The Decomposition of Methyltrichlorosilane in Hydrogen and Helium
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THOMAS H. OSTERHELD AND MARK D. ALLENDORF Sandia National Laboratories, Mail Stop 9052 Livermore, CA 94551-0969
ABSTRACT Experimental measurements of the decomposition of methyltrichlorosilane (MTS), a common silicon carbide precursor, in a high-temperature flow reactor are presented. Methane, hydrogen chloride, and silicon tetrachloride are observed as products of the decomposition. Trapping experiments with acetylene and ethylene also detected SiC13 as a decomposition product. Upper limits on the concentrations of any CH 3 Cl, HSiC13 , H2 SiC12 , or H2 C=SiC12 which might form are provided. Quantitative measurements of product branching and MTS decomposition rates are presented. The results suggest a radical-chain mechanism for the decomposition in hydrogen but not in helium.
INTRODUCTION Methyltrichlorosilane (MTS; CH 3 SiCI 3 ) is a commonly used precursor in the chemical vapor deposition/infiltration of silicon carbide (SiC) [1, 2]. The kinetics of SiC chemical vapor deposition from MTS are of considerable interest since computational models are needed to assist in the optimization and scale-up of new synthetic methods. Unfortunately, little is known about the gas-phase decomposition kinetics of this system. In an early study, Burgess and Lewis used a flow reactor to measure the temperature dependence (973-1023 K) of MTS decomposition in a hydrogen carrier gas at atmospheric pressure [3]. Recently, we published a theoretical study on the kinetics of MTS decomposition using RRKM theory [4]. The results show that Reaction 1 (M is a third body) is the dominant unimolecular decomposition pathway and suggest that the rate constant obtained by Burgess and Lewis overestimates the unimolecular MTS decomposition rate by a factor of 5-7 under the conditions of their experiment (1 atm, 973-1023 K). CH 3 SiC13 + M -- CH 3 + SiCI3 + M
(1)
Clearly, more experimental work is needed to establish the decomposition kinetics and reaction pathways for MTS. In this paper, we report measurements using a high-temperature flow reactor (HTFR) coupled with mass spectrometric detection to characterize the decomposition of MTS in both helium and hydrogen carrier gasses. The objectives of this study are: 1) to identify intermediates and products of MTS decomposition, and 2) to obtain quantitative information concerning the rates of decomposition. In addition, we report limited measurements of the decomposition of MTS in hydrogen carrier gas, the chemistry of which appears to be considerably more complex than that observed in helium carrier gas. *Work supported by the Advanced Industrial Materials Program of the U.S. Dept. of Energy Office of Industrial Technologies. 27
Mat. Res. Soc. Symp. Proc. Vol. 363 C 1995 Materials Research Society
EXPERIMENTAL METHODS Reactions in the HTFR occur within a graphite tube with an inner diameter of 5.0 cm and a length of 100 cm. The tube is enclosed within a water-cooled, insulated vacuum chamber. Three independently controlled graphite heating elements surround the tube and heat the gases flowin
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