Electron density modulation in a pulsed dual-frequency (2/13.56 MHz) dual-antenna inductively coupled plasma discharge

The electron density, n_e, modulation is measured experimentally using a resonance hairpin probe in a pulsed, dual-frequency (2/13.56 MHz), dual-antenna, inductively coupled plasma discharge produced in argon-C₄F₈ (90–10) gas mixtures. The 2 MHz power is pulsed at a frequency of 1 kHz, whereas 13.56 MHz power is applied in continuous wave mode. The discharge is operated at a range of conditions covering 3–50 mTorr, 100–600 W 13.56 MHz power level, 300–600 W 2 MHz peak power level, and duty ratio of 10%–90%. The experimental results reveal that the quasisteady state n_e is greatly affected by the 2 MHz power levels and slightly affected by 13.56 MHz power levels. It is observed that the electron density increases by a factor of 2–2.5 on increasing 2 MHz power level from 300 to 600 W, whereas n_e increases by only ∼20% for 13.56 MHz power levels of 100–600 W. The rise time and decay time constant of n_e monotonically decrease with an increase in either 2 or 13.56 MHz power level. This effect is stronger at low values of 2 MHz power level. For all the operating conditions, it is observed that the n_e overshoots at the beginning of the on-phase before relaxing to a quasisteady state value. The relative overshoot density (in percent) depends on 2 and 13.56 MHz power levels. On increasing gas pressure, the n_e at first increases, reaching to a maximum value, and then decreases with a further increase in gas pressure. The decay time constant of n_e increases monotonically with pressure, increasing rapidly up to 10 mTorr gas pressure and at a slower rate of rise to 50 mTorr. At a fixed 2/13.56 MHz power level and 10 mTorr gas pressure, the quasisteady state n_e shows maximum for 30%–40% duty ratio and decreases with a further increase in duty ratio.