is not possible without robust peak deconvolution

2100 2150 2200 2250 2300 2350 2400 2450 2500

eV

MoL SK


Take-away points

• Energy resolution

• Laboratory noise impacts performance

• Speed matters: faster acquisition = worse resolution




• Low energy part of the spectrum is affected more dramatically than the moderate to higher part of the

spectrum

• Energy resolution specifications up at Mn –ka (5.9 keV) are not reflective of the performance in the low

energy spectrum.

• Sensitivity

• Detection to B isn’t always detection to B


• The technology for light element detection exists today. You need to specifically ask & 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


• EDS detector have made many advances over the last several years

• 3 main drivers in specifying an EDS detector

• Energy resolution @ Mn k-alpha

• Sensitive to

• Solid angle

• Actually important

• Energy resolution in low energy

• Actual sensitivity of light element

• Actual throughput

• Most important: Peak deconvolution and net counts mapping

• The most important factor is careful discernment of the EDS detector and the

overall EDS system

choosing the right eds detector - thermo scientific

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