PPT-SEM & TEM.pdf for material science and engineering students

best for characterization is my future

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5. Scanning electron microscope
SIMS is a technique used to analyze compositions of thin films and surfaces by using a focused
primary ion beam to sputter the surface of a sample and analyze the ejected secondary ions.



















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XPS is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can
identify the elements that exist within a material (elemental composition) or are covering its surface, as
well as their chemical state, and the overall electronic structure and density of the electronic states in the
material.



















Fig. Electromagnetic spectrum showing the size of the wavelength used in the light, scanning (SEM), and
transmission electron microscope (TEM)
Electron microscopies such as SEM and TEM are scientific instruments that use a beam of
energetic electrons to examine objects on a very fine scale.

Through the use of electron microscopy, we can see things that we would not normally be
able to see with our naked eyes and has greater magnification than light microscopy. This
required 10,000x plus magnifications which was not possible using current optical
microscopy.




















Table: Comparison of various characteristics of the SEM with light and transmission
electron microscope

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Image
formation
Light from the source
is scattered by the
sample surface and
redirected by the
objective lens to form
an image onto the
retina of the human
eye. The image can
also be displayed on
an electronic display
Electrons originating from the source
travel in vacuum within a column lined
with electromagnetic lenses which
focus these electrons into a small probe
on the surface of the specimen.
Electron-specimen interaction results in
information emanating from the
specimen which is passed through
detectors and reconstituted as an image
on an electronic display
Electrons originating from the
source travel in vacuum within
a column. Electron beam
passes through a thin foil of
sample and then focused and
magnified by electromagnetic
lenses to form an image on a
fluorescent screen or
transferred to an electronic
display
Type of
image
Real image. Color
images. Images
formed using visible
light can be observed
directly by the human
eye
Processed/ reconstituted image.
Grayscale images (black and white).
Images formed with electrons cannot
be observed directly by humans
The real image is projected
onto the screen which can be
observed by the human eye.
Grayscale images
Specimen
preparation
Required
Can be omitted (based on specimen
type)
Required, tedious
Specimen
thickness
Thin, bulk Bulk
Thin (Electron transparent, 100
nm)
Depth of
field
Small 15 μm (at 4X)
0.2 μm (1000X)
Large (3-D like images) 4 mm (at 10X)
0.5 μm (500,000X)
Small
Interpretati
on of
images
Easy Moderate Difficult



















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Fig.Secondary electron images of tin balls showing good contrast at low to very high
magnifications (100,000x to 1,000,000x)



















There are two basic models of the electron microscopes: Scanning electron
microscopes (SEM) and transmission electron microscopes (TEM).

In a SEM, the secondary electrons produced by the specimen are detected to generate
an image that contains topological features of the specimen. The image in a TEM, on
the other hand, is generated by the electrons that have transmitted through a thin
specimen.

SEM is an instrument which is used to study the morphology of the sample at
higher magnification. In this technique of characterization, high energy beam of
electrons is used to scan sample surface.

The electron beam and constituent atoms of the sample interact with each other that
produce signal containing information about the sample.

Many signals described below are generated by the interaction of the electron
beam with the specimen. Each of these signals is sensitive to a different aspect of
the specimen and give a variety of information about the specimen.

•
Secondary Electron (SE),
• Back Scattered Electron (BSE),
• X- Ray,
• Auger Electron (AE)
• etc
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Characteristic information of SEM

1.Topography: it is the surface feature of an object or “how it looks,” its texture,
direct relation between these features and materials properties.

2.Morphology: it refers to the shape and size of the particles making up the objects,
direct relation between these structures and materials properties

3.Composition: the elements and compounds that the object is composed of and the
relative amounts of them, direct relationship between composition and materials
properties.

4.Crystallographic information: it refers to how the atoms are arranged in the
object, direct relation between these arrangements and material properties.

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Image formation in the SEM is dependent on the acquisition of signals produced
from the electron beam and specimen interactions. These interactions can be divided
into two major categories: elastic interactions and inelastic interactions.

Elastic scattering results from the deflection of the incident electron by the
specimen atomic nucleus or by outer shell electrons of similar energy. This kind of
interaction is characterized by negligible energy loss during the collision and by a
wide-angle directional change of the scattered electron.

Incident electrons that are elastically scattered through an angle of more than 90°
are called backscattered electrons, and yield a useful signal for imaging the
sample.

Inelastic scattering occurs through a variety of interactions between the incident
electrons and the electrons and atoms of the sample, and results in the primary
beam electron transferring substantial energy to that atom.

The amount of energy loss depends on whether the specimen electrons are excited
singly or collectively and on the binding energy of the electron to the atom. As a
result, the excitation of the specimen electrons during the ionization of specimen
atoms leads to the generation of secondary electrons (SE), which are conventionally
defined as possessing energies of less than 50 eV and can be used to image or
analyze the sample.



















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In addition to those signals that are utilized to form an image, a number of other
signals are produced when an electron beam strikes a sample, including the emission
of characteristic x-rays, Auger electrons, and cathodoluminescence.
Fig. Schematic showing elastic scattering where
beam electrons change direction without losing
any appreciable energy upon interaction with the
Coulombic field of the specimen material
Fig. Schematic diagram showing inelastic
scattering where beam electron knocks out
an orbital electron belonging to the
specimen losing energy in the process



















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In most cases when incident electron strikes the specimen surface, instead of being
bounced off immediately, the energetic electrons penetrate into the sample for
some distance before they encounter and collide with a specimen atom.

The primary electron beam produces what is known as a region of primary
excitation, from which a variety of signals are produced. The size and shape of
this zone is largely dependent upon the beam electron energy and the atomic
number, and hence the density of the specimen.

The variation of interaction volume with respect to different accelerating voltage
and atomic number. At certain accelerating voltage, the shape of interaction volume
is “tear drop” for low atomic number specimen and hemisphere for specimens of
high atomic number.

The volume and depth of penetration increase with an increase of the beam energy
and fall with the increasing specimen atomic number because specimens with
higher atomic number have more particles to stop electron penetration. One
influence of the interaction volume on signal acquisition is that use of a high
accelerating voltage will result in deep penetration length and a large primary
excitation region, and ultimately cause the loss of detailed surface information of the
samples.

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Figure: Scanning electron micrographs of a CaF2 close-packed opal structure, which are taken
under different accelerating voltages: (a) 1 kV and (b) 20 kV
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1.Secondary Electrons

The most widely used signal produced by the interaction of the primary electron
beam with the specimen is the secondary electron emission signal. When the
primary beam strikes the sample surface causing the ionization of specimen atoms,
loosely bound electrons may be emitted and these are referred to as secondary
electrons.

As they have low energy, typically an average of around 2–5 eV, they can only
escape from a region within a few nanometers of the material surface. So secondary
electrons accurately mark the position of the beam and give topographic
information with good resolution. 90% of secondary electrons have energies less
than 10 eV; most, from 2 to 5 eV.

Produced by inelastic interactions of high energy electrons with valence electrons
of atoms in the specimen which cause the ejection of the electrons from the atoms.

After undergoing additional scattering events while traveling through the specimen,
some of these ejected electrons emerge from the surface of the specimen.

Being low in energy they can be bent by the bias from the detector and hence even
those secondary electrons which are not in the ‘line of sight’ of the detector can be
captured.



















2. Backscattered Electrons (BSEs)

BSEs provide both compositional and topographic information in the SEM.

A BSE is defined as one which has undergone a single or multiple scattering events
and which escapes from the surface with an energy greater than 50 eV.

The elastic collision between an electron and the specimen atomic nucleus causes
the electron to bounce back with wide-angle directional change. Roughly 10–50%
of the beam electrons are backscattered toward their source, and on an average these
electrons retain 60–80% of their initial energy.

Elements with higher atomic numbers have more positive charges on the nucleus,
and as a result, more electrons are backscattered, causing the resulting
backscattered signal to be higher.

The backscattered yield, defined as the percentage of incident electrons that are
reemitted by the sample, is dependent upon the atomic number of the sample,
providing atomic number contrast in the SEM images. For example, the BSE yield is
~6% for a light element such as carbon, whereas it is ~50% for a heavier element
such as tungsten or gold.

Due to the fact that BSEs have a large energy, which prevents them from being
absorbed by the sample, the region of the specimen from which BSEs are produced
is considerably larger than it is for secondary electrons. For this reason the lateral
resolution of a BSE image is considerably worse (1.0 μm) than it is for a secondary
electron image (10 nm).



















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3. Characteristic X-rays
Another class of signals produced by the interaction of the primary electron beam with the specimen is
characteristic x-rays. The analysis of characteristic x-rays to provide chemical information is the most
widely used micro analytical technique in the SEM.

When an inner shell electron is displaced by collision with a primary electron, an outer shell electron
may fall into the inner shell to reestablish the proper charge balance in its orbitals following an
ionization event. Thus, by the emission of an x-ray photon, the ionized atom returns to ground state.

In addition to the characteristic x-ray peaks, a continuous background is generated through the
deceleration of high-energy electrons as they interact with the electron cloud and with the nuclei of atoms
in the sample. This component is referred to as the Bremsstrahlung or Continuum x-ray signal.
BSEs are produced by elastic interactions of beam electrons with nuclei of atoms in the
specimen
Energy loss less than 1 eV
Scattering angles range up to 180°, but average about 5°
Many incident electrons undergo a series of such elastic event that cause them to be scattered
back out of the specimen
Bremsstrahlung (German pronunciation), from bremsen "to brake" randion) Strahlung "radiation"; i.e.,
"braking radiation" or "deceleration radiation", is electromagnetic radiation produced by
the deceleration of a charged particle when deflected by another charged particle, typically
an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into
radiation (i.e., a photon)



















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4. Auger Electrons

Auger electrons are produced following the ionization of an atom by the incident electron
beam and the falling back of an outer shell electron to fill an inner shell vacancy. The excess
energy released by this process may be carried away by an Auger electron.

This electron has a characteristic energy and can therefore be used to provide chemical
information. Because of their low energies, Auger electrons are emitted only from near the
surface. They have escape depths of only a few nanometers and are principally used in surface
analysis.

5. Cathodoluminescence

Cathodoluminescence is another mechanism for energy stabilization following beam specimen
interaction. Certain materials will release excess energy in the form of photons with infrared,
visible, or ultraviolet wavelengths when electrons recombine to fill holes made by the
collision of the primary beam with the specimen.

These photons can be detected and counted by using a light pipe and photomultiplier similar
to the ones utilized by the secondary electron detector. The best possible image resolution
using this approach is estimated at about 50 nm.

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In BSE image, Ni precipitates in much brighter than Al
2
O
3,
as comparison, in SE image, Ni and
Al
2
O
3
.
BSE image of Al
2
O
3
-Ni composite
SE image of Al
2
O
3
-Ni composite



















Working principles of SEM
The overall design of an electron microscope is similar to that of a light microscope. In the
electron microscope, the light is substituted with electrons and the glass lenses are substituted
with electromagnetic/electrostatic lenses.
The following figure shows a simplified schematic diagram of a SEM. The electrons produced
by the electron gun are guided and focused by the magnetic lenses on the specimen.
Figure: A simplified schematic diagram of a scanning electron microscope.



















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SEM requires the proper coating of
the sample with platinum, gold or
carbon using a high vacuum sputter
coater. This increases the release of
SE from the sample.

For conductive materials, coating
procedure is not required.

If non-conducting samples are
analyzed using a SEM, electrons
build up on the surface and cause
scattering of the electron beam,
which further interferes with the
analysis and clear topographic is
not created.

Thus, fixation, dehydration and
conductive coating of the sample
are necessary.




















1.A column which generates a beam of
electrons.

2.A specimen chamber where the electron
beam interacts with the sample.

3.Detectors to monitor the different signals
that result from the electron
beam/sample interaction.

4. A viewing system that builds an image
from the detector signal.

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1.Electron Guns

Modern SEM systems require that the electron gun produces a stable electron beam with high
current, small spot size, adjustable energy, and small energy dispersion. Several types of electron
guns are used in SEM system and the qualities of electrons beam they produced vary
considerably. The first SEM systems generally used tungsten or lanthanum hexaboride (LaB6)
cathodes.
Condenser Lenses

The electron beam will diverge after passing through the anode plate from the emission source.
By using the condenser lens, the electron beam is converged and collimated into a relatively
parallel stream. A magnetic lens generally consists of two rotationally symmetric iron pole
pieces in which there is a copper winding providing magnetic field. There is a hole in the center
of pole pieces that allows the electron beam to pass through. A lens-gap separates the two pole
pieces, at which the magnetic field affects (focuses) the electron beam. The position of the focal
point can be controlled by adjusting the condenser lens current.
Objective Lenses

The electron beam will diverge below the condenser aperture. Objective lenses are used to focus
the electron beam into a probe point at the specimen surface and to supply further
demagnification. An appropriate choice of lens demagnification and aperture size results in a
reduction of the diameter of electron beam on the specimen surface (spot size), and enhances the
image resolution.



















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Transmission Electron Microscope
The Transmission Electron Microscope (TEM) was the first type of Electron Microscope to
be developed and is patterned exactly on the Light Transmission Microscope except that a
focused beam of electrons is used instead of light to “see through” the specimen.

The basic structure of TEM is similar to that of optical microscopes, replacing a light
bulb with an electron gun, and glass lenses with magnetic lenses.


TEM is used to reveal sub-micrometre, internal fine structure in solids. Materials scientists
tend to call this microstructure while bioscientists usually prefer the term ultrastructure.

The amount and scale of the information which can be extracted by TEM depends critically on
four parameters:

• the resolving power of the microscope (usually smaller than 0.3 nm);
•the energy spread of the electron beam (often several eV);
• the thickness of the specimen (almost always significantly less than 1 μm),
•the composition and stability of the specimen.
A Transmission Electron Microscope works in the same way as a slide projector. In the
projector, a beam of light transmits through the slide resulting in some parts of the light beam
being transmitted onto parts of the slide, and the beam transmission is then projected onto a
viewing screen which forms an enlarged image of the slide.

In the TEM, a beam of electrons (like the light) transmits through the specimen (like the
slide), and then the transmission is projected onto a phosphor screen for the user to see.



















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Ruska was also aware that magnetic fields could affect electron trajectories, possibly focusing
them as optical lenses do to light. After confirming these principles through research, he set
out to design the electron microscope.

Ruska had deduced that an electron microscope would be much more powerful than an
ordinary optical microscope since electron waves were shorter than ordinary light waves and
electrons would allow for greater magnification and thus to visualize much smaller structures.
In the advancement of materials science and engineering, it is necessary to observe, analyze
and understand the phenomena occurring on a small size scale.
The first TEM was built by Max Kroll and Ernst Ruska in 1931, with this group developing
the first TEM with resolution power (about 0.05nm) greater than that of light in 1933 and the
first commercial TEM in 1939.
In a TEM, a high-energy (» 200 keV) electron beam is transmitted through the specimen.
During transmission, the electrons interact with the specimen, giving rise to signals containing
information about the internal structure and chemistry of the specimen.

Electron diffraction patterns and lattice images are two forms of data which give an insight of
crystallographic information in TEM.

Lattice images are interference patterns between the direct beam and diffracted beams,
viewed in direct space, and are obtained by high-resolution TEM (HRTEM) imaging.

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Fig. Schematic comparing the modes of image formation in the light, transmission, and scanning
electron microscopes.



















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Principle of TEM

Illumination: source is a beam of high velocity electrons accelerated under vacuum,
focused by condenser lens (electromagnetic bending of electron beam) onto
specimen.

Image formation: loss and scattering of the electrons by individual parts of the
specimen . Emergent electron beam is focused by objective lens. Final image on a
fluorescent screen for viewing.

Beam of electrons is transmitted through an ultra-thin specimen. An image is formed
from the interaction of the electrons transmitted through the specimen.

The image is magnified and focused onto an imaging device, such as a fluorescent
screen, on a layer of photographic film, or to be detected by a sensor such as a CCD
camera.



















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TEM gives the following information:

1.Topography
The surface feature of an object
2. Morphology
The shape and size of the particles
3. Composition
The elements and compounds that the object is composed
4. Crystallographic information
How the atoms are arranged in the object
TEM offers variety of information obtained from different modes such as bright field (BF)
and dark field (DF) imaging, selected area diffraction (SAD) and high resolution lattice
imaging (HRTEM).

BF and DF imaging are used to characterize defects and domain structures. SAD allows
reconstructing the reciprocal space and in that way obtain information about the crystal
structure and identifying different phases.

HRTEM allows under certain conditions to directly visualize the projected crystal potential.

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1. Bright field: Contrast - is 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".

2. Diffraction contrast: Contrast is formed by elastically scattered electrons - Incident
electrons that are scattered (deflected from their original path) by atoms in the specimen in an
elastic fashion (no loss of energy). Samples can exhibit diffraction contrast, whereby the
electron beam undergoes Bragg’s scattering which in the case of a crystalline sample, disperses
electrons into discrete locations in the back focal plane. Utilization – electrons passing through
at a similar angle are scattered, 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 examined.

3. Electron energy loss: Contrast is formed by inelastically scattered electrons. Utilization :
The inelastic loss of energy by the incident electrons is characteristic of the elements that were
interacted with. These energies are unique to each bonding state of each element and thus can be
used to extract both compositional and bonding.



















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Selected area
diffraction (SAD)

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Advantage
•High resolution and magnification
•Give information of element and compound
•Detailed image obtained
Disadvantage
•Expensive
•Images are black and white
•Instrument in very large
Limitation
•Samples are limited to those that are electron transparent
•Difficult to handle
•Tricky sample preparation

Sample preparation
Sample preparation is the most important part of the TEM characterization. As the
electrons transmit through the specimen, the specimen has to thin down to the electron
transparency (< 100 nm) for conventional TEM and even lesser.