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