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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 53 where P is the specific power deposition, No is the ambient gas density, Ï is the gas replenishment time (either by gas flow or surface reactions), and âε is the energy deposition required for molecular dissociation. If η << 1, then the gas is lightly dissociated and F-P reactions are likely the most important. If η ⥠1, then P-P reactions are likely the most important. Finally, if η >> 1, then S-S reactions most likely dominate. The availability of reaction rate coefficients for thermal neutral chemistry for plasma deposition systems is in some instances very good. For example, the SiH4/Si2H6/H2 system has been studied extensively in the context of plasma enhanced and thermal chemical vapor deposition, and compendia of rate coefficients have been assembled for use in models.18 The major uncertainties in these databases are in the formation and reaction of higher silanes . These reactions are particularly important with respect to particle formation. The situation is similar for databases for plasma enhanced chemical vapor deposition (PECVD) of SiO2 using SiH4/O2/N2O mixtures. The gas phase chemistry of this system has recently been addressed in the context of modeling remote PECVD of SiO219 and thermal CVD.20 In this system, the mechanisms for the deposition pathways are still uncertain, largely due to the uncertainties in surface reaction rate coefficients. This uncertainty reflects back on the "goodness" of the gas phase database. For example, it has been proposed that Si-O bonds are formed on the growing film21 and that precursors are dominantly fragments of SiH4 and the oxygen donor. If this is the case, the gas phase database is in moderately good shape. A competing school of thought proposes that Si-O bonds are formed in the gas phase and that silanols (SiHnOm) are the deposition precursors. If this is the case, the gas phase database is not adequate since the major uncertainty in the database results from uncertainties in the formation and reactions of silanols. The status of the database for Si3N4 deposition using SiH4/N2/NH3 mixtures is similar to that for SiO2 and is in fairly good shape.22 The deposition precursors for this system, however, are uncertain, and that situation again affects the "goodness" of the database. It has been proposed that the Si-N bonds form on the surface of the growing film for SH4/N2 mixtures, whereas they form in the gas phase for SiH4/NH3 mixtures.23 In the former case, the present status of the database is fairly good. In the latter case, rate coefficients for formation and reactions of the proposed deposition precursors, amino-silanes (SiHn(NH2)m), are largely unknown. Databases for deposition of dielectrics using gases other than SiH4 are currently inadequate. For example, deposition of SiO2 at low temperatures is often performed using TEOS (Si(C2H5O)4); however, the gas phase chemistry is virtually unknown. In all cases, the experimentally derived databases for reactions of vibrationally and electronically excited species range from very poor to nonexistent. Databases for etching chemistries are less complete than those for deposition, largely because of the lack of experimentally derived rate coefficients. The situation is improving based largely on rapid advances in computational chemistry, which have enabled accurate calculation of rate coefficients in ground and vibrationally excited states. A compendium of experimental and computed rate coefficients for the CF4/CHF3/H2/O2 system has recently been assembled for use at high pressure.24 An assessment of neutral chemistry databases for selected chemistries is given in Table 6.2. Excited State Chemistry and Penning Ionization Internal energy (either electronic or vibrational) of reactants increases the amount of energy available and therefore may bridge or reduce activation energy barriers. In plasmas, significant fractions of the atoms and molecules can have internal activation energy. To first order, one might simply reduce the activation energy barrier of the process by the amount of the internal energy. This, however, is a questionable practice, according to Armentrout.25 Electronic energy may not couple directly into the reaction coordinate, and the reaction efficiencies of ground and excited states are often very different even after the effect of total energy has been accounted for.
