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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 50 The energy dependence and product channels of cross sections for these reactions are required. Measurements of these cross sections for systems of interest to plasma processing are in general sparse; however, definitive experiments for a subset of the gases of interest have been performed by Armentrout et al.10 for energies of 0 to 80 eV. (See Figure 6.2.) These systems include The large variety of ion-molecule systems and possible reaction channels puts a large premium on developing scaling laws for these interactions. In this regard Armentrout et al.11 have proposed scaling laws for cross sections of endothermic ion molecule collisions. Ion-Ion Neutralization Ion-ion neutralization processes are collisions between positive and negative ions that result in the neutralization of both reactants. The rates of these reactions scale with the square of the plasma density, while the magnitude of Coulomb cross sections scales inversely with energy. Therefore ion-ion neutralization cross sections are required for thermal energies, but are not particularly important for superthermal energies. The total rate of ion-ion neutralization reactions will usually be ignorably small in the sheaths where positive ion energies are large and negative ion densities are small. The distribution of products should be identified. Figure 6.2 Cross section set for impact of on SiF4. (Reprinted, by permission, from E.R. Fisher and P.B. Armentrout, Chem. Phys. Lett. 179:435 (1991). Copyright © 1991 by Elsevier Science - NL.)
