We illustrate the input and output of the various Pre-processors and the example problem with an analysis of the deposition of Si3 N4. The gas-phase reaction mechanism contains a detailed description of NH3 decomposition (about which there is much published information), two reactions describing decomposition, and three cross-reactions. At the low pressures we consider, the gas-phase decomposition of reactants is slow. The surface reaction mechanism contains six steps describing the overall conversion of 3 SiF4 and 4 NH3 molecules to 3 Si(d) and 4 N(d) and 12 HF.
Note: The surface reaction mechanism is from a preliminary analysis at one temperature, and thus we have not supplied any activation energies. Also, the thermodynamic data in the mechanism is contrived and should not be used in other contexts. As such, this mechanism should only be considered as illustrative and not as a source of kinetic data on the Si3 N4 system.
The input to the sample problem gives the initial pressure as 2.63 × 10-3 atm (2 Torr) and temperature 1713 K. In this problem the temperature is fixed, but the pressure will increase as 12 moles of HF are produced for every 7 moles of reactant destroyed. The input gas-phase mole fractions represent a 6:1 ratio of NH3 to SiF4. Initial site fractions of the surface species came from a steady-state analysis of the system (not discussed here). The initial activities of the two bulk species are set to 1. The area to volume ratio is 6 (a cubic box).
The print-out from the sample problem shows the initial conditions followed by print-outs of the concentrations at subsequent states of the deposition. The reactants SiF4 and NH3 are seen to be depleted and the product HF forms. In this fixed-volume system the pressure rises (discussed above). There is a net decrease in the gas density as the heavy Si and N atoms are lost from the gas into the bulk.