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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 55 As a consequence of past activities in generating databases for combustion, lasers, and atmospheric chemistry, techniques are now available for measuring or calculating many of the missing ion and neutral chemistry rate coefficients and cross sections. It is important to leverage that existing capability to address the database needs discussed here. A task equal in importance to the generation of rate coefficients and cross sections is the construction of "reaction mechanisms." In many cases these mechanisms may consist of reduced reaction sets that address the conditions of interest but may not be applicable to, for example, higher pressures. In constructing them, sensitivity analyses will identify key reaction sets or sequences that are particularly important and for which data may be lacking. In all cases, the choice of the systems of interest must be aligned with the current planning of the semiconductor industry, as indicated in the Semiconductor Industry Association's roadmap.28 Ion Processes The ion-molecule database is fragmentary for most of the chemistries of interest. The majority of ion- molecule reaction rate coefficients that are now available were not produced specifically for the semiconductor manufacturing industry, but rather for upper atmospheric chemistry, high-energy radiation chemistry, and gas- discharge or e-beam excited lasers. The techniques developed for generating those databases may be applied to the systems of interest with little new invention. Data for ion-molecule reactions in which either reactant is a radical are generally missing from existing databases and are of crucial importance to plasma processing. In principle, existing techniques can be applied to producing these data; however, new invention may be required to produce pure radical sources. In all cases, rate coefficients for the distribution of and branching ratios for products, and for reactions with radical species, are the least well known and therefore deserve particular attention. Electron-ion (dissociative) recombination and ion-ion neutralization are also classes of reactions for which experimental techniques exist to measure rate coefficients in the systems of interest. The major gap in the database is the identity of the products. New techniques may be necessary to resolve the identity and branching ratios of the products. Neutral Chemistry The database for neutral chemistry is fairly complete for select deposition systems, and significantly less complete for most etching systems. Incomplete data on rate coefficients for those systems are found predominantly among reactions of electronically, vibrationally, or translationally hot species, and among reactions in the fall-off pressure regime. Given thermodynamic properties, vibrational frequencies, and so on, computational methods are available to generate many of these rate coefficients, particularly for translationally and vibrationally hot species, and for the fall-off pressure regime. There is much greater uncertainty in calculations for reactions of electronically excited species. Applying and improving computational methods to fill the gaps in the neutral chemistry database are to be encouraged. These activities should be coordinated with a less exhaustive experimental program that provides high confidence and well-characterized rate coefficients for validating and benchmarking the computationally derived values. THERMOCHEMICAL DATA Thermochemical data describe the initial and final states of a chemical reaction in equilibrium with a thermal bath. Such data are essential for benchmarking (testing) models of chemical reactions, e.g., testing the validity of theoretical potential energy surfaces used in reaction models. These data are also29 used to reduce the complexity of possible reaction schemes used in chemical kinetics models by screening out energetically unfavorable reactions. In addition, in some cases it is possible to estimate reaction rate coefficients using thermochemical kinetics and transition state theory.
ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 56 Thermochemical data of interest for plasma processing include homogeneous reaction energies, entropies, and energy levels for chemical reactions; molecular dissociation, chemi-ionization, and negative ion formation for ground and excited states of neutral and ionic species; heterogeneous reaction energies, such as heats of desorption; and solid state and thin film quantities related to annealing and transport of impurities and defects. In plasma processing discharges, significant deviations from local thermal equilibrium can exist even for neutral species, and this complicates the use of thermochemical information. Thermochemical properties are often of direct use in the modeling of thermal chemical processing systems, where the kinetic and internal energies of the reacting species are more properly described by a local temperature. Most of the currently available thermochemical databases have been developed to serve other fields, such as rocket fuel combustion, upper atmosphere chemistry, and pollution abatement. These databases do not include many of the more reactive gas-phase species of interest for plasma processing. One of the best known sources of thermochemical data, the JANAF Thermochemical Tables,30 provides data on molecular geometries, vibrational frequencies, heat capacities, entropies, enthalpies, and equilibrium constants. The extensive polynomial fits to the temperature-dependent thermochemical data are not usually applicable to models of plasma processing systems. The only relevant compiled sources of thermochemical data found for solids or gas-solid interfaces of interest in plasma processing are represented by the listings of thermochemical properties for solid silicon, carbon, and boron in the JANAF tables. Particularly needed are data on heats of desorption for various adsorbed and absorbed species. Understanding of surface thermochemistry, although not well enough developed to provide an exhaustive compilation, would benefit from data compilations and evaluations to establish the state and availability of the database. Although the first step in this process is to assemble and disseminate the currently available data, the value of evaluated databases must be emphasized. Data and data gathering techniques in original journal articles generally need to be evaluated further by critical comparison with other data and other techniques. Thermochemical data for gas phase etching-related compounds, such as SinClm, CnClm, BnClm, and their derivatives, are generally available. One concern is that there are significant differences (10 kcal/mol or 0.5 eV) among heats of formation of some of the SiClx molecules and ions (P.B. Armentrout, University of Utah, private communication, 1995). Thermochemical data for gas phase compounds related to amorphous hydrogenated silicon, such as SinHm, and their derivatives are also generally available. No reviews or compilations of data are known to the panel, however, regarding the thermochemistry of these species on semiconductor surfaces. Theoretical calculations of thermochemical data have become very useful for certain classes of compounds, such as organic molecules and small inorganic molecules with atoms from the first and second rows of the periodic table.31 In addition, recent work has suggested that the reactions of fluorinated hydrocarbons with oxygen, for example, can be fruitfully analyzed using ab initio techniques coupled with transition state theory.32 One possibility to improve the database is to develop multiple-species discharge systems for reaction measurements, such as the flowing afterglow technique. This apparatus was successfully applied to determining the energetics and kinetics of ion-molecule and radical reactions of interest to upper atmosphere and pollution chemistry. The principal effort here would have to be placed on surface reactions and properties using plasma- generated species. Currently, experiments on surface reactions are based largely on thermal processes, which generally address incident species at energies below about 700 K. The calculation of therrnochemical properties of ground state species using quantum chemistry and empirical approaches has reached the point of considerable utility for estimation of unknown values. In some cases, however, discrepancies with experimental data emphasize the need for additional refinements to the techniques. Although the overlap with thermochemistry data requirements is limited, it is desirable to maintain a collaboration with the proposed data generation efforts for chemical vapor deposition (CVD).33
