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Suggested Citation:"Neutral Chemistry." 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 51

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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 51 Cross sections for these long-range Coulomb collisions are not particularly sensitive to the internal structure of the reactants, and therefore they are amenable to scaling laws based on the electron affinity and ionization potential of the reactants. For example, semiempirical cross sections for ion-ion neutralization at low pressure have been proposed by Moseley et al.12 Electron-Ion Recombination Collisions between electrons and positive ions resulting in neutralization of the ion constitute electron-ion recombination. The rates of these reactions scale with the square of the plasma density. Since electron-ion recombination results from a long-range Coulomb force, the cross section often scales inversely with energy. The total rate of electron-ion recombination reactions will therefore usually be ignorably small in the sheaths, and their cross sections (or rate coefficients) are required only in the thermal energy range. At the plasma densities and temperatures of interest for plasma processing, radiative and collisional radiative recombination are not important. Dissociative recombination of molecular ions is the only significant volumetric recombination process. The distribution of neutral products should be identified, as well as their translational energies. Cross sections for these processes are widely scattered in the literature, with few examples for systems of interest to plasma processing. Recent compendia and reviews of cross sections and rate coefficients can be found in Mitchell13 and Adams.14 The temperature dependencies of these processes are also important.15 Ion-Neutral and Neutral-Neutral Excitation Inelastic collisions between ions and neutral species that result in excitation, dissociation, or ionization of the neutral target without transfer of charge are classified as ion-neutral excitation collisions. Hot atom collisions on neutrals can similarly instigate excitation, ionization, or dissociation. Since there is a threshold energy associated with these collisions, they are important only for superthermal ions and usually only in the sheath regions. Similar reactions involving hot atoms may occur throughout the bulk plasma. There is fragmentary data scattered through the literature for energetic ion-neutral and neutral-neutral excitation collisions. Phelps16 has compiled and assessed complete cross section sets (see Figure 6.3) for ion and high-energy neutral impact reaction mechanisms for Neutral Chemistry Many neutral chemistry databases have been developed for combustion and atmospheric chemistry?17 Processes for evaluation of those databases are well established, and so evaluation is not addressed here. Instead, the panel assesses the status of the availability of rate coefficients and proposes methods to address unmet needs. It is important to emphasize that these needs are better stated in terms of mechanisms, as opposed to a collection of rate coefficients.

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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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