MODELING ELECTRONEGATIVE PLASMA DISCHARGES

A macroscopic analytic model for a three-component electronegative plasma has been developed. Assuming the negative ions to be in Boltzmann equilibrium, a positive ion ambipolar diffusion equation is found. The electron density is nearly uniform, allowing a parabolic approximation to the plasma profile to be employed The resulting equilibrium equations are solved analytically and matched to an electropositive edge plasma. The solutions are compared to a simulation of a parallel-plane rf driven oxygen plasma for two cases: (1) p=50 mTorr, n(e0)=2.4 x 10(15) m-3, and (2) 10 mTorr, n(e0) = 1.0 x 10(16) m-3. In the simulation, for the low power case (1), the ratio of negative ion to electron density was found to be alpha0 almost-equal-to 8, while in the higher power case alpha0 almost-equal-to 1.3. Using an electron energy distribution that approximates the simulation distribution by a two-temperature Maxwellian, the analytic values of alpha0 are found to be close to, but somewhat larger than, the simulation values. The average electron temperature found self-cosistently in the model is close to that in the simulation. The results indicate the need for determining a two-temperature electron distribution self-consistently within the model.