Use of emissive probes in high pressure plasma

The characteristics of emissive probes in unmagnetized high pressure (≤1 Torr) argon and helium plasmas, produced by inductively coupled 13.56 MHz rf power, are studied. A procedure is given for interpreting emissive probe current–voltage (I–V) characteristics. The I–V curves indicate the amplitude of the rf fluctuation of the plasma potential. They also show ionization near the emissive probe when the potential drop between the emissive probe and the plasma potential is more than the ionization potential. Experiments show that when the temperature of the emissive probe wire and/or the neutral pressure is increased, ionization becomes significant. An increase in the local ion density due to the additional ionization was demonstrated by the I–V curves of an emissive probe and a nearby Langmuir probe. A simple procedure is presented for interpreting these results.