Plasma-CVD

The deposition of graphite can also be obtained by plasma CVD, with the following characteristics f l 

In a plasma-activated reaction, the substrate temperature can be considerably lower than in thermal CVD. This allows the coating of thermally sensitive materials. The characteristics, and properties of the coating are similar to those of coatings deposited at higher temperatures ( 1000°C). 

Plasma activation is also used extensively in the deposition of polycrystalline diamond and diamond-like carbon (DLC). It is reviewed in more detail in Chs. 13 and 14. 

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). 

Plasma CVD was first developed in the 1960s for semiconductor applications, notably for the deposition of silicon nitride. The number and variety of applications have expanded greatly ever since and it is now a major process on par with thermal CVD. 

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Two maj or contributors to this rapid growth are plasma CVD and metallo-organic CVD (MOCVD). Both are extensively reviewed in this new edition. Likewise, the growing importance of CVD in the production of semiconductor and related applications is emphasized with a systematic and detailed analysis of the role of CVD in this field. 

Metallo-organic CVD (MOCVD) and plasma CVD are developing rapidly, not only in the semiconductor-microelectronic area but also in hard coatingsfor erosion andwearapplicationssincethelower deposition temperature now permits the use of a broader spectrum of substrates. Special emphasis hasbeen given to these two areas in this second edition of the CVD Handbook (see Ch. 4 and 5). 

Describe the various processes and equipment used in R D and production such as thermal CVD, plasma CVD, photo CVD, MOCVD, and others. 

The various CVD processes comprise what is generally known as thermal CVD, which is the original process, laser and photo CVD, and more importantly plasma CVD, which has many advantages and has seen a rapid development in the last few years. The difference between these processes is the method of applying the energy required for the CVD reaction to take place. 

Advantages of Plasma CVD. As shown in Table 5.4, with plasma CVD, a deposit is obtained at temperatures where no reaction whatsoever would take place in thermal CVD. This is its major advantage since it permits the coating of low-temperature sub-... 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

Plasma CVD tends to create undesirable compressive stresses in the deposit particularly at the lower frequencies. This may not be a problem in very thin films used in semiconductor applications, but in thicker films typical of metallurgical applications, the process is conducive to spalling and cracking. 

Overall the advantages of plasma CVD are considerable and it is used in an increasing number of applications as will be shown below. 

The following table lists examples of plasma CVD materials and their applications.P iP ]... 

In addition to the thermal CVD systems mentioned above, molybdenum is deposited by plasma CVD using Reaction (3) in hydrogen.Annealing is required to remove incorporated carbon and oxygen. 

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

To deposit diamond by CVD, the carbon species must be activated since, at low pressure, graphite is thermodynamically stable and without activation only graphite would be formed. Activation is obtained by two energy-intensive methods high temperature and plasma. CVD processes based on these two methods are continuously expanded and improved and new ones are regularly proposed. 

In addition to the thermal CVD reactions mentioned above, plasma CVD is used for the low temperature deposition of boron.i l... 

Amorphous Silicon. Amorphous silicon is generally deposited by Reaction (4) at a deposition temperature of 560°C and at low pressure (ca. 1 Torr).P l Helium RF plasma CVD is also commonly used, especially in the production of solar photovoltaic devices. 

Crystalline deposits are obtained with Reaction (2) in the temperature range of 350-400°C at low pressure (< 1 Torr).P9] At still lower temperature (< 330°C) and moderate pressure (20-50 Torr), an amorphous germanium deposit is obtained. With Reaction (2), germanium is obtained by plasma CVD at 450°C and by laser CVD at 340°C.[31][32]... 

Groner, P., Gimzewski, J., and Veprek, S., Boron and Doped Boron First Wall Coatings by Plasma CVD, / Nucl. Mater., 103(l-3) 257-260(1981)... 

The useful temperature range is lower than that of Reaction (1) with 800°C being typical. A pressure of approximately 10 Torr is typical, although atmospheric pressure can also be used.P l Plasma CVD has been used with Reactions (2) and (3) to deposit SiC at considerably lower temperatures (200-500°C).P l... 

Jiang,X., andKlages, C. P, SynthesisofDiamond SiCComposites Films by Microwave-Assisted Plasma CVD, App. Phys. Lett., 61(14) 1629-31 (Oct 1992)... 

Coatings and monolithic components of h-BN are usually produced by CVD by the reaction of a boron halide with ammonia. MOCVD and plasma-CVD are also used. The reaction of boron trichloride and ammonia is as follows ... 

Deposition at low temperature (200 00°C) is possible by plasma-CVD from the reaction of ammonia and a metallo-organic precursor tetrakis(dimethylamido)silicon, Si(NMe2)4. The films are essentially featureless. 

Saitoh, H., et al., Synthesis of C-BN Film by Thermally Activated RF Plasma CVD Method, Japan New Diamond Forum, pp. 57-59, New Diamond (1988)... 

Kiermasz A., and Beekman, K., Plasma CVD of Silicon HitndiQf Semiconductor International,. 108-111 (June 1990)... 

Common deposition reactions are based on the combination of silane with various oxidizers, either as thermal CVD or plasma CVD as follows 1... 

Fluorinated Silicon Oxide. The introduction of fluorine in the ratio of 2 to 14 at.% lowers the dielectric constant, which is reported as low as 3.0. This is a major factor in the design of dielctric films. The CVD of these fluorinated compounds is accomplished by plasma-CVD and usually with Sip4 as a fluorine source. Also available are fluorinated compounds, such as fluorotriethoxysilane (FTES), l,2bis(methyldifluorosilyl)ethane, and 2,5disilahexane.P2]... 

Plasma CVD and thermal laser CVD are also used particularly in the deposition of GaAs. The formation of epitaxial GaAs at 500°C and polycrystalline GaAs at 185°C has been reported,... 

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