KINETICS OF LASER-PHOTOCHEMICAL DEPOSITION BY GAS-PHASE DISSOCIATION

The effects of reaction and transport kinetics on deposition resulting from laser-induced gas-phase photodissociation were investigated using numerical and analytical models. Deposition rates and deposit profiles for a laser beam focused onto a substrate in a chamber were determined assuming production of metal atoms in the gas phase by a single-photon dissociation mechanism and free-molecular and diffusive transport to chamber surfaces. The predictions of the model computations were compared to experimental and theoretical results from the literature. The effect of total pressure on deposition rates and profiles depends strongly on the sticking coefficient. With a sticking coefficient of unity, deposition rates and profiles do not depend strongly on the total pressure or transport regime. However, when the sticking coefficient is 0.01, the deposit profile flattens dramatically at pressures below about 1 atm. The conditions and consequences of gas-phase reactant depletion due to diffusional limitations were also demonstrated. The extent of gas-phase reactant depletion can be predicted by a single dimensionless group; however, transport of the reaction product must also be considered to predict deposition rates. At high laser power or total pressure, reactant depletion leads to reduced deposition rates and flatter deposit profiles, and severe depletion can produce volcano-shaped deposits.