THE CVD OF SILICON CARBIDE

Silicon carbide (SiC) is a major industrial material with a considerable number of applications. CVD plays a significant role in its development and production, SiC is a covalent carbide with two phases a and [3. The phase of major interest here is pSiC, which has a cubic zinc blend structure. It is the one reported here. 

Beta SiC (PSiC) has good chemical resistance, particularly to oxidation owing to the formation of a thin adherent and protective film of silicon dioxide on the surface. Its characteristics are summarized in Table 9.6. 

Color colorless to yellow if pure, brown if doped with boron, nitrogen or aluininum X-ray Density (g/cm3) 3.214 Melting Point 2545°C at 1 atm. 2830°C at 35 atm. 

Dielectric Constant 300K 9.72 Electrical Resistivity (pQcm) lO to 10  

Debye Temperature 1430K Energy Gap (eV) 2.6 ExitonEnergy Gap (eV) 4.2K 2.39 Superconductive Transition Temperature 5K Refractive Index, n. (Na) 2.48  

Silicon carbide (SiC) is a major industrial material with many applications. CVD plays a major role in its development and production. 

A common CVD reaction is the decomposition of methyl trichlorosilane (MTS) at 900-1400°C (optimum 1100 C) and 1-6 kPa in a hydrogen atmosphere  

The deposition rate and the crystallite size increase with increasing partial pressure of MTS. 

Another common deposition system is the reaction of silane with a hydrocarbon such as propane or benzene at 800°C and 1 kPa in the following simplified forms l ll ll l 

Plasma CVD has been used with reactions 6 and 7 to deposit SiC at considerably lower temperatures (20Q-500 C).P 1 

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Uses Sterically hindered reducing agent to install highly hindered di-t-butylsilylene group for protection of 1,2 and 1,3-diols upon thermolysis, as source of silicon for the CVD of silicon carbide... 

The formation of silicon carbide whiskers can occur through one of three general methods vapor-solid (VS), chemical vapor deposition (CVD), vapor-liquid-solid (VLS). The processes vary widely in the raw... 

This work reports the development of a polymeric/sol-gel route for the deposition of silicon carbide and silicon oxycarbide thin films for applications such as heat-, corrosion-, and wear-resistant coatings, coatings on fibers for controlling the interaction with the matrix in ceramic matrix composites, or films in electronic and optoelectronic devices. This method, in which the pre-ceramic films are converted to a ceramic coating either by a conventional high temperature annealing or by ion irradiation, is alternative to conventional methods such as chemical or physical vapor deposition (CVD, PVD), molecular beam epitaxy, sputtering, plasma spray, or laser ablation, which are not always practical or cost efficient. 

A review article on the CVD processes used to form SiC and Si3N4 by one of the pioneers in this area, Erich Fitzer [Fitzer, E., and D. Hegen, Chemical vapor deposition of silicon carbide and silicon nitride—Chemistry s contribution to modem silicon ceramics, Angew. Chem. Int. Ed. Engl, 18, 295 (1979)], describes the reaction kinetics of the gas-phase formation of these two technical ceramics in various reactor arrangements (hot wall, cold... 

The adsorption of hydrocarbon molecules on Si surfaces is an interesting topic of study under various viewpoints. For example, a thin hydrocarbon film coating Si may be applied as a low dielectric in microelectronics and may passivate the surface if covalent bonds are formed between Si atoms and the adsorbate species. Further, unsaturated hydrocarbons play an important role as precursor species for chemical vapor deposition (CVD) of diamond - like films on the Si surface, and of silicon carbide (SiC). 

Another CVD type silicon carbide fiber available commercially, is called Sigma fiber. Sigma fiber filament is a continuous silicon carbide monofilament obtained by CVD on a trmgsten substrate. Figure 6.19 gives the flow diagram for the fabrication of this process. 

Pyrolytic BN was deposited on Nicalon NL202 silicon carbide yarns at 1000-1200°C to improve aerodynamic resistance and oxidation behavior of silicon carbide. Yarns were fed into a CVD furnace at a rate of 0.01 m/s. The pBN was made by... 

F. Christin, R. Naslain, and C. Bernard, A thermodynamic, and experimental approach of silicon carbide-CVD application to the CVD infiltration of porous carbon-carbon composites. Proc. 7th Int. Conf. Chem. Vap. Deposition, pp. 499-514 (1979). 

Chin et al. [16] developed the chemical composition-process relationships in CVD-SiC from H2-CH3SiCl3. The phase relationships given in Fig. 10 indicate that the two parameters of supersaturation and temperature are joined by the mass transport requirements in terms of the rate of deposition of silicon carbide. In this case, the rate is also dependent on the hydrogen concentration as it is required in the reaction to form SiC and is the carrier gas for the reactants. At low hydrogen levels, pyrolytic carbon is formed. 

Christin F, Naslain R, Bernard C (1979) A thermodynamic and experimental approach of silicon carbide CVD. Application to the CVD-infiltration of porous carbon composites. In Sedgwick TO, Lydtin H (eds) Proceedings of the 7th international conference on chemical vapour deposition. Electrochemical Society, Pennington, NJ, pp499-514... 

CVD was also used to grow CNTs on microscale ceramic fibers consisting of silicon carbide or alumina [272]. Xylene was employed as the carbon source, and ferrocene (FefCTHs ) as the catalyst. The temperature during CVD was 700 °C. Generally, the deposition time was 10-20 min, and 10-gm-thick layers were formed. 

Figure 6.14. Stability diagrams for a carbide composite showing calculated and actually observed depositions at the same conditions, (a) Calculated stability diagram. The indicated compounds are expected to be in equilibrium with a gas mixture of the tetrachlorides of silicon and titanium (with hydrogen and methane) at different concentration ratios. A composite of SiC and TiC is predicted to be stable only below 1440°C. (b) The solids deposited under different reaction conditions (the experimentally observed stability diagram). A composite of SiC and TiC is formed only at temperatures over 1550°C From T. Goto and T. Hirai. Proc. of the 10th International Conference CVD (1987), p. 1070. Reproduced by permission of the Electrochemical Society, Inc.

Also noted is the rapid expansion of a number of materials produced by CVD, which include copper, tungsten, diamond, silicon carbide, silicon nitride, titanium nitride, and others. The coverage of the chemistry and deposition techniques of these materials has been greatly expanded. 

Chemical vapor deposition (C VD) is a versatile process suitable for the manufacturing of coatings, powders, fibers, and monolithic components. With CVD, it is possible to produce most metals, many nonmetallic elements such as carbon and silicon as well as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. This technology is now an essential factor in the manufacture of semiconductors and other electronic components, in the coating of tools, bearings, and other wear-resistant parts and in many optical, optoelectronic and corrosion applications. The market for CVD products in the U.S. and abroad is expected to reach several billions dollars by the end of the century. 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

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). 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

CVD is a maj or process in the production of thin films of all three categories of electronic materials semiconductors, conductors, and insulators. In this chapter, the role of CVD in the fabrication of semiconductors is reviewed. The CVD production of insulators, conductors, and diffusion barriers is reviewed in the following chapter. The major semiconductor materials in production or development are silicon, germanium, ni-V and II-VI compounds, silicon carbide, and diamond. 

CVD is used in the industrial production of inorganic structural fibers such as boron and silicon carbide. Boron fibers are, in... 

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