Plasma assisted CVD

A plasma is an efficient way to dissociate gas molecules to produce non-equilibnum concentrations of gas-phase species, such as the high concentrations of atomic hydrogen needed for diamond growth. Plasmas can be generated by a number of energy sources (microwave, radio-frequency, or direct-current electric fields), and can be either cold (non-isothermal, or nonequilibrium plasmas) or hot (isothermal, or equilibrium plasmas). The major characteristics of these plasmas are summarized in Table 3. 

Plasma type non-isothermal fnon-eauilibrium) isothermal (equilibrium) 

P AC VD (Figs. 2b-g) may be further subdivided into the following four major types in terms of the methods with which plasmas are generated and the characteristics of plasmas, as listed in Table 4. 

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. 

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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). 

In plasma-assisted CVD, an electrostatically or electromagnetically induced plasma discharge is carried out in a low pressure system. The result is that the process may be operated at a considerably lower temperature. This has been employed in the deposition of SiOj and Si3N4 in the production of heat-sensitive microelectronic circuits. 

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). 

The region of our ne versus T plot in which plasma-assisted CVD reactors function is also shown by the shaded area. Clearly, this only represents a very small region of the total space within which a reactor could operate. In the future, operation of CVD reactors in other regions of this plot may lead to CVD... 

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

Stiegler, J., Lang, T., Nygard-Ferguson, M., Von Kaenel, Y. and Blank, E. (1996), Low temperature limits of diamond film growth by microwave plasma-assisted CVD. Diam. Relat. Mater., 5(3-5) 226-230. 

Microwave plasma-assisted CVD No external heating required... 

Subsequent preliminary comparative studies of the behavior of an SiC based layer on Ta, Mo, Ti and steel substrates showed that better mechanical stability was obtained with a coating deposited on tantalum. This element was consequently considered to make PFCVD deposit/interlayer/steel stacks. Tantalum can be produced by physical vapor deposition (PVD), at variable thickness, with reproducible morphology. Note that preparation by chemical vapor deposition with or without plasma assistance (CVD or PECVD) is possible at low temperature but would require an optimization study in order to be compatible with the deposition conditions of the silicon carbide layer, the aim being to increase the mechanical stability. 

In general, several possible chemical reactions can occur in a CVD process, some of which are thermal decomposition (or pyrolysis), reduction, hydrolysis, oxidation, carburization, nitridization and polymerization. All of these can be activated by numerous methods such as thermal, plasma assisted, laser, photoassisted, rapid thermal processing assisted, and focussed ion or electron beams. Correspondingly, the CVD processes are termed, thermal CVD, plasma assisted CVD, laser CVD and so on. Among these, thermal and plasma assisted CVD techniques are widely used, although polymer CVD by other techniques has been reported. ... 

Plasma Assisted CVD (also known as Plasma Enhanced CVD or PECVD)... 

In addition to microelectronic and optical applications, polymers deposited using thermal and plasma assisted CVD are increasingly being used in several biomedical applications as well. For instance, drug particles microencapsulated with parylenes provide effective control release activity. Plasma polymerized tetrafiuoroethylene, parylenes and ethylene/nitrogen mixtures can be used as blood compatible materials. An excellent review of plasma polymers used in biomedical applications can be found in reference 131. 

Yttria thin films can also be deposited on Si substrates from Y(hfac)3 and Y(thd)3 by oxygen plasma-assisted CVD. It is found that with Y(hfac)3 the appropriate thin films were contaminated with fluorine, leading to unexceptional electrical properties. As discussed in Section V.A.4, next to yttrium oxide, SiOi and yttrium silicate are formed on the substrate surface. Pre-nitridation of the silicon surface impedes the reaction with the substrate. 

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. 

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... 

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