Cascade arc plasma jet

Figure 16.8 Typical emission spectra of nitrogen in low temperature cascade arc plasma jets (a) N2 in helium plasma jet, (b) N2 in argon plasma jet.

Following the work of Holland and Ojha several other methods for preparation of a-C H were developed, for example sputter deposition [7, 8], ion plating [8, 9], cascade arc plasma jet [10], and simultaneous operation of a RF and a micro-wave (2.45 GHz) discharge [11]. 

With low-pressure cascade arc, plasma formation (ionization/excitation of Ar) occurs in the cascade arc generator, and the luminous gas is blown into an expansion chamber in vacuum. The majority of electrons and ions are captured by the anode and the cathode, respectively, of the cascade arc generator, and there is no external electrical field in the expanding plasma jet. Consequently, the photon-emitting excited neutrals of Ar cause the majority of chemical reactions that occur in the plasma jet. The luminous gas coming out of the nozzle interacts with gases existing in the space into which it is injected or the surface that is placed to intercept the jet. 

Figure 16.16 Dependence of the normalized deposition rate of butane cascade arc plasmas on the parameter W FM)d FM). The deposition rates were obtained at an axial position of 27.5 cm from the luminous gas jet inlet.

An argon-hydrogen plasma is created in a dc thermal arc (cascaded arc) operated at high pressure 0.5 bar) [556, 559. 560] (the cascaded arc is also employed in IR ellipsometry, providing a well-defined source of intense IR radiation see Section 1.5.4 [343]). As the deposition chamber is at much lower pressure (0.1-0.3 mbar), a plasma jet is created, expanding into the deposition chamber. Near the plasma source silane is injected, and the active plasma species dissociate the silane into radicals and ions. These species can deposit on the substrate, which is positioned further downstream. 

The luminous gas phase created by a special mode of DC discharge recognized as the low-pressure cascade arc torch (LPCAT) provides an especially important case for understanding the fundamental aspects of the luminous gas phase. The luminous gas phase in form of luminous gas jet stream or torch are created by blowing out DC discharge into an expansion chamber in vacuum. The luminous gas jet of Ar mainly consists of photon-emitting excited neutral species of Ar, which is certainly not the plasma of classical definition. The core of LPCAT is the tip of injection nozzle however, it is not the core of electrical discharge. 

In comparison with conventional electrical discharge processes, LPCAT is a very different process in that its activation of carrier gas and the creation of polymerizable species by the activated carrier gas are temporally and spatially separated. When discharge power is applied to the cascade arc generator, the plasma of carrier gas (usually argon) is produced in the cascade arc column and the luminous gas phase is blown into a vacuum chamber where monomers are introduced. The deactivation of the reactive species, some of which lead to the creation of polymerizable species in the luminous gas phase, occurs within the relatively narrow beam of an argon luminous gas jet. The higher the flow rate of Ar, the narrower is the beam and the longer the luminous gas flame. 

Fig. 12 The optical emission spectra of argon plasma jets (A) with addition of 10 seem nitrogen and 10 seem hydrogen, (B) with addition of 60 seem nitrogen and 2.7 seem hydrogen, and (C) with pure nitrogen addition, 60 seem nitrogen. The cascade arc conditions are 2000 seem argon, 0.64 kW, and 75 Pa.

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