THE DYNAMICS OF HOT-F ATOMS IN LOW-PRESSURE PLASMA-ETCHING REACTORS

The time dependent distribution function of translationally energetic F atoms produced by electron impact dissociative collisions with CF4 in a low pressure CF4 plasma is determined from the solution of the Boltzmann equation. The hot F atom distribution function departs from equilibrium owing to the strength and energy distribution of the source atoms and is driven to equilibrium via F-CF4 elastic collisions. The extent of the departure from equilibrium depends on the rate of production of hot F atoms by reaction and the rate of thermalization by elastic collisions. The time dependent hot F distribution function is determined from the Boltzmann equation for an initial non-equilibrium distribution, with and without a steady source. The differential elastic collision cross section for F-CF4 collisions, that defines the collision operator in the Boltzmann equation, is calculated quantum mechanically. The phase shifts are determined with the WKB approximation for an appropriate Lennard-Jones 6-12 potential for the F-CF4 interaction. The results of these calculations suggest that hot neutral F radicals may contribute to the etching process and be competitive with ion activated etching. The enhancement of reaction rates owing to energetic F atoms is also determined and compared with the corresponding thermal rates.