A Monte Carlo simulation of laser ablation during the laser pulse: Cl/sub 2/(s) ablation dynamics for neutral beam etching

Summary form only given. Two requirements for any neutral beam source for etching applications are that the beam be highly directed (that there be little spread to the beam) and that the directed flux of etching precursors be sufficiently high to achieve a reasonable etch rate. Laser ablation of a target material such as frozen Cl/sub 2/ may meet these requirements. To investigate the utility of this scheme, we have been developing a plasma chemistry-fluid model for a chlorine ablation plume expanding into vacuum. A number of challenges exist in this sort of model. One being how to handle the plume/vacuum interface and the other being how to determine an initial condition that will couple easily to a fluid simulation. The latter, the topic of this presentation, deals with the problem that the plume, initially, resembles a critical fluid possessing very high densities and very small dimensions. Further, the critical temperature of chlorine is low suggesting that significant electron production in the plume comes after plume creation initially by photoionization after which the laser couples to the plume by other mechanisms. A Monte Carlo simulation has been developed as a means of calculating an initial condition for the post-laser pulse plume dynamics fluid model. This simulation includes the influence of the laser plume coupling on material removal at the surface (shadowing), provides an estimate of the initial electron density and spatial distribution in the plume and initial fractional dissociation rate. The spatial distribution of the electron density (=3 eV) at 40 ns for a 100 mJ pulse is shown.