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Scanning electron microscope
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. The electron beam is generally scanned in a raster scan pattern, and the beam's position is combined with the detected signal to produce an image. SEM can achieve resolution better than 1 nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet conditions (in environmental SEM), and at a wide range of cryogenic or elevated temperatures.
The most common SEM mode is detection of secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons that can be detected depends, among other things, on the angle at which beam meets surface of specimen, i.e. on specimen topography. By scanning the sample and collecting the secondary electrons with a special detector, an image displaying the topography of the surface is created.
==History==
An account of the early history of SEM has been presented by McMullan. Although Max Knoll produced a photo with a 50 mm object-field-width showing channeling contrast by the use of an electron beam scanner, it was Manfred von Ardenne who in 1937 invented〔von Ardenne M. Improvements in electron microscopes. , convention date (Germany) 18 February 1937〕 a true microscope with high magnification by scanning a very small raster with a demagnified and finely focused electron beam. Ardenne applied the scanning principle not only to achieve magnification but also to purposefully eliminate the chromatic aberration otherwise inherent in the electron microscope. He further discussed the various detection modes, possibilities and theory of SEM, together with the construction of the first high magnification SEM. Further work was reported by Zworykin's group,〔Zworykin VA, Hillier J, Snyder RL (1942) A scanning electron microscope. ASTM Bull 117, 15–23.〕 followed by the Cambridge groups in the 1950s and early 1960s〔Oatley CW, Nixon WC, Pease RFW (1965) Scanning electron microscopy. Adv Electronics Electron Phys 21, 181–247.〕〔Wells OC (1957) The construction of a scanning electron microscope and its application to the study of fibres. PhD Dissertation, Cambridge University.〕 headed by Charles Oatley, all of which finally led to the marketing of the first commercial instrument by Cambridge Scientific Instrument Company as the "Stereoscan" in 1965 (delivered to DuPont).
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