THE SCANNING ELECTRON MICROSOPE (SEM)....
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A scanning electron microscope is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample.
THE SCANNING ELECTRON MICROSOPE (SEM)....
- 2. THE SCANNING ELECTRON MICROSOPE PRINCIPAL •The basic principle is that a beam of electrons is generated by a suitable source, typically a tungsten filament or a field emission gun. •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. •Then the beam scans the surface of the specimen. Electrons are emitted from the specimen by the action of the scanning beam and collected by a suitably-positioned detector.
- 3. Basic components are as following: • Electron gun (Filament). • Condenser lenses • Objective ApertureScan • coilsChamber • Detectors • Computer hardware and software CONSTRUCTION
- 4. ELECTRON GUNS ▸ Electron guns are typically one of TWO types 1. THERMIONIC GUNS2 2. FIELD EMISSIONGUNS THERMIONIC GUNS: ► Which are the most common type, apply thermal energy to a filament to coax electrons away from the gun and toward the specimen under examination. ► Usually made of tungsten, which has a high melting point FIELDEMISSIONGUNS: ► Create a strong electrical field to pull electrons away from the atoms they are associated with. ►Electron guns are located either at the very top or at the very bottom of an SEM and fire a beam of electrons at the object under
- 5. CONDENSER LENSES ►Just like optical microscopes, SEMs use Condenser lenses to produce clear and detailed images. • The Condenser lenses in these devices, however, work differently. ►For one thing, they aren't made of glass. ► Instead, 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.
- 6. •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 metal containing four holes. ►By moving the arm in and out different sized holes can be put into the beam path. ► An aperture holder: This arm holds a thin metal strip with different sized holes that line up with the larger holes. ► The metal strip is called an Aperture strip.
- 7. •CAHMBER ► The sample chamber of an SEM is where researchers place the specimen that they are examining. ►Because the specimen must be kept extremely still for the microscope to produce clear images, the sample chamber must be very sturdy and insulated from vibration. ►In fact, SEMs are so sensitive to vibrations that they are often installed on the ground floor of a building. ► They also manipulate the specimen, placing it at different angles and moving it so that researchers don't have to constantly remount the object to take different
- 8. •VACUUM CHAMBER SEMs require a 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.
- 9. •DETECTORS ► Various types of detectors are there in SEM. ►These devices detect the various ways that the electron beam interacts with the sample object. ►For instance, Everhart-Thornley detectors register secondary electrons, which are electrons dislodged from the outer surface of a specimen. These detectors are capable of producing the most detailed images of an object's surface. ►Other detectors, such as backscattered electron detectors and X-ray detectors, can tell researchers about the composition of a substance.
- 10. •BACKSCATTERED ELECTRONS (BSE) A fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 180° the electron can escape from the surface- Fewer BSE than SE High energy electrons
- 12. Thank you…