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

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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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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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