1SUNMOON UNIVERSITY
A method to increase FOV in an e-beam
microcolumn
Om Krishna Suwal, Ph.D.
Department of Physics and Nanoscience
Sun Moon University
2SUNMOON UNIVERSITY
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
3SUNMOON UNIVERSITY
Microcolumn
Cathode
Source lens
Deflector
Einzel lens
Sample
~ 3-10 mm
Optical axis for e-beam
Source size, do
Probe size, d1
2o
21
𝑑12=(𝑀𝑑0 )2+𝑑𝑑
2 +𝑑𝐶𝑠
2 +𝑑𝐶𝑐
2Resolution
T.H.P. Chang et al. (1989)
d1 = Probe sizeM = Column magnificationd0 = virtual source sizedd= 1.5/1VV = Beam energy= 0.25 Cs
= Cc 1 V/VCs = Spherical aberration coefficientCc = Chromatic aberration coefficient
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Microcolumn Features and Benefits
Miniaturized Electron Optical system Portable electron source Mini-SEM (Desktop SEM)
Low energy e-beam Electron energy 100 eV – 2 keVLess sample damages due to e-beam
Microfabricated ColumnsMEMS technology applicable Low cost and High yield
Arrayed Beam OperationParallel beam operationHigh throughput with low beam voltageShort stage traveling
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Microcolumn Assembly
H.S. Kim et al. / Microelectronic Engineering 83 (2006) 962–967
Emitter
Source Lens
Deflectors
Einzel lens
Schematic Shield microcolumn
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Einzel Lens Assembly
3 mm8 mm
~0.5
mm
/
1.5
mm
= 200 m / 100 m
E1
E2
E3
Si Pyrex
Pyrex 150 m / 500 m
Source or Einzel lens structural cross section
Emitter
Source Lens
Einzel Lens
Deflector
Grid (sample)
3 - 10 mm
Microcolumn cross section
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Fabrication Process
Si wafer
a. Boron doping and oxidation
c. DRIE Si trenches
d. SiO2 deposition
b. Front side lithography
Si
PR SiO2
Boron doped Si
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Fabrication Process cont…
e. Back side patterning
g. SiO2 strip off and doping
10 mm
Si
PR SiO2
Boron doped Si
f. Back side Si etching
view
view
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Si Membrane-Pyrex Bonding
A
HT
Membrane detachment
SiPyrex
Hot plate
Si membrane
Pyrex
A Pyrex bonded membrane
Interface of bonded Si-Pyrex
10 ㎛SiPyrex
Pyrex
Si
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Multiple Lens Alignments
Las
er
Las
er
Las
erSi
Pyrex
L2L1 L3
22.5o 45o
Top view of the assembled lens
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Modes of OperationSource lens
focusing
Sample
S1
S3S2
D1
E1E2E3
D2
VS1VS2
Einzel lens focusing
Sample
VS1
VE2
S1
S3S2
D1
E1E2E3
D2
Source + Einzel lens focusing
Sample
VS1VS2
VE2
S1
S3S2
D1
E1E2E3
D2
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Simulation of e-beam trajectory and electric potential along optical axis
Gap = 250 m, S2 = -240 V, E2 = 500 V
Gap = 150 m, S2 = -240 V, E2 = 500 V
Gap =500 m, S2 = -240 V, E2 = 500 V
WD
WD
WD
100 200 300 400 5001100
1200
1300
1400
1500
1600
Wor
king
Dis
tanc
e (
m)
Gap between electrodes (m)
S2 & E2 focusing
= 200 m = 500 m
50100 20
-300VS1
S2S3 D1 D2 E1 E3
E2Module Electric Potential distribution at Einzel lens system
Gap = 150 m Gap = 500 m
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Scan Range Dependency on WD and Gap (Source Lens Focusing )
200 m
S2 : -269 V, 20 nA Deflector 100 V, E1 : 0.02 nA, E2 : 0 V
Narrow gap
500 m
S2 : -252 V, 20 nA Deflector 100V, E1 : 0.02 nA, E2 : 0 V
Wide gap
2 4 6 8200
300
400
500
600
700
800
-200 V -300 V -400 V Wide gap
Sca
n le
ngth
(m
)
WD (mm)
Tip voltage
Narrow gap
S2 focusing mode
100 200 300 400 500250
300
350S2 focusing mode
Sca
n R
ange
(um
)
Gap between electrodes (um)
Gap = 150 m, S2 = -245 V, D = 40 V, E2 = 0 V
Gap = 250 m, S2 = -245 V, D = 35 V, E2 = 0 V
Gap = 500 m, S2 = -245 V, D = 30 V, E2 = 0 V
Source lensEinzel lens
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Image Quality Dependency on Gap and Scan Range with Tip Voltage (Einzel Lens Focusing )
100 m
Narrow gap
Tip Voltage = -300V; S1 Current = 10 nAS1 Voltage = -79V; E2 Voltage = 411 V
-400 -200
250
300
350
400
450
Experiment Linear fitting
Sca
n R
ange
(m
)
Vtip (V)
Wide gap Einzel lensE2 focusing mode
E1 positions
Wide Narrow
-200V -250V -300V -350V
-400V -450V -500V
Wide gap
200 m
Tip Voltage=-300V; S1 Current=0.5uAS1 Voltage=-89V; E2 Voltage=357V
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Combined Source and Einzel Lens Focusing
100 200 300 400 500
50
100
150
Sca
n R
an
ge
(u
m)
Gap between elctrodes (um)
Combined S2 and E2 focusing mode
SEM Image
Tip S1 S2 E2 Mag
V nA V nA V nA V nA V
A
-300
-140 -270
100
-267 25 50 10 10B -141 -214 -267.1 24 150
5
4.5C -140 -220 -265 17 200 4.5D -143 -222 -264 27 250 4.5E -145 -232 -265 28 270 2F -147 -231 -265 29 289.8 1
A B C
D E F
SEM Images from narrow gap lens system
Operation parameters
Simulation of E-beam trajectory for combined S2 and E2 focusing mode
Gap = 500 m, S2 = -240 V, D = 40 V, E2 = 570 V
Gap = 150 m, S2 = -240 V, D = 55 V, E2 = 500 V
Gap = 250 m, S2 = -240 V, D = 50 V, E2 = 480 V
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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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