Local equilibrium phase diagrams: SiC deposition in a hot wall LPCVD reactor
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Ching Yi Tsai Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0219 (Received 7 January 1994; accepted 13 April 1994)
Traditional CVD phase diagrams, which neglect the depletion effects in a hot wall reactor and assume the gaseous species concentrations at the substrate are the same as input concentrations, are at best valid for a cold wall reactor. Due to the constant change of gaseous species concentration along the length of the reactor, traditional CVD phase diagrams do not accurately predict the phases in the deposit on the substrate in a hot wall CVD system. In this paper, a new approach to calculate the local equilibrium CVD phase diagrams at the substrate is presented by coupling the depletion effects in a hot wall reactor to the equilibrium thermodynamic computer codes SOLGASMIX-PV. The deposition of SiC using the gas system of methyltrichlorosilane (MTS)-hydrogen (H2) under low pressure was chosen for this study. Differences between the new CVD phase diagrams and the traditional phase diagrams for this gas system are discussed. The calculated CVD phase diagrams are also compared with the experimental data both from our own experiment and from the literature. The local equilibrium phase diagrams predicted the deposition of a single phase of SiC much better than those without the consideration of the depletion effects. The experimental regions for depositing single phase SiC are larger than the calculated local phase diagrams. This is attributed to the higher linear velocity of the gas flux under low pressure and the polarity of the Si carrying intermediate species.
I. INTRODUCTION Chemical vapor deposition (CVD) working under reduced pressure conditions and a hot wall reactor is currently used in the semiconductor industry to manufacture microelectronic devices because of its high wafer throughput rate, improved layer thickness homogeneity, and the elimination of contamination from the susceptor by using very low deposition pressure.1 Although lots of models have been developed to study the CVD phase diagrams, all of the calculated phase diagrams are based on the cold wall assumptions in which the precursor concentrations at the substrate are assumed to be the same as those at the inlet of the reactor.2"4 For a hot wall reactor, the temperature profile is continuously increasing from the inlet of the reactor to the position where the substrates are located. Thus it is expected that the precursors will undergo some chemical reactions (i.e., depletion effects) along the reactor, which exhaust the precursor concentrations and generate some intermediate gaseous species. Moreover, the other kinetic
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J. Mater. Res., Vol. 9, No. 8, Aug 1994
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factors such as thermal diffusion and thermal convection of the gas flux were also believed to play a role in the depletion effects. Therefore, with the constant changes in the re
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