Secondary Plasma Reactor

In such a reactor, two kinds of electrode pairs exist. One is the pair represented by electrodes 1 and 2. Another is the pair represented by electrodes 2 and 3. Electrodes 1 and 3 are both hot electrodes therefore, there is the possibility that glow discharge could be created between 1 and 2 or between 2 and 3. Glow discharge in such a situation depends on the breakdown voltage of two gas phases, which is dependent on a parameter given by the product of system pressure p and the distance d between two electrodes (see breakdown voltage in Chapters 14 and 15). 

The secondary plasma created in a secondary plasma reactor at pressure 200 mtorr 

In the case of oxygen plasma treatment, the chemically reactive species created in the primary plasma diffuse to the dark space and react with the surface of substrate. Since the substrate surface is not directly exposed to plasma, which contains various energetic species such as electrons, ions, excited neutrals, and photons in different energy levels, the damage, which might be caused by these energetic species, could be avoided. Furthermore, the line-of-sight treatment does not occur in the dark space, and the more uniform treatment of complex shape substrate could be obtained. This is the main reason why most plasma treatment equipment employs secondary plasma treatment or remote plasma treatment. 

The primary glow discharge reactor in this configuration could be used for plasma coating in a low-pressure regime, in principle however, in consideration of flow pattern in such a multiple shelves reactor and electrical field in multiple electrodes, it is difficult to achieve uniform coatings in such a mode of operation. 

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It probably is necessary to describe the reactor, in particular the electrode arrangement, in some detail because it is a key factor to understanding primary plasma and secondary plasma. A typical secondary plasma reactor contains six sets of electrode assembly and five spaces for substrate trays (between electrode assemblies). The first two sets of electrodes and a space between them are shown schematically in Figure 18.1. The distance between the hot electrode 1 and the ground electrode 2 (the gap between electrode 1 and electrode 2, and also 3 and 4) is relatively small, i.e., approximately 3 cm. The distance between two sets of electrode assembly (distance between electrodes 2 and 3) is much greater, i.e., 25-30 cm. 

Figure 18.3 Schematic representation of glow develops at low pressure in a secondary plasma reactor.

The treatment by secondary plasma reactor utilizes chemically reactive species created in glow discharge without influences of electron and ion bombardments and luminous gas phase. In-glow LPCAT treatment, on the other hand, utilizes luminous gas phase without the influence of ion and electron bombardment, and chemically reactive species are created on PTFE by energy transfer from the luminous gas phase. Thus, surface treatment by secondary plasma works only with gases that produce relatively long-lived chemically reactive species. Most secondary plasma treatments appear to be surface modifications by air or oxygen. 

In the Tokamak fusion reactor depicted in Fig. 21.9, electric current to the poloidal coils on the primary magnetic transformer generates the axial current in the secondary plasma composed of deuterium and tritium ions. These ions are heated to ignition temperature and then the reaction becomes self-sustaining. The toroidal field coil suspends the plasma away from the metal conducting walls. Contact with the wall would both cool the plasma below ignition temperature and contaminate the plasma with heavy ions. The relevant reactions are given below. 

In order to create glow discharge in the basket, it is necessary to insert the hot electrode coaxially placed in the center of the basket and ground the reactor and the tumbler basket as the counterelectrodes. Without the hot center electrode, glow discharge will develop between the reactor wall and the metal basket, i.e., in the volume (Fi — F2) outside of the basket, and develop the same situation for the secondary plasma discussed in Chapter 18, which could be good for surface treating... 

Elowever, as stated in Section 10.2,2 [222], a secondary plasma could be generated in the center of the substrate between the main plasma ball and the substrate holder as well as the outer rim of the substrate holder. The substrate holder has a complicated shape and was precoated with diamond film, and the 3-inch Si(lOO) substrate had been carburized prior to the BEN treatment. As a result, the diamond nucleation occurred both in the center and the peripheral of the substrate. Thus, the position of the secondary plasma depends on the geometry of the reactor chamber. 

At normal deposition pressures, the mean free path of the gas molecules is 10" -10" cm and is much smaller than the dimensions of the reactor, so that many intermolecular collisions take place in the process of diffusion to the substrate. An understanding of the growth is made particularly difficult by these secondary reactions. In a typical low power plasma, the fraction of molecular species that is radicals or ions is only about 10" , so that most of the collisions are with silane. An important process is the formation of larger molecules, for example... 

The a.c. radiofrequency power (1.2 meg. cycle/sec.) to the A- and B-type reactors was supplied by a modified C-12 Radyne plasma generator of IKW rated output. The modification consisted of placing a secondary winding within the tank coil of the generator. The power output was tapped off from this winding. Measurement of the power dissipated in the discharge was achieved by determining the power factor... 

Fig. 11 shows the range of kinetic energies and densities of particles typically present in plasma processing reactors [39]. The non-equilibrium nature of the discharge is evident from the fact that different species have different energies (and temperatures). Ions bombarding the cathode (box C) and the secondary electrons... 

Growth processes should be conducted in chemically clean environments. There should be no interaction of the plasma with any element of the reactor, because the plasma can etch materials and transport them to the growing substrate. In general, any etching should be eliminated, and only diamond growth should occur. Hot filaments are not advised for homoepitaxial growth, due to contamination of the diamond by materials from the filament. Dust particles originating inside the system should be eliminated because they can provide secondary nucleation sites. 

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