Electron source size

Table 3.1 Electron source size, electron energy in the storage ring and maximum operating current for some synchrotron facilities where IR microspectroscopy beamlines are under operation or construction ( ) of low fS values...

Synchrotron center Energy (GeV) Maximum operating current (mA) Horizontal electron source size (p.m) Vertical electron source size (p.m)... 

For a somewhat more accurate estimate of the apparent source size in particular emission and observation conditions, numerical methods of Fourier optics can be used. In this framework, the effective source size can be obtained either by backpropagation of the wavefront (at a specific wavenumber) to the source position, or by simulating the radiation focusing at optical magnification equal to 1. To illustrate this, we have considered two cases an IR beamline at the NSLS (0.8 GeV storage ring) normalized at 1000 mA (electron source size = 550 p,m horizontal, 70 (im vertical) and an IR beamline at SOFFIT, (2.75 GeV - electron source... 

Mechanical Stability.—Finally, we note mechanical constraints on system construction which may be more severe in the case of STEM than CTEM. In either type of instrument the user is accustomed to extreme mechanical stability in the specimen stage region exposure times measured in seconds at resolutions of 2 A or so require drift and vibrational stability in the stage at a commensurate level. In the STEM, however, an extra constraint exists. As a consequence of passing through an aperture the emitted electron beam from the field emission tip, the effective electron source size... 

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 irradiating X-ray beam cannot be focussed upon and scanned across the specimen surface as is possible with an electron beam. Practical methods of small-spot XPS imaging rely on restriction of the source size or the analysed area. By using a focussing crystal monochromator for the X-rays, beam sizes of less than 10 pm may be achieved. This must in turn correspond with the acceptance area and alignment on the sample of the electron spectrometer, which involves the use of an electron lens of low aberration. The practically achievable spatial resolution is rarely better than 100 pm. A spatial resolution value of 200 pm might be regarded as typical, and it must also be remembered that areas of up to several millimetres in diameter can readily be analysed. 

CNTs are also valuable as field emitters because they have a small virtual source size [30], a high brightness, and a small positive temperature coefficient of resistance [31]. The latter means that they can run hot under high emission currents, but not go into thermal runaway. Emission from nanotubes can be visualized by electron holography in a TEM [32],... 

Tab. 13.1 Performances of different types of electron guns source size (r), reduced brightness (B), energy spread (AE), stability, and operating temperature.

Consider the following experiment. An electron source emits, at fixed known time t0, electrons with an uncertainty in position Ajcq 100A, which, according to Heisenberg uncertainty relations, corresponds to a minimum velocity dispersion Avo 10 6 cm. A millisecond later, the initial wavepacket will have spread to a size of about 10 m. Let us now suppose that from each electron wavepacket one cuts a piece of size 8xq 100 A (see Fig. 25). 

Two types of ion source produce high enough brightness (> 106 A/cm2.ster., 20 keV) for them to be considered for semiconductor fabrication applications the field ion source (56) and the liquid metal source (57,58). The field ion source produces relatively small energy spread (<3 eV) and when combined with a short focal length (< 1 cm) electrostatic focusing system should be able to produce beam sizes as small as 10 nm with adequate current (10-11 amp) for laboratory microfabrication experiments. As with field emission electron sources, the field ion source only produces a limited total current and the maximum beam current is limited to about 1 10 amp. 

Consider a system consisting of a metal corroding in an electrolyte. The corrosion process involves a metal-dissolution deelectronation (anodic) reaction at electron-sink areas on the metal and an electronation (cathodic) reaction at electron-source areas. (This picture is applicable to a metal s corroding by a Wagner-Traud mechanism provided one imagines the sink and source areas shrunk to atomic-sized dimensions and considers the situation at one instant of time.)... 

To illustrate the importance of emission source, synthetic goethite particles deposited on 0.1 -pm pore-size polycarbonate filters (150,000 x mag.) were imaged using both a JEOL 6310 and a JEOL 6320F microscope equipped with thermoionic LaB6 and field-emission (EE) electron sources, respectively (Figure 11.8). The... 

Specimen illumination. To obtain a relatively coherent electron beam a small source size is required. Thus, a small condenser aperture is used, and LaBe and pointed filaments are preferable to the standard hairpin filament, although not essential. The illumination is focused onto the specimen using the second condenser lens, and exposure times should be only a few seconds. 

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