A method for measuring electron temperature and ion density with immunity to RF fluctuation and ion current

Abstract

A measurement method immune to radio frequency (RF) fluctuations is proposed for obtaining electron temperature and plasma density in RF discharges. The self-bias voltage formed by applying a square voltage to a floating planar probe and its fundamental frequency current are measured to obtain electron temperature and plasma density. To investigate the change in electron temperature due to RF distortion, the case with and without RF filters is compared, and our method is least affected by RF fluctuations compared to the conventional methods: electron energy probability function (EEPF) and floating harmonic method (FHM). When the RF powers and the gas pressures change, the electron temperature and the ion density measured from our method agree well with those measured from the FHM. Although our method and the EEPF are slightly different due to the depletion of the EEPF at high energy (near the floating potential), the trends of the three methods (our method, FHM, and EEPF) agree well under all conditions. In our method, the electron temperature was investigated with and without correction for the increase in the ion current at probe tip radii of 5 and 1 mm. When correcting the increase in ion current due to the sheath expansion, the electron temperature is not overestimated and does not change in the planar probe with a small radius. This can be useful in plasma monitoring system where an RF filter cannot be installed, or the probe tip must be made small.