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Chemkin 4.1.1 Chapter 4: Materials Problems The chem.inp file includes 3 elements, 11 gas-phase species, and 9 reversible gasphase reactions. The gas-phase reactions are reversible decomposition reactions of various chlorosilanes and chlorinated disilanes. These reactions cause the conversion of some of the initial chlorosilane starting material to these other gasphase species, which can be significant because the less-chlorinated molecules have higher surface reactivities. These reactions are written as unimolecular decomposition reactions at their high-pressure limit, so this reaction mechanism would tend to overstate the importance of gas-phase chemistry if it were used at lower total pressures. The surf.inp file has 3 surface species: open silicon surface sites, hydrogen-covered sites, and chlorine-covered sites, plus solid silicon as a bulk phase. There are 10 surface reactions, all written as irreversible reactions with placeholders for the thermodynamic data of the surface species. These reactions include the dissociative adsorption of SiCl 3 H, SiCl 2 H 2 and SiCl 4 , which result in the formation of deposited silicon and hydrogenated/chlorinated silicon surface species. These are “lumped reactions” to the extent that the initial adsorption event, plus the successive transfer of H and Cl atoms from the initially adsorbed Si to other surface Si atoms, are all described as one step. The physical interpretation of this “lumping” is that adsorption of the gas-phase species is assumed to be slow compared to subsequent transfer of H and Cl atoms on the surface. For a chemically balanced reaction, this is written as involving 4 open sites, which by default would make the reaction fourth order in open sites, so the coverage dependence option has been used to set the kinetics to a more reasonable first-order dependence. Other reactions include the dissociative adsorption and associative desorption of H 2 , HCl and SiCl 2 , plus the dissociative adsorption of HSiCl. In many cases, the rate parameters are based on experimental surface-science studies, in other cases they are the result of fitting a model to experimental silicon deposition rate data from a specific CVD reactor. 4.4.4 Alumina ALD This chemistry set describes the atomic layer deposition (ALD) of alumina from trimethylaluminum (TMA) and ozone. This mechanism is deliberately simplistic for illustration purposes only. It demonstrates one way of describing the ALD of alumina, but it should generally be considered as illustrative only and not used as a source of 35. “Chemical Kinetics for Modeling Silicon Epitaxy from Chlorosilane”, P. Ho, A. Balakrishna, J. M. Chacin, A. Thilderkvist, B. Haas, and P. B. Comita, in “Fundamental Gas-Phase and Surface Chemistry of Vapor-Phase Materials Synthesis”, T. J. Mountziaris, M. D. Allendorf, K. F. Jensen, R. K. Ulrich, M. R. Zachariah, and M. Meyyappan, Editors, PV 98-23, p. 117-122, Proceedings of the 194th Meeting of the Electrochemical Society, 11/1-6/98, The Electrochemical Society Proceedings Series, Pennington, NJ (1999). © 2007 Reaction Design 156 RD0411-C20-000-001
Chapter 4: Materials Problems <strong>Tutorials</strong> <strong>Manual</strong> kinetic data for this process. This mechanism is designed to deposit stoichiometric alumina, with three oxygen atoms being deposited for each two aluminum atoms. If the goal of a simulation was to track the elemental make-up of a material with a wide range of possible compositions, the choice of surface species and reactions would have to be quite different. The fact that the different chemicals are separately pulsed into the system, rather than being mixed in the gas, prevents many possible gas-phase reactions from occurring in this process. The gas-phase chemistry described in the chem.inp file has 5 elements: Al, C, H, O, and Ar; and 6 gas-phase species: AlMe 3 , O, O 2 , O 3 , C 2 H 6 and Ar. There are only two gas phase reactions; the collisionally-induced decomposition of ozone and the reaction of O atoms with ozone to form molecular oxygen. The rate parameters are from Benson and Axworthy. 36 This mechanism does not include reactions for TMA decomposition because it was determined to be too slow at the temperatures of interest. But if such reactions were included, they would be listed in the same chemistry input file. The temporal separation of the gas mixtures is accounted for by omitting reactions between gas-phase species that would not be present in the reactor at the same time, such as reactions between TMA and O atoms. A reaction mechanism for alumina CVD, in contrast, would need to include all such reactions. The surface chemistry described in the surf.inp file defines three surface species: O(S), ALME2(S) and ALMEOALME(S); where the last species occupies two surface sites, plus the bulk alumina AL2O3(B). There are only three surface reactions, each of which represents several elementary surface processes. The first surface reaction represents the dissociative adsorption of TMA on the oxygenated surface species O(S) to form the ALME2(S), combined with the recombination of two methyl groups and desorption of an ethane molecule. This reaction has been given a moderately high sticking coefficient of 0.1, and is written in terms of a half ethane molecule in order to have a balanced reaction that is first order in TMA. This reaction only occurs in the presence of TMA, and will terminate when all of the O(S) surface species have reacted. The second surface reaction describes gas-phase oxygen atoms reacting with the two ALME2(S) to form the ALMEOALME(S) species and a gas-phase ethane molecule. The third surface reaction describes an O atom reacting with the ALMEOALME(S) species to deposit AL2O3(B), regenerate O(S), and form gas-phase ethane, where fractional molecules are used to write balanced reactions. Oxygen atoms are expected to be very reactive, so these reactions have been given high sticking coefficients of 1.0. These reactions only occur in the presence of O atoms generated from ozone decomposition, and will terminate when all the methylated 36. S. W. Benson and A. E. Axworthy, Jr., J. Chem. Phys., 26:1718 (1957). RD0411-C20-000-001 157 © 2007 Reaction Design
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Chemkin 4.1.1 Contents Table of Con
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Chemkin 4.1.1 Tables List of Tables
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Tutorials Manual 4.1.1 Figures List
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List of Figures Tutorials Manual 2-
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List of Figures Tutorials Manual 4-
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Chemkin 4.1.1 1 1 Introduction The
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Chemkin 4.1.1 2 2 Combustion in Gas
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Chapter 2: Combustion in Gas-phase
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Chemkin 4.1.1 3 3 Catalytic Process
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