An investigation of the temporal evolution of plasma potential in a 60 MHz/2 MHz pulsed dual-frequency capacitively coupled discharge

摘要

Using an electron-emitting probe, time-resolved plasma potential (V _p) measurements are carried out in a pulsed dual-frequency 60 MHz/2 MHz capacitively coupled discharge. The discharge is produced using argon gas at two chosen pressures of 20 and 40 mTorr, a duty ratio of 50% and pulse powers ranging from 100 to 500W. The pulsing frequency is set at 1kHz. The plasma potential measurements are carried out at 10mm above the centre of the substrate. It is observed that V _p follows the source discharge pulse voltage and remains positive during the whole pulse cycle. Without substrate biasing, the prominent features observed in the V _p profile, under all the operating conditions, remain similar; however, the magnitude of V _p increases with the applied RF source power. For further analysis, three distinguishable features in the V _p profile, a transient spike at the beginning of the discharge pulse, a stable 'on-phase' and a 'stable-off' phase, are identified. For typical operating conditions (20 mTorr and 500 W), the transient spike in V _p of ～30 V appears for 30 μs, then it attains a stable value of ～12V during the rest of the pulse on-period. V _p decreases up to ～3V as the pulse is switched off. It is also observed in this study that a continuous wave RF biasing of the substrate significantly modulates the plasma potential evolution, specifically when the pulse is switched off and the magnitude of modulation depends on the substrate biasing power. The temporal evolution of electron temperature derived from the plasma and floating potentials is also reported.