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Scanning Electron Microscopy

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Scanning Electron Microscopy is a category of the electron microscope, which uses highly focused beams of strong or high-energy electrons in order to generate diverse types of signals on the entire surface of the solid specimens. The high-energy electrons interact actively with the solid specimen atoms thus producing clear signals containing detailed information of the sample such as chemical composition, texture (morphology), orientation of sample materials, electrical conductivity of the sample, and crystalline structure. SEM  produces many types of electron signals such as characteristics X-rays, secondary electrons, cathodoluminescence (light), back-scattered (BSE) electrons, transmitted electrons, and specimen current, which result from,  active interactions of electrons beams with specimen atoms near or  at the sample surface (Joseph & Linda 2003). SEM technology, especially the secondary electrons imaging (SEI), is highly effective as it can be able to produce extremely high-resolution specimen images revealing extremely small details even of size lower than 1 nanometer (nm). In addition, SEM is even capable of analyzing selected points on the sample an approach extremely useful in semi-quantitatively or qualitatively analysis. In order to understand Scanning Electron Microscopy (SEM), its concepts such as the basics of SEM as well as the Structure and Working of SEM are carefully analyzed.

The SEM uses focused beams of many electrons to create the specimen image and gain information regarding its composition and structure. The most basic steps involved in the SEM are: A unique stream of strong electrons are produced by the electron guns in the high-vacuum chamber, the stream with positive electrical potentials, is then accelerated towards a solid specimen while being focused and confined using magnetic lenses and metal apertures into a very thin, highly focused, and monochromatic beam. The solid sample is highly irradiated by this beam and interactions quickly occur inside this highly irradiated sample thus affecting the beam of electrons. These effects and unique interactions are detected and quickly transformed into the specimen image.

The structure of SEM comprises many parts, which work together, to achieve the overall objectives of this machine. The first and most basic part of SEM structure is the Electron Gun. This part is paramount as it is the only source of the electrons in SEM. In most cases, Electron Gun is a unique v-shaped filament manufactured using tungsten or LAB6 (lanthanum hexaboride) cathodes that are perfectly wreathed with the Wehnelt cap also referred to as Wehnelt electrode (Patrick 2009). The other part of the SEM structure is the condenser lenses. It has two-condenser lenses referred to as first and second condenser lens respectively, which are used to condense the electron beams flowing from the electron gun. In addition, it has deflection coils, which are used to deflect the electron beams flowing from the electron gun. Objective lenses are also found in the SEM structure. In addition, it has electron detectors that are used to detect various types of signals such as X-ray detectors, and Backscatter electron detectors. It also has a vacuum pump which is used to create a vacuum in the SEM. Vacuum in the specimen’s chamber creation is crucial because it aids in the only specimens pretreatments necessary for this process which is the metal coating of the specimens. This unique procedure is only done in a complete vacuum evaporator and lasts for approximately 15 minutes.

The standard working of SEM is unique and intriguing. A finely and highly-focused beam of electrons scanned across the entire sample surface results in generation of back-scattered electrons, characteristic X-rays, and secondary electrons. These electron signals are thoroughly collected by electron detectors in order to form clear images of the specimen displayed on the screens made of cathode ray tubes (Goldstein, Newbury, & Lifshin, 1981). All the features obtained from the SEM images are then analyzed to determine their elemental compositions and other features. Data output in SEM is carefully generated in the real time on the Cathode Ray Tube (CRT) monitor. The spectra and images obtained in the screen scan be easily printed, recorded, or even emailed.

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