Simulations of step responses of electronegative radio-frequency capacitively coupled discharges

The transient response of electronegative radio-frequency glow discharges is important for process control, charge free etching, and highly selective etch applications. The step response of typical electronegative process gases (silane at 1 Torr and chlorine at 100 mTorr) is studied using a drift-diffusion model for silane and a three-moment model for chlorine. The silane simulations include a blocking capacitor whereas the chlorine results do not. For the silane results with a blocking capacitor in series with the plasma, it is found that there are three types of transients. Depending on the final steady-state value of the source rf voltage, the step response can be characterized either by smooth transitions in the number densities of species in the discharge from one steady state to the next, temporary extinction of the discharge or a discharge mode characterized by temporary extinction and reignition of the discharge. In the case of silane definite thresholds separate the phenomena. The step response of the chlorine discharges is always characterized by a smooth transition from one steady state to the next. Smooth transitions from one steady state to the next in the case of step decreases in the source voltage are possible since decrease of the negative ion density in the bulk is controlled by ion-ion recombination. It appears that the temporary extinction of the discharge and natural pulsed steady state is the consequence of how the voltage is divided between the gap and the blocking capacitor during the transient and the fact that the attachment coefficient becomes larger than the ionization coefficient at low values of reduced electric field. (C) 1998 American Institute of Physics. [S0021-8979(98)08716-7]