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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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 2 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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 3 CiMe
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 Aïcha Hessler-Wyser 4 CiMe
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 Aïcha Hessler-Wyser 5 CiMe
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 Aïcha Hessler-Wyser 6 CiMe
a. SEM principle
Energy spectrum of electrons leaving the sample
Auger (secondary)electrons
Back scatered electrons BSE
Secondary electrons SE
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 7 CiMe
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")
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 8 CiMe
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).
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 9 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 10 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 11 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 12 CiMe
c. Electron probe and resolution
Resolving power ("resolution"): Rayleigh criterium
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 13 CiMe
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 Aïcha Hessler-Wyser 14 CiMe
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 Aïcha Hessler-Wyser 15 CiMe
c. Electron probe and resolution
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 16 CiMe
c. Electron probe and resolution
probe size and resolution (no noise)
particles 100 nm diam., probe dia 2 nm
particles 50 nm diam., probe dia 2 nm
particles 25 nm diam., probe dia 2 nm
model 100 nm diam. particles
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 17 CiMe
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 50 nm diam., probe diam 2 nm
particles 25 nm diam., probe diam 2 nm
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 18 CiMe
c. Electron probe and resolution
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 19 CiMe
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 Aïcha Hessler-Wyser 20 CiMe
c. Electron probe and resolution
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
microscope photonique
20 KV1
10
100
0.001 0.01
dia
mètr
e (
nm
)
Ouverture (mrad)
10pA
1pA
dch
dsph
dd
microscope photonique
100pA
1 kV1
10
100
0.001 0.01
dia
mètr
e (
nm
)
Ouverture (mrad)
100pA10pA
1pA
dch
dsph
dd
microscope photonique
5 kV
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 21 CiMe
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 Aïcha Hessler-Wyser 22 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 23 CiMe
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 Aïcha Hessler-Wyser 24 CiMe
c. Electron probe and resolution
Short focus length… or… FEG?
1
10
100
0.001 0.01
dia
mètr
e (
nm
)
Ouverture (mrad)
100pA
10pA
1pA
dch
dsph
dd
microscope photonique
20 KV1
10
100
0.001 0.01
dia
mètr
e (
nm
)
Ouverture (mrad)
100pA
10pA
1pA
dch
dsph
dd
microscope photonique
20 kV
1
10
100
0.001 0.01
dia
mètr
e (
nm
)
Ouverture (mrad)
100pA
10pA
1pA
dch
dsphd
d
microscope photonique
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 25 CiMe
c. Electron probe and resolution
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 26 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 27 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 28 CiMe
d. Depth of field
Effect of working distance (WD) and aperture on depth of field
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 29 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 30 CiMe
d. Depth of field
Other examples
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 31 CiMe
d. Depth of field
Effect of the objective aperture diameter
30 !m
50 !m
100 !m
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 32 CiMe
d. Depth of field
Measuring depth of field: stereoscopy
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 33 CiMe
e. Stereoscopy
The 3rd dimension: stereoscopic vision, anaglyphs
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 34 CiMe
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 35 CiMe
e. Stereoscopy
3-D
rec
on
stru
ctio
n (
an
ag
lyp
h)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 36 CiMe
e. Stereoscopy
3-D reconstruction (pseudo-perspective)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 37 CiMe
e. Stereoscopy
3-D
rec
on
stru
ctio
n (
gre
y le
vels)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 38 CiMe
e. Stereoscopy
3-D
rec
on
stru
ctio
n (
false
co
lors
)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 39 CiMe
e. Stereoscopy
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 40 CiMe
e. Stereoscopy
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 41 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 42 CiMe
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 Aïcha Hessler-Wyser 43 CiMe
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 Aïcha Hessler-Wyser 44 CiMe
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 Aïcha Hessler-Wyser 45 CiMe
f. Electron-matter interaction volume
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 46 CiMe
f. Electron-matter interaction volume
Penetration and back- scattering 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%
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 47 CiMe
f. Electron-matter interaction volume
Penetration and back- scattering 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
Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte-Carlo simulation):
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 48 CiMe
f. Electron-matter interaction volume
Penetration and back- scattering vs elements (Z) Vacc = 5 kV = cte
Depth of electron penetration vs Z and yield of electron backscattering BSE (Monte-Carlo simulation):
200 nm
C 5 keV
Cu 5 keV
200nm
U 5 keV
200 nm
BSE=8%
BSE=33% BSE=47%
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 49 CiMe
f. Electron-matter interaction volume
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 50 CiMe
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 Aïcha Hessler-Wyser 51 CiMe
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 Aïcha Hessler-Wyser 52 CiMe
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)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 53 CiMe
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)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 54 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 55 CiMe
g. SE and BSE
Contaminated area around a soldering spot
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 56 CiMe
g. SE and BSE
Toner particle (penetration in light material)
SE 28 kV BSE
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 57 CiMe
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
(adapted from D.C. Joy Hitachi News 16 1989)
Relative yield of SE vs angle of incidence on the sample surface !
I "( ) = I0# "( ) $I 0( )cos"
+
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 58 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 59 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 Aïcha Hessler-Wyser 60 CiMe
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 Aïcha Hessler-Wyser 61 CiMe
h. Contrast
Size and edge effects
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 62 CiMe
h. Contrast
Size and edge effects
(From L. Reimer, Image Formation in Low-Voltage Scanning Electron
Microscopy, (1993))
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 63 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 64 CiMe
h. Contrast
What does it suggest? Which objective information?
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 65 CiMe
h. Contrast
What does it suggest? Which objective information?
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 66 CiMe
h. Contrast
What does it suggest? Which objective information?
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 67 CiMe
pyramid?
Detector ? Detector ?
etch-pit?
h. Contrast
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 68 CiMe
(from L.Reimer, Image formation in the low-voltage SEM)
Change in SE contrast with the voltage
h. Contrast
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 69 CiMe
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.
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 70 CiMe
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)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 71 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 72 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 73 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 74 CiMe
i. Examples
Contrast reversal in BSE mode at low accelerating voltage
Ag Au Cu Si
from L.Reimer, Image formation in low-voltage SEM
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 75 CiMe
j. Charging effects
Fiberglass on epoxy
1 kV
5 kV, low current
5 kV, high current
(by courtesy of B. Senior CIME/EPFL)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 76 CiMe
j. Charging effects
fiberglass on epoxy Which polarity ??????
Improving SE contrast at low voltage
by courtesy B. Senior/CIME
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 77 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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 78 CiMe
j. Charging effects
Charging-up on a mask for microelectronic
Vacc >> E2 Vacc"E2
(SiO2 substrate, photoresist, SE mode)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 79 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 80 CiMe
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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 81 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
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 82 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 Aïcha Hessler-Wyser 83 CiMe
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)
Intensive SEM/TEM training: SEM Aïcha Hessler-Wyser 84 CiMe
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 Aïcha Hessler-Wyser 85 CiMe
j. Charging effects
Surface potential (voltage) contrast
(from Golstein et al, Practical SEM (1975))