The document describes a study investigating the effect of the gap between electrodes in an einzel lens on the field of view (FOV) of an e-beam microcolumn. A microcolumn was fabricated using MEMS processes to create thin silicon membrane lenses with adjustable gaps by selecting different Pyrex thicknesses during bonding. The performance of the microcolumn was analyzed for different lens gap sizes and operation modes. It was found that a narrow gap lens performed best in source lens focusing mode with a larger scan range, while a wide gap lens gave better image quality in einzel lens focusing mode due to less interference with the deflector electric field. An optimized gap of around 250-300 microns was determined to provide a good balance of scan range

1. A method to increase FOV in an e-beam
microcolumn
Om Krishna Suwal, Ph.D.
Department of Physics and Nanoscience
Sun Moon University
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SUNMOON UNIVERSITY

2. Outline
 Objective:


To analyze the influence of Einzel lens structure on FOV
Outline of presentation:

Microcolumn – introduction

MEMS fabrication technique of lens

Assembly of microcolumn with various gap between electrodes

Analysis of FOV for narrow and wide gap of electrodes and
microcolumn operation mode at various tip voltages and working
distances

E-beam trajectory, electric field analysis following computer
simulation
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3. Microcolumn
Source size, do
Cathode
Source lens
2o
Deflector
~ 3-10 mm
Optical axis
for e-beam
Einzel lens
Probe size, d1
21
Sample
2
Resolution 𝑑1 =
𝑀𝑑0
2
+ 𝑑2 + 𝑑2 𝑠 + 𝑑2 𝑐
𝐶
𝐶
𝑑
d1 = Probe size
M = Column magnification
d0 = virtual source size
dd= 1.5/1V
V = Beam energy
3
𝑑 𝐶 𝑠 = 0.25 Cs 1
𝑑 𝐶 𝑐 = Cc 1 V/V
Cs = Spherical aberration coefficient
Cc = Chromatic aberration
coefficient
T.H.P. Chang et al. (1989)
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4. Microcolumn Features and Benefits
Miniaturized Electron Optical system
 Portable electron source
 Mini-SEM (Desktop SEM)
Low energy e-beam
Electron energy 100 eV – 2 keV
Less sample damages due to e-beam
Microfabricated Columns
MEMS technology applicable
Low cost and High yield
Arrayed Beam Operation
Parallel beam operation
High throughput with low beam voltage
Short stage traveling
SUNMOON UNIVERSITY
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5. Microcolumn Assembly
Emitter
Source Lens
Deflectors
Einzel lens
Schematic
Shield microcolumn
H.S. Kim et al. / Microelectronic Engineering 83 (2006) 962–967
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6. Einzel Lens Assembly
 = 200 m / 100 m
~0.5 mm /
1.5 mm
E1
Pyrex 150 m
/ 500 m
E2
E3
Emitter
3 mm
8 mm
Si
Source Lens
Pyrex
Source or Einzel lens
structural cross section
3 - 10
mm
Deflector
Einzel Lens
Microcolumn cross section
SUNMOON UNIVERSITY
Grid
(sample)
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7. Fabrication Process
a. Boron doping and oxidation
Si wafer
b. Front side lithography
c. DRIE Si trenches
d. SiO2 deposition
Si
Boron doped Si
PR
SiO2
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8. Fabrication Process cont…
e. Back side patterning
f. Back side Si etching
v
i
e
w
g. SiO2 strip off and doping
10 mm
v
i
e
w
Si
Boron doped Si
PR
SiO2
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9. Si Membrane-Pyrex Bonding
A
Pyrex
Si
Hot plate
A Pyrex bonded
membrane
Pyrex
HT
Membrane detachment
Si membrane
Si
Pyrex
Si
Pyrex
10 ㎛
Interface of bonded Si-Pyrex
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10. Multiple Lens Alignments
Pyrex
Si
L1
L2
Laser
Laser
Laser
22.5o 45o
L3
Top view of the
assembled lens
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11. Modes of Operation
Source lens
focusing
VS1
VS2
S1
S2
S3
Source + Einzel
lens focusing
Einzel lens
focusing
VS1
S1
S2
S3
VS1
VS2
S1
S2
S3
D1
D1
D1
E1
E2
E3
E1
E2
E3
E1
E2
E3
D2
D2
Sample
VE2
VE2
D2
Sample
Sample
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12. Simulation of e-beam trajectory and electric potential
along optical axis
Module
-300V S2
S1
S3
100
D1
20
50
D2
 = 500 m
Electric Potential distribution at Einzel lens system
E2
E1 E3
Gap =
150 m
Gap =
500 m
 = 200 m
Gap = 150 m, S2 = -240 V, E2 = 500 V
WD
Gap = 250 m, S2 = -240 V, E2 = 500 V
WD
Gap =500 m, S2 = -240 V, E2 = 500 V
WD
Working Distance (m)
1600
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1500
S2 & E2 focusing
1400
1300
1200
1100
100
12
200
300
400
500
Gap between electrodes (m)

13. Scan Range Dependency on WD and Gap
(Source Lens Focusing )
Narrow gap
Wide gap
800
200 m
S2 : -269 V, 20 nA
Deflector 100 V,
E1 : 0.02 nA, E2 : 0 V
Scan length (m)
700
500 m
S2 : -252 V, 20 nA
Deflector 100V,
E1 : 0.02 nA, E2 : 0 V
600
Tip voltage
S2 focusing mode
-200 V
-300 V Narrow gap
-400 V
Wide gap

500
400
300
200
2
4
6
8
WD (mm)
350
S2 focusing mode
Gap = 250 m, S2 = -245 V, D =  35 V, E2 = 0 V
Gap = 500 m, S2 = -245 V, D =  30 V, E2 = 0 V
Scan Range (um)
Gap = 150 m, S2 = -245 V, D =  40 V, E2 = 0 V
300
250
100
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Source lens
Einzel lens
200
300
400
500
Gap between electrodes (um)
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14. Image Quality Dependency on Gap and
Scan Range with Tip Voltage (Einzel Lens Focusing )
-200V
Wide gap
-250V
-300V
Narrow gap
-350V
100 m
200 m
Tip Voltage=-300V; S1 Current=0.5uA
S1 Voltage=-89V; E2 Voltage=357V
-400V
-450V
-500V
450
E1 positions
Wide gap Einzel lens
E2 focusing mode
Wide
400
Scan Range (m)
Tip Voltage = -300V; S1 Current = 10 nA
S1 Voltage = -79V; E2 Voltage = 411 V
Narrow
350
300
Experiment
Linear fitting
250
-400
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Vtip (V)
-200
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15. Combined Source and Einzel Lens Focusing
SEM Images from narrow gap lens system
A
B
Simulation of E-beam trajectory for
combined S2 and E2 focusing mode
C
Gap = 150 m, S2 = -240 V, D =  55 V, E2 = 500 V
D
E
Gap = 250 m, S2 = -240 V, D =  50 V, E2 = 480 V
F
Gap = 500 m, S2 = -240 V, D =  40 V, E2 = 570 V
Combined S2 and E2 focusing mode
Operation parameters
SEM
Image
Tip
nA
V
A
-140
B
S2
Mag
nA
V
nA
V
-270
-267
25
50
10
10
-141
-214
-267.1
24
150
4.5
-140
-220
-265
17
200
4.5
-143
-222
-264
27
250
E
-145
-232
-265
28
270
2
F
-147
-231
-265
29
289.8
1
D
-300
nA
E2
V
C
V
S1
100
SUNMOON UNIVERSITY
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4.5
Scan Range (um)
150
100
50
100
200
300
400
500
Gap between elctrodes (um)
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16. Summary
 Fabrication of thin Si membrane lens ->
gap can be easily adjusted by selecting the proper Pyrex thickness.
 MEMS process such as Deposition, DRIE, Etching, bonding etc.
 Investigated the performance of a microcolumn
depending on gap between lenses and operation mode
 Source Lens Focusing mode : Larger scan range for narrow gap
 Einzel Lens Focusing mode : Lower image quality with narrow gap
due to the interference of electric field with deflector
 Two Lens Focusing mode (use source and Einzel lens simultaneously) :
Scan range of narrow gap lens is smaller than that of wide gap lens
 For the optimized scan range and good image quality,
adjustment of lens gap is required, tentatively 250 or 300 m.
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17. THANK YOU
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