PACVD

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

Thermal CVD, reviewed above, relies on thermal energy to activate the reaction, and deposition temperatures are usually high. In plasma CVD, also known as plasma-enhanced CVD (PECV) or plasma-assisted CVD (PACVD), the reaction is activated by a plasma and the deposition temperature is substantially lower. Plasma CVD combines a chemical and a physical process and may be said to bridge the gap between CVD andPVD. In this respect, itis similar to PVD processes operating in a chemical environment, such as reactive sputtering (see Appendix). 

Two principal classes of diamond film deposition have been developed (I) PACVD (plasma-assisted chemical vapor deposition) and (2) IBED (ion-beam-enhanced deposition). 

Analysis of chemical composition is important because large percentages of hydrogen can be incorporated in the films, especially with PACVD techniques. This can cause a wide variety of hydrogenated structures. 

Diamond Hints, although not approaching bulk diamond, are harder than most refractory nitride and carbide thin films, which makes them attractive for tribological coatings. Transparency in the visible and infrared regions of the optical spectrum can be maintained and index-of-refraction values approaching that of bulk diamond have been measured. Electrical resistivities of diamond films have been produced within the full range of bulk diamond, and thermal conductivities equivalent to those of bulk diamond also have been achieved. As substrates for semiconductor electronic devices, diamond films can be produced by both the PACVD and IBRD techniques. 

PACVD or PECVD plasma-assisted or plasma-enhanced chemical vapor deposition... 

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. 

Plasma-enhanced (PECVD) or plasma-assisted (PACVD) CVD, (see chapters in Refs. 5,14, and 15), constitute a smaller category of CVD processes that also involves a variety of reactor designs. In these systems, a plasma is... 

Figure 1.3 clearly demonstrates the luminous gas phase created under the influence of microwave energy coupled to the acetylene (gas) contained in the bottle. This luminous gas phase has been traditionally described in terms such as low-pressure plasma, low-temperature plasma, nonequilibrium plasma, glow discharge plasma, and so forth. The process that utilizes such a luminous vapor phase has been described as plasma polymerization, plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), plasma CVD (PCVD), and so forth. 

The material deposition that occurs in the low-pressure electrical discharge has been discussed under various terminologies such as plasma polymerization (PP), plasma-enhanced chemical vapor deposition (PECVD), plasma-assisted chemical vapor deposition (PACVD), plasma chemical vapor deposition (PCVD), and so forth [1]. However, none of these terminologies seems to represent the phenomenon adequately. The plasma aspect in the low-pressure discharge is remote, although it plays a key role in creating the environment from which material deposition occurs to the extent that no chemical reaction occurs without the plasma. In this sense, PECVD and PACVD could be out of the context in many cases in which nothing happens without plasma. In such cases, PP or PCVD would describe the phenomenon better. If the substrate was not heated substantially above the ambient temperature, the use of PECVD or PACVD should be avoided. 

In order to find the domain of LCVD, it is necessary to compare various vacuum deposition processes chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma chemical vapor deposition (PCVD), plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), and plasma polymerization (PP). All of these terms refer to methods or processes that yield the deposition of materials in a thin-film form in vacuum. There is no clear definition for these terms that can be used to separate processes that are represented by these terminologies. All involve the starting material in vapor phase and the product in the solid state. 

PP is a CVD process in which the chemically reactive species are created by plasma. Terms such as plasma-assisted CVD (PACVD) and plasma-enhanced CVD (PECVD) are inadequate to describe PP. Unless the substrate temperature is raised... 

PECVD or PACVD Plasma and substrate surface Substrate surface in the same chamber Ts > Tv AE>0... 

PECVD or PACVD Electrical discharge and heat Gas and substrate surface Coupled... 

Figure 2. Schematic diagram of various CVD techniques for diamond synthesis, (a) HFCVD (b) MW PACVD (c) ECR MW PACVD (d) DC PACVD (e) RF PACVD (0 DC thermal plasma CVD (g) RF thermal plasma CVD (h) flame (combustion) CVD.l (Reproduced with permission.)...

In these P ACVD methods, gas compositions are similar to those used in the HFCVD,t J although gas flow rates in the HFCVD are up to a factor of 10 lower than in some ofthe PACVD techniques. Substrate temperatures are determined by power densities, placement of substrates, and water cooling rates of substrates, usually maintained at about 800-1100°C. Gas-phase temperatures depend on the method used, i.e., whether the plasma is isothermal or non-isothermal. 

Research on MW PACVD began from the pioneering work of Kamo et The MW PACVD (Tables 1 and 2) is, aside from the HFCVD, the most frequently used method for diamond growth and also the most extensively studied process. The intensive use of the MW PACVD is prompted by the following factors ... 

In spite of some ofthe advantages ofthe MW PACVD over other CVD methods, notably its stability, the deposition rates currently achievable in the... 

A common DC glow discharge PACVD system may be varied by operating the plasma at higher pressures and powers at which a DC arc discharge between the electrodes can be produced. In 1988, Kurihara et al.t l first reported the use of the DC plasma arc-jet CVD method, in which... 

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