3. Scanning Electron Microscopy - Semantic Scholar · 2018-10-12 · 3. Scanning Electron Microscopy Dr Aïcha Hessler-Wyser Bat. MXC 134, Station 12, EPFL+41.21.693.48.30. Centre

Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 1 CiMe

3. Scanning Electron Microscopy

Dr Aïcha Hessler-Wyser

Bat. MXC 134, Station 12, EPFL+41.21.693.48.30.

Centre Interdisciplinaire de Microscopie Electronique

CIME


Outline

a.  SEM principle b.  Detectors c.  Electron probe and resolution d.  Depth of field e.  Stereoscopy f.  Electron-matter interaction volume g.  Secondary and back-scattered electrons h.  Contrasts i.  Examples j.  Charging effects


Outline

This chapter will describe the principle of a scanning electron microscope (SEM). We will start with a description of the detectors allowing signal detection, the formation of an electron probe and its influence on the spatial resolution. Then we will will define the depth of field and see how to control it, how to do stereoscopy. In order to understand the image formation and the contrasts observed on a picture, there will be considerations about the electron-matter interaction volume, and then an explanation of the origin of the secondary and back-scattered electrons (SE and BSE). This will allow us to analyse the different possible contrasts of a SEM picture, including artefacts. We will end with application examples.


a. SEM principle

•  Image formed step by step by the sequential scanning of the sample with the electron probe

•  Image acquisition as numerical data

• Bulk sample

•  Imaging the sample «!surface!» (from 1 nm to "1 #m depth depending on the analysed signal

• Contrast is due to secondary electrons (SE) emission or back scattered elctrons (or sometimes to photons, RX, absorbed current)

• Resolution: 1 nm to 10 nm


a. SEM principle

Response to incident electrons:

!  Secondary electrons SE topography, low energy "0-30 eV

!  Backscattered electrons BSE atomic number Z, energy " eV0

!  Auger Electrons : not detected in conventional SEM, surface analysis

!  Cathodoluminescence: photons UV, IR, vis

!  Absorbed current, electron-holes pairs creation, EBIC

!  plasmons

!  Sample heating (phonons)

!  Radiation damages: chemical bounding break, atomic displacement out of site (knock-on)


a. SEM principle

Energy spectrum of electrons leaving the sample

Auger (secondary)electrons

Back scatered electrons BSE

Secondary electrons SE


a detector (eye, photographic plate, video

camera...

an illumination section (lenses, apertures...)

a magnification section (lenses, apertures...)

A "light" source

b. Detectors

a sample (+ a "goniometer")


b. Detectors

Everhardt-Thornley detector: for SE and BSE

SE: the positive collector voltage (" +200 à +400V) attracts the SE toward the detector, the 10kV post acceleration give them enough energy to create a bunch of photons for each SE.

BSE: a negative collector polarisation ("-100V) repels the SE and the only BSE emitted in the narrow cone to the scintillator are detected (low collection efficiency = poor S/N ratio).


b. Detectors

BSE detectors

BSE Robinson detector: a large scintillator collects the BSE and guides more or less efficiently the light to a photomultiplicator

! large collection angle ! works at TV frequency

BSE semiconductor detector: a silicon diode with a p-n junction close to its surface collects the BSE (3.8eV/e--hole pair)

! large collection angle ! slow (poor at TV frequency)

! some diodes are split in 2 or 4 quadrants to bring spatial BSE distribution info


a detector (eye, photographic plate, video

camera...

an illumination section (lenses, apertures...)

a magnification section (lenses, apertures...)

a sample (+ a "goniometer")

A "light" source

c. Electron probe and resolution



Resolution in probe mode (SEM, STEM) !  Spherical aberration

!  Chromatic aberration

!  Diffraction (Airy, Rayleigh)

!  Brilliance ! conservation

!  Combinaison

3sph sd C != "

dd = 0.61!

n "sin#

dch = Cch!EE

+ 2 !II

" # $

% & ' (

dg =4 I! 2"

#1$ 1

10

100

0.001 0.01

SEM classique LaB6 courte focale

dia

mètr

e (

nm

)

Ouverture (mrad)

Vacc

=20 kV, !E=1.5 eV, "=1.105 eV

Csph

=17 mm, Cch

=9 mm

100pA

10pA

1pA

dc h

dsph

dd

microscope photonique

diam

ètre

(nm

) A/cm2sr Probe with coherent source: see Mory C, Cowley J M,

Ultramicroscopy 21 1987 171

Incoherent source

dech = dg2 + dsph

2 + dch2 + dd

2



Resolving power ("resolution"): Rayleigh criterium



SEM: Limiting parameters on resolving power " with SE 1.  High magnification

The probe size (generation of SE1)

r#dprobe

2.  The volume of interaction (generation of SE2+SE3 from BSE): energy and atomic number influence

3.  Low magnification The screen (or recording media) pixel size dscreen

r#dscreen/magnification



How to increase resolving power?

•  Reduce the probe current at constant dose

•  Increase exposure time t

•  Reduce probe size •  Decrease spot size •  Increase accelerating voltage

•  Reduce volume interaction •  Reduce accelerating voltage

•  Reduce Csph •  Short focus lenses: •  in-lens, semi in-lens, Snorkel

•  Increase brillance •  Field emission gun: •  Cold emission, thermal assisted,

Schottky effect

•  Reduce Csph and increase brillance

•  Dedicated columns: Gemini, XL30, …



SEM: Effet of current, probe diameter and image acquisition time

1 nA/160 s10 pA/160 s10 pA/10 s 100 pA/160 s

500nm

good resolution, but statistical noise

smal loss of resolution, still less

statistical noise

very few statistical noise, but high resolution loss!

Good resolution, less statistical noise



probe size and resolution (no noise)

particles 100 nm diam., probe dia 2 nm



model 100 nm diam. particles



Probe size and resolution (with noise) (with noise)

model 100 nm diam. particles

Particles100 nm diam.,

probe diam 2 nm

particles 50 nm diam., probe diam 2 nm

particles 25 nm diam., probe diam 2 nm



Current/probe diameter

Thermionique source: spherical aberration is the most important

Field emission source: gun aberrations and chromatic aberration are more important

Imax =3! 2

16"Csph

#2 3d8 3

Imax = cd2 3

Tiré de L.Reimer, SEM










Schottky effect





Thermionic SEM : low voltage?

Modern SEM short focus length: Csph=17 mm, Cch=9 mm, $E=1.5 eV, !=1.105 A/cm2sr

1

10

100

0.001 0.01

dia

mètr

e (

nm

)

Ouverture (mrad)

100pA

10pA

1pA

dch

dsph

dd


20 KV1

10

100

0.001 0.01

dia

mètr

e (

nm

)

Ouverture (mrad)

10pA

1pA

dch

dsph

dd


100pA

1 kV1

10

100

0.001 0.01

dia

mètr

e (

nm

)

Ouverture (mrad)

100pA10pA

1pA

dch

dsph

dd


5 kV










Schottky effect




SEM: résolution

Interaction volume versus E0

Penetration depth in Cu as a function of incident energy E0 and proportion of BSE (Monte-Carlo simulation)

Cu 20keV

1µ

Cu 5keV

1µ

Cu 1keV

Cu 1keV

1µ

Z = cte










Schottky effect





Short focus length… or… FEG?

1

10

100

0.001 0.01

dia

mètr

e (

nm

)

Ouverture (mrad)

100pA

10pA

1pA

dch

dsph

dd


20 KV1

10

100

0.001 0.01

dia

mètr

e (

nm

)

Ouverture (mrad)

100pA

10pA

1pA

dch

dsph

dd


20 kV

1

10

100

0.001 0.01

dia

mètr

e (

nm

)

Ouverture (mrad)

100pA

10pA

1pA

dch

dsphd

d


20 kV

Snorkel Regular focus length FEG

Csph=1.7 mm, Cch=1.9 mm Csph=17 mm, Cch=9 mm Csph=17 mm, Cch=9 mm

!=1.105A/cm2sr, $E=1.5 eV !=1.105A/cm2sr, $E=1.5 eV !=1.107A/cm2sr, $E=0.4 eV



Resolution loss at low voltage

100

50

10

5

10.5 1 2 5 10 20 30

FE LaB6

W

Tension d'accélération (kV)

Rés

olut

ion

(nm

)

Basse tension/haute résolution: - observation de la surface réelle - échantillons non-métallisés - faible endommagement dû au faisceau

Haute tension/haute résolution: - effets de bord - détails fins non-résolus - fort endommagement dû au faisceau

1985

2000 Shorter objective lens focal length

and Cs


Short questions 1. What is a condensor lens for?

a)  To create the image of the sample b)  To reduce the size of the electron source

2. Which parameters influence the resolution in SEM? a)  The size of the probe b)  The electron current c)  The electron energy d)  The aquisition device

e)  The wave length f)  The lens aberration

3. How to reduce the probe size? a)  By reducing the electron energy b)  By reducing the apperture size c)  By increasing the Working Distance

d)  By removing the spherical aberration

4. How to reduce the interaction volume? a)  By reducing the electron current b)  By reducing the electron energy


d. Depth of field

Depth of field as a function of dprobe

hprof .champ =max2pixel"image "

G1!

2dsonde1!

"

# $

% $

h

h

2dA

2%&

dA

The depth of field is the depth for which the image is focussed The depth of field increases when % decreases.

•  Increase the working distance

•  Reduce objective aperture size


d. Depth of field

Effect of working distance (WD) and aperture on depth of field


d. Depth of field

10mm

1mm

100µm

10µm

1µm

0.1µm

10µm 1µm 100nm 10nm

10 102 103 104

1nm

105

SEM

LM!=500nm

10mrad

1mrad

0.1mrad

Résolution "

Pro

fondeur

de c

ham

p h

Grandissement (grossissement) G

Light bulb filament


d. Depth of field

Other examples


d. Depth of field

Effect of the objective aperture diameter

30 !m

50 !m

100 !m


d. Depth of field

Measuring depth of field: stereoscopy


e. Stereoscopy

The 3rd dimension: stereoscopic vision, anaglyphs



e. Stereoscopy

3-D

rec

on

stru

ctio

n (

an

ag

lyp

h)


e. Stereoscopy

3-D reconstruction (pseudo-perspective)


e. Stereoscopy

3-D

rec

on

stru

ctio

n (

gre

y le

vels)


e. Stereoscopy

3-D

rec

on

stru

ctio

n (

false

co

lors

)


e. Stereoscopy


e. Stereoscopy


f. Electron-matter interaction volume

Elastic interaction

Total kinetic energy and momentum are constant Eel + Eat = cte

The light electron interacts with the electrical field in the heavy atom: Rutherford scattering. Only little energy is transferred, the electron speed does not change significantly in amplitude but only in direction (elastic scattering).

Elastic interaction:

Energy transfer from the electron

to the target

100 kV 1000 kV

angle de diffusion

C Au C Au

0.5° 0.5 meV 0.03 meV 9 meV 0.5 meV

10° 0.15 eV 9 meV 2.7 eV 0.17 eV

90° 10 eV 0.6 eV 179 eV 11 eV

180° 20 eV 1.2 eV 359 eV 22 eV



Inelastic interaction

part of the total kinetic energy is dissipated (energy loss)

! vibration in molecules or crystals (phonons "meV-100meV) !  collective oscillations of electrons (plasmons "10 eV) !  intra- et interband transitions ("mev-"1 eV) !  inner shell atom ionisation ("50 to150 keV < eV0) !  bond breaking " eV, atom displacement " 10-30 eV (requires Vacc 100kV...1MV, no longer SEM!)



Mean free path

Elastic cross-sections 'el and mean free path (el, total (elastic+inelastic) mean free path (t and electron range R The mean free path is the average path that an electron does before having interaction with an atom



Monte-Carlo simulations

Electron Flight Simulator ($$$ Small World / D. Joy) –  old… DOS !!!! –  http://www.small-world.net

Single Scattering Monte Carlo Simulation (Freeware) –  "Monte Carlo Simulation" Mc_w95.zip –  by Kimio KANDA –  http://www.nsknet.or.jp/~kana/soft/sfmenu.html

CASINO (Freeware)

–  " monte CArlo SImulation of electroN trajectory in sOlids " –  by P. Hovongton and D. Drouin –  http://www.gel.usherbrooke.ca/casino/What.html



Number/Energy of backscattered electrons by Monte-Carlo simulations

30 kV 3 kV 1 kV

BSE 5% BSE 8% BSE 10%

C

BSE 52% BSE 43% BSE 41%

W



Penetration and backscattering vs elements (Z) Vacc = 20kV = cte

Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte-Carlo simulation):

1µ&

C 20 keV

BSE=6%

BSE=33%

1µ&

U 20 keV

BSE=50%

Cu 20 keV

1µ&

BSE=33%



Penetration and backscattering vs elements (Z)

10 nm

C 1keV

U 1keV 10 nm

Cu 1keV

10 nm

BSE=14%

BSE=34% BSE=44%

Vacc = 1 kV = cte




Penetration and backscattering vs elements (Z) Vacc = 5 kV = cte


200 nm

C 5 keV

Cu 5 keV

200nm

U 5 keV

200 nm

BSE=8%

BSE=33% BSE=47%



Penetration and backscat-tering vs energy(E) Z = cte Depth of electron penetration in Cu vs energy E0 and yield of electron backscattering BSE (Monte-Carlo simulation):

BSE=33%

Cu 20 keV

1#m

Cu 5 keV 1#m

Cu 1 keV Cu 1keV

1#m


g. SE and BSE

"true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3

Various SE types from • SE1: incident probe • SE2: BSE leaving the sample • SE3: BSE hitting the surroundings

although this signal is gathered around the probe, its intensity is only attributed to the pixel corresponding to the actual probe position

x0,y0


g. SE and BSE

"true" secondary electrons SE1 and "converted BSE" secondaries SE2+SE3

The SE signal always contain a high resolution part (SE1 from the probe) and an average (low resolution) part

from SE2+SE3!


total

total

total

SE1

SE2

g. SE and BSE

Relative contribution of SE1 and SE2 (+SE3) vs primary energy

The total intensity (green and brown) is attributed to the (x,y) pixel, here at 0 nm on this 1-D model

(adapted from D.C. Joy Hitachi News 16 1989)


g. SE and BSE

Yield for SE and BSE emission per incident electron vs atomic number Z

):

*:& SE: low or no chemical contrast but for light elements the topographical contrast will dominate on rough surfaces

ISE =Ipe$* +ISE3 =Ipe(*pe+*pe$)+*sur$))

with * the total SE yield, *pe the yield for SE1 and *sur the SE3 yield for materials surrounding the sample (pole-pieces...)

SE1 SE2 SE3

Al Ni

0.11

0.28

(SE1)

BSE: chemical contrast for all the elements (sensitivity #DZ=0.5) A fast way to phase mapping

IBSE=Ipe$) with Ipe the intensity of the primary beam, ) the BSE

yield

sample surface polished (no topography) and perpendicular to the incident beam direction (intermediate energy E0 # 15 keV)


g. SE and BSE

Dust on WC (different Z materials)

SE 25 kV BSE

low Z material flat material rough material low Z material

thin low Z material


g. SE and BSE

Contaminated area around a soldering spot


g. SE and BSE

Toner particle (penetration in light material)

SE 28 kV BSE


g. SE and BSE

Topographical contrast in SE mode Effet de l'inclinaison de la surface

+

I0 I(+)

#1-10nm

penetration depth ("range") >>SE escape length


Relative yield of SE vs angle of incidence on the sample surface !

I "( ) = I0# "( ) $I 0( )cos"

+


h. Contrast

SE and BSE topography contrast For one position (x,y) of the electron probe: BSE escape from a "pear" volume around the probe position SE1 escape from a thin layer under the entrance surface of the probe SE2 escape from a thin layer under the escape surface of BSE

contrast = 2(I1-I2)/(I1+I2)

ISE(0°)=IPE!*=IPE!10% ISE(40°) = IPE!*!1/cos40°=IPE!13% SE1 contrast = 26%

IBSE(0°)=IPE!)=IPE!31% IBSE(40°)=IPE!37% BSE contrast = 18%

ISE2+3 = IBSE!*= IPE!)!* & ISE2+3(0°) = IPE!37%!10%= IPE!3.1% out of 10% ISE2+3(40°) = IPE!37%!10%=IPE!3.7% out of 13%

BSE topographical contrast is not negligible! Chemical contrast is well observed only on polished samples

IBSE 31% IBSE 37%

incidence normale +=0° incidence +=40°

Ni Ni


h. Contrast

Topographical contrast at low energy Effect of the incidence angle



h. Contrast

Size and edge effects

Do not forget, in SEM: The signal is displayed at the probe position, not at the actual SE production position!!!

intensity profile on image


h. Contrast



h. Contrast


(From L. Reimer, Image Formation in Low-Voltage Scanning Electron

Microscopy, (1993))


h. Contrast

Comparison of SE and BSE contrast modes SE BSE

ET detector

(0V)

+200V

BSE are absorbed

The observator looks down to the column and the "light" seems to come from the Everhardt-Thornley detector.

backscattered and transmitted e- create SE, some of them are driven to the ET detector by the electric field

The trajectories of BSE are not strongly affected by the electrical field, most BSE miss the detector


h. Contrast

What does it suggest? Which objective information?


h. Contrast



h. Contrast



pyramid?

Detector ? Detector ?

etch-pit?

h. Contrast


(from L.Reimer, Image formation in the low-voltage SEM)

Change in SE contrast with the voltage

h. Contrast


h. Contrast

SE, 5 kV SE, 30 kV

An example: a fracture in Ni-Cr alloy

Contraste enhancement at low voltage: less delocalization by SE2.


h. Contrast

SEM: Effect of the accelerating voltage on penetration and SE signal

20 kV: strong penetration, SE3 is a much larger signal than SE1/SE2. It reveals the copper grid under the C film via the electron backscattering, but the structure of the film itself is hidden

2 kV: low penetration, only a few electrons reach the copper grid and most of the SE3 are produced in the C film together with SE1/SE2. The C film and its defects become visible

(from D.C. Joy Hitachi News 16 1989)


i. Examples

Tin grains on a thin carbon film (TEM supporting grid) HRSEM 25 kV 1 nm nominal resolution left: SE right: scanning transmitted electrons (STEM)

(from B. Ocker, Scanning Microscopy 9 (1995) 63…) SE: e-/e- coulombian

STEM: Rutherford (e-/electric field in atom)

Physical limit to the imaging in secondary electron mode


The average grain size looks larger in SE (12.3 nm) than in STEM (9.1 nm) "Delocalisation": the elastic scattering in STEM (Rutherford) occurs at a much closer distance from the atom nucleus than the inelastic coulombian e-/e-interaction required to eject a SE

(from B. Ocker, Scanning Microscopy 9 (1995) 63…)

Physical limit to the imaging in secondary electron mode

i. Examples


i. Examples

AlxGa1-xAs/GaAs "quantum wire" (2-D quantum well)

(by courtesy of Dr. K. Leifer, IPEQ/EPFL)

GaAs x=0.55

x=0.20

SE mode image on a cleaved surface. The SE2 (BSE chemical) contrast dominates this image in absence of topographical contrast (SE1=cte)

QW


i. Examples

Contrast reversal in BSE mode at low accelerating voltage

Ag Au Cu Si

from L.Reimer, Image formation in low-voltage SEM


j. Charging effects

Fiberglass on epoxy

1 kV

5 kV, low current

5 kV, high current

(by courtesy of B. Senior CIME/EPFL)


j. Charging effects

fiberglass on epoxy Which polarity ??????

Improving SE contrast at low voltage

by courtesy B. Senior/CIME


j. Charging effects

Total yield for electron emission (SE + BSE) on insulators

E1 and E2 are critical energies where 1 electron leaves the surface for each incident electron: neutrality when eVacc= E2 charging-up disappears! eVacc= E1 is unstable, eVacc= E2 is stable Caution: E1 and E2 are specific to the material, but also change with the incidence angle +!

1

0 1000 2000 3000 énergie keV

taux

!≈0°

!>0°

!>>0°

E1 E2

Caution: this simple (simplist!) model is not quantitative for insulators because charge implantation and removal depends of the scanning speed and precise sample geometry


j. Charging effects

Charging-up on a mask for microelectronic

Vacc >> E2 Vacc"E2

(SiO2 substrate, photoresist, SE mode)


j. Charging effects

Charging-up on spherical silica particles

5 kV

TV scan slow scan

1.5 kV at 1.5 kV, close to the neutrality point, particles recover their sphere contrast

charges at the particle surface lead to anomalous contrast as a flying saucer


j. Charging effects

Observation of insulating samples

Charging-up is reduced or even cancelled when working at E2

Charging-up may be cancelled under partial atmosphere in a "low vacuum" or "low pressure" SEM, ESEM

–  Caution the "skirt" (incident electrons from the probe are scattered out of it by the atmosphere

–  reduced resolution and contrast –  delocalized microanalysis (may

attain mm!)

Cliché Kontron (Kuschek)pour CIME


j. Charging effects

Contrast reversal in SE mode close to the neutrality point

SiO2-Cr mask for TEG-FET transistors production

3.0 kV 1.8 kV

Cr (E2~1.8keV) SiO2 (E2~3.0keV)

Cliché Kontron (Kuschek)pour CIME


j. Charging effects

Some values of the neutrality E2 energy

E2: upper neutrality energy

Em: maximum emission

energy

*m: maximum yield at Em

adapted from: E. Plies, Advances in Optical and Electron Microscopy,13 (1994) p 226


j. Charging effects

Charging-up of an insulating particle of dust

Negative charges left on the particle create an electric field that repells the SE toward the substrate around the dust

0V

obj p

ole-

piec

e

(adapted from L. Reimer Scanning Electron Microscopy)


j. Charging effects

Extreme charging-up: electrons are reflected by the sample and hit the microscope sample chamber!!!

+ -

- - -

- -

- - ET

(adapted from Philips Bulletin)


j. Charging effects

Surface potential (voltage) contrast

(from Golstein et al, Practical SEM (1975))

Documents

3. Scanning Electron Microscopy - Semantic Scholar · 2018-10-12 · 3. Scanning Electron Microscopy Dr Aïcha Hessler-Wyser Bat. MXC 134, Station 12, EPFL+41.21.693.48.30. Centre