Reactant properties, high pressure chemical effects

The reactant gases, also referred to as precursor molecules, are chosen to react and produce a specific film. Properties necessary for a good precursor include thermal stability at its vaporization temperature and sufficient vapor pressure (at least —125 Pa) at a reasonable temperature (—300°C) for effective gas phase delivery to the growth surface. In addition, the molecules must be obtainable at high purity and must not undergo parasitic or side reactions which would lead to contamination or degradation of the film (10). Examples of the classes of precursor molecules (e.g., hydrides, halides, carbonyls, hydrocarbons, and organ-ometallics) and the types of chemical reaction (pyrolysis, oxidation/hydrolysis, reduction, carbidization/nitridation, and disproportionation) are summarized in Table 1.3. 

Pressure is a fundamental physical property that affects various thermodynamic and kinetic parameters. Pressure dependence studies of a process reveal information about the volume profile of a process in much the same way as temperature dependence studies illuminate the energetics of the process (83). Since chemical transformations in SCF media require relatively high operating pressures, pressure effects on chemical equilibria and rates of reactions must be considered in evaluating SCF reaction processes (83-85). The most pronounced effect of pressure on reactions in the SCF region has been attributed to the thermodynamic pressure effect on the reaction rate constant (86), and control of this pressure dependency has been cited as one means of selecting between parallel reaction pathways (87). This pressure effect can be conveniently evaluated within the thermodynamic framework provided by transition state theory, which has often been applied to reactions in solutions (31,84,88-90). This theory assumes a true chemical equilibrium between the reactants and an activated transition... 

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