2018 HM-Transmission electron microscope

Transmission Electron
Microscopy
DR. HARSH MOHAN
DEPARTMENT OF PHYSICS
M.L.N. COLLEGE
YAMUNA NAGAR
(HARYANA)

Transmission Electron
Microscopy (TEM): Principle,
Instrumentation and
Applications

Microscopy
• main branches: optical, electron and scanning
probe microscopy.
• Optical and electron microscopy involves the
diffraction, reflection, or refraction of radiation
incident upon the subject of study, and the
subsequent collection of this scattered
radiation in order to build up an image.
• Scanning probe microscopy involves the
interaction of a scanning probe with the
surface or object of interest.

• Optical or light microscopy involves passing
visible light transmitted through or reflected from
the sample through a single or multiple lenses to
allow a magnified view of the sample.
• The resulting image can be detected directly by
the eye, imaged on a photographic plate or
captured digitally.
• The single lens with its attachments, or the system
of lenses and imaging equipment, along with the
appropriate lighting equipment, sample stage and
support, makes up the basic light microscope.
Optical microscopy - definition

Optical microscopy - magnification

Resolution (not magnification!) is the ability to
separate two objects optically
Unresolved
Partially resolved
Resolved

Optical microscopy - limitations
OM can only image dark or strongly
refracting objects effectively
.
Out of focus light from points outside the
focal plane reduces image clarity.
Compound optical microscopes are limited
in their ability to resolve fine details by the
properties of light and the refractive
materials used to manufacture lenses. A
lens magnifies by bending light..

Optical microscopes are restricted in their ability to
resolve features by a phenomenon called diffraction
which, based on the numerical aperture AN of the
optical system and the wavelengths of light used (λ),
sets a definite limit (d) to the optical resolution.
Assuming that optical aberrations are negligible, the
resolution (d) is given by:
In case of λ = 550 nm (green light), with air as medium,
the highest practical AN is 0.95, with oil, up to 1.5.
Due to diffraction, even the best optical microscope is
limited to a resolution of around 0.2 micrometres

What is a Microscope?
• A tool that magnifies and improves resolution
of the components of a structure
• Has three components:
• sources of illumination,
• a magnifying system,
• detectors.

2018 HM-Transmission electron microscope

Scanning
Electron
Microscope
What is scale all about?

Remember that there are 1000 micrometers (µm) in 1 mm and
1000 nanometers (nm) in 1 µm.
The human eye can separate 0.2 mm at a normal viewing
distance of 25 cm
The light microscope can separate 0.2 µm (0.002mm)
depending on wavelength of light used
Electrons have a smaller wavelength than light therefore
provide the highest resolving power – about 2 nm
(0.000002mm)

Electron Microscope vs. Optical Microscope
• Electron vs. Photon
Electron: charged, has rest mass, not visible
Photon: neutral, has no rest mass, visible at the
wavelength ~ 400 nm-760 nm.
Because of these differences, the microscope construction will also be different
(first one built in 1931 by Ruska and Knoll) (Leeuwenhoek in 17th
century)
What is the common property?

16
The Compound Microscope
• Magnification
–Multiply magnifying power of the
objective lens X magnifying power of the
eyepiece lens
• Mechanical system
–Supports the microscope
• Optical system
–Illuminates object
–Passes light through a series of lenses

17
• Optical instrument
– Lens or combination of lenses
– Magnify, resolve fine details
• Earliest methods for examining physical
evidence
• Magnified image = virtual image
• Image viewed directly = real image.

Sources of Illumination
• Light microscopes use a beam of light for
illumination and include fluorescence and
confocal microscopes
• Electron microscopes use electrons as a source
of illumination and include transmission and
scanning electron microscopes.

A short history
• TEM constructed in 1931
• Von Ardenne first STEM in 1938 by rastering
the electron beam in a TEM
• Zworykin et al. 1942, first SEM for bulk
samples
• 1965 first commercial SEM by Cambridge
Scientific Instruments
Resolution at that time ~ 50 nm <-> Today < 1 nm
Morphology only at that time <-> Today analytical instrument

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
29
Electron Microscopy
• beams of electrons
are used to
produce images
• wavelength of
electron beam is
much shorter than
light, resulting in
much higher
resolution

Types of Electron Microscope
• Transmission Electron Microscope (TEM) uses a
wide beam of electrons passing through a thin
sliced specimen to form an image. This
microscope is analogous to a standard upright or
inverted light microscope
• Scanning Electron Microscope (SEM) uses
focused beam of electrons scanning over the
surface of thick or thin specimens. Images are
produced one spot at a time in a grid-like raster
pattern.

31
The Transmission Electron
Microscope
• electrons scatter when they pass
through thin sections of a specimen
• transmitted electrons (those that do
not scatter) are used to produce
image
• denser regions in specimen, scatter
more electrons and appear darker

32

33
EM

Copyright © 2010 Pearson Education, Inc. Figure 3.10a
Transmission Electron Microscopy (TEM)
 Ultrathin sections of
specimens
 Light passes through
specimen, then an
electromagnetic lens,
to a screen or film
 Specimens may be
stained with heavy
metal salts

Copyright © 2010 Pearson Education, Inc. Figure 3.10a
 10,000–100,000×; resolution 2.5 nm

36
Specimen Preparation
• analogous to procedures used for
light microscopy
• for transmission electron
microscopy, specimens must be cut
very thin
• specimens are chemically fixed and
stained with electron dense material

37
Other preparation methods
• shadowing
– coating specimen with a thin film of a
heavy metal
• freeze-etching
– freeze specimen then fracture along
lines of greatest weakness (e.g.,
membranes)

Electron Microscopy - definition and types
• developed in the 1930s that use electron beams instead of light.
• because of the much lower wavelength of the electron beam
than of light, resolution is far higher.
TYPES
• Transmission electron microscopy (TEM) is principally
quite similar to the compound light microscope, by sending an
electron beam through a very thin slice of the specimen. The
resolution limit (in 2005) is around 0.05 nanometer.
• Scanning electron microscopy (SEM) visualizes details on
the surfaces of cells and particles and gives a very nice 3D
view. The magnification is in the lower range than that of the
transmission electron microscope.

• beam of electrons is transmitted through a specimen, then an
image is formed, magnified and directed to appear either on a
fluorescent screen or layer of photographic film or to be
detected by a sensor (e.g. charge-coupled device, CCD camera.
• involves a high voltage electron beam emitted by a cathode,
usually a tungsten filament and focused by electrostatic and
electromagnetic lenses.
• electron beam that has been transmitted through a specimen
that is in part transparent to electrons carries information
about the inner structure of the specimen in the electron beam
that reaches the imaging system of the microscope.
• spatial variation in this information (the "image") is then
magnified by a series of electromagnetic lenses until it is
recorded by hitting a fluorescent screen, photographic plate, or
CCD camera. The image detected by the CCD may be
displayed in real time on a monitor or computer.

The resolution is proportional to the
wave length!
Electron equivalent wavelength and accelerating voltage
The dualism wave/particle is quantified by the
De Broglie equation:
λ = h/p = h/mv
λ : wavelength; h: Planck constant; p:
momentum

At the acceleration voltages used in TEM, relativistic effects
have to be taken into account (e.g. E>100 keV)
λ = h / [2m0eV (1 + eV/2m0/c2
)]1/2
The energy of accelerate electrons is equal to their kinetic
energy:
E = eV = m0v2
/2
V: acceleration voltage
e / m0 / v: charge / rest mass / velocity of the electron
These equations can be combined to calculate the wave
length of an electron with a certain energy:
p = m0v = (2m0eV)1/2
λ = h / (2m0eV)1/2
(≈ 1.22 / V1/2
nm)

Resolution
Can increase the resolution by:

Light and Electron Microscopes
• Lenses are
used to
control a
beam of
illumination,
magnify, and
direct an
image to a
detector

Principal features of an optical microscope, a transmission electron microscope
and a scanning electron microscope, drawn to emphasize the similarities of
overall design.
Comparison of OM,TEM and SEM
OM TEM SEM
Magnetic
lenses
detector
CRT
Cathode Ray
Tube
Light source
Source of
electrons
Condenser
Specimen
Objective
Eyepiece
Projector Specimen

TEM
 Transmission Electron
Microscope
 Illumination source is
beam of electrons from
tungsten wire
 Electromagnetic lenses
perform same function
as glass lenses in LM
 Higher resolution and
higher magnification of
thin specimens

FEI Tecnai 20
For TEM, since the electrons
need to penetrate the specimen, it
must be very thin (< 100 nm)

Comparison of TEM and LM
a. Similarities (Arrangement and function of components are similar)
1) Illumination system: produces required radiation and directs it onto the
specimen. Consists of a source, which emits the radiation, and a
condenser lens, which focuses the illuminating beam (allowing variations
of intensity to be made) on the specimen.
2) Specimen stage: situated between the illumination and imaging
systems.
3) Imaging system: Lenses which together produce the final magnified
image of the specimen. Consists of i) an objective lens which focuses the
beam after it passes through the specimen and forms an intermediate
image of the specimen and ii) the projector lens(es) which magnifies a
portion of the intermediate image to form the final image.
4) Image recording system: Converts the radiation into a permanent
image (typically on a photographic emulsion) that can be viewed.

Comparison of TEM and LM
b. Differences
1) Optical lenses are generally made of glass with fixed focal lengths whereas magnetic lenses are
constructed with ferromagnetic materials and windings of copper wire producing a focal length which
can be changed by varying the current through the coil.
2) Magnification in the LM is generally changed by switching between different power objective lenses
mounted on a rotating turret above the specimen. It can also be changed if oculars (eyepieces) of
different power are used. In the TEM the magnification (focal length) of the objective remains fixed
while the focal length of the projector lens is changed to vary magnification.
3) The LM has a small depth of field, thus different focal levels can be seen in the specimen. The large
(relative) depth of field in the TEM means that the entire (thin) specimen is in focus simultaneously.
4) Mechanisms of image formation vary (phase and amplitude contrast).
5) TEMs are generally constructed with the radiation source at the top of the instrument: the source is
generally situated at the bottom of LMs.
6) TEM is operated at high vacuum (since the mean free path of electrons in air is very small) so most
specimens (biological) must be dehydrated (i.e. dead !!).
7) TEM specimens (biological) are rapidly damaged by the electron beam.
8) TEMs can achieve higher magnification and better resolution than LMs.
9) Price tag!!! (100x more than LM)

© OCR 2016
Comparing light and electron microscopes
Light microscope Electron microscope
Magnification x1500 X500,000
Resolution 250nm 0.25nm
Type of radiation used Light Electrons
Focussed by Glass lenses Electromagnets
Type of material that can
be viewed
Living/moving/dead/abiotic Dead/abiotic
Size Small and portable Large and static
Preparation and cost of
material
Cheap and easy Difficult and expensive

•All rays from a point in the object are gathered by the lens and
converge to a point in the image.
•All parallel rays are focused in the focal plane.
•The back focal plane of the objective lens contains groupings of
rays that have left the object at the same angle.
•The back focal plane contains the diffraction pattern of the sample.
•Diffraction pattern and image are both formed in the imaging
process
•The intermediate lens is then focused on either the image plane
(for the image), or the back focal plane (for the diffraction pattern).

•Advantages and disadvantages of Dark Field Imaging :-
•Advantages
•Provides high contrast for examining molecules with
very low contrast such as DNA.
•For crystalline objects, specific diffraction spots can be
selected in the back focal plane of the objective lens in
order to form a dark field image only from the electrons
scattered by a chosen set of crystal planes.
•Disadvantages
•More difficult to focus and correct for astigmatism since
phase contrast is not present.
•Image brightness is low, since the objective aperture
transmits only a small fraction of the scattered beam,,
•Longer exposure times needed to get good
photographic images.
•Consequently, specimens are subjected to greater
levels of radiation damage.
Dark Field – Advantages and Disadvantages

Imaging Modes
•Two principle modes of TEM operation, A – Projecting the diffraction pattern,
B – Projecting the image.
• The intermediate lens selects either the Back Focal Plane or the image plane of
the objective lens.

Bright field/dark field depends on the aperture position. Modern way for
diffraction tilts the beam instead of moving the aperture
TEM imaging modes

Magnification in TEM
Depends on the magnification, some lens may not be used
Mob × Mint × Mproj = Total Mag

Electron Beam and Specimen Interactions
Electron/Specimen InteractionsSources of Image Information
(1-50KeV)
Electron Beam Induced Current (EBIC)

Polymer crystalline
structure and
morphology
Polymer crystalline
structure and
morphology
ApplicationApplication
Distribution and size of
micropores
Distribution and size of
micropores
Polymer compositionPolymer composition Distribution of dispersed
phase
Distribution of dispersed
phase
lattice distortionlattice distortion

 ExamplesExamples
Graphene
Carbon
nanotube
PE/SWNT NHSK

Transmission Electron Microscopy
(TEM)
Black Ant
House Fly
Human red blood cells
Human stem
cells
Neurons
CNS
Neuron growing on astroglia
House Fly

Transmission Electron Microscopy
(TEM)
View inside cell via sectionsView inside cell via sections
magnification 120,000 Xmagnification 120,000 X
50,000X50,000X

Conventional TEM Micrographs
Skin
Bacteria in cell
Apoptosis
Chloroplast
Collagen Virus in cell

Scanning Electron Microscopy (SEM)

Scanning electron microscopy is used for inspecting topographies of specimens at
very high magnifications using a piece of equipment called the scanning electron
microscope. SEM magnifications can go to more than 300,000 X but most
semiconductor manufacturing applications require magnifications of less than 3,000
X only. SEM inspection is often used in the analysis of die/package cracks and
fracture surfaces, bond failures, and physical defects on the die or package surface.
During SEM inspection, a beam of electrons is focused on a spot volume of the
specimen, resulting in the transfer of energy to the spot. These bombarding
electrons, also referred to as primary electrons, dislodge electrons from the
specimen itself. The dislodged electrons, also known as secondary electrons, are
attracted and collected by a positively biased grid or detector, and then translated
into a signal.
To produce the SEM image, the electron beam is swept across the area being
inspected, producing many such signals. These signals are then amplified, analyzed,
and translated into images of the topography being inspected. Finally, the image is
shown on a CRT.

• The energy of the primary electrons determines the quantity of
secondary electrons collected during inspection. The emission of
secondary electrons from the specimen increases as the energy of
the primary electron beam increases, until a certain limit is reached.
Beyond this limit, the collected secondary electrons diminish as the
energy of the primary beam is increased, because the primary beam
is already activating electrons deep below the surface of the
specimen. Electrons coming from such depths usually recombine
before reaching the surface for emission.
•
• Aside from secondary electrons, the primary electron beam results
in the emission of backscattered (or reflected) electrons from the
specimen. Backscattered electrons possess more energy than
secondary electrons, and have a definite direction. As such, they
can not be collected by a secondary electron detector, unless the
detector is directly in their path of travel. All emissions above 50 eV
are considered to be backscattered electrons.

• Backscattered electron imaging is useful in distinguishing one
material from another, since the yield of the collected backscattered
electrons increases monotonically with the specimen's atomic
number. Backscatter imaging can distinguish elements with atomic
number differences of at least 3, i.e., materials with atomic number
differences of at least 3 would appear with good contrast on the
image. For example, inspecting the remaining Au on an Al bond pad
after its Au ball bond has lifted off would be easier using backscatter
imaging, since the Au islets would stand out from the Al background.
•
• A SEM may be equipped with an EDX analysis system to enable it
to perform compositional analysis on specimens. EDX analysis is
useful in identifying materials and contaminants, as well as
estimating their relative concentrations on the surface of the
specimen.

• type of electron microscope capable of producing high-
resolution images of a sample surface.
• due to the manner in which the image is created, SEM
images have a characteristic 3D appearance and are
useful for judging the surface structure of the sample.
Resolution
• depends on the size of the electron spot, which in turn
depends on the magnetic electron-optical system which
produces the scanning beam.
• is not high enough to image individual atoms, as is
possible in the TEM … so that, it is 1-20 nm

Advantages of Using SEM over OM
Magnification Depth of Field Resolution
OM 4x – 1000x 15.5mm – 0.19mm ~ 0.2mm
SEM 10x – 3000000x 4mm – 0.4mm 1-10nm
The SEM has a large depth of field, which allows a large amount of the
sample to be in focus at one time and produces an image that is a good
representation of the three-dimensional sample. The SEM also produces
images of high resolution, which means that closely features can be
examined at a high magnification.
The combination of higher magnification, larger depth of field, greater
resolution and compositional and crystallographic information makes the
SEM one of the most heavily used instruments in research areas and
industries, especially in semiconductor industry.

Scanning Electron Microscope
– a Totally Different Imaging Concept
• Instead of using the full-field image, a point-to-
point measurement strategy is used.
• High energy electron beam is used to excite the
specimen and the signals are collected and analyzed
so that an image can be constructed.
• The signals carry topological, chemical and
crystallographic information, respectively, of the
samples surface.

Principles of SEM
Magnification?
Resolution?

Image Formation in SEM
beam
e-
Beam is scanned over specimen in a raster pattern in synchronization with
beam in CRT.
Intensity at A on CRT is proportional to signal detected from A on specimen
and signal is modulated by amplifier.
A
A
Detector
Amplifier
10cm
10cm
M= C/x

92
The Scanning Electron Microscope
• (SEM) bombards a specimen with a beam of
electrons instead of light
• Produces a highly magnified image from 100x to
100,0000
• Depth of focus 300X better than optical systems at
similar magnification
• Bombardment of the specimen’s surface with
electrons
– Produces x-ray emissions
– Characterize elements present in the material under
investigation

• An electron gun produces a beam of electrons that
scans the surface of a whole specimen.
• Secondary electrons emitted from the specimen
produce the image.
Figure 3.9b

Beam passes down the
microscope column
Electron beam now tends to
diverge
But is converged by
electromagnetic lenses
Cross section of
electromagnetic
lenses
Electron beam
produced here
Sample
Diagram of Scanning Electron Microscope or SEM
in cross section - the electrons are in green

96

What is SEM
Scanning electron microscope (SEM) is a microscope that uses electrons
rather than light to form an image. There are many advantages to using the
SEM instead of a OM.
The SEM is designed
for direct studying of
the surfaces of solid
objects
Cost: $0.8-2.4M
Column
Sample
Chamber
TV Screens

A Look Inside the Column
Column

A more
detailed
look
inside
Source: L. Reimer,
“Scanning Electron
Microscope”, 2nd
Ed.,
Springer-Verlag, 1998, p.2
Electron Gun
e-
beam
α

Cathode Ray Tube (CRT) accelerates electrons towards the
phosphor coated screen where they produce flashes of light
upon hitting the phosphor. Deflection coilsDeflection coils create a scan
pattern forming an image in a point by point manner

Image Magnification
Example of a series of increasing magnification (spherical lead particles imaged
in SE mode)

How an Electron Beam is Produced?
• Electron guns are used to produce a
fine, controlled beam of electrons
which are then focused at the
specimen surface.
• The electron guns may either be
thermionic gun or field-emission gun

Electron beam Source
W or LaB6 Filament
Thermionic or Field Emission Gun

Thermionic Emission Gun
• A tungsten filament
heated by DC to
approximately 2700K or
LaB6 rod heated to around
2000K
• A vacuum of 10-3
Pa (10-4
Pa for LaB6) is needed to
prevent oxidation of the
filament
• Electrons “boil off” from
the tip of the filament
• Electrons are accelerated
by an acceleration voltage
of 1-50kV
-
+

Source of Electrons
T: ~1500o
C
Thermionic Gun
W and LaB6 Cold- and thermal FEG
Electron Gun Properties
Source Brightness Stability(%) Size Energy spread Vacuum
W 3X105
~1 50µm 3.0(eV) 10-5
(τ )
LaB6 3x106
~2 5µm 1.5 10-6
C-FEG 109
~5 5nm 0.3 10-10
T-FEG 109
<1 20nm 0.7 10-9
(5-50µm)
E: >10MV/cm
(5nm)
Filament
W
Brightness – beam current density per unit solid angle

Electron Gun
W hairpin
LaB6 crystal
FEG

Thermionic Sources
Increasing the filament current will increase the beam current but
only to the point of saturation at which point an increase in the
filament current will only shorten the life of the emitter

Beam spot image at different stage of heating

Magnetic Lenses
• Condenser lens – focusing
determines the beam current
which impinges on the sample.
• Objective lens – final probe
forming
determines the final spot size of
the electron beam, i.e., the
resolution of a SEM.

Electromagnetic Lenses
An electromagnetic lens is essentially soft iron core wrapped in
wire
As we increase the current in the wire we increase the strength
of the magnetic field
Recall the right hand rule electron will move in a helical path
spiralling towards the centre of the magnetic field

Why Need a Vacuum?
When a SEM is used, the electron-optical column and
sample chamber must always be at a vacuum.
1. If the column is in a gas filled environment, electrons will be
scattered by gas molecules which would lead to reduction of the
beam intensity and stability.
2. Other gas molecules, which could come from the sample or the
microscope itself, could form compounds and condense on the
sample. This would lower the contrast and obscure detail in the
image.

The Condenser Lens
• For a thermionic gun, the diameter of
the first cross-over point ~20-50µm
• If we want to focus the beam to a size
< 10 nm on the specimen surface, the
magnification should be ~1/5000, which
is not easily attained with one lens (say,
the objective lens) only.
• Therefore, condenser lenses are added
to demagnify the cross-over points.

The objective lens aperture
Aperture in SEM: either to limit the amount of electrons or enhance contrast

How Is Electron Beam Focused?
A magnetic lens is a solenoid designed to produce
a specific magnetic flux distribution.
p
q
Magnetic lens
(solenoid)
Lens formula: 1/f = 1/p + 1/q
M = q/pDemagnification:
(Beam diameter)
F = -e(v x B)
f ∝ Bo
2
f can be adjusted by changing Bo, i.e., changing the current through
coil.

The Condenser
Lens
Demagnification:
M = f/L

C1 controls the spot size
C2 changes the convergence of the beam
Condenser-lens system
The condenser aperture must be centered

The Objective Lens
• The objective lens
controls the final
focus of the electron
beam by changing the
magnetic field strength
• The cross-over image is
finally demagnified to
an ~10nm beam spot
which carries a beam
current of
approximately 10-9
-10-
10-12
A.

The Objective Lens - Focusing
• By changing the
current in the
objective lens, the
magnetic field
strength changes
and therefore the
focal length of
the objective lens
is changed.
Out of focus in focus out of focus
lens current lens current lens current
too strong optimized too weak
Objective
lens

Electron Detectors and Sample Stage
Objective
lens
Sample stage

Topographical Contrast
Topographic contrast arises because SE generation depend on the
angle of incidence between the beam and sample.
Bright
Dark
+200V
e-
lens polepiece
SE
sample
Everhart-Thornley
SE Detector
Scintillator
light pipe
Quartz
window
+10kV
Faraday
cage
Photomultiplier
tube
PMT

Electron beam – Specimen Interaction. Note the two types
of electrons produced.

Electrons from the focused beam interact with the sample
to produce a spray of electrons up from the sample. These
come in two types – either secondary electrons or
backscattered electrons.
As the beam travels across (scans across) the sample the
spray of electrons is then collected little by little and forms
the image of our sample on a computer screen.
We can look more closely at these two types of electrons
because we use them for different purposes.

+
-
Inelastic scattering
+
-
Elastic scattering
Energy of electron from beam is
lost to atom
An incoming electron rebounds
back out (as a backscattered
electron)
A new electron is knocked
out (as a secondary
electron)

• Secondary Electrons:
Source
Caused by an incident electron passing "near" an atom in the specimen, near
enough to impart some of its energy to a lower energy electron (usually in the K-
shell). This causes a slight energy loss and path change in the incident electron and
the ionization of the electron in the specimen atom. This ionized electron then
leaves the atom with a very small kinetic energy (5eV) and is then termed a
"secondary electron". Each incident electron can produce several secondary
electrons.
Utilization
Production of secondary electrons is very topography related. Due to their low
energy, 5eV, only secondaries that are very near the surface (< 10 nm) can exit the
sample and be examined. Any changes in topography in the sample that are larger
than this sampling depth will change the yield of secondaries due to collection
efficiencies. Collection of these electrons is aided by using a "collector" in
conjunction with the secondary electron detector. The collector is a grid or mesh
with a +100V potential applied to it which is placed in front of the detector,
attracting the negatively charged secondary electrons to it which then pass
through the grid-holes and into the detector to be counted.

A conventional secondary electron detector is positioned off to the
side of the specimen. A faraday cage (kept at a positive bias) draws
in the low energy secondary electrons. The electrons are then
accelerated towards a scintillator which is kept at a very high bias
in order to accelerate them into the phosphor.

The position of the secondary electron detector also affects
signal collection and shadow. An in-lens detector within the
column is more efficient at collecting secondary electrons that
are generated close to the final lens (i.e. short working distance).

Secondary Electron Detector
Side Mounted In-Lens
What are the differences between these two images?

• Backscattered Electrons:
Formation
Caused by an incident electron colliding with an atom in the
specimen which is nearly normal to the incident's path. The
incident electron is then scattered "backward" 180 degrees.
Utilization
The production of backscattered electrons varies directly with
the specimen's atomic number. This differing production
rates causes higher atomic number elements to appear
brighter than lower atomic number elements. This interaction
is utilized to differentiate parts of the specimen that have
different average atomic number.

The most common design is a four quadrant solid state detector that is positioned
directly above the specimen
Backscatter Detector

Example of an image using a scanning electron microscope and
secondary electrons
Here the contrast of these grains is all quite similar.
We get a three-dimensional image of the surfaces.

Grain containing
titanium so it is
whiter
Grain containing
of silica so it is
darker
Example of an image using a scanning electron microscope and
backscattered electrons
Here the differing contrast of the
grains tells us about composition

So how does this work – telling composition from
backscattered electrons?
The higher the atomic number of the atoms the more
backscattered electrons are ‘bounced back’ out
This makes the image brighter for the larger atoms
Titanium – Atomic
Number 22
Silica – Atomic Number
14

+
-
Inelastic scattering
If the yellow electron falls
back again to the inner
ring, that is to a lower
energy state or valence,
then a burst of X-ray
energy is given off that
equals this loss.
This is a characteristic
packet of energy and can
tell us what element we
are dealing with
Understanding compositional analysis using X-rays and the
scanning electron microscope

Backscattered Electrons (BSE)
BSE are produced by elastic interactions of beam electrons with nuclei of
atoms in the specimen and they have high energy and large escape depth.
BSE yield: η=nBS/nB ~ function of atomic number, Z
BSE images show characteristics of atomic number contrast, i.e., high
average Z appear brighter than those of low average Z. η increases with tilt.
Primary
BSE image from flat surface of an Al
(Z=13) and Cu (Z=29) alloy

Effect of Atomic Number, Z, on
BSE and SE Yield

Interaction Volume: I
The incident electrons do not go along a
straight line in the specimen, but a zig-zag
path instead.
Monte Carlo simulations of 100 electron trajectories
e-

Interaction Volume: II
The penetration or,
more precisely, the
interaction volume
depends on the
acceleration voltage
(energy of electron)
and the atomic
number of the
specimen.

Escape Volume of Various SignalsEscape Volume of Various Signals
• The incident electrons interact with specimen
atoms along their path in the specimen and
generate various signals.
• Owing to the difference in energy of these
signals, their ‘penetration depths’ are
different
• Therefore different signal observable on the
specimen surface comes from different parts
of the interaction volume
• The volume responsible for the respective
signal is called the escape volume of that
signal.

If the diameter of primary
electron beam is ~5nm
- Dimensions of escape
zone of
Escape Volumes of Various Signals
•Secondary electron:
diameter~10nm; depth~10nm
•Backscattered electron:
diameter~1µm; depth~1µm
•X-ray: from the whole
interaction volume, i.e., ~5µm
in diameter and depth

Electron Interaction Volume
5µm
a b
a.Schematic illustration of electron beam interaction in Ni
b.Electron interaction volume in polymethylmethacrylate
(plastic-a low Z matrix) is indirectly revealed by etching
Pear shape

Magnification
The magnification is simply the ratio of the length of the scan C on the
Cathode Ray Tube (CRT) to the length of the scan x on the specimen. For a
CRT screen that is 10 cm square:
M= C/x = 10cm/x
Increasing M is achieved by decreasing x.
M x M x
100 1 mm 10000 10 µm
1000 100 µm 100000 1 µm
Low M
Large x
40µm
High M
small x
7µm
2500x 15000x
1.2µm
e-
x

Resolution Limitations
Ultimate resolution obtainable in an SEM image can be
limited by:
1. Electron Optical limitations
Diffraction: dd=1.22λ/α
for a 20-keV beam, λ =0.0087nm and α=5x10-3
dd=2.1nm
Chromatic and spherical aberrations: dmin=1.29λ3/4
Cs
1/4
A SEM fitted with an FEG has an achievable resolution of ~1.0nm at 30 kV
due to smaller Cs (~20mm) and λ.
2. Specimen Contrast Limitations
Contrast dmin
1.0 2.3nm
0.5 4.6nm
0.1 23nm
0.01 230nm
3. Sampling Volume Limitations (Escape volume)

How Fine Can We See with SEM?
• If we can scan an area with width 10 nm
(10,000,000×) we may actually see
atoms!! But, can we?
• Image on the CRT consists of spots called
pixels (e.g. your PC screen displays
1024×768 pixels of ~0.25mm pitch)
which are the basic units in the image.
• You cannot have details finer than
one pixel!

Resolution of Images: I
• Assume that there the screen can display 1000
pixels/(raster line), then you can imagine that
there are 1000 pixels on each raster line on the
specimen.
• The resolution is the pixel diameter on
specimen surface.
P=D/Mag = 100um/Mag
P-pixel diameter on specimen surface
D-pixel diameter on CRT, Mag-magnification
Mag P(µm) Mag P(nm)
10x 10 10kx 10
1kx 0.1 100kx 1

• The optimum condition for imaging is when
the escape volume of the signal concerned
equals to the pixel size.
Resolution of Images: II

• Signal will be weak if escape volume,
which depends on beam size, is smaller
than pixel size, but the resolution is still
achieved. (Image is ‘noisy’)
Resolution of Images: III

Resolution of Images: IV
• Signal from different pixel will overlap
if escape volume is larger than the
pixel size. The image will appeared
out of focus (Resolution decreased)

Resolution of Images: V
Pixel diameter on Specimen
Magnification µm nm
10 10 10000
100 1 1000
1000 0.1 100
10000 0.01 10
100000 0.001 1
In extremely good SEM, resolution can be a few nm. The
limit is set by the electron probe size, which in turn depends
on the quality of the objective lens and electron gun.

Depth of Field
D = (µm)
AM
4x105
W
To increase D
Decrease aperture size, A
Decrease magnification, M
Increase working distance, W (mm)
Depth of Field

Image Contrast
Image contrast, C
is defined by
SA-SB ∆S
C= ________
=____
SA SA
SA, SB Represent signals
generated from two
points, e.g., A and B, in
the scanned area.
In order to detect objects of small size and low contrast in an SEM it is
necessary to use a high beam current and a slow scan speed (i.e., improve
signal to noise ratio).
SE-topographic and BSE-atomic number contrast
SE Images

162

The Scanning Electron Microscope is analogous to the
stereo binocular light microscope because it looks at
surfaces rather than through the specimen.

Main Applications
• Topography
The surface features of an object and its texture
(hardness, reflectivity… etc.)
• Morphology
The shape and size of the particles making up the
object (strength, defects in IC and chips...etc.)
• Composition
The elements and compounds that the object is
composed of and the relative amounts of them
(melting point, reactivity, hardness...etc.)
• Crystallographic Information
How the grains are arranged in the object
(conductivity, electrical properties, strength...etc.)

SE Images - Topographic Contrast
The debris shown here is an oxide fiber
got stuck at a semiconductor device
detected by SEM
1µm
Defect in a semiconductor device Molybdenum
trioxide crystals

BSE Image – Atomic Number Contrast
BSE atomic number contrast image showing a niobium-rich
intermetallic phase (bright contrast) dispersed in an alumina matrix
(dark contrast).
Z (Nb) = 41, Z (Al) = 13 and Z(O) = 8
Alumina-Al2O3
2µm

Can you see the
difference?
TEM SEMLIGH
T

We learn more from mistakes than successes…

2018 HM-Transmission electron microscope

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