Temporal evolution of electron density in a low pressure pulsed two-frequency (60 MHz/2 MHz) capacitively coupled plasma discharge

Time-resolved electron density, n(e), is measured in a low pressure pulsed two-frequency capacitively coupled plasma discharge sustained in Ar and in Ar/CF4/O-2 (80 : 10 : 10) gas mixture using a floating resonance hairpin probe. The top electrode is powered by 60 MHz in pulse mode and the bottom electrode is powered by 2 MHz in continuous wave mode. The dependence of time-resolved n(e) on the low frequency (LF) and high frequency (HF) power levels, operating gas pressure, pulse repetition frequency (PRF) and duty cycle are investigated. It is found that the steady state n(e) in the long on-phase is greatly influenced by the HF power level and slightly affected by the LF power level in both Ar and Ar/CF4/O-2 plasma. The decay time of n(e) is slow (similar to 30-90 mu s) in the case of Ar plasma and strongly depends on the LF power level, whereas in the case of Ar/CF4/O-2 gas mixture it is very fast (similar to 15 mu s) and marginally dependent on LF power level. In Ar plasma the steady state n(e) is increasing with a rise in operating gas pressure, however, in Ar/CF4/O-2 plasma it first increases with gas pressure reaching to the maximum (at 20 mTorr) value and then decreases. The pressure dependence of decay time constant mimics the pressure variation of steady state ne. Furthermore, it is observed that the on-phase electron density is greatly affected by changing the PRF and duty cycle. This effect is more prominent in Ar/CF4/O-2 plasma when compared to Ar discharge. In addition, n(e) is observed to overshoot the steady state densities in the beginning of the on-phase in Ar/CF4/O-2 gas mixture, but this effect is either small or absent in the case of Ar plasma.