Electron Optics - Lawrence Technological University

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Quantrainx50
Module 3.1 Electron
Optics
1-2011
Confidential
SEM Main Components
2
2
Wehnelt cylinder
or FEG unit
Electron Gun
Condenser lenses
Demagnification system
Scan generator
Scan generator
Scan Unit
Objective and
Stigmation lenses
Focus Unit
Electron detector
Detecting Unit
SEM Main Components
Wehnelt cylinder
FEG
3
3
Electron
ElectronGun
Gun
Electron Gun Emitters
• Tungsten filament (W)
• Lanthanum Hexaboride filament (LaB6)(obsolete)
• Cerium Hexaboride (CeB6)
• Field emission filament (FEG)
4
4
Electron Gun Animation *
* Video courtesy of Oxford Instruments
5
5
Electron Source Properties
• Current density (brightness)
ą
• Emission current
• Stability of source
• Lifetime of filament
• Design of electron source assembly
ip
• Ease of operation
• Costs involved
do
specimen
6
6
Emission Area For Tungsten (W)
Filament heating supply
Filament
Wehnelt cap
Cross-over plane
Anode
High voltage
supply (200 v- 30 kV)
7
7
70
A
Bias on Wehnelt Cap
Optimum bias voltage
Low bias voltage
Equipotential
lines of the
Voltage Field
0
+
0
+
High
emission
large spot
8
8
High bias voltage
0
+
No emission
Sufficient
emission
small spot
Emission : Autobias control
Bias 255 ……………………………….. Bias 1
110
µA
90
µA
1 kV
30 kV
Autobias keeps emission between 90-110 µA for all kV
9
9
9
W Filament Saturation
emission current
Saturation point
False peak / Misalignment
filament current
10
10
Tungsten Filament
11
11
High Resolution, High Brightness FEG
source…
Normalized Brightness (-)
Maximum probe current (nA)
Life time (hrs)
Tungsten
1
2000
60-200
LaB6
10
500
200-1000
FEG
1000
100
> 10000
Beam current stability (10 hrs)
Resolution 30kV (nm)
Resolution 1kV (nm)
<1%
3.0
25
<1%
2.0
15
<0.4%
1.2
3.0
20
900
26000
Cost source (USD)
12
12
XL Schottky FEG Theory
o
o
• The Boersch Effect
• A) Perfect beam: no
interactions
• B) Random beam: one
dimension
• C) Random beam: two
dimensions
o
o
o
o
o
o
o
o
o
dimensional
14
14
o
o
o
o
o
o
o
o
o
oo
o
o
o
o
• It is actually three
A
o
o
B
C
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.
15
15
o
o
o
o
oo
o o
oo
o
o
Lens Defects
optical axis
Aperture
image plane
Spherical
aberration
16
16
Chromatic
aberration
Diffraction
Spherical Aberrations
Disc of Least
Confusion
• Electrons entering into a lens at different points get
focused at different points
17
17
Chromatic Aberrations
Disc of Least
Confusion
• Electrons of differing energies will be focused at
different places
18
18
Diffraction
• The wave nature of electrons cause
diffraction limitations
19
19
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.
20
20
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
21
21
FEG Column Principle Diagram
10KV
Drift
Space
(Coulomb Tube)
C1
Gun Alignment Coils
C2
Objective Aperture
Scan Coils
Objective Lens
22
22
FEG gun (electron source)
23
23
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
Emission Area for FEG
Filament heating supply
Extractor
system
C1 static lens
150 A
Anode
High voltage
supply (200 v- 30 KV)
24
24
Schottky Gun Design
• Fil = Filament current
input
(2.4
Ampere)
Fil
• S = Suppressor (-500V)
• E = Extractor (+5000V)
• C1 = Electrostatic
Condenser lens
S
E
E
C1
25
25
Schottky Tip design
• M = Tip module
• W = Welded
tungsten Tip
M
• Fil = Tungsten
wire filament
T
• T = Sharpened
Tip
• Zr = Zirconium
reservoir
26
26
W
Fil
Zr
FEG Startup Steps
• Warmstart / Coldstart
• Gun conditioning
27
27
Beam Menu
Final operation status
28
28
FEG Column Double condenser lens
• Extraction voltage changes
not necessary, beam current
is set by condenser lenses
C1
• C1 is electrostatic
• C2 is electromagnetic
• Variable lens strengths:
A = high beam current mode
C2
B = low beam current mode
• Final beam energy 30keV
down to 200eV
A
29
29
B
FEG Column Double Condenser Lens
• Extraction voltage changes
not necessary, beam current
is set by condenser lenses
C1
• C1 is electrostatic
• C2 is electromagnetic
Internal
Spray
Aperture
• Variable lens strengths:
A = high beam current mode
C2
B = low beam current mode
• Final beam energy 30keV
down to 200eV
A
30
30
B
FEG Column
C1
• Different paths for low
and high beam current
conditions through the
coulomb tube, but
common path to
objective
C2
Deceleration
Lens
Aperture
31
31
Small Spot
Large Spot
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
32
32
Probe Current for FEG
33
33
Beam Current:
Spotsize 30kV
20kV
10kV
5kV
2kV
1kV
500V
1
21 p
13 p
8p
5p
2.5 p
1.4 p
0.7 p
2
44 p
40 p
33 p
25 p
13 p
7p
4p
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
FEG Spot Size (nM)
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).
34
34
SEM Main Components
Wehnelt cylinder
Electron Gun
Condenser lenses
Demagnification
Demagnificationsystem
system
Scan generator
35
35
Scan Unit
Magnification
L
M=L/l
L
l
***-important
36
36
Scan Size Vs. Magnification
• Low Mag.
• Med Mag.
• Hi Mag.
37
37
Magnifying Your Sample on
Quantax50
l
L
L_
M=
l
38
38
Low Magnification
Scan Here
Display Here
39
39
Intermediate Magnification
Scan Here
Display Here
40
40
Higher Magnification
Scan Here
Display Here
41
41
Scan Size Vs. Magnification
• The viewed area (L) is fixed
• The smaller the area scanned on the sample
results in higher viewed magnification
42
42
A Focused Vs. An Unfocused Beam
43
43
The Crossover point on the Beam is of a
Finite Size
I = Beam Current
D= Spot Size
ą
44
44
= Measurement of the ‘cone’
Current Density
β
4 X I
Amps
=
(π
X do
X
a
) Cm 2 Steradians
• Current Density remains constant through the optical path of the
electron beam
45
45
Current Density (remove constants)
β
I
Amps
=
(
do
) Cm 2
• Current and Spot size are directly proportional
46
46
Resolution
resolved
unresolved
The resolution of the microscope
is a measure of the smallest separation
that can be distinguished in the image
47
47
The Diameter of the Electron Beam Must Be Smaller Than
the Feature to Be Resolved
48
48
The Electron Beam Scans From Left to
Right
• There can be
from 512 to
4096 scan lines,
at all
magnifications
49
49
The Electron Beam Spot Size Must Be Smaller
Than the Features Being Resolved
• The ideal spot
size
50
50
Too Large of Spot Size Looks Out of
Focus
• Too big of spot
size creates an
out of focus
image
51
51
Scan Size Vs. Magnification
• Spot size for low
mag is not
acceptable for
higher mag
***-important
52
52
Scan Size Vs. Magnification
• Spot size for
medium mag is
not acceptable
for highest mag
***-important
53
53
Obtaining an Image
• The SEM operator needs to do two things:
1- find the correct focus
2- determine the correct spot size
54
54
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
55
55
Electro-magnetic Condenser Lens
emetal
jacket
copper
windings
Air
gap
56
56
Optic axis
Cross-over
Condenser Lens Action on Beam
Electron beam In
Condenser lens
Electron spray
Aperture
Electron beam Out
58
58
Condenser Lens Action on Beam
• Decreased lens current
creates more beam
current
59
59
Condenser Lens Action on Beam
• Increased lens
current creates less
beam current
60
60
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
61
61
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
62
62
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
63
63
SEM Main Components
64
64
Wehnelt cylinder
Electron Gun
Condenser lenses
Demagnification system
Scan generator
Scan Unit
Objective and
Stigmation lenses
Focus Unit
Focusing the Beam Onto the
Sample Uses the Objective
Lens
final lens
aperture
objective lens
pole piece
sample
65
65
Focusing the Beam Onto the Sample
final lens
aperture
objective lens
pole piece
sample
66
66
Focusing the Beam Onto the Sample
final lens
aperture
objective lens
pole piece
sample
67
67
Focusing the Beam Onto the Sample
final lens
aperture
objective lens
pole piece
sample
68
68
Working Distance (WD)
objective lens
final lens
aperture
pole piece
OWD
FWD
specimen
69
69
Synchronizing Stage Height With WD
Z
WD
Z
specimen
70
70
specimen
WD
WD Vs. Gas Path Length(GPL)
Hi-Vac
Final Lens Pole Piece
EDS
WD
71
71
GPL
WD Vs. Gas Path Length(GPL)
Hi-Vac
Intermediate Vacuum
Final Lens Pole Piece
EDS Cone(8mm)
EDS
GPL= 2MM
72
72
WD= 10 mm
Using the EDS Cone..
Low noise EDS Mapping
in Low-vacuum with use of
EDS Cone
73
73
Focus and Stigmation
• Focusing brings the beam crossover up or down
• Stigmation controls the ovalness of the beam
74
74
Astigmation Is an Un-oval Beam
75
75
Astigmatism
76
76
disc of least confusion
magnified point source
Astigmatism...Continued
You have to
see it to
believe it…
77
77
SEM Main Components
Wehnelt cylinder
Electron Gun
Condenser lenses
Demagnification system
Scan generator
Scan Unit
Objective and
Stigmation lenses
Specimen + detector
78
78
Detector Unit
Detecting
Different Types of Electron
Detectors
A detector is a detector to the
SEM
SEM
Electron Detector
79
79
:
Quantax
50
High Vacuum Everhardt-Thornley
Secondary Electron Detector
Light guide
Faraday cage
(-250 - +250 V)
Phosphorous
screen (Al-coated)
( +10 kV)
glass target
Scintillator
Photomultiplier
80
80
Solid State Backscattered Detector
Base plate
+++++++++++++
----------------------
Semiconductor
Silicon dead layer
Surface electrode
Backscattered electrons
81
81
The Solid State BSD
82
82
The Gaseous Analytical Detector (GAD)
83
83
Low voltage high Contrast Detector
(vCD)
Base plate
+++++++++++++
----------------------
Semiconductor
Silicon dead layer
Surface electrode
Backscattered electrons
84
84
The best imaging conditions at LV Low KeV: flat cone
short beam gas path length, low pressures and long
amplification path
Electron
beam
Detected
electron
signal
EDX
Detector
5 mm WD
Sample
85
85
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
86
86
The Large Field (LF) Detector
87
87
Gaseous Secondary Electron Detector
Primary beam
GSED
Collection area at high
positive voltage
non-conductive specimen
88
88
Detected
electron
signal
Signal amplification by
gas ionisation
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
89
89
GSED (Gaseous Secondary Electron
Detector)
90
90
Available SE Gas Amplification Detectors &
Cones
LFD
91
91
Low
KV
Cap
X-Ray
Cone
GSED
GBSD
HighVac / LowVac: LF-GSE + SS-BSE
BSE
LFD
Changing modes without detector change
92
92
Low kV imaging with Low KV Cap
Low
KV
Cap
LF-Detector + Low KV Cap
93
93
LFD
X-Ray Cone
X-Ray
Cone
LFD
LF-Detector + X-Ray cone: no BSE detection
94
94
Gaseous
Analytical
Detector
GAD
LFD
•
The GAD is a
SS-BSED + X-Ray cone
•
Optimised low vacuum microanalysis and imaging
(SE and BSE) at the analytical WD
•
95
Minimum Magnification 250 x
95
GBSD (Gaseous Backscattered Electron)
Detector
96
96
The GBSD
BSE Converter Plate
+
+
-
+
- -
Buried Signal Track
PLA
BSE Generated by
Primary Beam
97
97
SE 3
SE Collection Grid
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
98
98
GBSD Optimized for High Pressures
Signal vs Pressure
1.2
B
C
Signal (Arbitrary)
1
0.8
0.6
0.4
0.2
0
0
2
4
6
Pressure
99
99
8
10
Oil in Water
Secondary
Mode
100
100
Backscattered Mode
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
101
101
Hot Stage “Hook” (ESD)
102
102
Hot Stage ‘Hook” and Detector
103
103
Through The Lens Detector (TLD)
TLD
PMT
E.T. SED
Specimen
104
104
Scintillator-type Backscattered
Detector (Robinson & Centaurus)
P-scintillator
through light guide to
Photomultiplier tube
specimen
Aluminium
105
105
Cathodoluminescence Detector
Polished Aluminium
specimen
106
106
Light guide
Photomultiplier
Electron Backscatter Pattern (EBSD)
Detector
Primary Beam
Final Lens
BSE
EBSD
107
107
EBSD Applications
1  m = 5 0 s te p s
OIM from 1000 Å PVD Copper Damascene lines
108
108
Specimen Current Detector
iPC
iBSE
iSE
iSC
specimen
109
109
Electron Beam Induced Current (EBIC)
PE
P
N
P
SCA
110
110
CCD Camera - Quantax50 View
E.T. SED
LFD
BSD
Sample
As
viewed
from
under
the EDS
detector
111
111
The end QUANTRAINx50 3.2PPT- Optics
112
112

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