Computer modeling of the interaction of a laser-ablated plume with an ambient background gas

It is now well established that thin films of a wide variety of materials can be deposited by ablation of a target material by a laser. Here, expansion of the laser-ablated plume in vacuum and in a background gas is comparatively studied using continuum hydrodynamics models, molecular gas dynamics models, and a recently developed multiple scattering model which combines continuum hydrodynamics and inter-species collisions. Continuum hydrodynamics models and molecular gas dynamics models predict, for the most part, that background plasma would reach the deposition substrate (or an ion probe placed at the same distance away from the target) first. On the other hand, the multiple scattering model shows that a component of the plume can indeed reach the substrate at vacuum speed, followed by a second plume component which is more or less slowed down by the presence of the background gas depending on its ambient pressure. Quantitative fits to the experimental data have been obtained with this multiple scattering model for expansion of Silicon in Helium and in Argon. The successful application of the multiple scattering model serves to explain the phenomenon of `plume splitting' which is frequently observed in laser ablation processes for thin film deposition.