Tungsten electron sources

Operating conditions Electron sources Tungsten, LaB6, PEG Tungsten, LaB, FEG... 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

There are three major types of electron sources thermionic tungsten, LaBg, and hot and cold field emission. In the first case, a tungsten filament is heated to allow... 

Using an electron-gun source, tungsten atoms were reacted with benzene, toluene, or mesitylene at 77 K, to form the expected (arene)2W complex (42) in a yield of 30%, compared with the —2% yield from the previously published, bis(benzene)W synthesis (32). These arene complexes are reversibly protonated, to give the appropriate [(T7-arene)2WH] species. By using the same technique, the analogous, niobium complexes were isolated (43). 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

An electron gun (see Section 5.1.3) provides the requisite electron source for AES, and may consist of a tungsten or a LaB6 cathode, or a Field Emission source. The latter provide the brightest beams, and beam widths as narrow as lOnm permit... 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

Figure 14.5. Representative plots of the contrast transfer function (CTF) as a function of spatial frequency, for two different defocus settings (0.7 and 4.0 fxm underfocus) and for a field emission (light curve) or tungsten (dark curve) electron source. AH plots correspond to electron images formed in an electron microscope operated at 200 kV and with objective lens aberration coefficients, Cg =...

An electron source. or electron gun traditionally the most frequent is the triode gun comprising a tungsten wire cathode heated to around 2 700 K. The electrons emitted by the cathode are accelerated by the electric field set up between this negatively polarised filament and the earthed anode. This gun can provide an electron beam of 30 keV with a current density per unit solid angle (also called bright-... 

The electron probe is the reduced image of an electron source supplied by a gun. The tungsten filament thermionic emission gun is currently the most suitable source for X-ray microanalysis. It is not very expensive to run, can be easily aligned and offers satisfactory stability. It delivers high current intensities (1 to 1000 nA) for probe diameters of the order of a micrometre. 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W)... 

Electron sources used in EBL exposure tools are similar to those used in conventional electron microscopes. They can be divided into two main groups— thermionic or field emission—depending on the way in which they emit electrons. The sources that rely on the emission of electrons from a material that is heated to a temperature at which electrons are emitted from the surface are referred to as thermionic sources. These sources are fabricated from materials such as tungsten, thoriated tungsten, or lanthanum hexaboride (LaBe). ... 

Table 2 summarizes some important characteristics of three electron sources currently used in SEMs. Clearly the tungsten field emitter is most attractive, due to its high brightness and low energy spread. There are, however, a number of drawbacks associated with this type of source. In the first place, a gun vacuum of 10 torr is required, which can result in problems when examining a specimen which outgasses. The emission current is unstable. The maximum... 

For the primary energy range 5-10 keV used in conventional AES, the electron emitter is thermionic, usually a hot tungsten filament, and focusing of the electron beam is carried out electrostatically. Typically, such an electron source would be able to provide a spot size on the specimen of about 0.5 pm at 10 keV and a beam current of about 10 A. The beam can normally be rastered over the specimen surface, but such a source would not be regarded as adequate for SAM. Sources for SAM may be of either the thermionic or the field emission type, the latter being partic-... 

Thermionic cathodes consist of a directly heated tungsten hairpin cathode at = 2500 -3000 K, or an indirectly heated pointed rod of lanthanum or cerium hexaboride (LaB, CeB(,) at 1400 - 2000 K. The electrons must overcome the work function of 4.5 eV (W) or 2.7 eV (LaBfe) by thermal activation (Fig. 78, curve a). Between the cathode at the potential -V and the grounded anode, a negatively biased Wehnelt electrode forms a crossover of diameter 20-50 pm (W) or 10-20 pm (LaBe) as an effective electron source. The emitted electrons show an energy spread A = 1 - 2 eV (W) or 0.5- 1 eV (LaBft). A measure of the quality of an electron gun is the axial gun brightness [i ... 

Schottky emission cathodes consist of zirconium-doped tungsten tips, with a radius of about 0.5 -1 pm, coated with a ZrO layer. This layer decreases the work function from 4.5 to 2.7 eV. A Schottky emission cathode works with a higher electric field strength at the tip that decreases the work function by A

Schottky effect. Fig. 78. curve b) and concentrates the emission at the tip with a virtual electron source having a diameter of 15-20 nm. However, the electrons still have to... 

Electron Gun and Optics. The electron source is usually a tungsten filament source, although field emission guns ate also employed for high-resolution work. The electrons are accelerated to an energy between 1 and... 

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