Download - EDS Energy Dispersive Spectroscopy
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EDS
Energy Dispersive Spectroscopy
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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
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Introduction to the EDS System
Hardware Software
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Hardware Schematic
Monitor (MCA Display)
HPComputer
EDAMIII
PCI
Dewar
Preamp
SEMColumn
Pole Piece
SampleStage
Chamber
Detector
Window
Collimator
FET
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Processing Schematic
SpectrumInterpretation
SignalProcessing
SignalDetection
X-RaySignal
Beam-SpecimenInteraction
ElectronBeam
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X-Ray Signal Generation
Signal Origin Spatial Resolution Directionality of Signals Analysis of Rough Surfaces or
Particles
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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
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Atomic Number Order for the K Series Peaks
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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.
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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.
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SE vs BSE Images
SE -- Edge effect, charge sensitive, very little Z contrast.
BSE --Z contrast dominates, no edge effect, no charging seen.
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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.
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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
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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
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Spectrum Anomalies
Absorption of x-rays
Detector
Electron Beam
Fluorescence
X-rays
Interactionvolume
Specimen Matrix
Backscatterelectrons
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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)
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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.
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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.
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EDS Instrumentation & Signal Detection
X-Ray Detectors The Detector Efficiency Geometrical Efficiency Signal Processing The Signal Processor Energy Resolution Collimation
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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
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Detector EfficiencyWindow Transmission Capabilities
I / Io = e -( t)
Where :
I = Final Intensity
Io = Initial Intensity
= mass absorption coefficient
= density
t = thickness
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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%
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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
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Absorption evidence in Spectra
The background is lower on the high-energy side due toabsorption in the sample.
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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.
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The Preamplifier
Detector
Reset
FET
C Output50 ns/x-ray event
Ultimate peak measurement time will be about 50 us (1000x 50 ns)
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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
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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.
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Multichannel Analyzer
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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)
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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
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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.