Sem md ppt

Scanning Electron Microscopy
M.Sc Material Science & Engineering
JIMMA UNIVERSITY
MScE6409 – Electron Spectroscopy
and Microscopy

What are Electron Microscopes?
Electron Microscopes are scientific instruments that use a beam of
energetic electrons to examine objects on a very fine scale.
Electron microscopes work by using an electron beam instead of visible
light and an electron detector instead of our eyes.
An electron beam allows us to see at very small scales because electrons
can also behave as light.
It has the properties of a wave with a wavelength that is much smaller
than visible light (a few trillionths of a meter).
With this wavelength we can distinguish features down to a fraction of a
nanometer.
Electron Microscopy

10,000x plus magnification, not possible using optical microscopes.
Uses electromagnetic lenses, electrons and fluorescent screen to produce
image
Resolution increased 1000 fold over bright field microscope to about 0.3
nm (1 X 10-9)
Magnification increased to 100,000x
Two types of electron microscopes
1. Scanning Electron Microscope (SEM)
2. Transmission Electron Microscope (TEM)
Principles of Electron Microscopy

What is SEM?
Working principles of SEM
Major Components and their functions
Electron beam – specimen interactions
Detection of the sample surface
Energy Dispersive X-ray Spectroscopy (EDS)
Characterization
Applications
Scanning Electron Microscopy

SEM uses a very fine probe of electrons
focused at the surface of the specimen
and scanned across it in a raster or
pattern of parallel lines.
It is a type of electron microscope
capable of producing high resolution
images of a sample surface (as low as
10nm or 100Å) in vacuum.
First constructed by Knoll and von
Ardenne in Germany, 1930s.
Commercially available in 1963 with
improvements made by McMullan and
Oatley at Cambridge University.
What is SEM?

The basic principle is that a beam of electrons is generated by a
suitable source, typically a tungsten filament (thermionic gun) or a
field emission gun (FEG).
The electron beam is accelerated through a high voltage (e.g. 20 kV)
and pass through a system of apertures and electromagnetic lenses to
produce a thin beam of electrons.
Working Principles of SEM

Working Principles of SEM
A beam of electrons scans the surface of a sample (Figure). The
electrons interact with the material in a way that triggers the emission
of secondary electrons.
These secondary electrons are captured by a detector, which forms an
image of the surface of the sample.
The direction of the emission of the secondary electrons depends on
the orientation of the features of the surface.
There, the image formed will reflect the characteristic feature of the
region of the surface that was exposed to the electron beam.

Major Components and their Functions

A tungsten (W) thermionic
filament is used as electron
gun.
Other examples: Lanthanum
or Cerium Hexaboride
When high voltage is applied
to tungsten filament which
produces the steady stream
of electrons.
Electron gun located at the
very top of a SEM fire a beam
of electrons at the object
under examination
Electron Gun

Thermionic Gun
makes use of a filament that is heated
up to make the electrons overcome the
work function
Field Emission Gun
makes use of two anodes: (1) to extract
electrons from the filament, (2) to
accelerate the electrons to the operating
voltage
Types of Electron Gun

1. Thermionic electron gun
Thermionic filaments are made of tungsten (W) in the form of a v-shaped wire.
Thermionic emission is the liberation of electrons from an electrode by virtue of its
temperature (releasing of energy supplied by heat).
This occurs because the thermal energy given to the carrier overcomes the work
function of the material.
The charge carriers can be electrons or ions, and in literature are sometimes
referred to as thermions.
The classical example of thermionic emission is that of electrons from a hot
cathode into a vacuum (also known as thermal electron emission or the Edison
effect) in a vacuum tube.
The hot cathode can be a metal filament e.g. Tungsten. Vacuum emission from
metals tends to become significant only for temperatures over 1,000 K (730 °C).

2. Field Emission Gun (FEG)
For a field emission source, a fine, sharp, single crystal tungsten tip is employed.
An FEG emitter gives a more coherent beam and its brightness is much higher than
the tungsten filament.
Electrons are emitted from a smaller area of the FEG source, giving a source size of
a few nanometers, compared to around 50 μm for the tungsten filament. This
leads to greatly improved image quality with the FEG source.
Comparison of Thermionic (tungsten) and Field Emission Gun

There are two types of FEG sources: Cold and Schottky FEGs
1. Cold emission source, heating of the filament is not required as it operates at
room temperature. However, this type of filament is prone to contamination and
requires more stringent vacuum conditions (10-8 Pa, 10-10 torr).
Regular and rapid heating (‘flashing’) is required in order to remove contamination.
The spread of electron energies is very small for a cold field emitter (0.3 eV) and the
source size is around 5 nm.
2. Thermal and Schottky sources, operate with lower field strengths. The surface of
the tungsten (W) tip is covered with a thin layer of zirconium oxide (ZrO) to further
decrease the potential barrier.
The Schottky source is also heated and dispenses zirconium dioxide onto the
tungsten tip to further lower its work function.
The Schottky source is slightly larger, 20–30 nm, with a small energy spread (about
1 eV).
Types of FEG Sources

SEMs use lenses to
produce clear and
detailed images.
They are not made of
glass but rather magnets
which are capable of
bending the path of
electrons.
The lenses focus and
control the electron
beam, ensuring that the
electrons end up
precisely on the desired
surface.
Condenser – Lens system

It accommodates the
specimen holder and
mechanisms for
manipulating it and
detectors for the various
emissions.
It must be very sturdy and
insulated from vibration
since the specimen must
be kept extremely still for
the microscope to produce
clear images.
SEM operates in a vacuum
to avoid interference of the
suspended particles with
the movement of incident
and reflected electron
beams.
Sample Chamber

Electron Collection System
It contains devices
detect the various ways
that the electron beam
interacts with the
sample object
The detector common to
all SEM systems is based
on the design of Everhart
and Thornley, which
detects secondary and
backscattered electrons

Electron beam – Specimen interactions
The result of the primary beam hitting the specimen is the formation of a
teardrop shaped reaction vessel (Fig.).
The reaction vessel by definition is where all the scattering events are
taking place. An increase in the topography will increase the surface area
of the reaction vessel resulting in more signal (Fig.).
Figure. Formation of teardrop shaped
reaction vessel
Figure. Surface area of teardrop
shaped reaction vessel

Six or more different events occur in the reaction vessel. These events
include:
1. Backscattered electrons. A primary beam electron may be scattered in
such a way that it escapes back from the specimen but does not go
through the specimen.
Backscattered electrons are the original beam electrons and thus, have a
high energy level, near that of the gun voltage.
Operating in the backscattered imaging mode is useful when relative
atomic density information in conjunction with topographical information
is to be displayed.

2. Secondary electrons are generated when a primary electron dislodges a
specimen electron from the specimen surface.
Secondary electrons have a low energy level of only a few electron volts,
thus, they can only be detected when they are dislodged near the surface of
the reaction vessel. Therefore, secondary electrons cannot escape from
deep within the reaction vessel.
Two of the foremost reasons for operating in the secondary electron
imaging mode are to obtain topographical information and high resolution.
3. X-rays. When electrons are dislodged from specific orbits of an atom in
the specimen, X-rays are omitted.
Elemental information can be obtained in the X-ray mode, because the X-ray
generated has a wavelength and energy characteristic of the elemental
atom from which it originated.

4. Cathode Luminescence. Some specimen molecule's florescence when
exposed to an electron beam. In the SEM, this reaction is called cathode
luminescence.
The florescence produces light photons that can be detected. A compound or
structure labeled with a luminescent molecule can be detected by using
cathode luminescence techniques.
5. Specimen Current. When the primary electron undergoes enough scattering
such that the energy of the electron is decrease to a point where the electron is
absorbed by the sample, this is known as specimen current.
6. Transmitted electrons. If the specimen is thin enough, primary electrons
may pass through the specimen. These electrons are known as transmitted
electrons and they provide some atomic density information.
The atomic density information is displayed as a shadow. The higher the atomic
number the darker the shadow until no electrons pass through the specimen.

Detection of Sample Surface
Secondary electrons emitted from the specimen are detected using a
scintillator-photomultiplier, "Everhart-Thornley," detector.
Low energy secondary electrons are emitted from the sample in all directions.
They are gathered by a charged collector grid (or cage), which can be biased
from -50 to +300 V. This draws the secondary electrons towards the scintillator.

The scintillator is a thin plastic disk coated with a short-persistence
phosphor that is highly efficient at converting the energy contained in the
electrons into ultraviolet light photons (4000 Å).
The response time of the phosphor is fast and permits high resolution
scanning.
The outer layer of the scintillator is coated with a thin layer [10-50 nm] of
aluminum, positively biased at approximately 10 KeV, which accelerates the
electrons to the scintillator surface.
The charged collector grid, in addition to collecting secondary electrons
from the sample, helps to alleviate some of the negative effects of the
scintillator aluminum layer bias, which can actually distort the incident
beam.

The topographical aspects of a secondary electron image depend on how
many of electrons actually reach the detector.
When the incident electron beam intersects the edges of topographically
high portions of a sample at lower angles, it puts more energy into the
volume of secondary electron production.
Thus, high points produce more secondary electrons, generating a larger
signal.
Faces oriented towards the detector also generate more secondary
electrons.
Secondary electrons that are prevented from reaching the detector do not
contribute to the final image and these areas will appear as shadows or
darker in contrast than those regions that have a clear electron path to the
detector.

Signals are collected by appropriate detector resulting signal is amplified and
displayed.
The topographic information contained in the image is straightforward.

1. Reduce the specimen into a size that fits the stub
2. Coat non-conducting specimens using either a vacuum evaporator or
sputter coater. This step increases primary and secondary emissions,
which enhances image quality.
For conventional imaging in the SEM, specimens must be electrically
conductive, at least at the surface, and electrically grounded to prevent
the accumulation of electrostatic charge (i.e. using metal coating).
Sample Preparation

Non-conducting materials are usually coated with an ultra-thin coating of
electrically conducting material, including gold, gold/palladium alloy,
platinum, platinum/palladium, iridium, tungsten, and chromium.
The recommended metals and sputter coating thickness are given below
for tungsten and FEG sources:
Metals:
Au, Au/Pd (Tungsten source)
Pt, Pd/Pt, Ir, W (FEG source)
Thickness:
5-10 nm for low magnification
2-3 nm for high resolution, the thinner the better
Sample Preparation

Type of Specimen Cutting Coating
Metallic  
Ceramic, conducting  
ceramic, non-conducting  
Semiconductor  
Polymer & polymer-matrix
composites  
Particles and fibers  
Biological and botanical
specimens  
Sample Preparation

Energy Dispersive X-ray spectroscopy
Energy-dispersive X-ray spectroscopy (EDS) is an analytical technique used for the
elemental analysis or chemical characterization of a sample. It relies on an
interaction of some source of X-ray excitation and a sample.
Its characterization is in large part to the fundamental principle that each element
has a unique atomic structure allowing a unique set of peaks on its electromagnetic
emission spectrum.

A sample is excited under high energy of electron
beam, the inner shell of electrons is ejected to
vacuum creating a vacancy in that shell.
Electrons from the outer shell jump into the
vacant site for filling the inner shell.
During this process, the sample fluoresces X-ray of
energy same as the energy difference between
the initial state and final state.
Energy Dispersive X-ray spectroscopy
Since each atom has its unique and discretized energy levels, the X-Ray
fluorescence is also characteristic of that atom.
Energy dispersive X ray spectroscopy is a technique that detects the X-ray
fluorescence to characterize the elements present in a material.
The most common detector used is Si(Li) detector cooled to cryogenic
temperatures with liquid nitrogen. Now, newer systems are often equipped
with silicon drift detectors (SDD) with Peltier cooling systems.

Figure. EDS spectrum of the mineral crust of the vent shrimp. Most of these
peaks are X-rays given off as electrons return to the K electron shell.(K-
alpha and K-beta lines) One peak is from the L shell of iron.
EDS Spectrum

Advantages of SEM
• Specimen size (small)
• Samples must be conductive, otherwise non-conductive material
should be metal coated
• Only surface morphology can be obtained
Limitations of SEM
1. SEM has a large depth of field, which allows more of a specimen to be in
focus at one time
2. SEM also has much higher resolution, so closely spaced specimens can
be magnified at much higher levels
3. Because the SEM uses electromagnets rather than lenses, the researcher
has much more control in the degree of magnification
All of these advantages, as well as the actual strikingly clear images, make
the SEM one of the most useful instruments in research today

SEM has a variety of applications in a number of scientific and industry-
related fields, especially where characterizations of solid materials is
beneficial.
In addition to topographical, morphological and compositional
information, an SEM can detect and analyze surface fractures, provide
information in microstructures, examine surface contaminations, reveal
spatial variations in chemical compositions, provide qualitative chemical
analyses and identify crystalline structures.
In addition, SEMs have practical industrial and technological applications
such as semiconductor inspection, production line of miniscule products and
assembly of microchips for computers.
SEMs can be as essential research tool in fields such as life science, biology,
gemology, medical and forensic science, metallurgy.
Applications

1. Image morphological of samples
2. Image compositional and some bonding differences
3. Examine wet and dry samples while viewing them (only in an
ESEM)
4. View frozen material (in a SEM with a cryostage)
5. Generate X-rays from samples for microanalysis (EDS)
6. View/map grain orientation/crystallographic orientation and
study related information like heterogeneity
Applications

Fractography
Biology
Crystal Structure
Surface Morphology
Applications

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