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EDS Energy Dispersive Spectroscopy

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Page 1: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

EDS

Energy Dispersive Spectroscopy

Page 2: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Background Theory Introduction to the EDS System

– Hardware & Software X-Ray Signal Generation

– Signal Origin, Spatial Resolution, Direction of Signal, Sample Surface

EDS Instrumentation & Signal Generation– Detector and geometry efficiency, Signal

processing, Energy Resolution, Collimation

Page 3: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Introduction to the EDS System

Hardware Software

Page 4: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Hardware Schematic

Monitor (MCA Display)

HPComputer

EDAMIII

PCI

Dewar

Preamp

SEMColumn

Pole Piece

SampleStage

Chamber

Detector

Window

Collimator

FET

Page 5: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Processing Schematic

SpectrumInterpretation

SignalProcessing

SignalDetection

X-RaySignal

Beam-SpecimenInteraction

ElectronBeam

Page 6: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

X-Ray Signal Generation

Signal Origin Spatial Resolution Directionality of Signals Analysis of Rough Surfaces or

Particles

Page 7: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Bohr Model of the Atom (a simplified view) ---where X rays come from

M

L

L

K

K

Real life spectra are more complex because there are multiple orbitals (esp. for the L, M and N orbitals). L-series spectra in EDS can have 6 or 7 peaks.

Nucleus

Page 8: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Atomic Number Order for the K Series Peaks

Page 9: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Chart of Lines visible 0-10 kV

K Lines - Be (Z=4) to Ga (Z=31) L Lines - S (Z=16) to Au (Z=79) M Lines - Zr (Z=40) to the highest

occurring atomic numbers.

Every element (Z>3) will have at least one line viewable between0.1 and 10 keV. In some overlap conditions it might be necessary to examine the area between 10 and 20 keV.

Page 10: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Interaction Volume Regions

sebse

x-rays

samplesurface

primarybeam

This diagram is somewhat misleading. High-energy and low-energy x rays behave very differently (just like e-).

High energy x rays can not be excited at great depths. Low energy x rays can be excited at great depths, but will most likely be absorbed and will not escape.

Page 11: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

SE vs BSE Images

SE -- Edge effect, charge sensitive, very little Z contrast.

BSE --Z contrast dominates, no edge effect, no charging seen.

Page 12: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

X-Ray Spatial Resolution

Low Z

High Z

High kV Low kV

Spot size does not determine the reso-lution but kV and Z are more significant.

Page 13: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Signal Resolution Signal resolution (se) is determined by the width of the

electron beam (spot size) and is proportional to the signal depth.

sebse

x-rays

samplesurface

x-ray

bsese

Page 14: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Directionality of Signals

SE Signal - attracted to positive voltage on wire mesh network in front of detector.

BSE Signal - Detector is arranged to collect signals from a large, symmetrical area.

X-ray Signal - most directional of all signals, only one detector with no way to influence the trajectory of x-rays

Page 15: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Spectrum Anomalies

Absorption of x-rays

Detector

Electron Beam

Fluorescence

X-rays

Interactionvolume

Specimen Matrix

Backscatterelectrons

Page 16: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Directionality of X-ray Signal

A B C

DetectorDirection

samplestage/mount

Topography has a significant effect on spectrum count rate and on composition (take-off angle and absorption effects)

Page 17: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

BA C

A= Lower low end peaksB= NormalC= Higher low end peaks

Take-off angle is highest at C and lowest at A.

3 different spectra at 3 locations on the same particle with a uniform composition.

Page 18: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Effects of Tilt (FeCO3)

Peaks are autoscaled to the O K peak. Q: What if they were scaled to the background area? A: FeK same height, C K, O K and FeL would be higher at +30 degrees.

Page 19: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

EDS Instrumentation & Signal Detection

X-Ray Detectors The Detector Efficiency Geometrical Efficiency Signal Processing The Signal Processor Energy Resolution Collimation

Page 20: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

X-section of window & crystal (sapphire)

x-ray(photon)

microscopevacuum Detector

Vacuum

DetectorWindow

8u Be or 0.3u Polymer

+,-charges

Detector

SiLi

to preamplifier

(FET)

Metallization Layer,(85 angstroms) plusthe Si dead layer

-500 to 1000 volts

Page 21: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Detector EfficiencyWindow Transmission Capabilities

I / Io = e -( t)

Where :

I = Final Intensity

Io = Initial Intensity

= mass absorption coefficient

= density

t = thickness

Page 22: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Transmission of K x-rays through various windows

WindowType

B C N O F

8 micronBe

SUTW0.3micron

0%

25%

0%

85%

0%

42%

0%

60%

5%

70%

Page 23: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Mass Absorption Coefficient

0.284

Absorption edgeor critical excita-tion energy

(Kab)C

Ab

sorp

tio

n

X-ray Energy (keV)

C Ka Energy

N Ka Energy

Page 24: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Absorption evidence in Spectra

The background is lower on the high-energy side due toabsorption in the sample.

Page 25: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Solid Angle

= A/d 2

Where:

A= detector area, mm 2

d = the sample to detector distance

The solid angle (omega) is in steradians. Count rate at 70 mm scale setting = 1/4 that at 50 mm.

Page 26: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

The Preamplifier

Detector

Reset

FET

C Output50 ns/x-ray event

Ultimate peak measurement time will be about 50 us (1000x 50 ns)

Page 27: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Output signal of an X-Ray Event (or 3 events)

v

Voltage(mv)

Time

Multiple x-ray events too close to each other will be rejected.

Higher dead time (all rejected)

Lower dead time

Page 28: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Throughput Curves

0

1000

2000

3000

4000

5000

6000

7000

8000

0 5000 10000 15000 20000 25000 30000

Input CPS

Sto

red

CP

S

50 usec

100 usec

Lesson: High count rates and high dead times actually give fewer counts and poorer spectra. You might consider a faster time constant.

Page 29: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Multichannel Analyzer

Page 30: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Resolution EquationFWHM= SQRT[(FWHM)noise

2 + (2.35 FE)2]

Where:

F = fano factor= 0.11

E = energy of the x-ray, ev

= 3.8 ev/charge pair (Si), 2.96 ev/charge pair (Ge)

Page 31: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Resolution vs Energy for 70ev noise

0.00

50.00

100.00

150.00

200.00

250.00

0 10 20Energy, Kev

FW

HM

, e

v

FWHMMn

Page 32: EDS Energy Dispersive Spectroscopy. Background Theory u Introduction to the EDS System –Hardware & Software u X-Ray Signal Generation –Signal Origin,

Collimators

Be Window with no magnets (BSE do not penetrate)

SUTW or UTW Windowwith magnets (shown in yellow) to deflect BSE

If BSE reach the detector they will producebackground anomalies --a hump in thebackground at high energies.