EDS Energy Dispersive Spectroscopy

1
EDSEnergy Dispersive Spectroscopy
2
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

3
Introduction to the EDS System

• Hardware
• Software

4
Hardware Schematic
SEM Column
Monitor (MCA Display)
Dewar
FET
Preamp
Chamber
Pole Piece
HP Computer
Detector
EDAM III
Window
PCI
Sample Stage
Collimator
5
Processing Schematic
Spectrum Interpretation
Electron Beam
Signal Processing
Beam- Specimen Interaction
Signal Detection
X-Ray Signal
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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
La
Lb
Ka
Kb
Ma
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 (Z4) to Ga (Z31)
• L Lines - S (Z16) to Au (Z79)
• M Lines - Zr (Z40) to the highest occurring
  atomic numbers.

Every element (Zgt3) will have at least one line
viewable between 0.1 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
primary beam
sample surface
se
bse
This diagram is somewhat misleading. High-energy
and low-energy x rays behave very differently
(just like e-).
x-rays
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
Spot size does not determine the reso-lution but
kV and Z are more significant.
High Z
Low kV
High kV
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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.

x-ray
se
bse
sample surface
se
bse
x-rays
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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
Detector
Electron Beam
Backscatter electrons
Fluorescence
X-rays
Specimen Matrix
Interaction volume
Absorption of x-rays
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Directionality of X-ray Signal
Detector Direction
A B C
stage/mount
sample
Topography has a significant effect on spectrum
count rate and on composition (take-off angle
and absorption effects)
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B
A Lower low end peaks B Normal C Higher low
end peaks
C
A
3 different spectra at 3 locations on the same
particle with a uniform composition.
Take-off angle is highest at C and lowest at A.
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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)
-500 to 1000 volts
,- charges
x-ray (photon) microscope vacuum
to preamplifier (FET)
Detector Vacuum
Detector
SiLi
Detector Window 8u Be or 0.3u Polymer
Metallization Layer,(85 angstroms) plus the Si
dead layer
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Detector EfficiencyWindow Transmission
Capabilities

• I / Io e -(mr t)
• Where
• I Final Intensity
• Io Initial Intensity
• m mass absorption coefficient
• r density
• t thickness

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Transmission of K x-rays through various windows
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Mass Absorption Coefficient
N Ka Energy
Absorption edge or critical excita- tion energy
Absorption
(Kab)
C
C Ka Energy
0.284
X-ray Energy (keV)
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Absorption evidence in Spectra
The background is lower on the high-energy side
due to absorption in the sample.
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Solid Angle

• W 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
50 ns/x-ray event
C
Output
FET
Ultimate peak measurement time will be about 50
us (1000x 50 ns)
Reset
Detector
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Output signal of an X-Ray Event (or 3 events)
Higher dead time (all rejected)
Voltage (mv)
Lower dead time
v
Time
Multiple x-ray events too close to each other
will be rejected.
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Throughput Curves
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 Equation

• FWHM SQRT(FWHM)noise2 (2.35 FEe)2
• Where
• F fano factor 0.11
• E energy of the x-ray, ev
• e 3.8 ev/charge pair (Si), 2.96
  ev/charge pair (Ge)

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Resolution vs Energy for 70ev noise
Mn
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Collimators
Be Window with no magnets (BSE do not penetrate)
SUTW or UTW Window with magnets (shown in yellow)
to deflect BSE
If BSE reach the detector they will
produce background anomalies --a hump in
the background at high energies.