AC glow discharge

Figure 14. Deposition rate profiles on electrode and substrate for an AC glow discharge at a flow rate of 2 (STP)mL/min, Pji, = 60 mtorr with a magnetic peld...

Figure 1. Schematic of the arrangement of magnets behind an electrode. The annular zone of intense glow shown is obtained in an AF or AC glow discharge as described in the text and illustrated in Figure 2.

Figure 9-32. General schematie of a reactor for activation of polymer films by an industrial frequency AC glow discharge (1) gas control system (2) treatment zone (3) film-moving system.

For a dc or ac glow discharge, an increase of power is obtained by both an increase in potential drop between electrodes and an increase in current density on the electrodes. Both effects result in an increased density of energetic electrons and increase bombardment of the electrode by energetic ions. 

Plasma polymerization is usually carried out in a low pressure glow discharge sustained by either a dc or an ac electric field. Examples of the reactors used for this purpose are shown in Fig. 1. The simplest configuration involves a pair of circular parallel plate electrodes mounted inside a glass bell jar. The lower electrode usually serves as the substrate holder and is sometimes heated or cooled. Monomer is introduced through a feed tube and unconsumed monomer and gaseous products are withdrawn through a port in the base plate. 

The glow discharge initiated by an AC power source can be visualized by an alternating cathode and anode in the DC glow discharge, up to a certain frequency. 

The most common deposition technique is the glow-discharge decomposition (gdd) of volatile inorganic compounds - e.g., SiH4 to make a-Si H. Several variants are available where the ionized plasma is driven by a dc electric field (with the substrate on either the anode or the cathode), an ac field (60 Hz), an rf inductive, or rf capacitive (sometimes with a superposed dc bias). These techniques can be used in the presence of a magnetic field. The pressure of SiH4 is in the range 1 to 700 Pascal with the substrate at 200 300°C. Above 350°C, H2 evolves from the a-Si H layer ... 

The most important parameters in glow-discharge cleaning are the type of applied voltage (AC or DC), the value of discharge voltage, the current density, type of gas and gas pressure, duration of treatment, type of material to be cleaned, shape and arrangement of electrodes. 

The most important way of plasma generation, as a medium to conduct the polymerization, is the electric glow discharge. For that a direct (DC) or alternate (AC) current with frequencies ranging from 50 Hz to 5 GHz can be used. A t3q)ical plasma polymerization is conducted at low gas pressure of the order of 10 -10 Pa. The course of process in plasma may vary depending on the method of conducting the reaction. For this reason a distinction between the following is made. 

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