Confidential

Quantrainx50 Module 3.1 Electron

Optics1-2011

place photo here

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SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Detecting Unit

Wehnelt cylinderor FEG unit

Condenser lenses

Scan generator

Objective andStigmation lenses

Electron detector

Focus Unit

Scan generator

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SEM Main Components

Electron GunWehnelt cylinderFEG Electron Gun

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Electron Gun Emitters• Tungsten filament (W)• Lanthanum Hexaboride filament (LaB6)

(obsolete)• Cerium Hexaboride (CeB6)• Field emission filament (FEG)

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Electron Gun Animation *5

* Video courtesy of Oxford Instruments

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Electron Source Properties• Current density (brightness)• Emission current• Stability of source• Lifetime of filament• Design of electron source assembly• Ease of operation• Costs involved

ą

ip

do

specimen

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Emission Area For Tungsten (W)

Filament

Wehnelt cap

Anode

Cross-over plane

Filament heating supply

High voltage supply (200 v- 30 kV)

70 A

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8

Bias on Wehnelt Cap

Equipotential lines of the Voltage Field

High emissionlarge spot

Sufficient emission

small spot

Low bias voltage

0+

Optimum bias voltage

0+

High bias voltage

No emission

+0

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Bias 255 ……………………………….. Bias 1

110 µA

90 µA 1 kV 30 kV

Emission : Autobias control

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Autobias keeps emission between 90-110 µA for all kV

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10

W Filament Saturation

filament current

emis

sion

cur

rent

Saturation point

False peak / Misalignment

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Tungsten Filament

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High Resolution, High Brightness FEG source…

Tungsten LaB6 FEGNormalized Brightness (-) 1 10 1000Maximum probe current (nA) 2000 500 100Life time (hrs) 60-200 200-1000 > 10000Beam current stability (10 hrs) <1% <1% <0.4%Resolution 30kV (nm) 3.0 2.0 1.2Resolution 1kV (nm) 25 15 3.0Cost source (USD) 20 900 26000

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XL Schottky FEG Theory

• The Boersch Effect• A) Perfect beam: no

interactions• B) Random beam: one

dimension• C) Random beam: two

dimensions

• It is actually three dimensional

ooooooooo

oo

o

ooo

o

oo

oo

o o

o o

o oo o

A B C

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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.

o o o o o o

o o oo o o

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Lens Defects

image plane

Spherical aberration

Chromatic aberration

Aperture

Diffraction

optical axis

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Spherical Aberrations

• Electrons entering into a lens at different points get focused at different points

Disc of Least Confusion

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Chromatic Aberrations

• Electrons of differing energies will be focused at different places

Disc of Least Confusion

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Diffraction

• The wave nature of electrons cause diffraction limitations

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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.

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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

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FEG Column Principle Diagram

Scan Coils

Gun Alignment Coils

Objective Aperture

10KVDriftSpace(Coulomb Tube)

C1

C2

Objective Lens

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FEG gun (electron source)

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

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Emission Area for FEG

Extractor systemC1 static lens

Anode

Filament heating supply

High voltage supply (200 v- 30 KV)

150 A

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Schottky Gun Design

• Fil = Filament current input (2.4 Ampere)

• S = Suppressor (-500V)

• E = Extractor (+5000V)

• C1 = Electrostatic Condenser lens

S

E

C1

Fil

E

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Schottky Tip design• M = Tip module• W = Welded

tungsten Tip• Fil = Tungsten

wire filament• T = Sharpened

Tip• Zr = Zirconium

reservoir Zr

T

Fil

W

M

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FEG Startup Steps

• Warmstart / Coldstart

• Gun conditioning

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Beam Menu

Final operation status

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FEG Column Double condenser lens

• Extraction voltage changes not necessary, beam current is set by condenser lenses

• C1 is electrostatic• C2 is electromagnetic• Variable lens strengths:

A = high beam current modeB = low beam current mode

• Final beam energy 30keV down to 200eV

A B

C1

C2

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FEG Column Double Condenser Lens

• Extraction voltage changes not necessary, beam current is set by condenser lenses

• C1 is electrostatic• C2 is electromagnetic• Variable lens strengths:

A = high beam current modeB = low beam current mode

• Final beam energy 30keV down to 200eV

A B

C1

C2

InternalSpray

Aperture

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• Different paths for low

and high beam current conditions through thecoulomb tube, but common path to objective

Small Spot Large Spot

C1

C2

Aperture

DecelerationLens

FEG Column

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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

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Beam Current: Spotsize 30kV 20kV 10kV 5kV 2kV 1kV 500V

1 21 p 13 p 8 p 5 p 2.5 p 1.4 p 0.7 p

2 44 p 40 p 33 p 25 p 13 p 7 p 4 p

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

Probe Current for FEG

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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).

FEG Spot Size (nM)

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SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Wehnelt cylinder

Condenser lenses

Scan generator

Demagnification system

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Magnification

l

M=L/l

L

L

***-important

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Scan Size Vs. Magnification

• Low Mag.

• Med Mag.

• Hi Mag.

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Magnifying Your Sample on Quantax50

M= L

l

_

lL

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Low Magnification

Scan Here

Display Here

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Intermediate Magnification

Scan Here

Display Here

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Higher Magnification

Scan Here

Display Here

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• The viewed area (L) is fixed

Scan Size Vs. Magnification

• The smaller the area scanned on the sample results in higher viewed magnification

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A Focused Vs. An Unfocused Beam

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The Crossover point on the Beam is of a Finite Size

D= Spot Size

I = Beam Current

ą = Measurement of the ‘cone’

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Current Density

• Current Density remains constant through the optical path of the electron beam

=4 X I Amps

π X d o X a( ) Cm Steradians2β

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Current Density (remove constants)

• Current and Spot size are directly proportional

=I Amps

d o ( ) Cm 2β

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Resolution

The resolution of the microscope

is a measure of the smallest separation

that can be distinguished in the image

resolved unresolved

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The Diameter of the Electron Beam Must Be Smaller Than the Feature to Be Resolved

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The Electron Beam Scans From Left to Right

• There can be from 512 to 4096 scan lines, at all magnifications

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The Electron Beam Spot Size Must Be Smaller Than the Features Being Resolved

• The ideal spot size

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Too Large of Spot Size Looks Out of Focus

• Too big of spot size creates an out of focus image

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Scan Size Vs. Magnification

• Spot size for low mag is not acceptable for higher mag

***-important

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Scan Size Vs. Magnification

• Spot size for medium mag is not acceptable for highest mag

***-important

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Obtaining an Image• The SEM operator needs to do two

things:

1- find the correct focus

2- determine the correct spot size

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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

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Electro-magnetic Condenser Lens

metal jacket

copper windings

Optic axis

Air gap

Cross-over

e-

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Aperture

Condenser lens

Electron beam In

Electron spray

Electron beam Out

Condenser Lens Action on Beam

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Condenser Lens Action on Beam

• Decreased lens current creates more beam current

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• Increased lens current creates less beam current

Condenser Lens Action on Beam

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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

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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

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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

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SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Wehnelt cylinder

Condenser lenses

Scan generator

Focus UnitObjective andStigmation lenses

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Focusing the Beam Onto the Sample Uses the Objective Lens

objective lensfinal lensaperture

pole piece

sample

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Focusing the Beam Onto the Sample

pole piece

objective lensfinal lensaperture

sample

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Focusing the Beam Onto the Sample

pole piece

objective lensfinal lensaperture

sample

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Focusing the Beam Onto the Sample

objective lensfinal lensaperture

pole piece

sample

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Working Distance (WD)

OWDFWD

pole piece

objective lensfinal lensaperture

specimen

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Synchronizing Stage Height With WD

Z

Z

WD

WDspecimen specimen

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WD Vs. Gas Path Length(GPL)

EDS

WD

Final Lens Pole Piece

Hi-Vac

GPL

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WD Vs. Gas Path Length(GPL)

Final Lens Pole Piece

EDS

WD= 10 mmGPL= 2MM

EDS Cone(8mm)

Hi-Vac Intermediate Vacuum

7373

Low noise EDS Mapping in Low-vacuum with use of EDS Cone

Using the EDS Cone..

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Focus and Stigmation• Focusing brings the beam crossover up or

down

• Stigmation controls the ovalness of the beam

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Astigmation Is an Un-oval Beam

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Astigmatism

disc of least confusionmagnified point source

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Astigmatism...Continued

You have to see it to believe it…

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SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Detecting Unit

Wehnelt cylinder

Condenser lenses

Scan generator

Specimen + detector Detector

Objective andStigmation lenses

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Different Types of Electron Detectors

Electron Detector

SEM :Quantax50

A detector is a detector to the SEM

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High Vacuum Everhardt-Thornley Secondary Electron Detector

Photomultiplier

Light guide

glass target

Phosphorousscreen (Al-coated)( +10 kV)

Faraday cage(-250 - +250 V)

Scintillator

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Solid State Backscattered Detector

Backscattered electrons

Surface electrode

Silicon dead layer

SemiconductorBase plate

+++++++++++++----------------------

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The Solid State BSD

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The Gaseous Analytical Detector (GAD)

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Low voltage high Contrast Detector (vCD)

Backscattered electrons

Surface electrode

Silicon dead layer

SemiconductorBase plate

+++++++++++++----------------------

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The best imaging conditions at LV Low KeV: flat cone short beam gas path length, low pressures and long amplification path

Electron beam

EDX

Detector

Sample

Detected electron signal

5 mm WD

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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

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The Large Field (LF) Detector

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Gaseous Secondary Electron Detector

non-conductive specimen

GSED

Primary beam

Signal amplification by gas ionisation

Collection area at high positive voltage

Detected electron signal

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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

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GSED (Gaseous Secondary Electron Detector)

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Available SE Gas Amplification Detectors & Cones

Low KV Cap

GSEDLFD GBSDX-RayCone

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HighVac / LowVac: LF-GSE + SS-BSE

Changing modes without detector change

LFD

BSE

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LF-Detector + Low KV Cap

Low kV imaging with Low KV Cap

LFD

Low KV Cap

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X-Ray Cone

LF-Detector + X-Ray cone: no BSE detection

LFD

X-Ray Cone

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GaseousAnalyticalDetector

• The GAD is a SS-BSED + X-Ray cone• Optimised low vacuum microanalysis and imaging

(SE and BSE) at the analytical WD• Minimum Magnification 250 x

LFD

GAD

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GBSD (Gaseous Backscattered Electron) Detector

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The GBSD

---

BSE Converter Plate

BSE Generated by Primary Beam

PLA

SE Collection Grid

SE 3

Buried Signal Track

++

+

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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

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GBSD Optimized for High Pressures

Signal vs Pressure

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

Pressure

Sig

nal (

Arb

itrar

y)

BC

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Oil in Water

Secondary Mode

Backscattered Mode

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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

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Hot Stage “Hook” (ESD)

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Hot Stage ‘Hook” and Detector

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Through The Lens Detector (TLD)

Specimen

PMT

E.T. SED

TLD

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Scintillator-type Backscattered Detector (Robinson & Centaurus)

specimen

Aluminium

P-scintillator

through light guide toPhotomultiplier tube

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Cathodoluminescence Detector

Polished Aluminium Light guide

Photomultiplierspecimen

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Electron Backscatter Pattern (EBSD) Detector

Final Lens

Primary Beam

BSE

EBSD

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EBSD Applications

1m = 50 steps

OIM from 1000 Å PVD Copper Damascene lines

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Specimen Current Detector

iPC

iSE

iBSE

iSC

specimen

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Electron Beam Induced Current (EBIC)

PE

SCA

P N P

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CCD Camera - Quantax50 View

As viewed from under the EDS detector

LFD

E.T. SED

BSD

Sample

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The end QUANTRAINx50 3.2PPT- Optics