Plasma discharges

Lieberman M A and Liohtenberg A J 1994 Principles of Plasma Discharges and Materials Processing (New York Wiley)... 

The multiple energetic collisions cause molecules to break apart, eventually to form only atoms, both charged and neutral. Insertion of sample molecules into a plasma discharge, which has an applied high-frequency electric field, causes the molecules to be rapidly broken down into electronically excited ions for all of the original component atoms. 

This is the basic process in an inductively coupled plasma discharge (ICP). The excited ions can be examined by observing the emitted light or by mass spectrometry. Since the molecules have been broken down into their constituent atoms (as ions) including isotopes, these can be identified and quantified by mass spectrometry, as happens with isotope ratio measurements. 

Carbon with its wide range of sp bond hybridisation appears as the key element of a future nanotechnology. However, so far there is almost no control over the formation processes, and the structures of interest cannot be built at will. Tubes, for example, are produced under the very virulent conditions of a plasma discharge and one would like to have more elegant tools to manipulate the carbon structures, a task which remains a challenge for the future. 

In plasma-assisted CVD, an electrostatically or electromagnetically induced plasma discharge is carried out in a low pressure system. The result is that the process may be operated at a considerably lower temperature. This has been employed in the deposition of SiOj and Si3N4 in the production of heat-sensitive microelectronic circuits. 

Fakes DW, Newton JM, Watts JF, and Edgel MJ. Surface modification of contact lens co-pol3Tner by plasma-discharge treatments. Surf Interf Anal, 1987, 10, 416. 

M.A. Lieberman and A.J. Lichtenberg, Principles of plasma discharges and materials processing , John Wiley Sons, Inc., 1994. 

Type of plasma Discharge power Gas pressure Treatment time... 

The radio-frequency glow-discharge method [30-34] has been the most used method in the study of a-C H films. In this chapter, it is referred to as RFPECVD (radio frequency plasma enhanced chemical vapor deposition). Film deposition by RFPECVD is usually performed in a parallel-plate reactor, as shown in Figure 1. The plasma discharge is established between an RF-powered electrode and the other one, which is maintained at ground potential. The hydrocarbon gas or vapor is fed at a controlled flow to the reactor, which is previously evacuated to background pressures below lO"" Torr. The RF power is fed to the substrate electrode... 

ICP-MS Inductively coupled plasma Argon plasma discharge... 

Sample preparation for the common desorption/ionisation (DI) methods varies greatly. Films of solid inorganic or organic samples may be analysed with DI mass spectrometry, but sample preparation as a solution for LSIMS and FAB is far more common. The sample molecules are dissolved in a low-vapour-pressure liquid solvent - usually glycerol or nitrobenzyl alcohol. Other solvents have also been used for more specialised applications. Key requirements for the solvent matrix are sample solubility, low solvent volatility and muted acid - base or redox reactivity. In FAB and LSIMS, the special art of sample preparation in the selection of a solvent matrix, and then manipulation of the mass spectral data afterwards to minimise its contribution, still predominates. Incident particles in FAB and LSIMS are generated in filament ionisation sources or plasma discharge sources. 

The main detectors used in AES today are photomultiplier tubes (PMTs), photodiode arrays (PDAs), charge-coupled devices (CCDs), and vidicons, image dissectors, and charge-injection detectors (CIDs). An innovative CCD detector for AES has been described [147]. New developments are the array detector AES. With modem multichannel echelle spectral analysers it is possible to analyse any luminous event (flash, spark, laser-induced plasma, discharge) instantly. Considering the complexity of emission spectra, the importance of spectral resolution cannot be overemphasised. Table 8.25 shows some typical spectral emission lines of some common elements. Atomic plasma emission sources can act as chromatographic detectors, e.g. GC-AED (see Chapter 4). 

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. 

Fig. 20. SIMS profiles of total deuterium density across p-n junctions formed by implanting phosphorus into a (100) silicon water uniformly doped with 1 x 1017 boron atoms per cm3 for various times of deuteration at 150°C (Johnson, 1986a). The phosphorus profile is also shown and serves to locate the pre-deuteration depth of the junction at 0.5 Deuteration was from downstream gases from a plasma discharge (Johnson and Moyer, 1985).

Figure 21 shows, for specimens similar to those of Fig. 20, the SIMS profiles of total deuterium and the distribution of deuterium taking part in acceptor passivation as obtained by C-V profiling for two conditions of deuteration. Both involved one hour exposure to plasma discharge products at 200°C, but one was with zero bias applied to the junction during this time, the other with 10 V reverse bias. The main features of these... 

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