Plasma impedance and microwave interferometric measurements of electron concentrations in dual-frequency powered sulfur hexafluoride plasmas

Abstract

Recent literature shows that dual frequency rf power can give better characteristics in both plasma etching and plasma deposition. The key features of the powering system used in our experimental studies on a parallel plate plasma system is that the high frequency (HF) is an exact multiple of the low frequency (LF), and that the power from both is supplied to the same electrode. These features make it much easier to measure the plasma voltage and current waveforms with an oscilloscope. We examined 100 kHz plus 12.2 MHz discharges at 0.35 Torr with LF power from 0% to 100% of the total power of 0.05 to 0.6 W/cm2. The plasma impedance for the HF signal tended to be largest at times when the magnitude of the low frequency signal’s voltage was the largest which may be due to additional loss of electrons by extraction out of the plasma. Because of the time-varying HF impedance, we could alter when HF power was dissipated within the low frequency cycle by changing the matching network settings to effectively tune into the low or high impedance times. This effect shows that the process characteristics of the plasma might depend on the matching of the dual frequency systems for the same time-averaged HF power and LF power readings. In general as we increased the percentage of LF power, the electron concentrations decreased. No sudden changes of any plasma characteristics were observed while going through the full range of percentage LF power.