Radio-frequency thermal plasma

Continuous production of fullerenes was possible by pyrolysis of acetylene vapor in a radio-frequency induction heated cylinder of glassy polymeric carbon having multiple holes through which the gas mixture passes [44]. Fullerene production is seen at temperatures not exceeding 1500 K. The yield of fullerenes, however, generated by this method is less than 1%. A more efficient synthesis (up to 4.1% yield) was carried out in an inductively coupled radio-frequency thermal plasma reactor [45]. 

PACVD method is another typical way of making diamond films. Precursor gas molecules can be decomposed into radicals under the effect of plasma. There are three plasma sources commercially available (Davis 1993). Microwave plasma typically uses excitation frequencies of 2.45 GHz. Radio frequency (RF) plasma excitation typically employs frequencies of 13.56MHz (or less commonly 450kHz). Direct current plasmas can be run at low electric powers, named as cold plasma, or at high electric powers, which create an arc, named as thermal plasma. Microwave PACVD method is the most common one among the three methods. 

Besides the MPCVD reactors, other CVD reactors are also used for diamond deposition. They are hot filament, DC plasma, radio-frequency (rf) plasma, thermal rf plasma, plasma jet, and combustion CVD reactors. In the following, hot filament and DC plasma CVD reactors will be described, because they have been used for oriented growth of diamond. 

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. 

A large number of CVD diamond deposition technologies have emerged these can be broadly classified as thermal methods (e.g., hot filament methods) and plasma methods (direct current, radio frequency, and microwave) [79]. Film deposition rates range from less than 0.1 pm-h to 1 mm-h depending upon the method used. The following are essential features of all methods. 

The inductively coupled plasma source (Fig. 20.11) comprises three concentric silica quartz tubes, each of which is open at the top. The argon stream that carries the sample, in the form of an aerosol, passes through the central tube. The excitation is provided by two or three turns of a metal induction tube through which flows a radio-frequency current (frequency 27 MHz). The second gas flow of argon of rate between 10 and 15 L min-1 maintains the plasma. It is this gas stream that is excited by the radio-frequency power. The plasma gas flows in a helical pattern which provides stability and helps to isolate thermally the outside quartz tube. 

Non-thermal plasmas can be produced in a number of ways, including a variety of electrical corona discharges, radio frequency discharges, microwave discharges and electron beams. The most common NTP technologies for emission reduction in engine exhaust streams are the following. 

Atoms may be produced both in thermal and supersonic beams using the techniques of thermal dissociation [33] and dissociation by micro-wave [34] and radio frequency [35] discharges and by plasma sources [36]. Comparatively few reactions involving radicals have been studied in molecular beams, but sources have been developed that produce radicals by pyrolysis [37], reaction [25, 38, 39] and photolysis [40]. 

Atoms may be produced both in thermal and supersonic beams using the techniques of thermal dissociation [33] and dissociation by micro-wave [34] and radio frequency [35] discharges and by plasma sources... 

Reaction conditions similar to those in the DC thermal plasma CVD can be obtained in an inductively coupled, radio-frequency induced thermal plasma at atmospheric pressure, as shown schematically in Fig, 2g. This technique was first reported by Matsumoto et al. in 1987.Very high... 

Surface modification and metallization of poly(tetrafluoroethylene) (PTFE) has attracted a considerable attention from viewpoints of fundamental science and applied technology. PTFE has a low surface free energy while it shows excellent thermal and chemical stability. Therefore, the chemical and physical inertness of PTFE makes metallization an extremely difficult process. At present a chemical treatment using a sodium naphthalenide solution (1-4), a radio frequency plasma process (5-8), and electron/ion beam irradiation (9-//) have been employed for the modification of PTFE surface. 

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