Charged species profiles in oxygen plasma

The charged species profiles have been studied in oxygen plasma of asymmetrically driven capacitively coupled rf discharge. The electron (n_e), negative (n), and positive (p) ion densities have been measured by Langmuir probe in a wide range of pressures (3-30 Pa) and interelectrode distances (2-10 cm). At small powers (n_e<10¹⁵m^-3) it was obtained that the electronegativity is high (n/n_e∼10) and the electron density profile is flat in plasma bulk. The negative ion density profile is nearly flat in the ion-ion core, where n≫n_e, and falls off towards the plasma-sheath boundary with n≈0 with the spatial scale π√D_n/(2 γ_d), determined by the negative ion diffusion coefficient D_n and detachment frequency γ_d. As the pressure decreases, this spatial scale exceeds half of the interelectrode distance and the negative ion density profile tends to a parabolic one. The obtained spatial distributions of charged particles are compared with the results of self-consistent modeling by use of a fluid approach for ions, and kinetic equation for electrons. Numerical simulation results agree well with both theory predictions and measurement data.