Scanning Electron Microscope Definition
- A Scanning Electron Microscope uses focused beams of electrons to create an image of a specimen by scanning the surface area.
- Atoms of specimens are combined with the electron beams and form different types of signals, which contain data, which are related to the surface topography and composition of the sample.
- It can produce high-resolution, three-dimensional images of the specimen.
- These types of microscopes are used to examine the surface of microorganisms in great detail.
- In 1937, Manfred von Ardenne invented the first Scanning Electron Microscope.
Principle of Scanning Electron Microscope
The scanning electron microscope scans narrow tapered electron beams back and forth over the specimen. When the beam strikes a particular area, surface atoms discharge a tiny shower of electrons called secondary electrons.
Secondary electrons entering the detector strike a scintillator causing it to emit light flashes that a photomultiplier converts to an electrical current and amplifies.
The signal is sent to a cathode ray tube and produces an image. The number of secondary electrons reaching the detector depends on the nature of the specimen surface.
When the electron beam strikes a raised area a large number of secondary electrons in the detector. Thus, raised areas appear lighter on the screen and depression is darker.
Parts of Scanning Electron Microscope
Scanning electron microscope contain these following parts;
- Electron Source: A electron source help in the production of electron beams in SEM. There are present different types of electron sources are used in SEM such as;
- A Thermionic filament: It is a tungsten filament. When heated, it emits electron beams.
- A Field emission gun (FEG): It creates a strong electrical field, which pulls electrons away from their atoms.
- A Cerium Hexaboride cathode (CeB6): It provides ten-time brightness as compared to other electron sources.
- Electromagnetic Lens: It focuses the electron beams on the specimen from the source.
- Vacuum chamber: It prevents the intersection between an electron beam and air particles.
- Sample chamber and stage: It holds the specimen inside the vacuum.
- Computer: It controls the magnification power and the surface to be scanned.
- Secondary electron detectors: It detects the secondary electrons.
- BackScattered Electron (BSE) detector: It detects the backscattered electrons.
- Power Supplier: It Supply Power to the SEM.
Sample Preparation for Scanning Electron Microscope
1. Primary Fixation:
- This is done with ALDEHYDES (PROTEINS).
- This step will help to stabilize the ultrastructure of the specimen with the crosslinking the Proteins by glutaraldehyde and formaldehyde.
2. Secondary Fixation:
- This is done with the OSMIUM TETROXIDE (LIPIDS).
- This step prevents the Blipid membranes extraction during dehydration.
- It also increases sample conductivity and minimizes image distortions
- This is done by incubation of fixed specimen in solvents such as ETHANOL OR ACETONE.
- To remove the water from the specimen without shrinking it the Solvent concentration is increased gradually.
- In this method, the dehydration solvents are replaced with the Hexamethyldisilazane (HMDS) or liquid CO2 to prevent the artifacts and micro-ripping of the surface.
5. Mounting on a Stub:
- This is done with a sticky carbon dis, which mount the specimen on a metal stub.
- This step helps to increase the conductivity of the specimen.
- To increase further conductivity Silver-containing glue can be added to it.
6. Sputter Coating With Conductive Material:
- In this method, the specimen is coated with a conductive material to prevent the charge buildup on specimen surface.
Mechanism of Scanning Electron Microscope
- First of all, an electron source or electron gun located at the top of the SEM column is heated with high-voltage.
- As a result, it will release electron beams.
- Electron beams are now accelerated down the column and onto a series of electromagnetic lenses.
- These lenses and tubes are also called solenoids, because they are wrapped in a coil.
- These coils create fluctuations in the voltage, which results in increasing/decreasing the speed of electrons. Thus, how they create focused electron beams.
- This electron beam is focused onto a specimen.
- A computer is attached to the SEM, which controls the magnification power of the microscope and as well as determines the surface area to be scanned.
- The electron beams and atoms of the sample are combined, the rate is determined by the acceleration rate of incident electrons, which carry significant amounts of kinetic energy before focusing on the sample.
- When the incident electrons come in contact with the sample, energetic electrons are released from the surface of the sample. The scatter patterns made by the interaction yields information on the size, shape, texture, and composition of the sample.
- Different types of electrons are emitted from the sample after the interaction between electron beams and sample.
- An electron detector is placed over the sample, called BackScattered Electron (BSE) detector, which will detect backscattered electrons.
- The secondary electrons are detected using a Secondary Electron (SE) detector, which is placed at the side of the electron chamber. It will provide more detailed surface information.
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Application of Scanning Electron Microscope
- SEM is used in life science, biology, gemology, and medicine for research purposes.
- It is also used in forensic laboratories.
- In industries and technology, it is used for semiconductor inspection, production line of minuscule products, and assembly of microchips for computers.
- It is used to examine surface contaminants.
- It reveals spatial variations in chemical compositions.
- It provides qualitative chemical analyses and identifies crystalline structures.
- It provides information in microstructures.
- It can detect and analyze surface fractures.
Advantages of Scanning Electron Microscope
- It provides a 3D and topographical image of the specimen with great detail.
- Require less time.
- Require minimal preparation action of the sample.
- Modern SEMs produces portable digital data.
- SEM is easy to operate with proper training.
Disadvantages of Scanning Electron Microscope
- SEMs are a costly item.
- It is large in size, that is why required a room to operate SEM.
- The room should be free of electric, magnetic field, and vibration.
- Required a steady voltage.
- Required cool water.
- Required proper training to operate SEM.
- It is limited to solid, inorganic samples small enough to fit inside the vacuum chamber.
- It also carried a risk of radiation exposure.