FEG SEM in the Nova/Strata DBs
Nova NanoLab/Strata DB
FEGSEM
Source
Column
Chengge Jiao
Detectors
2
Source: FEG Electron Source
Source: thermionic and field emission source
W hair
pin
Work Function
LaB6 single
crystal <110>
Lanthanum hexabroide
CFE
<310> W
SE
<100> with ZrO
4.5
2.4
4.5
Lowered From 4.5 to 2.8
106
107
109
109
1
1
0.2 to 0.4
0.5 to 0.7
1.3 x 10-4
2 x 10 -7
5 x 10-11
5 x 10-9
40 - 100
500
> 5000
> 5000
Electrons need to have more energy than the work function
(WF) to leave the emitter.
Their energy depends on the temperature, so heating can
be used to cause emission (thermionic emitter like W or
LaB6).
(eV)
Brightness
(A/cm2/Sr)
Energy Spread
(eV)
Operating Pressure
(mbar)
Life time
(hours)
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The WF depends on the material, so choosing a material
with a low WF helps emission (LaB6 or ZrO layer on FEG
emitter).
It also depends on the crystal orientation (FEG).
The Schottky effect is that the WF becomes lower when a
strong electric field is applied on the emitting surface (used
in the FEG).
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Source: Schottky emitter
Source: FEG filament temperature
As soon as the temperature is high
enough, the Zirconium Oxide will
melt and flow to the tips’s end.
WF: Tungsten
A Schottky emitter is
a FEG with a lower
working function
working function by
ZrO. A temperature is
needed to melt the
ZrO. The Shottky
emitter starts with the
emission as soon as
the ZrO has reached
the tip.
Temperature:
1800K
WF: lowered
with ZrO
High Temperature: High emission
Warm field mitter.
mitter.
Tip diameter is
500nm
The working function is lowered now, so
the tip will start emitting when there is
an extraction field.
Too high temperature: ZrO evaporates before it
reaches the tip.
The emission increases when
increasing the operating temperature
of the tip: until approx. 1850K.
diffusion
5
ZrO bulb
Low Temperature: Low emission
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ZrO Flow
positive extraction field
for field emission
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The column: Vacuum
Source: Schottky Emitter
The operating vacuum for the Schottky emitter must be better
then 2 x 10-8 mbar; however the lower 10-9 range is
recommended.
The system vacuum is:
Upper IGP (2) vacuum: < 2x10-9 mbar
Lower IGP (1) vacuum: < 2x10-7 mbar
T
M = Tip module
W = Welded tungsten Tip
Fil = Tungsten wire
filament
T = Sharpened Tip
Zr = Zirconium reservoir
Zr
M
W
Fil
2 IGP’s cascaded: the lower IGP backs up the upper IGP
Vacuum is achieved by bake-out
Items baked out:
Emission Chamber outside, Tip module inside, Upper IGP
1 torr = 1.33 mbar = 133 Pa
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Schottky Gun Design
Where are the 6 lens ?
C1
F:
Filament current input (2.1
to 2.4 Ampere)
S:
Suppressor (-500V FEG or 950 V Sirion FEG)
E:
Extractor (+ 4 to 6 kV)
Gun alignment
C2
F
S
Deceleration
UHR/Intermediate
E
C1: Electrostatic condenser lens
Scan coil assembly
C1
UHR/Immersion
HR/normal
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The upper three lens: Condenser Optics - Spot Size
Dgun
source
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Column: the upper part of 3 lens
Dgun
C1
Coulomb
Tube
v1
C1
Gun Alignment Coils
C2
v2
b1
Objective Aperture
v2
C2
crossover
b2
Dxover
Spot 7
A high current
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b2
Scan Coils
Dxover
M = (b1/v1)·(b2/v2)
Dxover = Dgun·M
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Objective Lens
Spot 1
A low current
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Crossover position in aligned column
Modes of operation
X-over Dec. Lens
Mode I: Searching Mode/HR Mode
On a perfectly aligned system
the disk is cantered on the
aperture when the scan is zero,
i.e. the middle of the image,
except that the whole image is
shifted to the right. This is
because in the cross-over
mode, the SEM scancoils are
turned off. The image is
generated by the Gun-Shift coils
in the top of the column. The
shift coils are scanned slower
than the HR/AC/DC coils results
in the cross-over to the right.
0.3D
FLA
D
Upper scan
Lower scan
Final lens
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5 kV
Mode II: UHR Mode
Excellent imaging at low kV as the sample can be close to the
virtual lens which reduces Cc.
Use of highly efficient within-the lens detector with SE, BSE and
Mix capability.
Full resolution at tilted condition.
Easy and fast switching between modes.
Mode III: EDX Mode
A proper lens current remnance setting for EDX mode use
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Atomic number on the interaction volume
Beam energy on the interaction volume
1 kV
Large field of view for easy navigation.
Low magnetic field at the sample.
Ideal for magnetically sensitive samples.
30 kV
BCN, density 1.805
Zn, density 7.14
W, density 19.30
Monte Carlo electron trajectory simulations of interaction volume of different
materials at 5 kV beam energy
Monte Carlo electron trajectory simulations of interaction volume in
aluminum as a function of beam energy
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Acceleration voltages, spot sizes, apertures
Acc. voltage
Use
3-5 kV
5-10 kV
High resolution imaging, e-beam deposition
General imaging, light element X-ray mapping
10-30 kV
Heavy element X-ray mapping
Spot size
Use
1&2
1–3
4–7
Very high resolution (mag > 50k X)
Standard imaging
X-ray analysis, EBSD, e-beam deposition
HR and UHR mode
NOTE – the NOVA NanoLab / Strata GUI does not display spot sizes but the
approximate beam current at the respective spot.
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Aperture
Use
30 µm
30 µm
50 / 100µm
High resolution imaging - 5kV, spots 1,2,3
General use
X-ray mapping at low kV
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Intermediate Lens/Virtual objective aperture
Lower Three Lenses
HR Mode
X-over
5 micron
UHR Mode
EDX Mode
30 micron
Final Lens Aperture
Inter mediate lens
Intermediate Lens
UHR lens
Specimen
Internal Lens
External Lens
Intermediate/UHR lens combination: No need to change aperture when
switching between HR and UHR modes
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HR and UHR mode
HR and UHR mode
UHR mode
Immersion lens
Search lens
OWD 60° pole = 51mm
deflectors
OWD = 31mm
FWD = 5mm
OWD = 3mm
In UHR mode, the magnetic field does not stay inside the lens and the specimen
is in the hearth of the lens.
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Why HR mode is not suitable for XEDS analysis
EDX Mode
If we do XEDS in HR mode
The external lens
is activated but is
much weaker than
in UHR mode.
BSEs can escape and strike
EDX detector, causing too
high a count rate and dead
time
BSE can bounce off final lens
and re-enter the specimencreating
stray X-rays
Intermediate Lens
Intermediate Lens
TLD
TLD and SED can
be used.
BSE
Can be useful for
looking at
magnetic samples.
EDX
SED
Weak External Lens
Specimen
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Specimen
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Resolution / Probe current
EDX Mode
Spot Size
1kV
W D=2mm
nm
pA
2.5
1.4
2.5
7.4
2.7
30
2.8
120
3.1
460
3.6
1900
7200
Intermediate Lens
1
2
3
4
5
6
7
BSEs are trapped by the
immersion lens either in
UHR or EDX mode.
EDX
BSEs
5kV
WD=4mm
nm
pA
2.3
5
2.3
25
2.4
98
2.5
400
3.7
1600
4.7
6300
24000
15kV
W D=5mm
nm
pA
1.5
10
1.5
36
2.2
140
2.5
580
3.5
2200
5
8900
34000
30kV
W D=6.2mm
nm
pA
1.5
21
1.5
44
1.7
150
1.8
630
2
2400
3.8
9500
37000
NOTE – the NOVA NanoLab / Strata GUI does not display spot sizes but the
approximate beam current at the respective spot.
Objective Lens
Specimen
Low Voltage Limitations
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WD/kV limitations in Mode II
6
0A
Longer electron-electron interaction times and smaller electronelectron distances lead to higher statistical aberrations at low kV
re
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WD, mm
Chromatic aberration is more dominant at low voltages.
4
3
2
1
Re
co
25
1
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4
9
16
HT, kV
25
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Detector SFEG: HR mode SED detection
Normal Secondary Electron Detection
Why a through the lens detector?
light pipe2
Through the Lens Detector (TLD)
light pipe1
10 kV
The secondary electrons are trapped in the strong
magnetic field between lens and specimen in the
UHR mode. So, a detector which can detect
secondary electrons from the optical axis is need.
SE electrons are detected by a positive bias
(20~25 Volt) and split from the optical axis by a
special device: a deflector.
10 kV
visible light
Bias TLD
(-10V)
Bias SED (+250V)
SE electrons
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TLD Configuration
Detector SFEG: UHR detection SE mode
TLD secondary electron detection
Within-the-lens detector is part of
the final lens.Bias voltage between
-250V and +250V . Grid is -25V.
SE: positive suction tube voltage of 20
or 25 Volt.
BS: negative suction tube voltage.
Down hole: very positive suction tube
voltage (>150Volt).
Charge reduction: suction tube
voltage zero.
Visible light
SE electrons
-V
2V
-V
V
-2V
Illumination screen
10 kV
V
Illumination screen
10 kV
Bias TLD
(+20 or 25V)
Bias SED (-25V)
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EBID - Electron beam induced deposition
Detector SFEG: UHR detection BSE mode
TLD back-scatter detection
-V
V
-2V
2V
Electron beam
10 kV
Available metals
BSE
SE
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Comparison of electron and ion beam deposition
10 kV
V
-V
Bias TLD
(-250V)
Patterning capabilities using the electron beam in
conjunction with the gas injector system (GIS)
Bias SED (-25V)
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EBID - Electron beam induced deposition
Examples of electron beam
deposited patterns
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Ion beam
Pt, W, silicon oxide
Deposition rate
Low
High
Milling prior to
deposition
No
Yes
Ga inclusion
No
Yes
Purity of deposit
Process dependent
High
Min feature at HAR /
LAR
28 nm / 12 nm
40 nm / 20 nm
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Alignment order
Final Lens aperture strip manual mechanical position
alignment
81 Crossover holey point calibration
10 Source Tilt and Shift
44 UHR Lens Alignment
42 UHR Stigmator Alignment
43 UHR Image Shift Correction
45 HR image Shift Correction
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Final lens aperture
• Reduces coulomb interactions
• Electronic Beam Centering
• Silicon wafer design/Molybdenum coated
• 30 micron gives good all round operation
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Alignment function overview
Procedure
Function
5 – Emitter Startup
Enables electron gun switching on/off and to change gun
emission mode. In case of emergency shut down it
makes also possible to start IGP.
10 - Source Tilt and Shift
Corrects source tilt and shift for the whole range of the
accelerating voltages and spotsizes.
44 - UHR Lens Alignment
Eliminates image shift when focusing in UHR
(immersion) mode for the whole
range of the accelerating voltages and working
distances.
42 - UHR Stigmator Alignment
Alignment Eliminates image shift during normal stigmator
correction in UHR (immersion) mode for the whole range
of the accelerating voltages and working distances.
43 - UHR Image Shift Correction
Eliminates image shift during HV change in UHR
(immersion) mode for the whole range of the
accelerating voltages and working distances.
45 - HR image Shift Correction
Eliminates image shift during HV change in HR mode for
the whole range of the accelerating voltages.
13 - Stigmator alignment
Alignment Eliminates image shift during normal stigmator
correction in both HR and UHR modes.
17 - Stage Rotation Center
Corrects the center of rotation at any point on the
specimen by computer correction of the X, Y offset from
the stage mechanical center.
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Aligned system
When all alignments are done properly, the image
will stay in focus, its rotation will be corrected, and it
will not show a substantial image displacement when
you change kV and, or beam current. Further more
the Ion beam and Electron beam should be in
coincidence and report the same sample location
accurate for milling / imaging purposes.
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THE END
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