Determination of electron energy probability function in low-temperature plasmas from current – Voltage characteristics of two Langmuir probes filtered by Savitzky–Golay and Blackman window methods

Abstract

Acceptable data for electron energy probability function (EEPF) measurement in low-pressure plasmas require a small depletion in near-zero electron energy and a large dynamic range in the high-energy regime. The voltage drop across internal resistance and noise from the data acquisition system cause a rounding of the EEPF near the zero electron energy and a reduction in the dynamic range due to a low signal-to-noise ratio of the high-energy regime, respectively, leading to erroneous interpretation in the EEPF measurement. A digital smoothing filter can be employed to reduce the noise signal, but it can also cause additional depletion near the zero energy. In order to obtain reliable EEPF data, a novel technique is proposed using two Langmuir probes with differing collecting areas and the Savitzky–Golay and Blackman window methods. The technique enables the internal resistance effect to be removed using a slope of the current – voltage characteristic taken from the probes. In addition, the Savitzky–Golay and Blackman window methods in the technique apply separately to two regimes of the EEPF, i.e., the near-zero energy regime and the inelastic energy regime, because appropriate smoothing methods that minimize loss and distortion of information differ for the two regimes. This allows one to decrease the noise signal, minimizing the additional depletion near the zero energy. This technique improves the dynamic range of the EEPF from 30 to 55 dB to 65–105 dB and provides more accurate electron density and effective electron temperature from the EEPF, compared to those of the conventional technique.