Paper PS-ThA6
Transient Phenomena in Power Modulated Chlorine Plasma

Thursday, October 25, 2018, 4:00 pm, Room 104A

Power-modulated plasmas (i.e. rapid switching from high to low power) could have some potential advantages over conventional pulsed plasmas (where power is periodically turned completely off) for plasma processing in that a larger parameter space is available between fully pulsed and continuous power. In the present study, power at 13.56 MHz applied to a mostly chlorine inductively-coupled plasma was modulated between a high power and low power state. Time-resolved optical emission, Langmuir probe, and forward and reflected power measurements were performed. Two distinct types of transient phenomena were found upon switching from the high power to low power state. In a “normal” mode, electron temperature (T_e) remains constant, while electron and ion number densities (n_e and n_i⁺) and optical emission intensities smoothly drop to a level roughly equal to the fractional drop in power. In a second “anomalous” mode, n_e, T_e and optical emission intensities rapidly drop and stay low for an extended period before rising to values commensurate with the drop in power. Under many circumstances, a single delay time is found that depends on pressure and power duty cycle. In some cases, two delay times can be found, with subtle changes in matching network settings causing one delay time or the other. The anomalous mode can exhibit complex behavior such as two or three phases in the low power period, and periodicity at half the pulsing frequency. The anomalous mode will be discussed in terms of negative ion-driven instabilities.