Plasma chemistry at long mean-free-paths

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Abstract
A long mean-free-path (lmfp) model of transport of neutral particles has been developed and applied to the chemistry of a low neutral-pressure (≲2 mTorr) plasma etching system. In cylindrical geometry, using coordinates (r,z) while ignoring angle φ, a transition matrix is set up for an arbitrary mfp that indicates the fraction of the particles originating in the cell at (r′,z′) that experience their next collision in the cell at (r,z). This matrix can be iterated, allowing for chemical reactions, to obtain the steady state density of the neutral species. It can also provide angular distributions of neutrals; at present, their energy distribution is not kept track of, although it will be in the future. The method has been applied to an electron cyclotron resonance plasma in CF4. Using the measured electron energy distribution, the breakdown of CF4 into the species responsible for etching is calculated. Densities of CF4, CF3, CF2, and F are presented for a wide range of conditions.