Ignition Delay in a Pulsed Inductively Coupled Plasma (ICP) in Tandem with an Auxiliary ICP

Plasma ignition delays were observed in a ``main'' ICP, in tandem with an ``auxiliary'' ICP. The Faraday-shielded ICPs were separated by a grounded metal grid. Power (13.56 MHz) to the main ICP was pulsed with a frequency of 1 kHz, while the auxiliary ICP was operated in continuous wave (cw) mode. In chlorine plasmas, ignition delay was observed for duty cycles greater than 60{\%} and, in contrast to expectation, the delay was longer with increasing duty cycle up to $\sim$ 99.5{\%}. The ignition delay could be manipulated by changing the auxiliary and/or main ICP power. Langmuir probe measurements provided the temporal evolution of electron temperature, and electron and positive ion ($n_{+})$ densities. These measurements revealed that the plasma was re-ignited shortly after the decaying $n_{+}$ in the main ICP reached the density ($n_{+,aux})$ measured when only the auxiliary ICP was powered. At that time, the depressed electron density increased sharply resulting in plasma re-ignition. Plasma ignition delay occurred when the afterglow of the pulsed plasma was not long enough for $n_{+}$ to reach $n_{+,aux}$ during the afterglow. Besides Cl$_{2}$, plasma ignition delays were also observed in other electronegative gases (SF$_{6}$, CF$_{4}$/O$_{2}$ and O$_{2})$ but not in an electropositive gas (Ar).