Power plasma

An ICP-OES instrument consists of a sample introduction system, a plasma torch, a plasma power supply and impedance matcher, and an optical measurement system (Figure 1). The sample must be introduced into the plasma in a form that can be effectively vaporized and atomized (small droplets of solution, small particles of solid or vapor). The plasma torch confines the plasma to a diameter of about 18 mm. Atoms and ions produced in the plasma are excited and emit light. The intensity of light emitted at wavelengths characteristic of the particular elements of interest is measured and related to the concentration of each element via calibration curves. 

The lower electrode is coupled via a fl-type matching network to a 13.56-MHz generator. This network provides power matching between the RF power cable (50 2) and the plasma. Power levels are between 1 and 100 W, or between 6 and 600 mW/cm, using the area of the powered electrode. 

Of course, care should be taken when comparing the experimental data and the modeling results the model is an approximation, and the experiment has its uncertainties. The most important approximation is that the model is one-dimensional. A main uncertainty of the experiment is the relation between source power and plasma power (see Section 1.3.2.3). The source power is defined as the power delivered by the power generator. In experiments, the source power is a discharge setting (Table IV). The plasma power is the power dissipated by the plasma. As a rule, this plasma power is smaller than the source power, due to losses in the matching network, which matches the plasma impedance to the impedance of the source power generator, viz., 50 [180]. From comparison of experimental data... 

Pressure variation. In Figure 17 are shown the effects of total pressure on the relative pressures (i.e., the ratio of the partial pressure to the total pressure) of silane, hydrogen, and disilane (Fig. 17a) and on the deposition rate (Fig. 17b). The RF frequency is 50 MHz, and the plasma power is 5 W. The relative pressure of hydrogen slowly increases, and the relative pressure of silane slowly decreases, both in model as well as in experiment. This is caused by an increase in silane depletion at higher total pressures, which results from a higher power dissipation... 

FIG. 17. (a) The relative pressures (i.e.. the ratio of the partial pressure to the total pressure) of Ht. SiHa, and SiiHg, and (b) the deposition rate, as a function of total pressure at an RF frequency of 50 MHz and a plasma power of 5 W. Other discharge settings are given in Table IV. Modeling results are in dotted lines and open symbols, experimental data in solid lines and filled symbols. Note the sudden increase at 30 Pa. i.e.. the transition from the a- to the / -regime. (Compiled from G. J. Nienhuis, Ph.D. Thesis. Universiteit Utrecht. Utrecht, the Netherlands. 1998.)... 

The decrease of the silane partial pressure and the concomitant increase of the hydrogen partial pressure as a function of plasma power can be understood in terms of the increased electron density and electron energy. Both lead to a higher dissociation of silane and hydrogen. The silane radicals and atomic hydrogen thus... 

The partial pressure of disilane as a function of plasma power first increases due to the higher production of silane radicals. Above a certain power the increase in disilane dissociation is higher than the increase in disilane production hence the disilane partial pressure decreases again. Similar behavior has been obserbed by Kae-Nuneet al. [217]. 

Figure 3. Deposition rates (D) for organo-silicones, normalized to flow rate (F) and monomer molecular weight (M), as function of plasma power (P). Shaded develope 13.56 MH plasmas data points are for 2.45 (M plasmas at monomer pressure 0.1 Torr (A) 0.2...

Mendez et al. [231] showed in deposition experiments with RF plasma and without substrate bias that there is a coherence of the nano-cBN amount with the substrate temperature and the plasma power. Additionally a dependence on the substrate material used (e.g. Si or NaCl) could be found. Further... 

Fig. 8 Increase of wavelength (filled triangles) and amplitude (open circles) of the wrinkles with increasing plasma dose (oxygen plasma exposure time multiplied by plasma power). The values were evaluated by quantitative FFT-analysis applied to AFM height-images...

These and further economic requirements have dictated the trend towards reducing the reactor size for a given thermal power. In order to achieve an adequate increase of the plasma power density higher values of P (Eq. 8) have to be used (Eq.9). A convenient way to achieve this requirement is to use plasmas of a noncircular cross section (Fig. 7). 

Most reliable are the data on neutron fluxes which are determined by the plasma power density, reactor geometry and structural parameters. Most of the conceptual power reactor design studies contain a detailed neutronics analysis giving the energy as well as the spatial distribution of neutron fluxes. 

As stated above (see Chapter II.B), LII signals also contain information about the size distribution. To compare the influence of different plasma powers on primary particle diameters, different ways of size evaluation have been accomplished. It could be shown by assuming a monodisperse distribution that the mean primary particle diameter is 31 nm for 30 kW and 33 nm for 70 kW. In contrast, under the assumption of a log-normal distribution and by applying the two-decay time evaluation, the determination yields a different result which can be seen in Figure 15. Size distributions with median sizes of 17nm and 28 nm and standard deviations of 0.39 and 0.18 for 30 kW and 70 kW were observed, respectively. This indicates that in practical production systems, the evaluation of a mondisperse distribution is not sufficient. Unfortunately, the reconstruction of particle size distributions is relatively sensitive on... 

Figure 15 Primary particle size distributions for different plasma power (Sommer et al., 2004) (see Plate 18 in Color Plate Section at the end of this book).

If the plasma power is increased, the location where the plasma begins along the center axis moves upstream (closer to the injector tube of the torch) and the plasma temperature increases. Therefore, if the nebulizer gas flow rate were optimized at a power of 1.0 kW and the power were increased to 1.2 kW, ions would be produced farther from the sampling orifice. There would be more extensive diffusion of the ions before they reached the sampling orifice. This could be overcome by either moving the sampling orifice closer to the load coil or increasing the nebulizer gas flow rate. 

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