The characteristics of a VHF SiH4/H-2 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/H-2 plasma. In addition, the measured floating potential agreed with the theoretical value. (C) 2014 The Japan Society of Applied Physics