Explain about the scanning process and image formation of scanning electron microscope.
Scanning process and image formation of scanning electron microscope
In a typical scanning electron microscope, an electron beam is thermionically emitted through an electron gun fitted along with a tungsten filament cathode. Tungsten is usually used in thermionic electron guns since this has the highest melting point and lowest vapour pressure of each metal, thereby allowing this to be heated for electron emission, and due to its low cost. Many types of electron emitters comprise lanthanum hexaboride (LaB6) cathodes that can be used into a standard tungsten filament SEM when the vacuum system is upgraded and field emission guns (FEG) that may be of the cold-cathode type by using tungsten single crystal emitters or the thermally-assisted Schottky kind, by using emitters of zirconium oxide.
The electron beam, that typically has an energy ranging from small hundred eV to 40 keV, is focused through one or two condenser lenses to a spot around 0.4 nm to 5 nm into diameter. The beam passes by pairs of scanning coils or pairs of deflector plates into the electron column, classically in the last lens that deflect the beam into the x and y axes so that this scans in a raster fashion over a rectangular region of the sample surface.
While the primary electron beam interacts along with the sample, the electrons lose energy by repeated random scattering and absorption into a teardrop-shaped volume of the specimen termed as the interaction volume that extends from less than 100 nm to about 5 µm in the surface. There size of the interaction volume depends onto the specimen's density, the electron's landing energy and the atomic number of the specimen. The energy exchange among the electron beam and the sample results into the reflection of high-energy electrons through elastic scattering, emission of secondary electrons by inelastic scattering and the emission of electromagnetic radiation, all of which can be detected through specialized detectors. The beam current absorbed through the specimen can also be detected and used to make images of the distribution of specimen current. Electronic amplifiers of different types are used amplify the signals that are displayed as variations into brightness onto a cathode ray tube. The raster scanning of the cathode ray tube display is synchronised with which of the beam on the specimen in the microscope, and the resulting image is thus a distribution map of the intensity of the signal being emitted through the scanned area of the specimen. The representation may be captured through photography by a high resolution cathode ray tube, but into modern machines is digitally captured and displayed onto a computer monitor and saved to a computer's hard disk.