Charged species profiles in oxygen plasma

The charged species profiles have been studied in oxygen plasma of asymmetr ically driven capacitively coupled rf discharge. The electron (n(e)), negat ive (n), and positive (p) ion densities have been measured by Langmuir prob e in a wide range of pressures (3-30 Pa) and interelectrode distances (2-10 cm). At small powers (n(e)< 10(15) m(-3)) it was obtained that the electro negativity is high (n/n(e)similar to 10) and the electron density profile i s flat in plasma bulk. The negative ion density profile is nearly flat in t he ion-ion core, where n > n(e), and falls off towards the plasma-sheath bo undary with n approximate to 0 with the spatial scale pi root D-n/(2 gamma( d)), determined by the negative ion diffusion coefficient D-n and detachmen t frequency gamma(d). As the pressure decreases, this spatial scale exceeds half of the interelectrode distance and the negative ion density profile t ends to a parabolic one. The obtained spatial distributions of charged part icles are compared with the results of self-consistent modeling by use of a fluid approach for ions, and kinetic equation for electrons. Numerical sim ulation results agree well with both theory predictions and measurement dat a. (C) 2000 American Institute of Physics. [S0003-6951(00)00532-5].