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Suggested Citation:"Ion-Ion Neutralization." National Research Council. 1996. Database Needs for Modeling and Simulation of Plasma Processing. Washington, DC: The National Academies Press. doi: 10.17226/5434.
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Page 50

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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.)

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In spite of its high cost and technical importance, plasma equipment is still largely designed empirically, with little help from computer simulation. Plasma process control is rudimentary. Optimization of plasma reactor operation, including adjustments to deal with increasingly stringent controls on plant emissions, is performed predominantly by trial and error. There is now a strong and growing economic incentive to improve on the traditional methods of plasma reactor and process design, optimization, and control. An obvious strategy for both chip manufacturers and plasma equipment suppliers is to employ large-scale modeling and simulation. The major roadblock to further development of this promising strategy is the lack of a database for the many physical and chemical processes that occur in the plasma. The data that are currently available are often scattered throughout the scientific literature, and assessments of their reliability are usually unavailable.

Database Needs for Modeling and Simulation of Plasma Processing identifies strategies to add data to the existing database, to improve access to the database, and to assess the reliability of the available data. In addition to identifying the most important needs, this report assesses the experimental and theoretical/computational techniques that can be used, or must be developed, in order to begin to satisfy these needs.

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