Scanning Electron Microscopy

10 4 10 5 10 6 10 7 10 8 0 100 200 300 400 500 600 700

Intensity (arb. units)

Energy Loss (eV)

O-K

edge

Si L edge Incident Beam

Valence Excitations

Electron Energy Loss Spectrum of SiO

Path of the Electron Beam

SE1

SE1

SE2

SE2

BS2

BS2

Kanaya-Okayama Depth Penetration

Formula

R

R

=

=

______________

______________

0.89

0.89

(Z

(Z

ρ

ρ

)

)

0.0276 A E

0.0276 A E

1.67

1.67

μ

μ

m

m

R= Depth Penetration

R= Depth Penetration

A= Atomic Weight (g/mole)

A= Atomic Weight (g/mole)

E= Beam Energy (KV)

E= Beam Energy (KV)

Z= Atomic number

Z= Atomic number

ρ

ρ

= density (g/cm )

= density (g/cm )

2

2

The Affect of Accelerating Voltage

30KV

30KV

15KV

15KV

5KV

5KV

1KV

1KV

.5KV

.5KV

3.1

3.1

μ

μ

m

m

.99

.99

μ

μ

m

m

.16

.16

μ

μ

m

m

.01

_(100A)

.01

_(100A)

μ

μ

m

m

35 A

35 A

Depth Penetration in Iron

Depth Penetration in Iron

Primary Beam

Primary Beam

Interaction Volume vs Accelerating Voltage

25 kV

25 kV

15 kV

15 kV

5 kV

5 kV

Interaction Volume –Sample Composition

Silver

Silver

Carbon

Carbon

Iron

Iron

(20 kV incident beam in all 3 cases)

Secondary Electrons

SE1

SE1

SE2

SE2

SE3

SE3

final lens

final lens

specimen

specimen

BSE

BSE

Electron Interactions (Between Primary Beam and Sample)

• SE1- at point of primary interaction

• SE2- away from initial interaction point

• SE3- by BSE outside of sample

• BSE1- at point of primary interaction

Lateral Distribution of SE

SE1

SE1

SE2A

SE2A

SE2B

SE2B

SE Escape Depth

SE Escape Depth

Total Beam Penetration Volume

Total Beam Penetration Volume

SE1 > 100KX

SE1 > 100KX

SE2A

SE2A

-

-

50KX

50KX

SE2B< 15KX

SE2B< 15KX

Lateral Distribution of BSE

BS2A

BS2A

BS2B

BS2B

BS1

BS1

BS2A Escape Depth

BS2A Escape Depth

BS2B Escape Depth

One Primary Electron In Can Create Several

SEs Out at Low Accelerating Voltages

100

100

Angstroms

Angstroms

in

PE

out

SE

=

δ

David Muller 2008

Energy Distribution of Emitted Electrons

0

0

50

50

eV

eV

_{2 kV}

_{2 kV}

E

E

SE

SE

BSE

_BSE

Auger

Auger

# of electrons

# of electrons

collected

collected

electron energy

electron energy

in

PE

out

SE

=

δ

As-received samples are all coated with a carbon contamination layer

Overall scaling factor is from the different backscattering responses of the substrate

Secondary Electron Yields

Cleaned in-situ Carbon-contaminated

0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 KV EC 1 EC 2 KV

Charge Neutralization

δ

Incident Beam Voltage

Electron Yield

δ= # SE out / # Inc e

_in

Sample charges +ve

(increases landing energy Of incident electrons)

Sample charges –ve (reduces landing energy Of incident electrons)

Voltage Contrast with SE

The floating end of the via chain is bright because of trapped negative charge causes secondary electrons to be repelled.

The remainder of the chain is neutral, and thus darker.

(http://www.acceleratedanalysis.com/hepvc.html)

High Vacuum E.T. Secondary

Electron Detector

Photomultiplier

Light guide

glass target

Phosphorous

screen (Al-coated)

(10 kV)

Faraday cage

(-150 - +300 V)

Scintillator

Secondary Electron Detectors

Specimen

PMT

E.T. SED

TLD

Internal

Lens

Field-Free Operation

Immersion Lens

Small area, high resolution Large area, lower resolution

TLD in BSE Mode

Specimen

• Within-the-lens

detector is

part of the final

lens

• Bias voltage

down to -150V

_SE

SE3

BSE

Topography Affects Secondary Electron Emission

(Angle of Incidence)

Scanning Action of the

Electron Beam in a 3-D

Location of Detector Leads to Shadowing

A

A

B

B

C

C

+300 V

+300 V

SE

SE

-

-

detector

detector

What Is “Reality” in the SEM ?

Previous image turned upside down.

Edge Effects on a Sphere

Example of Sample charging in a

Secondary Electron Image

Charging is worse On this face

As more secondaries escape

A Line Profile on a Semi-conductor

Line

One Micron SiO2 in Si

One Micron SiO2 in Si

The E

The E

-

-

Beam line profile of the specimen

Beam line profile of the specimen

Where do you measure “One Micron” ?

A Line Profile on a Semi-conductor

Line

One Micron SiO2 in Si

One Micron SiO2 in Si

A 1 KV bean has minimal beam penetration and can give

A 1 KV bean has minimal beam penetration and can give

an image that is closer to ‘reality’.

an image that is closer to ‘reality’.

1KV

1KV

.99

A Line Profile on a Semi-conductor

Line

One Micron SiO2 in Si

One Micron SiO2 in Si

A 5KV beam penetrates deep into the specimen

A 5KV beam penetrates deep into the specimen

which gives the appearance of the peaks

which gives the appearance of the peaks

being closer together

being closer together

5KV

5KV

.74uM

Line Profiles on the Same Sample Can

Change with Accelerating Voltages

1KV

1KV

3KV

3KV

2KV

2KV

5KV

5KV

.99

.99

μ

μ

m

m

.92

.92

μ

μ

m

m

.85

.85

μ

μ

m

m

.74

.74

μ

μ

m

m

This was a 1

Secondary Electrons

SE1

SE1

SE2

SE2

SE3

SE3

final lens

final lens

specimen

specimen

BSE

BSE

Angular Distribution of BSE

• Normal angle of

incidence

• Greater angle of

incidence

Angular Distribution of BSE

• Contrary to SE images, BSE images can have

dark edges

Electron Emission Coefficient Vs.

Atomic Number at 20 KV

• Electron

Emission

Coefficient

• Atomic Number

Total

Total

BSE

BSE

SE

SE

20

20

80

80

1

1

.2

.2

SE Electron Emission Coefficient Vs

Atomic Number at Various KV

• Secondary

Electron

Emission

Coefficient

• Atomic Number

2KV

2KV

5KV

5KV

10KV

10KV

15KV

15KV

20KV

20KV

20

20

80

80

1

1

.2

.2

.2

.2

SE Emission Coefficient Vs. KV at

Various Atomic Numbers

• Secondary

Electron

Emission

Coefficient

• Accelerating Voltage in KV

5

5

15

15

25

25

1

1

.2

.2

.2

.2

AU

AU

AL

AL

C

C

Tilt Dependence of BSE

Reverse Biased S.E.D. Repulses

Secondary Electrons

--

150 V

150 V

SE

SE

-

-

detector

detector

A

A

B

B

C

C

Backscatter Electrons Ignore

the Bias

--

150 V

150 V

SE

SE

-

-

detector

detector

BSE

BSE

A

A

B

B

C

C

A Solid State BSD Can Image

Two Ways

• Elemental backscatter images are acquired by

adding detectors A+B.

• Topographical backscatter electron images

can be acquired by subtracting b from a

(A-B)

Solid State BSD

-ve Biased E-T Noisy Backscattered Signal

-ve Biased E-T Secondary Electron Signal

Grains in a Polished Fe-Si Alloy imaged by

Different SEM methods

Backscattered A-B “Topographic” Signal Backscattered A+B “Composition” Signal

Electron Channeling in a Crystal

Electron wave fields within a crystal for incident electron directions close to the Bragg angle qB. The vertical lines are the position of the Bragg reflecting atomic planes. From H. Niedrig, “Electron backscattering from thin films”, Journal of Applied Physics -- April 1982 -- Volume 53, Issue 4, pp. R15-R49

Electron Channeling in a Crystal

Electron Backscatter Diffraction Pattern of Germanium. Right –automatic indexing software matches the high symmetry zone axes and spacing between them to identify the crystal type and orientation. (University of Queensland, http://www.uq.edu.au/nanoworld/xl30_anl.html)

Electron Backscattering Diffraction Patterns

(EBSD or EBDP) for Orientational Imaging

Orientation Imaging Map (color shows grain orientation)

Boundary – Color shows angle of grain boundary

Sample Prep for EBSD

No pattern visible 3 micron diamond polish

Pattern Image Quality (IQ) = 25

1 micron alpha alumina

Pattern IQ = 177 10 minutes colloidal silica

Pattern IQ = 224 30 minutes colloidal silica