Liquid metal sources

Liquid Metal Sources. The source feed is a metal of low melting point - Ga and In are commonly employed. It is introduced as a liquid film flowing over a needle towards the tip whose radius is relatively blunt (10 pm). The electrostatic and surface tension forces form the liquid into a sharp point known as the Taylor cone. Here the high electric field is sufficient to allow an electron to tunnel from the atom to the surface, leaving the atom ionized. 

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. 

The most commonly used ion sources include duoplasmatrons, which are used to obtain an intense beam of micron size, and liquid metal sources, with a lower output level but providing a probe and thus a lateral resolution of a few nanometres. 

Figure 9 Comparison of In D (calculated) versus In D (measured) with data for cobalt n = 207) and tungsten n = 109), and using oxide-component-based compositional terms. Open symbols are 0.1 MPa experiments, and solid symbols are higher-pressure experiments. Dashed lines are 2cr errors on the regressions. The number of experiments used in the regression is indicated as n. M/LS refers to metal/liquid silicate for the partition coefficients, and includes experiments that have solid metal or liquid metal (source Righter and Drake, 1999).

Figure 14.12 Vaporization and surface tension y both involve breaking bonds, modelled through the quantity u AA- The slope of y x area (liquid metals. Source redrawn from SH Overbury, PA Bertrand and GA Somorjai, Chem Rev 75, S47-560 (1975). 

Record spectra using a pulse (1 ns, 12 kHz) liquid metal source ( Ga, 15 keV) operating in the bunched mode to provide good mass resolution (m/Am = 2000 measured at m/z 43). 

Computers will be integrated more and more into commercial SEMs and there is an enormous potential for the growth of computer supported applications. At the same time, related instruments will be developed and extended, such as the scanning ion microscope, which uses liquid-metal ion sources to produce finely focused ion beams that can produce SEs and secondary ions for image generation. The contrast mechanisms that are exhibited in these instruments can provide new insights into materials analysis. 

Fig. 3.2. Schematic diagram ofthe design and operation of a liquid-metal ion source (LMIS) [3.7] (a) metal ions (b) extractor (c) liquid-metal film (d) capillary tube (e) liquid metal (f) needle.

Element mapping with non-resonant laser- SNM S can be used to investigate the structure of electronic devices and to locate defects and microcontaminants [3.114]. Typical SNMS maps for a GaAs test pattern are shown in Fig. 3.43. In the subscript of each map the maximum number of counts obtained in one pixel is given. The images were acquired by use of a 25-keV Ga" liquid metal ion source with a spot size of approximately 150-200 nm. For the given images only 1.5 % of a monolayer was consumed -"static SNMS". 

The Centralized Reliability Data Organization (CREDO) is maintained at ORNL to provide a central computer-based source of accurate, timely data and information for use in reliability, availability and maintainability analyses of liquid metal reactors (LMRs). CREDO is a component-based system, that addresses a comprehensive list of 45 genetic components that are representative of all components found at LMRs. 

A minor source of metallic Ca is the recovery of the Ca that crystallizes from the liq Na produced in the electrochemical cell. This can be effected either by filtration of the liquid metal or by aleohol leaching of the residual Na-Ca sludge. 

As to expected performance, Figure 8.3 shows some (conservative) values for lateral and in-depth resolutions for various physical methods of analysis. Recent developments already allow better performance, e.g. a lateral resolution of about 0.1 irn with liquid metal ion sources in SIMS and small beams of 150 xm in diameter in [xXPS. 

Surface Ionization Sources. In this system, a low ionization potential atom (e.g. caesium) is adsorbed on a high work function metal (e.g. iridium). The temperature is raised so that the rate of desorption exceeds the rate of arrival of the atoms at the surface, and the Cs is then desorped as ions with very small energy spread (< 1 eY). The spot size - current characteristics of these sources lie between liquid metal and plasma discharge sources. 

Figure 4.3. Diagram of the extraction region of a liquid metal ion source.

Modern theories of electronic structure at a metal surface, which have proved their accuracy for bare metal surfaces, have now been applied to the calculation of electron density profiles in the presence of adsorbed species or other external sources of potential. The spillover of the negative (electronic) charge density from the positive (ionic) background and the overlap of the former with the electrolyte are the crucial effects. Self-consistent calculations, in which the electronic kinetic energy is correctly taken into account, may have to replace the simpler density-functional treatments which have been used most often. The situation for liquid metals, for which the density profile for the positive (ionic) charge density is required, is not as satisfactory as for solid metals, for which the crystal structure is known. 

SPMs such as AFM, FFM, and SSPM were performed with a SEIKO SPA-300 unit together with an SPI-3700 control station. Details for fluorescence microscopy by SNOAM based on a modified SEIKO SPA-300 unit with an SPI-3700 control station were reported previously [27-33]. Conventional fluorescence microscopy was carried out with a Nikon XF-EFD2 fluorescence microscope [40]. SIMS was performed with a Perkin Elmer PHI model 6600 SIMS system with a Ga liquid metal ion source (beam diameter ca. 80 nm). For mapping of F negative secondary ions, a width of ca. 50 pm was scanned with 256 lines. 

Alex Zettl and his colleagues at the University of California, Berkeley, have been active in this field. In 2005, Zettl and his team built an oscillator that is one of the smallest electric motors in the world. The motor works by driving some of the atoms of a tiny drop of liquid metal over to an adjacent and smaller drop. An electric current powers this movement. The most interesting phase of the operation occurs during the rebound, as the atoms return to the original source. This phase happens because of a molecular force known as surface tension. 

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