TARGET ABLATION CHARACTERISTICS DURING PULSED-LASER DEPOSITION OF THIN-FILMS

A theoretical study into the target ablation characteristics during pulsed laser deposition of thin films was performed. The optical properties (absorption coefficient, reflectivity), power density of the laser pulse and thermophysical properties of the material including thermal conductivity, heat capacity, etc., control the nature of the evaporation process. Based on the laser target interactions, two separate heating regimes, depending on the thermal diffusion distance and the absorption coefficient of the target, have been identified: (i) the surface heating regime in which the laser energy is deposited near the target surface, and (ii) the volume heating regime in which a volume of the solid material is heated by the laser pulse. In both these regimes, vaporization of the target can proceed either by planar surface etching or by volume etching in which solid material may be ejected from the target surface. The thermophysical and the optical properties of the target, and the laser parameters determine the specific nature of the ablation regime. Volume ablation is more likely to occur in the volume heating regime where target absorption coefficients for the incident laser beam are relatively small. The theoretical aspects of both these heating regimes are investigated in detail. The effect of plasma absorption in controlling the ablation characteristics is also theoretically investigated.