Cascade arc generator

The spatially decoupled activation and deactivation can be also seen in a mode of PP known as low-pressure cascade arc torch (LPCAT) polymerization), which is described in Chapter 16. The activation of a carrier gas (e.g., argon) occurs in a cascade arc generator, and the chemical activation of a monomer or a treatment gas takes place near the injection point of the argon torch in the deposition chamber. The material deposition (deactivation) occurs in the deposition chamber. This is the same situation as the HWCVD, except that the mode of activation is different. 

In cascade arc plasma polymerization, a monomer (or monomers) is introduced in the expansion chamber. Because of an extremely high velocity of gas injected from a small nozzle (e.g., 3 mm in diameter), the second gas injected into the expansion chamber in vacuum cannot migrate into the cascade arc generator. Thus, the activation of Ar in the cascade arc generator and deactivation of the excited neutral species of Ar in the expansion chamber, which activate the monomer introduced in the expansion chamber, are totally decoupled. LPCAT plasma polymerization occurs under such a spatially and temporally decoupled activation/ deactivation system. 

The ionization of Ar by high-energy electron, e, occurs in the cascade arc generator. 

In low-pressure cascade arc torch (LPCAT), the electrical power is applied in the cascade arc generator, in which only carrier gas, generally Ar, is activated to create luminous gas. The luminous gas created in the cascade arc generator is blown into the second expansion chamber, in which the monomer is introduced. Thus, the luminous gas of Ar neutrals primarily creates polymerizable species, and following these two steps should treat the deposition kinetics. Principles described in this chapter apply to each of the two steps. Details of deposition kinetics in LPCAT are described in Chapter 16. 

CASCADE ARC GENERATOR AND LOW-PRESSURE CASCADE ARC TORCH REACTOR... 

Plurals of integrated cascade arc generators, which are strategically placed on an expansion chamber, can be used to achieve wider and more uniform treatment. 

The relative motion of substrate with respect to the luminous gas jet is more or less mandatory for the uniform treatment. Figure 16.4 depicts a reactor equipped with three cascade arc generators, of which two are used to treat substrates placed on a rotating plate. 

Another potential mode of LPCAT processing is that the integrated cascade arc generator, such as that shown in Figure 16.2, is placed in a vacuum chamber held by a robot arm. In this mode, LPCAT jet could scan over a complex shaped substrate by the robotic operation. 

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. 

In the LPCAT process, only an inert gas such as Ar exists in the cascade arc generator, and DC voltage is applied between the cathode and the anode. Therefore, it is a DC discharge of Ar, but it occurs under much higher pressure than in most low-pressure DC discharges, and the gas travels very fast in one direction in the generator. The basic process of ionization of Ar takes place in the cascade arc generator, which can be depicted as follows. 

The cathode captures most of Ar" ", and the anode captures the majority of electrons. When the luminous gas phase created in the cascade arc generator is blown out of the nozzle, the majority of species in the luminous gas jet are excited Ar neutrals and some strayed electrons. 

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. 

In LPCAT process, only an inert gas, such as Ar, exists in the cascade arc generator, and DC voltage is applied... 

Fig. 4 The LTCAT model reactor with multiple cascade arc generators. (View this art in color at www.dekker.com.)...

Low-pressure cascade arc torch has the unique feature that excited neutral species of Ar that are created in the cascade arc generator are injected into the reaction chamber. The beams of Ar excited species can be used ... 

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