Chemical vapour deposition of silicon carbide by pyrolysis of methylchlorosilanes

Abstract

Silicon carbide (SiC) prepared by chemical vapour deposition (CVD) remains of importance for both structural [1, 2] and electronic [3, 4] applications. A variety of gaseous precursors has been used for SiC deposition under widely varying conditions of input gas composition, temperature and pressure [5]. For CVD of SiC, in general, a carrier gas is bubbled through a silicon containing liquid mixed with another stream of carrier gas. The necessary carbon is either contained in the chlorosilane or supplied by introducing a hydrocarbon. Methylchlorosilane (MTS: CH3SiCl3), as a precursor, is one of the most useful of the available chlorosilanes because it contains silicon and carbon in stoichiometric proportions [6]. Accordingly, it has been expected to give stoichiometric SiC deposition. Recently, from thermodynamic studies on the CVD of SiC in the MTS ‡ H2 gas system [7, 8], it has been shown that SiC is the only stable solid phase present in a wide range of temperature, pressure and concentration of the reactant. In experimental works, however, there have been many reports in the literature [9, 10] showing that excess Si is codeposited with SiC at lower temperature. Furthermore, in our previous work [11] conducted with the MTS ‡ H2 gas system in the temperature range 1000–1500 8C, we always found excess Si deposition at temperatures below 1400 8C. In order to obtain stoichiometric SiC deposition, we recently reported [12] the addition of propane (C3H8) as an excess carbon source to the MTS ‡ H2 gas system, which resulted in the deposition of stoichiometric SiC. Therefore, it is conceivable that silanes are more reactive with the substrate than hydrocarbon and supplying excess carbon is needed for stoichiometric SiC deposition. There are some precursors that supply excess carbon, such as dimethyldichlorosilane (DDS: (CH3)2SiCl2), trimethylchlorosilane (TCS: (CH3)3SiCl) and tetramethylsilane (TS: (CH3)4Si). These chlrorosilanes contain both silicon and carbon, and the ratios of C/Si in the molecules are 2, 3 and 4, respectively. In addition DDS, TCS and TS molecules decompose more easily than MTS and supply sufficient hydrocarbon above the substrate. The purpose of the work reported here was thus to prepare stoichiometric SiC deposit by supplying DDS, TCS or TS as a reactant and to investigate the change of the microstructure. The SiC coatings were deposited in a horizontal quartz reactor at 1000–1500 8C and under atmospheric pressure. The temperatures were measured with an optical pyrometer and corrected to approximate the temperature of substrate in accordance with a previous calibration with a thermocouple. Graphite plates and SiC-coated graphite were used as substrate and susceptor, respectively. The three types of methylchlorosilane (DDS, TCS and TS) were used and they were carried by hydrogen from the evaporator maintained at 0 8C. The concentration of methylchlorosilane was controlled with hydrogen. The total flow rate of X (X ˆ DDS, TCS or TS) ‡ H2 gas mixture was kept constant at 1600 standard cubic centimetres per minute (sccm). The growth rate was estimated by measuring the weight increase during deposition periods. The crystal structure was analysed by X-ray diffractometry (XRD) and the silicon content of the coating layer was determined by energy dispersive spectrometry (EDS) and Auger electron spectroscopy (AES). The surface morphology of the coating layer was investigated by scanning electron microscopy (SEM). Details of the experimental procedure were similar to those reported previously [11, 12]. The molar fraction dependence of the deposition rate is shown in Fig. 1. The results of previous work [11], carried out in the MTS ‡ H2 system, are also shown for comparison. The total flow rate of methylsilane was 1600 sccm and the temperature of the graphite substrate was 1300 8C. As shown in Fig. 1, the deposition rate increased linearly with increasing reactant concentration. Hunt and Stirl [13] have shown that the growth rate r is determined by the relationship expressed as r ˆ cE . P0 . TF, where cE, P0 and TF are deposition yields in experimental work, partial pressure of reactants and total system pressure, respectively. Thus, increase of the molar fraction (X/(X ‡ H2), X ˆMTS, DDS, TCS or TS) leads to linear increase in the growth rate. By using DDS (C/Si ˆ 2), a higher growth rate was achieved, while it decreased with TCS or TS. The temperature dependence of the deposition rate is shown in Fig. 2. In the MTS ‡ H2 system, as is