SEM and TEM: Part-II

By:

Saloni Sharma

Scanning Electron Microscope (SEM)

• The first Scanning Electron Microscope was initially made by Mafred von Ardenne in 1937 with an aim to surpass the transmission electron Microscope. He used high-resolution power to scan a small raster using a beam of electrons that were focused on the raster. He also aimed at reducing the problems of chromatic aberrations images produced by the Transmission electron Microscopes. More studies followed by scientists and research institutions such as Cambridge Scientific Instrument Company who eventually developed a fully constructed Scanning electron Microscope, in 1965 and named it a Stereoscan. 

Scanning Electron Microscope (SEM)

• Scanning Electron Microscope (SEM) is a type of electron microscope that scans surfaces of microorganisms that uses a beam of electrons moving at low energy to focus and scan specimens. The development of electron microscopes was due to the inefficiency of the wavelength of light microscopes. electron microscopes have very short wavelengths in comparison to the light microscope which enables better resolution power

�Principle of Scanning Electron Microscope (SEM)�

• Unlike the Transmission Electron Microscope which uses transmitted electrons, the scanning electron Microscope uses emitted electrons.
•  The Scanning electron microscope works on the principle of applying kinetic energy to produce signals on the interaction of the electrons.
• These electrons are secondary electrons, backscattered electrons, and diffracted backscattered electrons which are used to view crystallized elements and photons.
• Secondary and backscattered electrons are used to produce an image. The secondary electrons are emitted from the specimen play the primary role of detecting the morphology and topography of the specimen while the backscattered electrons show contrast in the composition of the elements of the specimen.

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Parts of Scanning Electron Microscope

The major components of the Scanning Electron Microscope include;

• Electron Source
• Lenses
• Scanning Coil
• Detector
• The display device
• Power supply
• Vacuum system

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• Electron Source – This is where electrons are produced under thermal heat at a voltage of 1-40kV. the electrons condense into a beam that is used for the creation of an image and analysis.
• There are three types of electron sources that can be used i. e
• Thermionic Gun(Tungsten filament)
• Lanthanum/Cerium hexaboride and
• Field emission gun (FEG)

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• Thermionic Gun:

It is the most common type of electron gun, which makes use of thermal energy to a filament to coax electrons away from the gun and towards the specimen under examination. The filament is usually made of Tungsten, which has high melting point. inside the microscope, this tungsten heats up at white hot temperatures, until it emits electrons. Under intense heat conditions, its lifetime is about 100 hours.

• Lanthnum/Cerium hexaboride cathode:

At ten times the brightness of tungsten, this electron source provides an improved signal-to-noise ratio, a better ratio, and has a lifetime of over 1,500 hours.

• Field Emission Gun:

It creates a strong electrical field to pull electrons away from the atoms they are associated with and generate high resolution images.

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The electron guns are located at the top of the microscope and fire a beam of electrons at the object under examination. It uses vacuum design. The electron source generates electrons at the top of the microscope’s column.

The anode plate has a positive charge, which attracts the electrons to form a beam.The electrons move to the next component of the microscope

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Condenser Lenses

It has several condenser lenses that focus the beam of electrons from the source through the column forming a narrow beam of electrons that form a spot called a spot size.

The condenser lenses are made of magnets capable of bending the path of electrons.

By doing so, the condenser lenses focus and control the electron beam, ensuring that the electrons end up precisely where they need to go.

The condenser lens controls the size of the beam, and determines the number of electrons in the beam. The size of the beam will define the resolution of the image.

Objective Aperture

• The objective aperture arm fits above the objective lens in the SEM.
• It is a metal rod that holds a thin plate of metals called aperture strip, containing four holes with different sizes that line up with the larger holes.
• By moving the strip in and out, different sized holes can be put into the beam.
• Apertures can also be used to control the size of the beam.

• Scanning Coil – They are used to deflect the beam over the specimen surface beam along x and y axes, to ensure it scans in a raster fashion over the surface of the sample.
• Objective Lenses-
• The objective lens is the last lens in the sequence of lenses and closest to the sample, it focuses the beam to a very small spot on the sample.
• Electrons cannot pass through glass, so SEM lenses are electromagnetic. They are made up of a coil of wires inside metal poles.
• When a current passes through these coils they generate a magnetic field.
• Electrons are highly sensitive to these magnetic fields, which therefore enables the lenses in the microscope to control them.
• Chamber-
• The sample chamber of SEM s the place where the researchers place the sample they are examining.
• Because the specimen must be kept extremely still for the microscope to produce clear images, the sample chamber must be sturdy and insulated from all vibrations. In fact SEM are so sensitive that they are installed at the ground flour of the building.
• They also manipulate the specimen, placing it at different angles and moving it so that the researchers don’t have to constantly remount the specimen to take different images.

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Signals from Sample

Signals from sample

• When electrons from the microscope interact with a sample, this can generate different kinds of other electrons, photons and irradiations.
• The two types of electrons essential for imaging are:
• Backscattered electrons (BSEs) and
• Secondary electrons (SEs).

Secondary electrons (SE)

• Secondary electrons are generated from the inelastic collision between incoming electrons and the loosely bound electrons of the sample.
• These are low energy electrons (10-50 eV).
• Inelastic interactions occur when an interaction causes a loss of energy of the primary electron.
• SE are generated close to the surface escape and are useful to give topographic information.

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Backscattered electrons (BSE)�

• Backscattered electrons are reflected back when the primary electron beam interacts with the sample object through elastic interactions.
• A fraction of the electrons is retarded by electromagnetic field of the nucleus and if the scattering angle is equal to 180⁰ the electron can escape the surface. These are high energy electrons.
• Elastic interactions occur when there is no loss of energy of the primary electron, and when this happens, the electrons can change direction but do not change their wavelength.
• BSE originate from deeper areas of the sample.There are fewer BSE than SE. 

• These devices are able to detect and differentiate the various ways the electron beam interact with the sample the secondary electrons, backscattered electrons, and diffracted backscattered electrons. The functioning of the detectors highly depends on the voltage speed, the density of the specimen. Various types of detectors are used in SEM.
• Typically, for SEs, this will be an Everhart-Thornley detector. This consists of a scintillator inside a Faraday cage. This detector is positively charged to attract SEs. These detectors are capable of producing most detailed images of sample surface.
• For detecting BSEs, the microscope will use solid state detectors, placed above the sample. These can give information about composition of the sample.
• Energy dispersive spectrometer can detect X-rays.

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Detectors

• Vacuum system
• SEM requires vacuum to operate.
• Without a vacuum, the electron beam generated by the electron gun would encounter constant interference from air particles in the atmosphere.
• Not only would these particles block the path of the electron beam they would also be knocked out of the air and onto the specimen, which would distort the surface of the specimen
• Like the transmission electron Microscope, the Scanning electron microscope should be free from vibrations and any electromagnetic elements.
• The display device (data output devices)
• Power supply

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Working of Scanning Electron Microscope

• The source of the electrons and the electromagnetic lenses are from tungsten filament lamps that are placed at the top of the column and it is similar to those of the transmission electron Microscope.
• The electrons are emitted after thermal energy is applied to the electron source and allowed to move in a fast motion to the anode, which has a positive charge.
• The beam of electrons activates the emission of primary scattered electrons at high energy levels and secondary electrons at low-energy levels from the specimen surface. The beam of electrons interacts with the specimen to produce signals that give information about the surface topography and composition of the specimen.

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• The specimen does not need special treatment for visualization under the SEM, even air-dried samples can be examined directly. However, microbial specimens need fixation, dehydration, and drying in order to maintain the structural features of the cells and to prevent collapsing of the cells when exposed to the high vacuum of the microscope.
• The samples are mounted and coated with thin layer of heavy metal elements to allow spatial scattering of electric charges on the surface of the specimen allowing better image production, with high clarity.
• Scanning by this microscope is attained by tapering a beam of electrons back and forth over a thin section of the microscope. When the electrons reach the specimen, the surface releases a tiny staw of electrons known as secondary electrons which are then trapped by a special detector apparatus.

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• When the secondary electrons reach and enter the detector, they strike a scintillator (a luminescence material that fluoresces when struck by a charged particle or high-energy photon). This emits flashes of light which get converted into an electric current by a photomultiplier, sending a signal to the cathode ray tube. This produces an image that looks like a television picture that can be viewed and photographed.
• The quantity of secondary electrons that enter the detector is highly defined by the nature of the specimen i.e raised surfaces to receive high quantities of electrons, entering the detector while depressed surfaces have fewer electrons reaching the surface and hence fewer electrons enter the detector.
• Therefore raised surfaces will appear brighter on the screen while depressed surfaces appear darker.

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�Applications of the Scanning Electron Microscope (SEM)�

• It is used in a variety of fields including Industrial uses, nanoscience studies, Biomedical studies, Microbiology
• Used for spot chemical analysis in energy-Dispersive X-ray Spectroscopy.
• Used in the analysis of cosmetic components which are very tiny in size.
• Used to study the filament structures of microorganisms.
• Used to study the topography of elements used in industries.

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Advantages of the Scanning Electron Microscope

• They are easy to operate and have user-friendly interfaces.
• They are used in a variety of industrial applications to analyze surfaces of solid objects.
• Some modern SEMs are able to generate digital data that can be portable.
• It is easy to acquire data from the SEM, within a short period of time of about 5 minutes.

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�Limitations of Scanning Electron Microscope�

• They are very expensive to purchase
• They are bulky to carry
• They must be used in rooms that are free of vibrations and free of electromagnetic elements
• They must be maintained with a consistent voltage
• They should be maintained with access to cooling systems

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Difference between SEM and TEM

Difference between SEM and TEM

Difference between SEM and TEM

Similarities between SEM and TEM

• Both TEM and SEM share several key components:
• Multiple electrostatic and electromagnetic lenses are involved in managing the trajectory and shape of the electron beam.
• An electron source.
• The sample chamber is placed under high-vacuum.
• Electron apertures.

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�Preparation of a Sample for SEM�

• First, it is important to consider the sample’s size, its shape and state, and whether it has conductive properties.
• Where a sample does not have conductive properties, it is required to coat it first using a sputter-coater.
• Conductive coats include gold, silver, platinum and chromium. As well as non-conductive materials, coating works for samples that will be sensitive to the electron beam, such as plastics.
• To ensure image clarity, we must ensure the sample to be clean.
• To maintain its structural details during the process, a fixative is used or where necessary, it is dehydrated it by application of alcohol.
• Before placing the sample in the vacuum environment of the microscope it must be completely dry. If it is not dry, water vapourization can obstruct the electron beam, affecting the clarity of the image.

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�Types of Analysis Can SEM Perform�

The main types of analysis that SEM can perform are:

• Backscattered electron detection BSE- generates imaging that carries information on the composition of a sample. BSE images provide valuable crystallographic, topographic and magnetic field information.

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• Energy-dispersive x-ray spectroscopy EDS- separates x-rays that are characteristic of different elements, helping to analyze the energy spectrum and chemical composition of materials.

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• Cathodoluminescence CL- produces high-resolution digital images of luminescent materials.

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• Electron backscatter diffraction- EBSD gives direct information about the crystalline structure and orientation of materials, and can perform analysis on polycrystalline aggregates.

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