Parallel-plate plasma reactor

The cold plasma is most often generated in laboratories and industry by an electric glow discharge xmder low pressure using various frequencies of the applied electric field audio frequencies (AF, mainly in the range of 10-50 kHz), radio frequencies (RF, mainly 13.56 MHz), and microwave frequencies (MW, mainly 2.45 GHz). Sometimes, a direct current (DC) discharge is also used. An example of typical parallel plate plasma reactor, one of those being used in our laboratoiy for deposition of thin films, is sketched in Fig. 2. 

Fig. 2. A sketch of a typical parallel plate plasma reactor.

Plasmas can be used in CVD reactors to activate and partially decompose the precursor species and perhaps form new chemical species. This allows deposition at a temperature lower than thermal CVD. The process is called plasma-enhanced CVD (PECVD) (12). The plasmas are generated by direct-current, radio-frequency (r-f), or electron-cyclotron-resonance (ECR) techniques. Eigure 15 shows a parallel-plate CVD reactor that uses r-f power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit siUcon nitride, Si N and glass (PSG) encapsulating layers a few micrometers-thick at deposition rates of 5—100 nm /min. 

Physical and Electrical Characteristics. The electrical potentials established in the reaction chamber determine the energy of ions and electrons striking the surfaces immersed in a discharge. Etching and deposition of thin films are usually performed in a capacitively coupled parallel-plate rf reactor (see Plasma Reactors). Therefore, the following discussion will be directed toward this configuration. 

Figure 1.2 Parallel Plate Plasma-Enhanced Chemical Vapor Deposition (PECVD) Reactor. Typical Parameters are Radio Frequency (rf) - 50 kHz to 13.56 MHz Temperature - 25 to 700°C Pressure - lOOmTorr to 2Torr Gas Flowrate - 200seem...

Figure 13 Schematic representation of a radial flow parallel plate plasma CVO reactor (48).

FIG. 4. Schematic representation (a) of a parallel-plate, capacitively coupled RF-discharge reactor, with unequal-size electrodes. The potential distribution (b) shows the positive plasma potential Vp and the negative dc self-bias voltage... 

The radio-frequency glow-discharge method [30-34] has been the most used method in the study of a-C H films. In this chapter, it is referred to as RFPECVD (radio frequency plasma enhanced chemical vapor deposition). Film deposition by RFPECVD is usually performed in a parallel-plate reactor, as shown in Figure 1. The plasma discharge is established between an RF-powered electrode and the other one, which is maintained at ground potential. The hydrocarbon gas or vapor is fed at a controlled flow to the reactor, which is previously evacuated to background pressures below lO"" Torr. The RF power is fed to the substrate electrode... 

Figure 8. Configurations for plasma etch reactors, (a) barrel or volume loaded 0)) parallel plate or surface loaded (c) downstream etcher.

Three different dry etch techniques were investigated isotropic O2 plasma etching in a Tegal 200 reactor, R.I.E. in a parallel-plate in-house modified Tegal AOO reactor and R.I.M. in a Veeco, Model RG-830. The conditions of operation for each system were as follows where time is the time to etch 1.2 p of fully cured polyimide. 

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. 

Silicon wafers were oxygen plasma-cleaned in a parallel plate reactor at 400 W power, 300 mTorr pressure for 5 min. The gas flow was set to 100 seem total,... 

Figure 7. Configuration for plasma etch and deposition reactors, (a) parallel-plate or surface-loaded design with wafers positioned horizontally (h) parallel-plate design with vertical electrodes with a furnace tube (c) external coupling, downstream (d) external coupling, (a Reproduced from reference 2. Copyright 1983 American Chemical Society, b-d Reproduced with permission from reference 46. Copyright 1983 American Society for Testing and Materials.)...

Figure 11 Geometries of plasma-assisted CVD reactors (A) parallel-plate discharge, (B) tube with capacitive coupling, (C) tube with inductive coupling.13...

The two reactors just described are parallel plate reactors. However, they are also cold wall reactors. In other words, the electrode holding the wafers is hot, but all other surfaces exposed to the plasma are cold, or at least not heated. This is done to minimize the deposition on other surfaces so that down time for cleaning can be kept as short as possible. 

Figure 15 Hot-wall, parallel-plate reactor for plasma-enhanced CVD. (Courtesy of Pacific Western Systems, Inc.)...

Current commercial plasma-enhanced CVD reactors operate with only two physical concepts. In one case, we have the inductively-excited discharge in a tube, which is used for plasma ashing of resist. The other is the parallel plate arrangement using high-frequency RF power to create a low-pressure glow discharge, where the wafers to be coated sit on one of the electrodes. 

One approach which can be taken for PECVD is the parallel plate reactor (see figure 8.4). The plasma can be created by either an RF or a DC discharge. When pure WF6 is used only etching of tungsten will occur according to the gas phase reaction ... 

Although a plethora of reactor configurations and methods for plasma generation exist (see Section 4), the parallel plate capacitively-coupled reactor (also called diode) shown in Fig. 3a is a typical example. A semiconductor wafer rests on one electrode... 

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