MODELING OF ELECTRONEGATIVE RADIOFREQUENCY DISCHARGES

A continuum model is presented for low-pressure, radio-frequency (RF) electronegative discharges commonly encountered in reactive ion etching and plasma-deposition applications. The model is based on the moments of the Boltzmann transport equations. Local and convective acceleration terms are retained in the momentum equations for the electrons and ions as it allows nonlocal transport in weakly collisionial regions. A stable numerical scheme to solve these equations is also presented. A chlorine discharge at 13.56 MHz is simulated here as a case study. The simulation results reproduce features observed experimentally in Cl2 discharges under similar conditions. Of particular importance is the simulated excitation and ionization waveforms. In the bulk, the waveforms peak twice per cycle, which is essentially due to the modulation of electron temperature; in the sheath regions, the waveforms peak only during the anodic part of the cycle when the electrons are accelerated toward the electrode. The time-averaged ionization, excitation, and attachment rate profiles exhibit peaks near each sheath/plasma interface. The energy of the ions striking the electrodes is computed self-consistently. The ratio of ion energy to the average potential difference between the plasma and the electrodes is found to be 0.33.