In samples predominantly composed of light elements, such as biological specimens, BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, which would otherwise be difficult or impossible to detect in secondary electron images. BSE images can provide information about the distribution, but not the identity, of different elements in the sample. However, BSE are often used in analytical SEM, along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. Since they have much higher energy than SEs, they emerge from deeper locations within the specimen and, consequently, the resolution of BSE images is less than SE images. Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. The signal from secondary electrons tends to be highly localized at the point of impact of the primary electron beam, making it possible to collect images of the sample surface with a resolution of below 1 nm. Consequently, SEs can only escape from the top few nanometers of the surface of a sample. Secondary electrons have very low energies on the order of 50 eV, which limits their mean free path in solid matter. Secondary electron detectors are standard equipment in all SEMs, but it is rare for a single machine to have detectors for all other possible signals. Various types of signals are produced including secondary electrons (SE), reflected or back-scattered electrons (BSE), characteristic X-rays and light ( cathodoluminescence) (CL), absorbed current (specimen current) and transmitted electrons. The signals used by a SEM to produce an image result from interactions of the electron beam with atoms at various depths within the sample. Principles and capacities Schottky-emitter electron source Electron–matter interaction volume and types of signal generated Further work was reported by Zworykin's group, followed by the Cambridge groups in the 1950s and early 1960s 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, which was delivered to DuPont. He further discussed the various detection modes, possibilities and theory of SEM, together with the construction of the first high resolution SEM. Ardenne applied scanning of the electron beam in an attempt to surpass the resolution of the transmission electron microscope (TEM), as well as to mitigate substantial problems with chromatic aberration inherent to real imaging in the TEM. 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 a microscope with high resolution by scanning a very small raster with a demagnified and finely focused electron beam. History Īn account of the early history of scanning electron microscopy has been presented by McMullan. Specimens are observed in high vacuum in a conventional SEM, or in low vacuum or wet conditions in a variable pressure or environmental SEM, and at a wide range of cryogenic or elevated temperatures with specialized instruments. Some SEMs can achieve resolutions better than 1 nanometer. The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector ( Everhart–Thornley detector). The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. von Ardenne's first SEM Operating principle of a Scanning Electron Microscope (SEM) SEM with opened sample chamber Analog type SEMĪ scanning electron microscope ( SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. Image of pollen grains taken on a SEM shows the characteristic depth of field of SEM micrographs M. Not to be confused with Scanning tunneling microscope.
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