Download 1)Scanning Electron Microscope (SEM)

Types of Electron Microscopes
1)Scanning Electron Microscope (SEM)
The SEM is a microscope that uses electrons instead of light to form an
image. The scanning electron microscope has many advantages over
traditional microscopes. The SEM has a large depth of field, which
allows more of a specimen to be in focus at one time. The SEM also has
much higher resolution, so closely spaced specimens can be magnified at
much higher levels. Because the SEM uses electromagnets rather than
lenses, the researcher has much more control in the degree of
magnification. All of these advantages, make the scanning electron
microscope one of the most useful instruments in research today.
In the SEM, we use much lower accelerating voltages to prevent beam
penetration into the sample since what we require is generation of the
secondary electrons from the true surface structure of a sample.
Therefore, it is common to use low KV, in the range 1-5kV for biological
samples, even though our SEMs are capable of up to 30 kV.
How does a SEM work?
A beam of electrons is produced at the top of the microscope by an
electron gun. electron gun fitted with a tungsten filament cathode.
Tungsten is normally used in electron guns because it has the highest
melting point and lowest vapour pressure of all metals, thereby allowing
it to be heated for electron emission, and because of its low cost. The
electron beam is focused by one or two condenser lenses to a spot about
0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning
coils or pairs of deflector plates in the electron column, typically in the
final lens, which deflect the beam in the x and y axes so that it scans in a
raster fashion over a rectangular area of the sample surface. When the
primary electron beam interacts with the sample, the electrons lose
energy by repeated random scattering and absorption within a teardropshaped volume of the specimen known as the interaction volume, which
extends from less than 100 nm to around 5 µm into the surface. The size
of the interaction volume depends on the electron's landing energy, the
atomic number of the specimen and the specimen's density. Once the
beam hits the sample, electrons and X-rays are ejected from the sample.
Detectors collect these X-rays, backscattered electrons, and secondary
electrons and convert them into a signal that is sent to a screen similar to
a television screen. This produces the final image.
Scheme of Scanning electron microscopy
Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) is a microscopy technique
where by a beam of electrons is transmitted through an ultra-thin
specimen, interacting with the specimen as it passes through. An image is
formed from the interaction of the electrons transmitted through the
specimen.
TEMs are capable of imaging at a significantly higher resolution than
light microscopes. This enables the instrument's user to examine fine
detail—even as small as a single column of atoms, which is tens of
thousands times smaller than the smallest resolvable object in a light
microscope. TEM forms a major analysis method in a range of scientific
fields, in both physical and biological sciences. TEMs find application in
cancer research, virology, materials science as well as pollution,
nanotechnology, and semiconductor research.
At smaller magnifications TEM image contrast is due to absorption of
electrons in the material, due to the thickness and composition of the
material. At higher magnifications complex wave interactions modulate
the intensity of the image, requiring expert analysis of observed images.
Alternate modes of use allow for the TEM to observe modulations in
chemical identity, crystal orientation, electronic structure and sample
induced electron phase shift as well as the regular absorption based
imaging.
Source formation
the TEM consists of an emission source, which may be a tungsten
filament, or a lanthanum hexaboride (LaB6) source. or a small spikeshaped filament. LaB6 sources utilize small single crystals. By connecting
this gun to a high voltage source (typically ~100–300 kV) the gun will,
given sufficient current, begin to emit electrons either by field electron
emission into the vacuum. Once extracted, the upper lenses of the TEM
allow for the formation of the electron probe to the desired size and
location for later interaction with the sample. the lens shape originating
due to the distribution of magnetic flux. The lenses of a TEM allow for
beam convergence, with the angle of convergence as a variable
parameter, giving the TEM the ability to change magnification simply by
modifying the amount of current that flows through the coil, quadrupole
or hexapole lenses. The quadrupole lens is an arrangement of
electromagnetic coils at the vertices of the square, enabling the generation
of a lensing magnetic fields, the hexapole configuration simply enhances
the lens symmetry by using six, rather than four coils. Typically a TEM
consists of three stages of lensing. The stages are the condensor lenses,
the objective lenses, and the projector lenses. The condensor lenses are
responsible for primary beam formation, whilst the objective lenses focus
the beam that comes through the sample itself. The projector lenses are
used to expand the beam onto the phosphor screen or other imaging
device, such as film. The magnification of the TEM is due to the ratio of
the distances between the specimen and the objective lens' image plane .
Display
Imaging systems in a TEM consist of a phosphor screen, which may be
made of fine (10–100 μm) particulate zinc sulphide, for direct
observation by the operator. Optionally, an image recording system such
as film based or doped YAG screen coupled . Typically these devices can
be removed or inserted into the beam path by the operator as required.
Components
A TEM is composed of several components, which include a vacuum
system in which the electrons travel, an electron emission source for
generation of the electron stream, a series of electromagnetic lenses, as
well as electrostatic plates. The latter two allow the operator to guide and
manipulate the beam as required. Also required is a device to allow the
insertion into, motion within, and removal of specimens from the beam
path. Imaging devices are subsequently used to create an image from the
electrons that exit the system.
1) Vacuum system
To increase the mean free path of the electron gas interaction, a standard
TEM is evacuated to low pressures, typically on the order of 10 −4 Pa. The
need for this is twofold: first the allowance for the voltage difference
between the cathode and the ground without generating an arc, and
secondly to reduce the collision frequency of electrons with gas atoms to
negligible levels—this effect is characterised by the mean free path. TEM
components such as specimen holders and film cartridges must be
routinely inserted or replaced requiring a system with the ability to reevacuate on a regular basis. As such, TEMs are equipped with multiple
pumping systems and airlocks and are not permanently vacuum sealed.
High-voltage TEMs require ultra-high vacuums on the range of 10−7 to
10−9 Pa to prevent generation of an electrical arc, particularly at the TEM
cathode. Poor vacuum in a TEM can cause several problems, from
deposition of gas inside the TEM onto the specimen as it is being viewed
through a process known as electron beam induced deposition, or in more
severe cases damage to the cathode from an electrical discharge. Vacuum
problems due to specimen sublimation are limited by the use of a cold
trap to adsorb sublimated gases in the vicinity of the specimen.
2) Specimen stage
TEM specimen stage designs include airlocks to allow for insertion of
the specimen holder into the vacuum with minimal increase in pressure in
other areas of the microscope. The specimen holders are adapted to hold a
standard size of grid upon which the sample is placed or a standard size
of self-supporting specimen. Standard TEM grid sizes are a 3.05 mm
diameter ring, with a thickness and mesh size ranging from a few to
100 μm. The sample is placed onto the inner meshed area having
diameter of approximately 2.5 mm. Usual grid materials are copper,
molybdenum, gold or platinum. This grid is placed into the sample
holder, which is paired with the specimen stage.
3)Apertures ‫ فتحة‬، ‫منفذ‬
Apertures are annular metallic plates, through which electrons that are
further than a fixed distance from the optic axis may be excluded. These
consist of a small metallic disc that is sufficiently thick to prevent
electrons from passing through the disc, whilst permitting axial electrons.
This permission of central electrons in a TEM causes two effects
simultaneously: firstly, apertures decrease the beam intensity as electrons
are filtered from the beam, which may be desired in the case of beam
sensitive samples. Secondly, this filtering removes electrons that are
scattered to high angles, which may be due to unwanted processes such as
spherical or chromatic aberration, or due to diffraction from interaction
within the sample.
Phase contrast
Crystal structure can also be investigated by high-resolution transmission
electron microscopy (HRTEM), also known as phase contrast. When
utilizing a Field emission source and a specimen of uniform thickness, the
images are formed due to differences in phase of electron waves, which is
caused by specimen interaction. , the image is not only dependent on the
number of electrons hitting the screen, making direct interpretation of
phase contrast images more complex. However this effect can be used to
an advantage, as it can be manipulated to provide more information about
the sample, such as in complex phase retrieval techniques.
What is the difference between SEM and TEM?
In SEM (Scanning Electron Microscopy) you look at either
backscattered
or
secondary
electrones
whereas
in
TEM
(Transmission Electron Microscopy) you look how much of your
electron beam makes it through the sample onto your phosphor
screen or film camera. Usually SEM is used for surface analysis
and TEM for analyzing sections.
Both SEM (scanning electron microscope/microscopy) and TEM
(transmission electron microscope/microscopy) refer both to the
instrument and the method used in electron microscopy.
There are a variety of similarities between the two. Both are types
of
electron
microscopes
and
give
the
possibility
of
seeing,
studying, and examining small, compositions of a sample. Both
also use electrons (specifically, electron beams), the negative
charge of an atom. Also, both samples in use are required to be
“stained” or mixed with a particular element in order to produce
images.
Images
produced
from
these
instruments
are
highly
magnified and have a high resolution.
However, an SEM and TEM also share some differences. The
method used in SEM is based on scattered electrons while TEM is
based on transmitted electrons. The scattered electrons in SEM are
classified as backscattered or secondary electrons. However, there
is no other classification of electrons in TEM.
The scattered electrons in SEM produced the image of the sample after
the microscope collects and counts the scattered electrons. In TEM,
electrons are directly pointed toward the sample. The electrons that pass
through the sample are the parts that are illuminated in the image.
The focus of analysis is also different. SEM focuses on the sample’s
surface and its composition. On the other hand, TEM seeks to see what is
inside or beyond the surface. SEM also shows the sample bit by bit while
TEM shows the sample as a whole. SEM also provides a threedimensional image while TEM delivers a two-dimensional picture.
In terms of magnification and resolution, TEM has an advantage
compared to SEM. TEM has up to a 50 million magnification
level while SEM only offers 2 million as a maximum level of
magnification. The resolution of TEM is 0.5 angstroms while
SEM has 0.4 nanometers. However, SEM images have a better
depth
of
field
compared
to
TEM
produced
images.
Another point of difference is the sample thickness, “staining,”
and preparations. The sample in TEM is cut thinner in contrast to
a SEM sample. In addition, an SEM sample is “stained” by an
element that captures the scattered electrons.
On the other hand, TEM requires the sample to be prepared in a
TEM grid and placed in the middle of the specialized chamber of
the microscope. The image is produced by the microscope via
fluorescent screens.
Another feature of SEM is that the area where the sample is placed can be
rotated in different angles. TEM was developed earlier than SEM.
Disadvantages of Electron Microscopy
Electron microscopes are very expensive to buy and maintain. They are
dynamic rather than static in their operation: requiring extremely stable
high voltage supplies, extremely stable currents to each electromagnetic
coil/lens, continuously-pumped high/ultra-high vacuum systems and a
cooling water supply circulation through the lenses and pumps. As they
are very sensitive to vibration and external magnetic fields, microscopes
aimed at achieving high resolutions must be housed in buildings with
special services.
The samples have to be viewed in a vacuum, as the molecules that make
up air would scatter the electrons. This means that the samples need to be
specially prepared by sometimes lengthy and difficult techniques to
withstand the environment inside an electron microscope. Recent
advances have allowed some hydrated samples to be imaged using an
environmental scanning electron microscope, but the applications for this
type of imaging are still limited.