confidential quantrainx50 module 3.1 electron optics 1-2011 place photo here

Confidential

Quantrainx50 Module 3.1 Electron

Optics1-2011

place photo here

2

2

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

3

3

SEM Main Components

Electron GunWehnelt cylinderFEG Electron Gun

4

4

Electron Gun Emitters

• Tungsten filament (W)

• Lanthanum Hexaboride filament (LaB6)(obsolete)

• Cerium Hexaboride (CeB6)

• Field emission filament (FEG)

5

Electron Gun Animation *

5

* Video courtesy of Oxford Instruments

6

6

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

7

7

Emission Area For Tungsten (W)

Filament

Wehnelt cap

Anode

Cross-over plane

Filament heating supply

High voltage supply (200 v- 30 kV)

70 A

8

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

9

9

Bias 255 ……………………………….. Bias 1

110 µA

90 µA 1 kV 30 kV

Emission : Autobias control

9

Autobias keeps emission between 90-110 µA for all kV

10

10

W Filament Saturation

filament current

emis

sion

cur

rent

Saturation point

False peak / Misalignment

11

11

Tungsten Filament

12

12

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

1414

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

1515

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

16

16

Lens Defects

image plane

Spherical aberration

Chromatic aberration

Aperture

Diffraction

optical axis

17

17

Spherical Aberrations

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

Disc of Least Confusion

18

18

Chromatic Aberrations

• Electrons of differing energies will be focused at different places

Disc of Least Confusion

19

19

Diffraction

• The wave nature of electrons cause diffraction limitations

20

20

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.

21

21

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

22

22

FEG Column Principle Diagram

Scan Coils

Gun Alignment Coils

Objective Aperture

10KVDriftSpace(Coulomb Tube)

C1

C2

Objective Lens

23

23

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

24

24

Emission Area for FEG

Extractor systemC1 static lens

Anode

Filament heating supply

High voltage supply (200 v- 30 KV)

150 A

2525

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

26

26

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

27

27

FEG Startup Steps

• Warmstart / Coldstart

• Gun conditioning

28

28

Beam Menu

Final operation status

29

29

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 mode

B = low beam current mode

• Final beam energy 30keV down to 200eV

A B

C1

C2

30

30

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 mode

B = low beam current mode

• Final beam energy 30keV

down to 200eV

A B

C1

C2

InternalSpray

Aperture

31

31

• Different paths for low

and high beam current

conditions through the

coulomb tube, but

common path to objective

Small Spot Large Spot

C1

C2

Aperture

DecelerationLens

FEG Column

3232

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

33

33

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

34

34

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)

35

35

SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Wehnelt cylinder

Condenser lenses

Scan generator

Demagnification system

36

36

Magnification

l

M=L/l

L

L

***-important

37

37

Scan Size Vs. Magnification

• Low Mag.

• Med Mag.

• Hi Mag.

38

38

Magnifying Your Sample on Quantax50

M= L

l

_

lL

39

39

Low Magnification

Scan Here

Display Here

40

40

Intermediate Magnification

Scan Here

Display Here

41

41

Higher Magnification

Scan Here

Display Here

42

42

• The viewed area (L) is fixed

Scan Size Vs. Magnification

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

43

43

A Focused Vs. An Unfocused Beam

44

44

The Crossover point on the Beam is of a Finite Size

D= Spot Size

I = Beam Current

ą = Measurement of the ‘cone’

45

45

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

46

46

Current Density (remove constants)

• Current and Spot size are directly proportional

=I Amps

d o ( ) Cm 2β

47

47

Resolution

The resolution of the microscope

is a measure of the smallest separation

that can be distinguished in the image

resolved unresolved

48

48

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

49

49

The Electron Beam Scans From Left to Right

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

50

50

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

• The ideal spot size

51

51

Too Large of Spot Size Looks Out of Focus

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

52

52

Scan Size Vs. Magnification

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

***-important

53

53

Scan Size Vs. Magnification

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

***-important

54

54

Obtaining an Image

• The SEM operator needs to do two things:

1- find the correct focus

2- determine the correct spot size

55

55

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

56

56

Electro-magnetic Condenser Lens

metal jacket

copper windings

Optic axis

Air gap

Cross-over

e-

58

58

Aperture

Condenser lens

Electron beam In

Electron spray

Electron beam Out

Condenser Lens Action on Beam

59

59

Condenser Lens Action on Beam

• Decreased lens current creates more beam current

60

60

• Increased lens current creates less beam current

Condenser Lens Action on Beam

61

61

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

62

62

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

63

63

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

64

64

SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Wehnelt cylinder

Condenser lenses

Scan generator

Focus UnitObjective andStigmation lenses

65

65

Focusing the Beam Onto the Sample Uses the Objective Lens

objective lensfinal lensaperture

pole piece

sample

66

66

Focusing the Beam Onto the Sample

pole piece

objective lensfinal lensaperture

sample

67

67

Focusing the Beam Onto the Sample

pole piece

objective lensfinal lensaperture

sample

68

68

Focusing the Beam Onto the Sample

objective lensfinal lensaperture

pole piece

sample

69

69

Working Distance (WD)

OWDFWD

pole piece

objective lensfinal lensaperture

specimen

70

70

Synchronizing Stage Height With WD

Z

Z

WD

WD

specimen specimen

71

71

WD Vs. Gas Path Length(GPL)

EDS

WD

Final Lens Pole Piece

Hi-Vac

GPL

72

72

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

Using the EDS Cone..

74

74

Focus and Stigmation• Focusing brings the beam crossover up or

down

• Stigmation controls the ovalness of the beam

75

75

Astigmation Is an Un-oval Beam

76

76

Astigmatism

disc of least confusionmagnified point source

77

77

Astigmatism...Continued

You have to see it to believe it…

78

78

SEM Main Components

Electron Gun

Demagnification system

Scan Unit

Detecting Unit

Wehnelt cylinder

Condenser lenses

Scan generator

Specimen + detector Detector

Objective andStigmation lenses

79

79

Different Types of Electron Detectors

Electron Detector

SEM

:Quantax50

A detector is a detector to the SEM

80

80

High Vacuum Everhardt-Thornley Secondary Electron Detector

Photomultiplier

Light guide

glass target

Phosphorousscreen (Al-coated)( +10 kV)

Faraday cage(-250 - +250 V)

Scintillator

81

81

Solid State Backscattered Detector

Backscattered electrons

Surface electrode

Silicon dead layer

SemiconductorBase plate

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

82

82

The Solid State BSD

83

83

The Gaseous Analytical Detector (GAD)

84

84

Low voltage high Contrast Detector (vCD)

Backscattered electrons

Surface electrode

Silicon dead layer

SemiconductorBase plate

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

85

85

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

86

86

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

87

87

The Large Field (LF) Detector

88

88

Gaseous Secondary Electron Detector

non-conductive specimen

GSED

Primary beam

Signal amplification by gas ionisation

Collection area at high positive voltage

Detected electron signal

89

89

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

90

90

GSED (Gaseous Secondary Electron Detector)

91

91

Available SE Gas Amplification Detectors & Cones

Low KV Cap

GSEDLFD GBSDX-RayCone

92

92

HighVac / LowVac: LF-GSE + SS-BSE

Changing modes without detector change

LFD

BSE

93

93

LF-Detector + Low KV Cap

Low kV imaging with Low KV Cap

LFD

Low KV Cap

94

94

X-Ray Cone

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

LFD

X-Ray Cone

95

95

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

96

96

GBSD (Gaseous Backscattered Electron) Detector

97

97

The GBSD

---

BSE Converter Plate

BSE Generated by Primary Beam

PLA

SE Collection Grid

SE 3

Buried Signal Track

++

+

98

98

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

99

99

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

na

l (A

rbitr

ary

)

BC

100

100

Oil in Water

Secondary Mode

Backscattered Mode

101

101

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

102

102

Hot Stage “Hook” (ESD)

103

103

Hot Stage ‘Hook” and Detector

104

104

Through The Lens Detector (TLD)

Specimen

PMT

E.T. SED

TLD

105

105

Scintillator-type Backscattered Detector (Robinson & Centaurus)

specimen

Aluminium

P-scintillator

through light guide toPhotomultiplier tube

106

106

Cathodoluminescence Detector

Polished Aluminium Light guide

Photomultiplierspecimen

107

107

Electron Backscatter Pattern (EBSD) Detector

Final Lens

Primary Beam

BSE

EBSD

108

108

EBSD Applications

1m = 50 steps

OIM from 1000 Å PVD Copper Damascene lines

109

109

Specimen Current Detector

iPC

iSE

iBSE

iSC

specimen

110

110

Electron Beam Induced Current (EBIC)

PE

SCA

P N P

111

111

CCD Camera - Quantax50 View

As viewed from under the EDS detector

LFD

E.T. SED

BSD

Sample

112

112

The end QUANTRAINx50 3.2PPT- Optics