There are three basic types of electron microscope, they are the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM) and Electron Microprobe (EMPA). These machines differ in the way they are constructed and the purpose for which they are used, however, they are all electron beam based instrumentation that work on similar principles.
The Transmission Electron Microscope (TEM) operation is the easiest in principle to understand. The TEM uses an electron beam and lenses (condenser lenses) to illuminate the sample, and the image is formed by focussing and collecting the electrons with another series of lenses (intermediate and objective lenses). The image in its simplest form is equivalent to a transmitted image from an optical microscope that many people have used in there school years. One of the large advantages of a TEM is that with the aid of the imaging lenses it is also possible to view the electrons diffracted by the sample and thus giving the user access to information about crystal structure. The resolution of a TEM is in the order of angstroms which ideally enables atomic resolution. This level of resolution is of course dependent on the sample; and it may not necessarily be achieved if the electron beam interaction with the sample is insufficient and there is insufficient contrast created. TEM's also can have attached to them Energy Dispersive X -ray (EDX) detects or Electron Energy Loss Spectrometers (EELS) and Image intensifier systems
The Scanning Electron Microscope (SEM) uses a similar system to the TEM to illuminate the sample with electrons, however the image forming system is more akin to the average television. To form an image in an SEM, the focused electron beam is driven across the sample in typically a square raster pattern. As the electron beam scans across the specimen a at each point where the electron beam interacts with the sample a variety of signals are produced these signals are collected and the magnitude of the signal is used to control the brightness of the viewing screen. This of course will only create an image if the viewing screen is scanningat exactly the same rate and is synchronised in relative position with the beam scanning the sample. Magnification in this instance is achieved, not by virtue of lenses but by ensuring that the absolute area scanned on the sample is smaller than the dimension of the scan used on the viewing screen. The resolution of an SEM is in the order of nanometres compared to an optical microscope resolution of micrometres. SEM's can also have attached to them Energy Dispersive X-ray (EDX) detectors, Wavelength Dispersive X-ray (WDX) detectors, Electron Scattering Backscattered Detectors (ESBD), Cathodoluminescence detectors and many more.
The Electron Microprobe Analyser (EMPA) is similar to an SEM, in that it uses a scanning system to form its images, however the EMPA is optimized for best performance at high electron beam currents of micro amperes (uA) rather than nano ampere (nA) currents used to image in the SEM. Similarly the EMPA also includes an optical microscope system for viewing the samples and is optimized to suit the detector geometry of the more sensitive WDX detectors. EMPA will also have attached EDX detectors.
Electron microscopes can utilize a number of systems to generate the electrons for the electron beam, the most commonly used system is a triode ion gun that incorporates a tungsten filament and uses thermionic emission, this has the advantage of being able to provide high electron beam currents and as such is used in the EMPA systems. However to improve imaging resolution at higher magnifications it is advantages to have more electrons originating from a smaller source, one system commonly employed to do this is the Lanthanum Hexa-boride (LaB6) gun, where emission is obtained from the tip of a single crystal of LaB6, or the field emission gun (FEG) which extracts their electrons from the tip of a single crystal of tungsten coated with zirconium oxide which lowers the work function and enables greater electron emission. The FEG has a brightness orders of magnitude greater than a tungsten emitter, whichimproves the signal to noise ratio of the image at higher resolutions.
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