Electron emission sources

The Lanthanum Hexaboride Electron Emission Source and the Vacuum Environment, Amray Technical Bulletin 112-277, Amray, Bedford, Mass., 1986, pp. 11-18. 

Some XPS valence band spectra of thick insulating samples have been obtained using a low energy electron emission source (flood gun) to neutralize the positive surface charge. This... 

Milne Wl, Teo KBK, ChhowaUa M, Amaratunga GAJ, Yuan J, Robertson J, et al. Investigating carbon materials for use as the electron emission source in a parallel electron-beam lithography system. Curr Appl Phys 2001 1 317-20. 

Division of Chemistry and Molecular Engineering and Center for Electro-and Photo Responsive Molecules, Korea University, Seoul 136-701 Korea (fax +82 2 921 6901 telephone +82 2 3290 3123 email jijin korea.ac.kr) School of Physics, Seoul National University, Seoul 151-747, Korea NCRI, Center for Electron Emission Source, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea... 

Solid state metal borides are characteristically extremely hard, involatUe, high melting and chemically inert materials which are industrially important with uses as refractory materials and in rocket cones and turbine blades, i.e. components that must withstand extreme stress, shock and high temperatures. The borides LaBg and CeB are excellent thermionic electron emission sources, and single crystals are used as cathode materials in electron microscopes (see Box 13.8). 

It should be noted that the work function of structural alloys is always less than the work function of the isolated component metals. For instance, the work functions of B and La are 4.5 and 3.3 eV, respectively - much higher than LaBg. The mechanisms responsible for the reduction of the work function are not completely understood, although it must depend on the three-dimensional arrangement and orbital overlap of electron rich/deficient atoms or ions near the surface of the solid. Once this mechanism is deciphered, many more materials will be isolated as effective candidates for electron emission sources. 

In order to measure the field emission characteristics, the Cgo/CNT composite, which was produced by the drawing process, was mounted on a copper grid using an adhesive and a silver paste and was used as an electron emission source (Fig. 14.21). Six readings of the emission current (I) in a sample for the applied voltage (V) were measured in a high vacuum chamber with a base pressure of approximately 6.5 x 10 ° Pa. The distance between the electrodes was fixed at 200 /im using a mica spacer. 

Crewe A V, Eggenberger D N, Wall J and Welter L M 1968 Electron gun using a field emission source Rev. Sol. Instrum. 39 576-86... 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

The overall function of the electron gun is to produce a source of electrons emanating from as small a spot as possible. The lenses act to demagnify this spot and focus it onto a sample. The gun itself produces electron emission from a small area and then demagnifies it initially before presenting it to the lens stack. The actual emission area might be a few pm in diameter and will be focused eventually into a spot as small as 1 or 2 nm on the specimen. 

The STEM instrument itself can produce highly focused high-intensity beams down to 2 A if a field-emission source is used. Such an instrument provides a higher spatial resolution compositional analysis than any other widely used technique, but to capitalize on this requires very thin samples, as stated above. EELS and EDS are the two composition techniques usually found on a STEM, but CL, and even AES are sometimes incorporated. In addition simultaneous crystallographic information can be provided by diffraction, as in the TEM, but with 100 times better spatial resolution. The combination of diffraction techniques and analysis techniques in a TEM or STEM is termed Analytical Electron Microscopy, AEM. A well-equipped analytical TEM or STEM costs well over 1,000,000. 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

These observations consummated in a growth model that confers on the millions of aligned zone 1 nanotubes the role of field emitters, a role they play so effectively that they are the dominant source of electron injection into the plasma. In response, the plasma structure, in which current flow becomes concentrated above zone 1, enhances and sustains the growth of the field emission source —that is, zone 1 nanotubes. A convection cell is set up in order to allow the inert helium gas, which is swept down by collisions with carbon ions toward zone 1, to return to the plasma. The helium flow carries unreacted carbon feedstock out of zone 1, where it can add to the growing zone 2 nanotubes. In the model, it is the size and spacing of these convection cells in the plasma that determine the spacing of the zone 1 columns in a hexagonal lattice. 

Combined Electron Impact and Field Emission Source. ... 

Fig 7 Combined electron impact and field emission source. 

Specimens for field emission sources are of a very fine needle shape, usually in the form of tungsten wire with a tip radius of <0.1 pm (Figure 5.4). Application of a potential of lkV thus generates a field of 106V/m which lowers the work function barrier sufficiently for electrons to tunnel out of the tungsten. FEG electron microscopes usually employ a gun potential of 3-4 keV. 

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... 

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