3. Scanning Electron Microscopy

3. Scanning Electron Microscopy Dr Aïcha Hessler-Wyser Bat. MXC 134, Station 12, EPFL+41.21.693.48.30. Centre Interdisciplinaire de Microscopie Elec...
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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

Intensive SEM/TEM training: SEM

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Outline a.  b.  c.  d.  e.  f.  g.  h.  i.  j. 

SEM principle Detectors Electron probe and resolution Depth of field Stereoscopy Electron-matter interaction volume Secondary and back-scattered electrons Contrasts Examples Charging effects

Intensive SEM/TEM training: SEM

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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.

Intensive SEM/TEM training: SEM

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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

Intensive SEM/TEM training: SEM

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)

Intensive SEM/TEM training: SEM

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a. SEM principle Energy spectrum of electrons leaving the sample Secondary electrons SE

Back scatered electrons BSE

Auger (secondary)electrons

Intensive SEM/TEM training: SEM

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b. Detectors

A "light" source

a detector (eye, photographic plate, video camera... a magnification section (lenses, apertures...)

a sample (+ a "goniometer")

an illumination section (lenses, apertures...)

Intensive SEM/TEM training: SEM

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b. Detectors Everhardt-Thornley detector: for SE and BSE

SE: BSE:

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. 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).

Intensive SEM/TEM training: SEM

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b. Detectors BSE detectors

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

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 ! works at TV frequency

! large collection angle ! slow (poor at TV frequency) ! some diodes are split in 2 or 4 quadrants to bring spatial BSE distribution info

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution

A "light" source

a detector (eye, photographic plate, video camera... a magnification section (lenses, apertures...)

a sample (+ a "goniometer")

an illumination section (lenses, apertures...)

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution Incoherent source

Resolution in probe mode (SEM,SEM STEM) classique LaB

6

! 

microscope photonique

Spherical aberration

d sph = Cs " ! 3 Chromatic aberration

" !E !I % dch = Cch $ + 2 '( # E I& ! 

Diffraction (Airy, Rayleigh)

! d d = 0.61 n"sin # ! 

10pA

1pA 10 dd

Brilliance ! conservation

dg = ! 

100pA

100

diamètre(nm) (nm) diamètre

! 

courte focale

4I

1 # ! 2" $

1

Combinaison 2 + d2 + d2 dech = dg2 + dsph ch d

dsph

0.001

0.01

Ouverture (mrad)

Vacc=20 kV, !E=1.5 eV, "=1.105 eV A/cm2sr

Probe with coherent source: see Mory C, Cowley J M, Ultramicroscopy 21 1987 171 Intensive SEM/TEM training: SEM

dc h

Csph=17 mm, Cc h=9 mm 11

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c. Electron probe and resolution Resolving power ("resolution"): Rayleigh criterium

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution 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

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution 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, …

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution SEM: Effet of current, probe diameter and image acquisition time

500nm

10 pA/10 s

10 pA/160 s

100 pA/160 s

1 nA/160 s

good resolution, but statistical noise

Good resolution, less statistical noise

smal loss of resolution, still less statistical noise

very few statistical noise, but high resolution loss!

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution probe size and resolution (no noise)

particles 25 nm diam., probe dia 2 nm model 100 nm diam. particles

particles 50 nm diam., probe dia 2 nm

particles 100 nm diam., probe dia 2 nm Intensive SEM/TEM training: SEM

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c. Electron probe and resolution Probe size and resolution (with noise) (with noise)

model 100 nm diam. particles Particles100 nm diam., probe diam 2 nm

particles 25 nm diam., probe diam 2 nm particles 50 nm diam., probe diam 2 nm

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution Current/probe diameter Thermionique source: spherical aberration is the most important I max =

3! 2 #2 3 "C sph d 8 3 16

Field emission source: gun aberrations and chromatic aberration are more important I max = cd 2 3 Tiré de L.Reimer, SEM Intensive SEM/TEM training: SEM

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c. Electron probe and resolution 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, …

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution Thermionic SEM : low voltage? microscope photonique

microscope photonique

100

10pA 1pA

10

10 dd

1

dc h

0.001

dsph

20 KV

0.01 Ouverture (mrad)

10pA

100

1pA

diamètre (nm)

diamètre (nm)

100

10pA

100pA diamètre (nm)

100pA

dd dch

1 0.001

microscope photonique

dsph

5 kV

0.01 Ouverture (mrad)

100pA

1pA dc h

10

1 0.001

dd

dsph

1 kV

0.01 Ouverture (mrad)

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

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c. Electron probe and resolution 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, …

Intensive SEM/TEM training: SEM

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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

Z = cte

1µ Cu 1keV

Cu 1keV

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution 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, …

Intensive SEM/TEM training: SEM

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Aïcha Hessler-Wyser

c. Electron probe and resolution Short focus length… microscope photonique

1pA

10 dd

microscope photonique

100pA

100 diamètre (nm)

diamètre (nm)

microscope photonique

20 kV

10pA

FEG? 100

10pA

diamètre (nm)

100pA

100

or…

1pA 10 dd

20 kV 100pA 10 10pA dc h 1pA

1

dch

0.001

dsph

0.01 Ouverture (mrad)

1

dc h

0.001

dsph

dd

dsph

1

20 KV

0.01 Ouverture (mrad)

0.001

0.01 Ouverture (mrad)

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

Intensive SEM/TEM training: SEM

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c. Electron probe and resolution Resolution loss at low voltage

Résolution (nm)

100

Shorter objective lens focal length and Cs

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

W

FE 10

LaB 6

1985

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

5

2000

1

0.5

1

2

5

10

20 30

Tension d'accélération (kV) Intensive SEM/TEM training: SEM

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Short questions 1. What is a condensor lens for? a)  b) 

To create the image of the sample To reduce the size of the electron source

2. Which parameters influence the resolution in SEM? a)  b)  c)  d) 

The size of the probe The electron current The electron energy The aquisition device

e)  f) 

The wave length The lens aberration

3. How to reduce the probe size? a)  b)  c)  d) 

By reducing the electron energy By reducing the apperture size By increasing the Working Distance By removing the spherical aberration

4. How to reduce the interaction volume? a)  b) 

By reducing the electron current By reducing the electron energy

Intensive SEM/TEM training: SEM

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d. Depth of field Depth of field as a function of dprobe

2%&

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

dA

h

h

2dA

h prof .champ

Intensive SEM/TEM training: SEM

" 2d 1 $ sonde = max # pixel ! "image " 1 $% 2 G !

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

Intensive SEM/TEM training: SEM

d. Depth of field Light bulb filament Résolution " 10µm 10mm

1µm

100nm

10nm

1nm

1m 0. d ra 1m d ra

100µm m

10 d ra

Profondeur de champ h

1mm

10µm

1µm

0.1µm 10

SEM LM !=500 nm

102

103

104

105

Grandissement (grossissement) G Intensive SEM/TEM training: SEM

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d. Depth of field Other examples

Intensive SEM/TEM training: SEM

d. Depth of field Effect of the objective aperture diameter 100 !m 50 !m 30 !m

Intensive SEM/TEM training: SEM

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d. Depth of field Measuring depth of field: stereoscopy

Intensive SEM/TEM training: SEM

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e. Stereoscopy The 3rd dimension: stereoscopic vision, anaglyphs

Intensive SEM/TEM training: SEM

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Intensive SEM/TEM training: SEM

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3-D reconstruction (anaglyph)

e. Stereoscopy

Intensive SEM/TEM training: SEM

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e. Stereoscopy 3-D reconstruction

(pseudo-perspective)

Intensive SEM/TEM training: SEM

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3-D reconstruction (grey levels)

e. Stereoscopy

Intensive SEM/TEM training: SEM

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3-D reconstruction (false colors)

e. Stereoscopy

Intensive SEM/TEM training: SEM

e. Stereoscopy

Intensive SEM/TEM training: SEM

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e. Stereoscopy

Intensive SEM/TEM training: SEM

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). 1000 kV

100 kV

Elastic interaction:

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

Energy transfer from the electron to the target

Intensive SEM/TEM training: SEM

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f. Electron-matter interaction volume 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!)

Intensive SEM/TEM training: SEM

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f. Electron-matter interaction volume 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

Intensive SEM/TEM training: SEM

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f. Electron-matter interaction volume 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 Intensive SEM/TEM training: SEM

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f. Electron-matter interaction volume Number/Energy of backscattered electrons by Monte-Carlo simulations W BSE 41%

BSE 43%

1 kV

BSE 52%

3 kV

30 kV

C BSE 10%

BSE 8%

Intensive SEM/TEM training: SEM

BSE 5%

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f. Electron-matter interaction volume Penetration and backscattering vs elements (Z)

BSE=6%

Vacc = 20kV = cte Depth of electron penetration vs Z and yield of electron backscattering BSE (MonteCarlo simulation):

1µ&

C 20 keV

BSE=33%

BSE=50%

1µ&

1µ&

U 20 keV

Intensive SEM/TEM training: SEM

Cu 20 keV

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f. Electron-matter interaction volume Penetration and backscattering vs elements (Z)

BSE=14%

10 nm

Vacc = 1 kV = cte Depth of electron penetration vs Z and yield of electron backscattering BSE (MonteCarlo simulation):

C 1keV

10 nm U 1keV

BSE=34%

10 nm

BSE=44%

Cu 1keV

Intensive SEM/TEM training: SEM

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f. Electron-matter interaction volume Penetration and backscattering vs elements (Z)

200 nm

BSE=8%

Vacc = 5 kV = cte Depth of electron penetration vs Z and yield of electron backscattering BSE (MonteCarlo simulation):

C 5 keV

BSE=33%

200 nm

200nm

Cu 5 keV

Intensive SEM/TEM training: SEM

BSE=47%

U 5 keV

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f. Electron-matter interaction volume Penetration and backscattering vs energy(E)

1#m

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

1#m

Cu 20 keV

1#m

Cu 5 keV

Cu 1 keV

Intensive SEM/TEM training: SEM

Cu 1keV

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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

Intensive SEM/TEM training: SEM

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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!

Intensive SEM/TEM training: SEM

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g. SE and BSE Relative contribution of SE1 and SE2 (+SE3) vs primary energy total total total SE2 SE1

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

(adapted from D.C. Joy Hitachi News 16 1989) 52

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g. SE and BSE Yield for SE and BSE emission per incident electron vs atomic number Z sample surface polished (no topography) and perpendicular to the incident beam direction (intermediate energy E0 # 15 keV)

):

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

IBSE=Ipe$) yield

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

*:& 0.28

ISE 0.11

with Ipe the intensity of the primary beam, ) the BSE

(SE1)

=Ipe$* +ISE3 =Ipe(*pe+*pe$)+*sur$)) SE1 SE2

Al

SE3

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

Ni

Intensive SEM/TEM training: SEM

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g. SE and BSE Dust on WC (different Z materials) flat material

rough material low Z material

thin

low Z material

low Z material

SE 25 kVBSE Intensive SEM/TEM training: SEM

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g. SE and BSE Contaminated area around a soldering spot

Intensive SEM/TEM training: SEM

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g. SE and BSE Toner particle (penetration in light material)

SE 28 kVBSE Intensive SEM/TEM training: SEM

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g. SE and BSE Topographical contrast in SE mode Effet de l'inclinaison de la surface penetration depth ("range") >>SE escape length

+

I0

I(+) +

#1-10nm

I (" ) = I0# (" ) $

I (0) cos"

Relative yield of SE vs angle of incidence on the sample ! surface (adapted from D.C. Joy Hitachi News 16 1989)

Intensive SEM/TEM training: SEM

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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

IBSE 31% Ni

IBSE 37% Ni

incidence normale +=0°

incidence +=40° 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% Aïcha Hessler-Wyser Intensive SEM/TEM training: BSE topographical contrast SEM is not negligible! Chemical contrast is well observed58 only on polished samples

CiMe

h. Contrast Topographical contrast at low energy Effect of the incidence angle

(adapted from D.C. Joy Hitachi News 16 1989) Intensive SEM/TEM training: SEM

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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

Intensive SEM/TEM training: SEM

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h. Contrast Size and edge effects

Intensive SEM/TEM training: SEM

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h. Contrast Size and edge effects

(From L. Reimer, Image Formation in Low-Voltage Scanning Electron Microscopy, (1993))

Intensive SEM/TEM training: SEM

h. Contrast Comparison of SE and BSE contrast modes SE

BSE

ET detector +200V

(0V) backscattered and transmitted e- create SE, some of them are driven to the ET detector by the electric field

BSE are absorbed The trajectories of BSE are not The observator looks down to the column and the strongly affected by the "light" seems to come from the Everhardt-Thornley electrical field, most BSE miss the detector. detector

Intensive SEM/TEM training: SEM

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h. Contrast

What does it suggest? Which objective information? Intensive SEM/TEM training: SEM

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h. Contrast

What does it suggest? Which objective information? Intensive SEM/TEM training: SEM

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h. Contrast

What does it suggest? Which objective information? Intensive SEM/TEM training: SEM

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h. Contrast Detector ?

Detector ?

etch-pit?

pyramid? Intensive SEM/TEM training: SEM

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h. Contrast Change in SE contrast with the voltage

(from L.Reimer, Image formation in the low-voltage SEM) Intensive SEM/TEM training: SEM

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h. Contrast Contraste enhancement at low voltage: less delocalization by SE2. An example: a fracture in Ni-Cr alloy

SE, 5 kV

Intensive SEM/TEM training: SEM

SE, 30 kV

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h. Contrast SEM: Effect of the accelerating voltage on (from D.C. Joy penetration and SE signal Hitachi News 16 1989)

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 Intensive SEM/TEM training: SEM

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

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i. Examples Physical limit to the imaging in secondary electron mode Tin grains on a thin carbon film (TEM supporting grid) HRSEM 25 kV 1 nm nominal resolution left: SE right: scanning transmitted electrons (STEM)

SE: e-/e- coulombian

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

Intensive SEM/TEM training: SEM

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(from B. Ocker, Scanning Microscopy 9 (1995) 63…) Aïcha Hessler-Wyser

CiMe

i. Examples Physical limit to the imaging in secondary electron mode (from B. Ocker, Scanning Microscopy 9 (1995) 63…)

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 Intensive SEM/TEM training: SEM

72

Aïcha Hessler-Wyser

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i. Examples AlxGa1-xAs/GaAs "quantum wire" quantum well)

(2-D

x=0.20

GaAs x=0.55

SE mode image on a cleaved surface. The SE2 (BSE chemical) contrast dominates this image in absence of topographical contrast (SE1=cte) (by courtesy of Dr. K. Leifer, IPEQ/EPFL)

QW Intensive SEM/TEM training: SEM

73

Aïcha Hessler-Wyser

CiMe

Aïcha Hessler-Wyser

CiMe

i. Examples Contrast reversal in BSE mode at low accelerating voltage

Si

Cu

Ag

Au

from L.Reimer, Image formation in low-voltage SEM Intensive SEM/TEM training: SEM

74

j. Charging effects 1 kV

Fiberglass on epoxy

5 kV, high current

5 kV, low current

(by courtesy of B. Senior CIME/EPFL) Intensive SEM/TEM training: SEM

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j. Charging effects Improving SE contrast at low voltage fiberglass on epoxy

Which polarity ??????

by courtesy B. Senior/CIME Intensive SEM/TEM training: SEM

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Aïcha Hessler-Wyser

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j. Charging effects Total yield for electron emission (SE + BSE) on insulators taux

E1 and E2 are critical energies where 1 electron leaves the surface for each incident electron: neutrality

!>>0° !>0° !≈0°

1

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 +!

E1

0

1000

E2

2000

3000

énergie keV

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

Intensive SEM/TEM training: SEM

77

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j. Charging effects Charging-up on a mask for microelectronic (SiO2 substrate, photoresist, SE mode)

Vacc >> E2 Intensive SEM/TEM training: SEM

Vacc"E2 78

Aïcha Hessler-Wyser

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j. Charging effects Charging-up on spherical silica particles slow scan

TV scan

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

5 kV

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

Intensive SEM/TEM training: SEM

79

Aïcha Hessler-Wyser

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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!)

Intensive SEM/TEM training: SEM

Cliché Kontron (Kuschek)pour CIME

80

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j. Charging effects Contrast reversal in SE mode close to the neutrality point SiO2-Cr mask for TEG-FET transistors production SiO2

Cr (E2~1.8keV)

(E2~3.0keV)

3.0 kV Intensive SEM/TEM training: SEM

1.8 kV 81

Cliché Kontron (Kuschek)pour CIME

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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

Intensive SEM/TEM training: SEM

82

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j. Charging effects obj pole-piece

Charging-up of an insulating particle of dust

0V

Negative charges left on the particle create an electric field that repells the SE toward the substrate around the dust (adapted from L. Reimer Scanning Electron Microscopy) Intensive SEM/TEM training: SEM

83

Aïcha Hessler-Wyser

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

- + -

ET

(adapted from Philips Bulletin) Intensive SEM/TEM training: SEM

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CiMe

j. Charging effects Surface potential (voltage) contrast

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

Intensive SEM/TEM training: SEM

85

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CiMe

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