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SCANNING ELECTRON MICROSCOPE
first SEM in 1938 by rastering
the electron beam.
Zworykin et al. 1942, first
SEM for bulk samples.
1965 first commercial SEM by
Topography- the surface features of an object or how it looks, its texture.
Morphology – the shape and size of the particles making up the object.
Composition - The elements and compounds that the object is composed
of and the relative amount of them.
Crystallographic information – How the atoms are arranged in the
COMPONENTS OF SEM
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a source (electron gun) of the electron beam
which is accelerated down the column .
a series of lenses which act to control the
diameter of the beam as well as to focus the beam
on the specimen;
a series of apertures which the beam passes
through and which affect properties of that beam;
an area of beam/specimen interaction that
types of signals that can be detectedand
processed to produce an image or spectra;
all of the above maintained at high vacuum
Electron beam-Sample interactions
• The incident electron beam is scattered in the sample, both
elastically and inelastically
• This give rise to various signals that we can detect.
• Interaction volume increases with increasing acceleration
voltage and decreases with increasing atomic number
Images: Smith College Northampton, Massachusetts
We want many electrons per time
unit per area (high current
density) and as small electron
spot as possible
• Thermionic Electron Gun(TEG):
electrons are emitted when a solid
– W-wire, LaB6-crystal
• Field Emission Guns (FEG): cold
guns, a strong electric field is
used to extract electrons
With field emission guns we get
a smaller spot and higher current
densities compared to thermionic
-Single crystal of W, etched to a thin tip
Single crystal of LaB6
Field emission tip
Secondary electron detector:
X-rays: Energy dispersive spectrometer (EDS)
Image: Anders W. B. Skilbred, UiO
• Chemical (corrosion!!) and thermal stability is necessary
for a well-functioning filament (gun pressure)
– A field emission gun requires ~ 10-10 Torr
– LaB6: ~ 10-6 Torr
• The signal electrons must travel from the sample to the
detector (chamber pressure)
– Vacuum requirements is dependant of the type of
HOW THE SEM WORKS?
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The SEM uses electrons instead of light to form
A beam of electrons is produced at the top of the
microscope by heating of a metallic filament.
The electron beam follows a vertical path through
the column of the microscope. It makes its way
through electromagnetic lenses which focus and
direct the beam down towards the sample.
Once it hits the sample, other electrons are
ejected from the sample. Detectors collect the
secondary or backscattered electrons, and
convert them to a signal that is sent to a viewing
screen similar to the one in an ordinary
television, producing an image.
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When the accelerated beam of electrons strike a specimen they
penetrate inside it to depths of about 1 μm and interact both
elastically and inelastically with the solid, forming a limiting
interaction volume from which various types of radiation
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The most common imaging mode collects low-
energy (<50 eV) Secondary electrons that are
ejected from the k-shell of the specimen atoms
by inelastic scattering interactions with beam
electrons. Due to their low energy, these electrons
originate within a few nanometers from the sample
Backscattered electrons (BSE) consist of high-energy electrons
originating in the electron beam, that are reflected or back-scattered
out of the specimen interaction volume by elastic
scattering interactions with specimen atoms. Since heavy elements
(high atomic number) backscatter electrons more strongly than
light elements (low atomic number), and thus appear brighter in
SE - DETECTOR
BSE v/s SE
SE produces higher resolution
images than BSE
Resolution of 1 – 2 nm is
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– Secondary electrons (SE): mainly topography
• Low energy electrons, high resolution
• Surface signal dependent on curvature
– Backscattered electrons (BSE): mainly
• High energy electrons
• “Bulk” signal dependent on atomic number
– X-rays: chemistry
• Longer recording times are needed
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The SEM image is a 2-D intensity map in the analog or
digital domain. Each image pixel on the display
corresponds to a point on the sample, which is proportional to
the signal intensity captured by the detector at each specific
Unlike optical TEM no true image exists in the SEM. It is
not possible to place a film anywhere in the SEM and record
The image is generated and displayed electronically. The
images in the SEM are formed by electronic synthesis, no
optical transformation takes place, and no real of virtual
optical images are produced in the SEM.
MAGNIFICATION IN THE SEM
• No optical transformation is responsible for image
magnification in the SEM.
• Magnification in the SEM depends only on the excitation of
the scan coils which determines the focus of the beam.
• The magnification of the SEM image is changed by
adjusting the length of the scan on the specimen (Lspec) for a
constant length of scan on the monitor (Lmon), which gives
the linear magnification of the image (M)
• M = Lmon/Lspec
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Some Comments on RESOLUTION
• Best resolution that can be obtained when size of the
electron spot is complimentary to the sample surface
– The introduction of FEG has dramatically improved the
resolution of SEM’s
• The volume from which the signal electrons are formed
defines the resolution
– SE image has higher resolution than a BSE image
• Scanning speed:
– a weak signal requires slow speed to improve signal-to-noise
– when doing a slow scan drift in the electron beam can affect
the accuracy of the analysis
• Since the SEM is operated under high which means that liquids
and materials containing water and other volatile components
cannot be studied directly. Also fine powder samples need to be
fixed firmly to a specimen holder substrate so that they will not
contaminate the SEM specimen chamber.
• Non-conductive materials need to be attached to a conductive
specimen holder and coated with a thin conductive film by
sputtering or evaporation. Typical coating materials are Au, Pt,
Pd, their alloys, as well as carbon.
• There are special types of SEM instruments such as VPSEM
and ESEM that can operate at higher specimen chamber
pressures thus allowing for non-conductive materials or even wet
specimens to be studied.
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• To image challenging samples such as:
– insulating samples
– vacuum-sensitive samples (e.g. biological samples)
– irradiation-sensitive samples (e.g. thin organic films)
– “wet” samples (oily, dirty, greasy)
• To study and image chemical and physical processes
in-situ such as:
– mechanical stress-testing
– oxidation of metals
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TUNNELING ELECTRON MICROSCOPE
The first electron microscope was
built 1932 by the German physicist
Ernst Ruska, who was awarded the
Nobel Prize in 1986 for its invention.
the first commercial TEM in 1939.
Typical accel. volt. = 100-400 kV
(some instruments - 1-3 MV)
The design of a transmission electron
microscope (TEM) is analogous to that of an
optical microscope. In a TEM high-energy
(>100 kV) electrons are used instead of photons
and electromagnetic lenses instead of glass
lenses. The electron beam passes an electron-
transparent sample and a magnified image is
formed using a set of lenses. This image is
projected onto a fluorescent screen or a CCD
camera. Whereas the use of visible light limits
the lateral resolution in an optical microscope to
a few tenths of a micrometer, the much smaller
wavelength of electrons allows for a resolution of
0.2 nm in a TEM.
Condenser system :(lenses &
illumination on specimen
Objective lens system: image-
forming lens - limits resolution;
aperture - controls imaging
Projector lens system: magnifies
image or diffraction pattern onto
Intermediate lens: transmitting or
magnifying the enlarge image.
Image contrast is obtained by interaction of the
electron beam with the sample. In the resulting TEM
image denser areas and areas containing heavier
elements appear darker due to scattering of the
electrons in the sample. In addition, scattering from
crystal planes introduces diffraction contrast. This
contrast depends on the orientation of a crystalline
area in the sample with respect to the electron beam.
As a result, in a TEM image of a sample consisting of
randomly oriented crystals each crystal will have its
own grey-level. In this way one can distinguish
between different materials, as well as image
individual crystals an crystal defects. Because of the
high resolution of the TEM, atomic arrangements in
crystalline structures can be imaged in large detail
High resolution TEM
image of a multi-
nanowire. The wire
consists of segments,
bounded by inner
RESOLUTION IN TEM
• In a TEM, a monochromatic beam of electrons is accelerated
through a potential of 40 to 100 kilovolts (kV) and passed
through a strong magnetic field that acts as a lens. The
resolution of a modern TEM is about 0.2 nm.
• More recently, advances in aberration corrector design have been
able to reduce spherical aberrations and to achieve resolution
below 0.5 Ångströms at magnifications above 50 million times.
= 0.61λ/β β= semi-collection angle of magnifying lens
λ= electron wavelength
67.0 sCr Best attained resolution ~0.07 nm
Cs = spherical aberration
Electron Energy Loss Spectroscopy (EELS)
When travelling through the sample the electrons may lose
energy due to (multiple) inelastic scattering events. The
amount of energy that is transferred from the incident electron
to the sample is dependent on the composition of the sample.
Because the primary beam of electrons has one well-defined
energy, the spectrum of the electrons that have passed the
sample contains chemical information on the irradiated area.
Quantification of the spectrum enables determination of (local)
concentrations of elements. The fine structure in the EELS
spectra provides information on the chemical binding of the
TEM tomography involves the acquisition of a large series of
images at many tilt angles of the sample with respect to the
electron beam. In analogy to the CT scanner used in medical
diagnostics, the acquired tilt-series is reconstructed into a 3-D
representation. This technique is especially useful in case of
studies on 3-dimensionally shaped objects, such as layers in
small pores and 3-D shapes of small objects
The 3-dimensional reconstruction of the morphology of a part of a
GaP-GaAs hetero-structured nanowire with 40 nm diameter.
SAMPLE PREPARATION IN TEM
TEM foil specimens were prepared by
mechanical dimpling down to 20 μm,
followed by argon ion milling operating
at an accelerating voltage of 5 kV and 10°
incidence angle, with a liquid nitrogen
cooling stage to avoid sample heating and
microstructural changes associated with
the annealing effect.
For TEM observations, thin samples are
required due to the important absorption
of the electrons in the material. High
acceleration voltage reduces the
absorption effects but can cause
radiation damage (estimated at 170 kV
for Al). At these acceleration tensions, a
maximum thickness of 60 nm is
required for TEM
CONTRASTS: Electrons that go through a sample
Formed by incident electrons that are
scattered by the atoms of the
specimen elastically. These electrons
can then be collated using magnetic
lenses to form a pattern of spots; each
spot corresponds to a specific atomic
spacing (a plane). This pattern can
then yield information about the
orientation, atomic arrangements
and phases present in the area being
BRIGHT FIELD CONTRAST
formed directly by occlusion
and absorption of electrons in
the sample. Thicker regions of
the sample, or regions with a
higher atomic number will
appear dark, whilst regions
with no sample in the beam
path will appear bright hence
the term "bright field".
LIMITATIONS OF THE TEM
• Sampling---0.3mm3 of materials: The higher the resolution the
smaller the analyzed volume becomes. Drawing conclusions from a single
observation or even single sample is dangerous and can lead to
completely false interpretations
• Interpreting transmission images---2D images of 3D
specimens, viewed in transmission, no depth-sensitivity.
• Electron beam damage and safety---particularly in polymer
and ceramics: The high energy of the electron beam utilized in electron
microscopy causes damage by ionization, radiolysis, and heating
• Specimen preparation---”thin” below 100nm
ADVANCES IN TEM
Using dedicated equipment,
it is possible to freeze 0.1 μm
thick water films and study
these films at -170˚C in the
TEM. This enables imaging
of the natural shape of
organic bilayer structures.
processes in a dispersion can
be studied. In addition, the
application of cryogenic
studies of beam-sensitive
Another way of obtaining
compositional as well as
using TEM is High
Angle Annular Dark
Field (HAADF) imaging.
For this application a
dedicated detector is used
that only collects
electrons that are
elastically scattered over
large angles by the
specimen. The intensity
that is detected is
dependent on the average
atomic number Z
ENERGY FILTERED TEM
A special filter on the TEM
allows for selection of a very
narrow window of energies
in the EELS spectrum.
Using the corresponding
electrons for imaging,
EFTEM is performed. As a
result, a qualitative
elemental map is obtained.
EFTEM is the only
chemical analysis procedure
in the TEM that does not
use a scanning beam. As a
consequence, it is much
SEM V/S TEM
• in SEM is based on scattered
• The scattered electrons in SEM
produced the image of the sample
after the microscope collects and
counts the scattered electrons.
• SEM focuses on the sample’s
surface and its composition.
• SEM shows the sample bit by
• SEM provides a three-
• SEM only offers 2 million as a
maximum level of
• SEM has 0.4 nanometers.
• TEM is based on transmitted
• In TEM, electrons are directly
pointed toward the sample.
• TEM seeks to see what is inside
or beyond the surface.
• TEM shows the sample as a
• TEM delivers a two-dimensional
• TEM has up to a 50 million
• The resolution of TEM is 0.5
Each microscope works is very different from another.
SEM scans the surface of the sample by releasing electrons and
making the electrons bounce or scatter upon impact. The
machine collects the scattered electrons and produces an image.
The image is visualized on a television-like screen. On the other
hand, TEM processes the sample by directing an electron beam
through the sample.
Images are also a point of difference between two tools. SEM
images are three-dimensional and are accurate representations
while TEM pictures are two-dimensional and might require a
little bit of interpretation.
In terms of resolution and magnification, TEM gains more
advantages compared to SEM.
The result is seen using a fluorescent screen
BRIEF HISTORY OF AFM
Atomic force microscopy (AFM) to investigate the electrically
non-conductive materials, like proteins.
In 1986, Binnig and Quate demonstrated for the first time
the ideas of AFM, which used an ultra-small probe tip at the
end of a cantilever (Phys. Rev. Letters, 1986, Vol. 56, p
In 1987, Wickramsinghe et al. developed an AFM setup with
a vibrating cantilever technique (J. Appl. Phys. 1987, Vol.
61, p 4723), which used the light-lever mechanism.
COMPARISON BETWEEN AFM AND ELECTRONIC
• Optical and electron microscopes can easily generate two
dimensional images of a sample surface, with a magnification as
large as 1000X for an optical microscope, and a few hundreds
thousands ~100,000X for an electron microscope.
• However, these microscopes cannot measure the vertical dimension (z-
direction) of the sample, the height (e.g. particles) or depth (e.g. holes,
pits) of the surface features.
• AFM, which uses a sharp tip to probe the surface features by raster
scanning, can image the surface topography with extremely high
magnifications, up to 1,000,000X, comparable or even better than
• measurement of an AFM is made in three dimensions, the horizontal
X-Y plane and the vertical Z dimension. Resolution (magnification)
at Z-direction is normally higher than X-Y plane.
ATOMIC INTERACTION AT DIFFERENT
• At very small tip-sample distances (a few angstroms) a
very strong repulsive force appears between the tip and
sample atoms. Its origin is the so-called exchange
interactions due to the overlap of the electronic orbitals at
atomic distances. When this repulsive force is predominant,
the tip and sample are considered to be in “contact”.
Attraction (Van der Waals):
• A polarization interaction between atoms: An
instantaneous polarization of an atom induces a
polarization in nearby atoms – and therefore an attractive
Different modes of tip-sample interaction when
The cantilever bends laterally due to a friction force between the
tip and the sample surfaces.
• Adhesion can be defined as “the free energy change to separate
unit areas of two media from contact to infinity in vacuum or
in a third medium”.
• In general, care has to be taken with the term adhesion, since it
is also used to define a force - the adhesion force. In addition to
the intrinsic adhesion between tip and sample, there is another
one from the capillary neck condensing between the tip and
water meniscus --- interference from the huminity.
Electromagnetic interactions between tip
• Electrostatic interaction: Caused by both the localized
charges and the polarization of the substrate due to the
potential difference between the tip and the sample. It has
been used to study the electrostatic properties of samples
such as charges on insulator surfaces or ferroelectric
• Magnetic interaction: Caused by magnetic dipoles both on
the tip and the sample.
AFM IMAGING MODES
Contact mode (left): the deflection of cantilever is kept
Non-contact mode (right): the tip is oscillated at the resonance
frequency and the amplitude of the oscillation is kept constant.
Tapping mode: somewhere between the contact and non-
Contact mode AFM consists of raster-scanning the
probe (or sample) while monitoring the change in
cantilever deflection with the split photodiode
detector. A feedback loop maintains a constant
cantilever deflection by vertically moving the
scanner to maintain a constant photo-detector
difference signal. The distance scanner moves
vertically at each x, y data point is stored by the
computer to form the topographic image of the
sample surface. This feedback loop maintains a
constant force during imaging.
Two contact scanning modes: Constant Height
and Constant Force
Constant-force mode Constant-height mode
Constant-force scan v/s constant-height
– Large vertical range
– Constant force (can
be optimized to the
– Requires feedback
– Slow response
– Simple structure (no
– Fast response
– Limited vertical
– Varied force
Tapping Mode AFM consists of oscillating the cantilever at its
resonance frequency (typically ~300kHz) and lightly
“tapping” the tip on the surface during scanning.
A feedback loop maintains a constant oscillation amplitude
by moving the scanner vertically at every x,y data point.
Recording this movement forms the topographical image.
The advantage of Tapping Mode over contact mode is that it
eliminates the lateral, shear forces present in contact mode.
This enables Tapping Mode to image soft, fragile, and
adhesive surfaces without damaging them, which can be a
drawback of contact mode AFM.
Comparison between the three scanning modes: damage to
Contact mode imaging (left) is heavily influenced by
frictional and adhesive forces, and can damage samples
and distort image data.
Non-contact imaging (center) generally provides low
resolution and can also be hampered by the contaminant
(e.g., water) layer which can interfere with oscillation.
Tapping Mode imaging (right) takes advantages of the
two above. It eliminates frictional forces by
intermittently contacting the surface and oscillating
with sufficient amplitude to prevent the tip from being
trapped by adhesive meniscus forces from the
Imaging by contact and non-contact
AFM IN LIQUID ENVIRONMENT
One extraordinary feature of AFM is to work in liquid environment. A key
point for liquid AFM is a transparent solid (usually glass) surface, which,
together with the solid sample surface, retains the liquid environment
whilst maintains stable optical paths for the laser beams. An optional O-
ring can be used to form a sealed liquid cell. Otherwise, the system can also
work in an “open cell” fashion.
Advanced imaging techniques of AFM
• Lateral force microscope (LFM) --- measures
lateral deflections, shows surface friction.
• Force modulation microscope (FMM) ---
detecting surface stiffness or elasticity;
• Phase mode imaging --- detecting surface
structure or elasticity property.
AFM v/s SEM
Compared with Scanning Electron Microscope, AFM provides
extraordinary topographic contrast direct height measurements
and un-obscured views of surface features (no coating is
SEM is conducted in a vacuum environment, and AFM is
conducted in an ambient or fluid environment
Si covered with GaP
SEM IMAGE AFM IMAGE
AFM v/s TEM
• Compared with two dimensional Transmission Electron
Microscopes, three dimensional AFM images are obtained.
• No expensive sample preparation in AFM is required as
compared to TEM and yield far more complete information
than the two dimensional profiles available from cross-