Scanning Electron Microscopy - Important Aspects
Certain issues with transmission electron microscopy such as phase-contrast imaging and the consequent aberrations produced due to imaging lenses, brought to birth scanning electron microscopy. It helped reduce the effect of such issues.
Important aspects of scanning electron microscopy are covered in this article.
In scanning electron microscopy, a narrow focused electron beam, generated through use of a tungsten element in a scanning electron microscope, is first accelerated through use of electromagnetic condenser lenses. After this, it is made to gradually scan a prepared specimen, just as a video camera scans an audience. The scanning is achieved through a beam deflector. Some of these electrons are absorbed by the specimen.
Primary back-scattered electrons, scattered secondary electrons, and characteristic X-rays are detected through an Everhart-Thornley detector and electronically processed into a digitally rasterized high resolution image. The secondary electrons are produced as a reaction to the impingement of the primary electron beam on the surface of the specimen. Above is the basic working fundamental and principle scanning electron microscopy is associated with. It is also the fundamental and principle scanning electron microscopyscanning electron microscopes work on. Through scanning electron microscopy, high image resolution in the sub-nanometer range is obtained. An advantage of scanning electron microscopy is that it produces a pseudo three dimensional image. A further advantage of scanning electron microscopy is that issues such as phase-contrast imaging and aberrations caused by imaging lenses are avoided. Another advantage is that specimens are not required to be thin. Such high resolution 3D images are highly conducive for analysis through use of an energy dispersive spectrometer. Energy dispersive spectrometers, in combination with scanning electron microscopes, are typically used for metallurgical/corrosion-failure analysis of steels. These spectrometers help provide elemental composition data of the specimen. An energy dispersive spectrometer detects both the number of emitted X-rays and their energy. These X-rays are emitted when the secondary electrons break free from atoms of a specific element within the specimen. The energy associated with an X-ray is a characteristic of a specific element. Today software supports automated spectrum analysis for quantitative and qualitative elemental composition of a composite material specimen. An important point here is that only elements can be known, but not the compounds of which they are part. For deducing compounds, experts are required. As in transmission electron microscopy, vacuuming the entire scanning electron microscope unit and avoiding the effects of mechanical vibration are standard precautions. Specimen preparation consists of cryofixation, drying, and coating the specimen with gold, chromium, or graphite carbon to prevent charging of the specimen and to facilitate conduction. An alternative is to surround the specimen with a low pressure gas (instead of vacuum) or to use a low accelerating voltage on the electron beam. In case of gas usage, the scanning electron microscopy is known as environmental SEM. Applications of scanning electron microscopy are in failure analysis of materials, die and package failures, and microanalysis. An SEM is cheaper and more compact than a TEM.
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