Confidential
Quantrainx50 Module 3.1 Electron
Optics1-2011
place photo here
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2
SEM Main Components
Electron Gun
Demagnification system
Scan Unit
Detecting Unit
Wehnelt cylinderor FEG unit
Condenser lenses
Scan generator
Objective andStigmation lenses
Electron detector
Focus Unit
Scan generator
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3
SEM Main Components
Electron GunWehnelt cylinderFEG Electron Gun
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Electron Gun Emitters
• Tungsten filament (W)
• Lanthanum Hexaboride filament (LaB6)(obsolete)
• Cerium Hexaboride (CeB6)
• Field emission filament (FEG)
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Electron Gun Animation *
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* Video courtesy of Oxford Instruments
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Electron Source Properties• Current density (brightness)
• Emission current
• Stability of source
• Lifetime of filament
• Design of electron source assembly
• Ease of operation
• Costs involved
ą
ip
do
specimen
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7
Emission Area For Tungsten (W)
Filament
Wehnelt cap
Anode
Cross-over plane
Filament heating supply
High voltage supply (200 v- 30 kV)
70 A
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8
Bias on Wehnelt Cap
Equipotential lines of the Voltage Field
High emissionlarge spot
Sufficient emission
small spot
Low bias voltage
0+
Optimum bias voltage
0+
High bias voltage
No emission
+0
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9
Bias 255 ……………………………….. Bias 1
110 µA
90 µA 1 kV 30 kV
Emission : Autobias control
9
Autobias keeps emission between 90-110 µA for all kV
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10
W Filament Saturation
filament current
emis
sion
cur
rent
Saturation point
False peak / Misalignment
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Tungsten Filament
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High Resolution, High Brightness FEG source…
Tungsten LaB6 FEGNormalized Brightness (-) 1 10 1000Maximum probe current (nA) 2000 500 100Life time (hrs) 60-200 200-1000 > 10000Beam current stability (10 hrs) <1% <1% <0.4%Resolution 30kV (nm) 3.0 2.0 1.2Resolution 1kV (nm) 25 15 3.0Cost source (USD) 20 900 26000
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XL Schottky FEG Theory
• The Boersch Effect
• A) Perfect beam: no interactions
• B) Random beam: one dimension
• C) Random beam: two dimensions
• It is actually three
dimensional
ooooooooo
oo
o
ooo
o
oo
oo
o o
o o
o oo o
A B C
1515
XL Schottky FEG Theory
• The Lateral Effect
• lateral trajectory displacement
• This effect results in a larger final spot
• The diameter of the circle of confusion due to this effect.
o o o o o o
o o oo o o
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Lens Defects
image plane
Spherical aberration
Chromatic aberration
Aperture
Diffraction
optical axis
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Spherical Aberrations
• Electrons entering into a lens at different points get focused at different points
Disc of Least Confusion
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Chromatic Aberrations
• Electrons of differing energies will be focused at different places
Disc of Least Confusion
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Diffraction
• The wave nature of electrons cause diffraction limitations
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XL Schottky FEG Theory
• Design Limitations
• Longer electron-electron interaction times and smaller electron-electron distances lead to higher statistical aberrations at low KV
• Chromatic aberration is more dominant at low voltages.
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XL Schottky FEG Theory
• Innovative solutions to reduce design limitations
• A Coulomb tube designed into the column to reduce aberrations and interactions by keeping a high beam energy in the tube
• Effective aperturing of the beam to remove those electrons not contributing to the probe
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FEG Column Principle Diagram
Scan Coils
Gun Alignment Coils
Objective Aperture
10KVDriftSpace(Coulomb Tube)
C1
C2
Objective Lens
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FEG gun (electron source)
Emitter Schottky Cold
Scource size 20nm 2nm
Beam current stability
<1%/hour decreases steadily 10-50%/hour
Flashing not required always needed (daily) depends on vacuum quality
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Emission Area for FEG
Extractor systemC1 static lens
Anode
Filament heating supply
High voltage supply (200 v- 30 KV)
150 A
2525
Schottky Gun Design
• Fil = Filament current input (2.4 Ampere)
• S = Suppressor (-500V)
• E = Extractor (+5000V)
• C1 = Electrostatic Condenser lens
S
E
C1
Fil
E
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Schottky Tip design
• M = Tip module
• W = Welded tungsten Tip
• Fil = Tungsten wire filament
• T = Sharpened Tip
• Zr = Zirconium reservoir Zr
T
Fil
W
M
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FEG Startup Steps
• Warmstart / Coldstart
• Gun conditioning
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Beam Menu
Final operation status
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FEG Column Double condenser lens
• Extraction voltage changes not necessary, beam current is set by condenser lenses
• C1 is electrostatic
• C2 is electromagnetic
• Variable lens strengths:
A = high beam current mode
B = low beam current mode
• Final beam energy 30keV down to 200eV
A B
C1
C2
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FEG Column Double Condenser Lens
• Extraction voltage changes not necessary, beam current is set by condenser lenses
• C1 is electrostatic
• C2 is electromagnetic
• Variable lens strengths:
A = high beam current mode
B = low beam current mode
• Final beam energy 30keV
down to 200eV
A B
C1
C2
InternalSpray
Aperture
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• Different paths for low
and high beam current
conditions through the
coulomb tube, but
common path to objective
Small Spot Large Spot
C1
C2
Aperture
DecelerationLens
FEG Column
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Comparison of Columns(20KV)
Spot W LaB6 FEG
5 1nA-100nM 2Na-59nM 2.4nA- 5nM
6 4nA- 200nM 8nA-100nM 9.5nA-10nM
7 16nA-400nM 30nA-200nM 35nA-20nM
8 64nA-800nM 100nA-400nM NA
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Beam Current: Spotsize 30kV 20kV 10kV 5kV 2kV 1kV 500V
1 21 p 13 p 8 p 5 p 2.5 p 1.4 p 0.7 p
2 44 p 40 p 33 p 25 p 13 p 7 p 4 p
3 154 p 148 p 130 p 98 p 53 p 30 p 16 p
4 625 p 617 p 538 p 398 p 211 p 116 p 62 p
5 2.41 n 2.39 n 2.11 n 1.57 n 840 p 464 p 249 p
6 9.54 n 9.45 n 8.37 n 6.27 n 3.37 n 1.86 n 1.00 n
7 36.9 n 36.5 n 32.4 n 24.3 n 13.1 n 7.24 n 3.89 n
Probe Current for FEG
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Spotsize 30kV 20kV 10kV 5kV 2kV 1kV 500V
1 0.4 0.4 0.4 0.5 0.5 0.5 0.6
2 0.6 0.7 0.8 1.0 1.2 1.3 1.3
3 1.0 1.3 1.7 2.1 2.4 2.5 2.6
4 2.1 2.6 3.4 4.1 4.8 5.0 5.2
5 4.1 5.0 6.7 8.2 9.5 10.0 10.4
6 8.2 10.0 13.4 16.4 19.0 20.0 20.7
7 16.0 19.6 26.3 32.3 37.4 39.4 40.9
*Source = 20KV and WD = 10mm (spot diameters in nm).
FEG Spot Size (nM)
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SEM Main Components
Electron Gun
Demagnification system
Scan Unit
Wehnelt cylinder
Condenser lenses
Scan generator
Demagnification system
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Magnification
l
M=L/l
L
L
***-important
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Scan Size Vs. Magnification
• Low Mag.
• Med Mag.
• Hi Mag.
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Magnifying Your Sample on Quantax50
M= L
l
_
lL
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Low Magnification
Scan Here
Display Here
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Intermediate Magnification
Scan Here
Display Here
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Higher Magnification
Scan Here
Display Here
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• The viewed area (L) is fixed
Scan Size Vs. Magnification
• The smaller the area scanned on the sample results in higher viewed magnification
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A Focused Vs. An Unfocused Beam
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The Crossover point on the Beam is of a Finite Size
D= Spot Size
I = Beam Current
ą = Measurement of the ‘cone’
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Current Density
• Current Density remains constant through the optical path of the electron beam
=4 X I Amps
π X d o X a( ) Cm Steradians2β
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Current Density (remove constants)
• Current and Spot size are directly proportional
=I Amps
d o ( ) Cm 2β
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Resolution
The resolution of the microscope
is a measure of the smallest separation
that can be distinguished in the image
resolved unresolved
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The Diameter of the Electron Beam Must Be Smaller Than the Feature to Be Resolved
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The Electron Beam Scans From Left to Right
• There can be from 512 to 4096 scan lines, at all magnifications
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The Electron Beam Spot Size Must Be Smaller Than the Features Being Resolved
• The ideal spot size
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Too Large of Spot Size Looks Out of Focus
• Too big of spot size creates an out of focus image
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Scan Size Vs. Magnification
• Spot size for low mag is not acceptable for higher mag
***-important
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Scan Size Vs. Magnification
• Spot size for medium mag is not acceptable for highest mag
***-important
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Obtaining an Image
• The SEM operator needs to do two things:
1- find the correct focus
2- determine the correct spot size
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Obtaining an Image
• Focusing moves the crossover point of the beam up and down, trying to place the focal point onto the sample
• Spot size controls the lateral size of the focused beam on the sample
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Electro-magnetic Condenser Lens
metal jacket
copper windings
Optic axis
Air gap
Cross-over
e-
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Aperture
Condenser lens
Electron beam In
Electron spray
Electron beam Out
Condenser Lens Action on Beam
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Condenser Lens Action on Beam
• Decreased lens current creates more beam current
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• Increased lens current creates less beam current
Condenser Lens Action on Beam
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Spot Size Summary
• Smaller spot sizes for higher magnification
• Larger spot size for x-ray analysis
• Too large of spot may result in a de-focused image
• Too small of spot may result in poor S/N
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How to get High Resolution (100.000 - 150.000x) (Tungsten)
• Use 20-30 kV
• Use spot 1
• Use WD 5 mm
• Tilt stage 10°
• Take BSE detector out
• Lock stage
• Use image definition of 1024x884 or 2048x1768
• Take 1 Frame, frametime min. 60 seconds
• Move to new area after focusing/stigmation
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Summary of Spot Size Affecting SEM Image
• The electron column is designed to produce smallest spot containing highest possible probe current
• Spot size limits minimum size of objects that can be separated
• Higher probe current improves the signal to background ratio
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SEM Main Components
Electron Gun
Demagnification system
Scan Unit
Wehnelt cylinder
Condenser lenses
Scan generator
Focus UnitObjective andStigmation lenses
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Focusing the Beam Onto the Sample Uses the Objective Lens
objective lensfinal lensaperture
pole piece
sample
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Focusing the Beam Onto the Sample
pole piece
objective lensfinal lensaperture
sample
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Focusing the Beam Onto the Sample
pole piece
objective lensfinal lensaperture
sample
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Focusing the Beam Onto the Sample
objective lensfinal lensaperture
pole piece
sample
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Working Distance (WD)
OWDFWD
pole piece
objective lensfinal lensaperture
specimen
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Synchronizing Stage Height With WD
Z
Z
WD
WD
specimen specimen
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WD Vs. Gas Path Length(GPL)
EDS
WD
Final Lens Pole Piece
Hi-Vac
GPL
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WD Vs. Gas Path Length(GPL)
Final Lens Pole Piece
EDS
WD= 10 mmGPL= 2MM
EDS Cone(8mm)
Hi-Vac Intermediate Vacuum
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Low noise EDS Mapping in Low-vacuum with use of EDS ConeLow noise EDS Mapping in Low-vacuum with use of EDS Cone
Using the EDS Cone..
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Focus and Stigmation• Focusing brings the beam crossover up or
down
• Stigmation controls the ovalness of the beam
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Astigmation Is an Un-oval Beam
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Astigmatism
disc of least confusionmagnified point source
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Astigmatism...Continued
You have to see it to believe it…
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SEM Main Components
Electron Gun
Demagnification system
Scan Unit
Detecting Unit
Wehnelt cylinder
Condenser lenses
Scan generator
Specimen + detector Detector
Objective andStigmation lenses
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Different Types of Electron Detectors
Electron Detector
SEM
:Quantax50
A detector is a detector to the SEM
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High Vacuum Everhardt-Thornley Secondary Electron Detector
Photomultiplier
Light guide
glass target
Phosphorousscreen (Al-coated)( +10 kV)
Faraday cage(-250 - +250 V)
Scintillator
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Solid State Backscattered Detector
Backscattered electrons
Surface electrode
Silicon dead layer
SemiconductorBase plate
+++++++++++++----------------------
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The Solid State BSD
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The Gaseous Analytical Detector (GAD)
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Low voltage high Contrast Detector (vCD)
Backscattered electrons
Surface electrode
Silicon dead layer
SemiconductorBase plate
+++++++++++++----------------------
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The best imaging conditions at LV Low KeV: flat cone short beam gas path length, low pressures and long amplification path
Electron beam
EDX
Detector
Sample
Detected electron signal
5 mm WD
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LF (Large Field) Detector
• Large field of view SE detector for LV based on gas amplification
• Excellent signal yield at low pressures
• Works from 0.5 to 1 Torr (2-3T with PLA)
• Detects primarily: SE1, SE2, SE3
• Not too sensitive to light or temperature
• Can be used with x-ray cone for low KeV or x-ray analysis
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The Large Field (LF) Detector
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Gaseous Secondary Electron Detector
non-conductive specimen
GSED
Primary beam
Signal amplification by gas ionisation
Collection area at high positive voltage
Detected electron signal
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GSED (Gaseous Secondary Electron Detector)• Second generation SE detector for ESEM based on gas
amplification
• Works from 0.5 to 20 Torr
• Not too sensitive to light or temperature
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GSED (Gaseous Secondary Electron Detector)
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Available SE Gas Amplification Detectors & Cones
Low KV Cap
GSEDLFD GBSDX-RayCone
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HighVac / LowVac: LF-GSE + SS-BSE
Changing modes without detector change
LFD
BSE
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LF-Detector + Low KV Cap
Low kV imaging with Low KV Cap
LFD
Low KV Cap
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X-Ray Cone
LF-Detector + X-Ray cone: no BSE detection
LFD
X-Ray Cone
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GaseousAnalyticalDetector
• The GAD is a
SS-BSED + X-Ray cone
• Optimised low vacuum microanalysis and imaging
(SE and BSE) at the analytical WD
• Minimum Magnification 250 x
LFD
GAD
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GBSD (Gaseous Backscattered Electron) Detector
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The GBSD
---
BSE Converter Plate
BSE Generated by Primary Beam
PLA
SE Collection Grid
SE 3
Buried Signal Track
++
+
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GBSD (Gaseous Backscattered Electron) Detector• Specialized detector allows BSE imaging at higher
pressures >4T
• SE & BSE detector for ESEM based on gas amplification
• Works from 4-10 Torr
• Detects SE or BSE Signal in a gas
• Not sensitive to light or temperature
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99
GBSD Optimized for High Pressures
Signal vs Pressure
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10
Pressure
Sig
na
l (A
rbitr
ary
)
BC
100
100
Oil in Water
Secondary Mode
Backscattered Mode
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When to use what detector…
Detector SE BSE Pressure Lowest kV X-ray Area
GSED YES NO 1.0-20T 3kV up BULK
LF/SS BSE YES YES .1-1.0T(.1-1.5 FEG) 5kV up BULK
LF/GAD YES YES 0.1-4T 3kV up POINT
GBSD YES YES 4-10T 10KV up BULK
ET SE/ SSBE YES YES Hi-VAC 1KV up POINT
ICD YES NO Hi-VAC no insert 1 KV with BD POINT
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Hot Stage “Hook” (ESD)
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Hot Stage ‘Hook” and Detector
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Through The Lens Detector (TLD)
Specimen
PMT
E.T. SED
TLD
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Scintillator-type Backscattered Detector (Robinson & Centaurus)
specimen
Aluminium
P-scintillator
through light guide toPhotomultiplier tube
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106
Cathodoluminescence Detector
Polished Aluminium Light guide
Photomultiplierspecimen
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107
Electron Backscatter Pattern (EBSD) Detector
Final Lens
Primary Beam
BSE
EBSD
108
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EBSD Applications
1m = 50 steps
OIM from 1000 Å PVD Copper Damascene lines
109
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Specimen Current Detector
iPC
iSE
iBSE
iSC
specimen
110
110
Electron Beam Induced Current (EBIC)
PE
SCA
P N P
111
111
CCD Camera - Quantax50 View
As viewed from under the EDS detector
LFD
E.T. SED
BSD
Sample
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The end QUANTRAINx50 3.2PPT- Optics