Field-emission sources

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 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... 

In order to increase the intensity and focus of electrons, a field emission source may be used. This consists of a single crystal tungsten or LaBeClOO) wire that is sharpened to a tip diameter of ca. 100 nm — 1 j,m. For crystalline tungsten, the axis is suitably aligned with respect to the optical axis of the microscope. For example, a beam with a diameter < 5 nm is possible from alignment of the filament planes perpendicular to (310) and (111). In addition to W and LaBe, a number of other materials are proposed for field emission applications, such as silicon, single-walled nanotubes,and ultrananocrystalline diamond (UNCD) or Cu/Li alloy films deposited onto sharpened tips.t ... 

It is well known that the radial breathing mode frequency is inversely related to tube diameter. For as-grown DWNTs (Fig. 9D), Raman peaks appear above 250 cm (corresponding to the inner shells of DWNTs), and below 250 cm (usually associated with the outer shells of DWNTs). We envisage this material to be useful in the fabrication of novel sensors, nanocomposites, field emission sources, nanobearings, nanotube bi-cables, and electronic devices. 

The SEM used for the work is a specially adapted ultra-high vacuum field emission source instrument (V.G. Microscopes Ltd.). 

Figure 1. Modes of operation of the cerl field emission source SEM with environmental cell. Key top, scanning electron microscopy, Auger electron spectroscopy, and argon ion etching and bottom, gas reaction cell configuration.

The composition of the deposited solder was measured by energy dispersive x-ray measurements using standards in a scanning electron microscope. For each deposit, four square regions measuring 1.5 mm per side were sampled. A scanning electron microscope with a field emission source was employed for the micrographs of the samples. X-ray diffraction measurements were also made on selected samples, in order to determine the structure and orientation of the deposits. 

Field emission source brightnesses are about 10 ° A m sr maximum current 0.1 pA. 

Thus we see that the field emission source permits many orders of magnitude more counts in any signal than the other sources, if ideal lenses are employed. In all cases, raising the acceleration voltage can increase the intrinsic brightness, by skewing the electron velocity distribution more along the optical axis. 

In summary, to produce a useful probe of subnanometer size, a high-brightness gun is essential together with optimised use of the probe-forming aperture to limit the aberration effects. Among the choice of the various electron sources, the field emission gun stands out. The penalty to be paid for a field emission source is the necessity to use ultra-high vacuum techniques. Such instruments were usually restricted to the dedicated STEM , but nowadays field emission sources are also popular in analytical TEMs. The improvement in the latter has blurred the distinction between dedicated STEM and TEM-STEM . 

Just as in ordinary vacuum technology the primary instrument for leak detection in a glass ultrahigh vacuum system is the Tesla coil. Care is necessary in sparking graded seals. In the vicinity of field emission sources, the induction coil should not be employed since the emitter may be destroyed by arcing. 

McCord and R.E.W. Pease, High resolution, low voltage probes from a field emission source close to the target plane, J. Vac. Sci. Technol. B 3, 198 (1985). 

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