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Polysilane Polycarbosilane Polysilazane Polysiloxane

Polysilane Polycarbosilane Polysilazane Polysiloxane

Scheme 20. General representation of organometallic precursors to silicon-containing ceramics.

Silicon carbide is an attractive material for demanding mechanical and high-temperature applications and is extensively used as an abrasive and erosive medium. In addition, it is tough, possesses low-friction characteristics, and is second only to diamond in wear resistance. Conventional SiC chemical vapor deposition processes generally utilize separate Si (e.g., SiH4, SiCl4, SiBr4) and C (e.g., CH4, C2H2, C3H8) sources and require temperatures in excess of 1000 °C for the phase formation. High temperatures cause significant deformation of released polycrystalline-Si microstructures, making postprocess SiC coating challenging [510]. Moreover, control over microstructure and composition is difficult at high temperatures. Such limitations clearly illustrate the need for low-temperature alternatives. Lee and co-workers have obtained epitaxial cubic-SiC films with the use of 1,3-disilabutane ([SiH2CH2]2, I, Scheme 21a) as a single precursor, at temperatures as low as 900 °C [511]. Polycrystalline SiC films were obtained at 650 °C. The molecular source, a liquid with high vapor pressure, simplifies the handling compared with conventional dual-source CVD, thereby eliminating the need for an elaborate gas-handling system and ensuring strict sto-ichiometry control. Using the same precursor, Maboudian et al. have deposited conformal SiC films on Si atomic force microscopy cantilevers at 780 °C. Figure 35 displays a cross section of a SiC film deposited with the use of a molecular precursor on a Si AFM cantilever; an excellent conformal coverage with no void formation at the SiC/Si interface was observed. The disilacyclobutane CVD process was also successfully used to coat released poly-Si microstructures (Fig. 34). This observation reveals the potential for integrating this process into the fabrication technology for microelec-tromechanical systems (MEMS) [512].

Although 1,3-disilabutane is an ideal SiC precursor, it is not suitable for large-scale applications because it is expensive. Similarly, the simple carbosilanes such as CH3SiCl3 afford little advantage in terms of either improved stoi-chiometry control or lowered deposition temperature [513]. Interrante et al. have used substituted disilacyclobutanes

Figure 34. SEM images of (a) the cross section of a 130-nm thick SiC film deposited on a Si AFM cantilever and (b) a released poly-Si lateral resonator coated with a thin SiC film. Reprinted with permission from [615], C. R. Stoldt, Sens. Actuators, A 97-8, 410 (2002). © 2002, Elsevier Science.

Figure 34. SEM images of (a) the cross section of a 130-nm thick SiC film deposited on a Si AFM cantilever and (b) a released poly-Si lateral resonator coated with a thin SiC film. Reprinted with permission from [615], C. R. Stoldt, Sens. Actuators, A 97-8, 410 (2002). © 2002, Elsevier Science.

[MeSi(H)-^-(CH2)2Si(Me)CH2SiH2Me] (II) and [Si(Me) HCH2]2 (III) (Scheme 21b, c) as alternative single precursors to SiC [514, 515]. Their results indicate that the decomposition chemistry is complicated in the case of II and III because of molecular rearrangments that adversely affect the product purity.

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