Radio frequency generated plasmas can be easily drawn out of the region enclosed by the coupling coil, while microwave generated plasmas tend to strongly localize in the region surrounded by the coupling cavity (Ref. 65). There are two distinct radio frequency arrangements that can be used for generating electrode discharge in gases, one involving the use of a self-exited power oscillator, and the other a driven oscillator-amplifier arrangement, which was found to work better (Ref. 28).
The successful operation of a discharge beam source depends strongly on the characteristics of its plasma. A detailed description is given in the paper by Sibener et. al. (Ref. 65) in 1979 who constructed a high pressure, radio frequency discharge nozzle beam source (plasma jet) for use with oxygen seeded in a rare gas to study the products, energetics, and reaction dynamics of atomic oxygen reactions in crossed beam scattering experiments. Their work significantly improved on previous work on such a source by Miller and Patch (Ref. 29) and Gorry et. al. (Ref. 30).
The production of a stable and efficiently coupled high pressure discharge for the generation of atomic species is much more difficult than the production of a low pressure (about 1 torr) discharge. Sibener et. al. identified three key elements to succeed in producing such a discharge. First, impedance matching of the plasma to the radio frequency (RF) power source as a function of gas pressure, temperature, and composition is required. Next it is essential that the plasma is localized just behind the orifice of the nozzle to prevent recombination of the atomic specie before the expansion occurs as well as to melt the nozzle. A melting nozzle leads to impurities in the beam and changes the beam characteristics with time due to a changing nozzle geometry. Even if the plasma is located behind the orifice of the nozzle the nozzle has to be cooled sufficiently to prevent melting and recombination (Ref. 31) at the nozzle walls. Last, the plasma discharge must operate in a stable, reproducible, and uniform discharge mode (no "bead" or "streamers") whose temperature is sufficient high to generate atomic species, but not high enough to melt the orifice of the nozzle.
Another important issue arises in semiconductor growth. Every given nozzle (inc. quartz) has a limited lifetime due to sputtering of the front surface of the nozzle (Ref. 65). This sputtering results when ions which leave the nozzle are accelerated back towards its front surface by the large RF fields present in that region. This situation is not desirable if a high purity beam is required for semiconductor growth. Also, some of the ions which emanate from the nozzle actually reach the substrate which causes ion damage. Therefore a deflecting field is needed to remove those ions from the beam. In the case of oxygen a deflection field of 5000 V/cm proved sufficient (Ref. 65).
Sibener et. al. obtained beam intensities of about 5x10^17 atoms sr^-1 s^-1 using argon as a the carrier gas and >=5x10^18 using helium, which were comparable in magnitude to other nozzle beam sources reported in the literature (Ref. 32). Based on previous observations concentrations of electronically exited atoms and molecules are likely to be present (Ref. 33, 34, 65, 35).
Sibener et. al. and others (Ref. 36,35) found that oxygen seeded in argon generates significantly hotter plasmas (T_o) and exhibits stronger power dependence than using helium as the carrier gas. He also found that gas temperature decreases with increasing pressure, but using argon as a carrier gas the plasma can change from the "hot" mode to an extremely hot "streamer mode", causing the nozzle to melt. The nozzle shape is an important factor in minimizing the probability to go into the "streamer" mode. Sibener et. al. data suggest that for the case of Oxygen seeded in Argon and Helium the dissociation fraction is only mildly dependent of RF power (slightly increasing with increasing power), but significantly higher using Argon as the carrier gas. Oxygen dissociation was between 40% to 90%, for Argon and Helium used as the carrier gas respectively, compared to 35% in previous works. On the other hand the velocity increases significantly with increasing RF-power, indicating that the effective plasma temperature does in fact rise with increasing RF-power.
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