THERMODYNAMIC APPROACH TO THE CHEMICAL-VAPOR-DEPOSITION PROCESS

The portion of the free energy change which is solely dissipated for deposition in the chemical vapor deposition (CVD) process is examined. The partial pressure of the species to be deposited in the supersaturated gas phase is determined by minimizing the Gibbs free energy without the condensed phase. The supersaturation ratio for deposition is defined as the ratio of this partial pressure to the equilibrium vapor pressure of its solid. The supersaturation ratio for deposition is the same as the activity when the solid phase is chosen as the standard state. It can be expressed in terms of the partial pressures of the other species which satisfy the law of mass action, allowing the estimation of the supersaturation ratio of the compound whose stoichiometry is not maintained in the vapor phase. This supersaturation ratio is consistent with the CVD phase diagram, being unity along the phase boundary and less than unity in the gas phase region. Based on this scheme, the line of the iso-supersaturation ratio for deposition or etching can be introduced to the conventional CVD phase diagram and the dependence of the CVD processing variables on the supersaturation can be assessed. By applying this scheme to the Si-Cl-H system, the supersaturations for deposition of silicon are evaluated with respect to temperature, pressure and composition.