Editing Scanning electron microscope (section)

==Scanning process and image formation==
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[[File:Schema MEB (en).svg|thumb|350px|Schematic of an SEM]]

In a typical SEM, an electron beam is [[thermionically]] emitted from an [[electron gun]] fitted with a tungsten filament [[cathode]]. Tungsten is normally used in thermionic electron guns because it has the highest melting point and lowest vapor pressure of all metals, thereby allowing it to be electrically heated for electron emission, and because of its low cost. Other types of electron emitters include [[lanthanum hexaboride]] ({{chem|LaB|6}}) cathodes, which can be used in a standard tungsten filament SEM if the vacuum system is upgraded, or field emission guns (FEG), which may be of the [[cold-cathode]] type using tungsten single crystal emitters or the thermally assisted [[Walter H. Schottky|Schottky]] type, that use emitters of tungsten single crystals coated in [[zirconium oxide]].

The electron beam, which typically has an [[energy]] ranging from 0.2 [[electronvolt|keV]] to 40 keV, is focused by one or two condenser lenses to a spot about 0.4&nbsp;nm to 5&nbsp;nm in diameter. The beam passes through pairs of [[scanning coils]] or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the ''x'' and ''y'' axes so that it scans in a [[Raster scan|raster]] fashion over a rectangular area of the sample surface.

[[File:Electron emission mechanisms.svg|thumb|Mechanisms of emission of secondary electrons, backscattered electrons, and characteristic X-rays from atoms of the sample]]

When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the '''interaction volume''', which extends from less than 100&nbsp;nm to approximately 5&nbsp;μm into the surface. The size of the interaction volume depends on the electron's landing energy, the atomic number of the specimen, and the specimen's density. The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering, the emission of secondary electrons by [[inelastic scattering]], and the emission of [[electromagnetic radiation]], each of which can be detected by specialized detectors. The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current. [[Electronics|Electronic amplifiers]] of various types are used to amplify the signals, which are displayed as variations in brightness on a computer monitor (or, for vintage models, on a [[cathode-ray tube]]). Each pixel of computer video memory is synchronized with the position of the beam on the specimen in the microscope, and the resulting image is, therefore, a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. Older microscopes captured images on film, but most modern instruments collect [[digital images]].

[[File:LT-SEM snow crystal magnification series-3.jpg|thumb|left|upright|Low-temperature SEM magnification series for a [[snow]] crystal. The crystals are captured, stored, and sputter-coated with platinum at cryogenic temperatures for imaging.]]

===Magnification===
Magnification in an SEM can be controlled over a range of about 6 [[order of magnitude|orders of magnitude]] from about 10 to 3,000,000 times.<ref>{{cite web| url = https://www.hitachi-hightech.com/eu/product_detail/?pn=em-su9000&version=#productSub-1| title = Ultra-high Resolution Scanning Electron Microscope SU9000}}</ref> Unlike optical and transmission electron microscopes, image magnification in an SEM is not a function of the power of the [[objective (optics)|objective lens]]. SEMs may have [[condenser (microscope)|condenser]] and objective lenses, but their function is to focus the beam to a spot, and not to image the specimen. Provided the electron gun can generate a beam with a sufficiently small diameter, an SEM could in principle work entirely without condenser or objective lenses. However, it might not be very versatile or achieve very high resolution. In an SEM, as in [[scanning probe microscopy]], magnification results from the ratio of the raster on the display device and dimensions of the raster on the specimen. Assuming that the display screen has a fixed size, higher magnification results from reducing the size of the raster on the specimen, and vice versa. Magnification is therefore controlled by the current supplied to the x, y scanning coils, or the voltage supplied to the x, y deflector plates, and not by objective lens power.