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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 52 Status of the Database The database requirements for neutral chemistry are both more lenient and more stringent than those for combus- tion and upper atmospheric chemistry. The gas temperatures of plasma processing systems are typically low, rarely exceeding a few hundred degrees above ambient temperature. Therefore gas phase chemical reactions that have significant activation energy barriers are not important. Plasma processing systems with high gas temperatures typically operate at low gas pressures, where the rates of gas phase chemical reactions are small compared to wall- activated chemistry. At the same time, there are large densities of molecules and atoms in plasmas that have internal energy (vibrational or electronic) or that are translationally âhotâ, and that therefore breach activation energy barri- ers. The low gas pressures used in tools with high plasma densities (ICP, ECR)may further restrict the number of reactions that one must address. Association reactions typically operate through a transition state that must be stabilized by colliding with a third body to complete the reaction. For example, the association reaction proceeds as The effective 2-body rate coefficient is If the operating pressure is sufficiently low so that the back reaction of Cl2* to 2Cl is fast compared to the rate Figure 6.3 of stabilizing collisions, then the effective rate of asso- Cross sections for excitation and ionization of H2 re- ciation is small and the reaction may be ignored. The sulting from impact of Ar + on H2 , and by impact of weakness in the existing databases for neutral combus- H2 on Ar. (Reprinted, by permission, from A.Phelps, tion and atmospheric chemistry is that they were intended J.Phys. Chem. Ref. Data 21:883 (1992). Copyright © to be used at high pressure, and therefore lack rate coef- 1992 by the American Institute of Physics and the ficients for the low pressure fail-off regime. American Chemical Society.) The important reactions may also depend on the oper- ating conditions of the reactor, such as power deposition and gas residence time. For purposes of discussion, it is useful to define three classes of species:feedstock (F), secondary (S), and primary (P). The feedstock species are those gases that flow into the reactor from the outside. Primary species, usually radicals, axe atoms or molecules that result from direct dissociation of the feedstock gases, by either electron impact or ion-molecule reactions. Secondary species are produced by reactions between primary species or of primary species with feedstock gases. The degree to which F-P, P-P, P-S, or S-S reactions dominate is largely a function of the degree of dissociation of the gas. A convenient measure of the degree of dissociation is
