Tsukasa Yamane

Characteristics of Very High Frequency Plasma Produced Using a Ladder-Shaped Electrode

A very high frequency (VHF) excited H2 plasma is produced with a ladder-shaped electrode consisted of 17 rods and the plasma parameters are measured with a movable Langmuir probe. When the discharge frequency (~120 MHz) of the RF power source is increased, the plasma density increases while the electron temperature decreases. The position of the RF connections and the shape of the rod, particularly at higher frequencies are found to affect the distribution of the ion saturation current. The uniformity of the ion saturation current of ±17.2% over a distance of 150 mm is achieved at 120 MHz.

Estimation of negative ions in VHF SiH4/H2 plasma

The characteristics of a VHF SiH4/H2 plasma (frequency: 60 MHz) at high pressures were examined as a function of silane concentration with a heated Langmuir probe. Anomalous reductions in electron saturation current were observed, suggesting the existence of many negative ions. An estimation of the concentration of negative ions was attempted using the sheath theory including negative ions. It was found that there exist H− and SiH3− ions as dominant negative ions in the VHF SiH4/H2 plasma. In addition, the measured floating potential agreed with the theoretical value.