Ab Initio Computation of Thermochemistry and Kinetics in the Oxidation of Gas Phase Silicon Species
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Ab Initio Computation of Thermochemistry and Kinetics in the Oxidation of Gas Phase Silicon Species Michael R. Zachariah and Wing Tsang Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Md 20899 Abstract Ab initio molecular orbital calculations coupled to RRKM reaction rate theory have been conducted on some important reactions involved in the oxidation of silane in a high-temperature/high H20 environment. The results indicate that H20 acts as an oxygen donor to SiH 2 to form H 3 SiOH or SiH 2 0. Subsequent reactions involve the formation of (HSiOOH, H2Si(OH) 2 , :Si(OH) 2 or SiO). In turn SiO polymerizes into planar rings, without an activation energy barrier. A list of calculated thermochemical data are also presented for a number of equilibrium species. Introduction While considerable effort has been expended using experimental and theoretical approaches to understand silane pyrolysis chemistry, relatively little attention has been paid to the analogous chemistry of silane oxidation. In addition to its importance in the microelectronics area, the oxidation chemistry is of importance to ceramic particle synthesis and fumed formation of silica powders for the fiber optics industry. In this paper we describe some ab initio molecular orbital computations for silane oxidation in a high humidity environment. Such environments are typically found in the combustion synthesis of ceramic powders [1] as well as SiO 2 deposition of thin films. FS _
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Species and reactions chosen for study were based on prior in-situ gas phase species measurements made in a nanophase Si/Si0 2 particle-forming
siH2
-
SiH3
reactor in conjunction with detailed
SiHH
•chemical 0
S
kinetic modeling [2-4]. These studies allowed us to determine which
OH
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s~io HS(H J
)
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H•i0H
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!important
H2Si(OH) 2
SiOn-mer
Si02
Figure 1. Primary pathways determined from detailed chemical kinetic modeling,
pathways required further more detailed analysis. Fig. 1 outlines the most pathways as determined by analysis. It is important to note that the mechanism shown here is based on the
"theabove
experimental conditions studied, where oxidation was taking place under essentially anaerobic conditions. This absence of molecular oxygen required that we speculate that water vapor was the primary donor of oxygen to silicon species through a variety of pathways. The purpose of this paper is to use highlevel theoretical computations to assess these assumptions.
Mat. Res. Soc. Symp. Proc. Vol. 282. ()1993 Materials Research Society
494
Comnutational Procedure All molecular orbital (MO) calculations employed the Gaussian90 [5] program for the determination of geometries, energies and vibrational frequencies. The results of these computations were used to obtain reaction rates using unimolecular reaction rate/transition state theory. Most of the geometries were computed at the MP2/6-31G* level, vibrational frequencies at the corresponding Hartree-Fock level (using the
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