Although the total current is lower than either the tungsten or the LaB 6 emitters, the current density is between 103 and 106 A/cm, making it hundreds of times brighter than a thermionic emission source. When the cathode forms a very sharp tip (typically 100 nm or less) and the cathode is placed at a negative potential with respect to the anode, so that the local field at the tip is very strong (greater than 107 V/cm), electrons can tunnel through the potential barrier and become free. al.).Īnother method for generating electrons is the field emission gun. Gun configuration for a LaB6 cathode (after Goldstein et. A schematic of the LaB 6 filament is shown in Figure 2. LaB 6 filaments tend to be an order of magnitude more expensive than tungsten filaments. The LaB 6 filament operates at approximately 2125☌, resulting in a brightness on the order of five times brighter than a tungsten filament under the same conditions. Lathanum hexaboride, commonly known as LaB 6, has been the best material developed to date for this application. A lower work function means more electrons at a given temperature, hence a brighter filament and higher resolution. The most straightforward method to achieve this goal is to find a material with a lower work function Ew. al.).Īs the need for higher resolution imaging increased, so did the need for brighter filaments. Schematic of a tungsten filament (self-biased) electron gun (after Goldstein et. A schematic diagram of a tungsten cathode is shown below (see Figure 1). At an emission of 1.75 A/cm 2, the lifetime of the tungsten filament is approximately 50 hours. With a bias voltage between 0 and 500 volts, the electrons can be accelerated toward the anode. The electrons will have a potential distribution of 0 to 2 volts. At this temperature, one can expect a current density of approximately 1.75 A/cm 2. The tip is heated to around 2400☌ by passing current through it. The tungsten cathode is a fine wire approximately 100mm in diameter that has been bent into the shape of a hairpin with a V-shaped tip. The three electron guns are described briefly below. the lanthanum hexaboride (LaB 6) cathode, and 3. There are basically three types of electron guns on today's transmission electron microscopes: 1. The TEM is probably one of the best positioned tools for examination of semiconductor structures for the foreseeable future. This trend is likely to continue as feature sizes shrink. Hirsh and his colleagues at Cambridge then developed the theory of electron diffraction contrast and documented his work in a large volume sometimes referred to as the "TEM Bible." The use of TEM for materials science work and semiconductor analysis has increased rapidly in the past two decades. In the late 1940's, Heidenreich first thinned specimens to electron transparency. After World War II, several manufacturers, including Hitachi, JEOL, Philips, and RCA began manufacturing electron microscopes. In 1936, the first electron microscope was built by Metropolitan-Vickers in the U.K. HistoryĪfter the theory of the wave nature of electrons was developed by Louis DeBroglie in 1925, researchers Knoll and Ruska proposed the idea of an electron microscope. Magnifications from about 100x up to several hundred thousand times can be achieved in the TEM. Dark-field images are formed from a single diffracted beam and are used to identify all the areas of a particular phase having the same crystalline orientation. An electron diffraction pattern can be generated from a particular area in a bright-field image (such as a particle or grain) by using a selected area aperture. Features in the sample that cause scattering have darker contrast in a bright-field image than those that cause little or no scattering. A bright-field image of the sample can be formed by looking at the straight-through, non-diffracted beam. These diffraction patterns can be used to identify unknown phases in the sample. In crystalline materials, the scattering takes the form of one or more Bragg diffracted beams, which are used to form a transmission diffraction pattern. The sample is illuminated with an almost parallel electron beam, which is scattered by the sample. Apertures are used to select among imaging modes and to select features of interest for electron diffraction work. The electron microscope consists of an electron gun, or source, and an assembly of magnetic lenses for focusing the electron beam. What is Transmission Electron Microscopy? OverviewĪ TEM (transmission electron microscope) uses a highly energetic electron beam (100 keV - 1 MeV) to image and obtain structural information from thin film samples. Semitracks' Online Training contains short courses and other material useful for any Engineer trying to learn new subjects or brush up on old ones. Are you looking for more information regarding this and other subjects? Look no further than Semitracks' Online Training.
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