Improvement of dynamic range of electron energy probability function from two asymmetrical collecting area probe data filtered by Savitzky-Golay and Blackman window methods

Abstract

The electron energy probability function (EEPF) measured by Langmuir probe is required to be reasonable in low energy regime and have large dynamic range (DR) in high energy regime to investigate the kinetics of low pressure plasma.1 However the internal resistance (R int ) in bias circuit of probe and the adaption of digital smoothing filter to increase DR destruct these requirements by distorting the EEPF in low energy regime.1,2 R int is sum of the resistances due to the chamber wall sheath and surface of chamber wall. The existence of R int gives distortion of measured EEPF in low energy regime by overestimating measured probe voltage. Adapting digital smoothing filter gives additional distortion of EEPF in low energy regime since it flattens the peak shape near zero electron energy. A new method is proposed to acquire EEPF which has reasonable value in low energy regime and large DR in high energy regime. The overestimated probe voltage is corrected by removing the effect of R int which is determined from two sets of plasma potential (V p ) and electron saturation current (I pe ∗). The Savitzky-Golay and Blackman window filters are adapted to the I-V characteristics of larger collecting area probe, which has larger signal-to-noise ratio. The two digital smoothing filters are optimized to maximize the strengths of each filter by considering the property of EEPF in low and high energy regime. The verification and capability evaluation of the proposed method are performed by comparing the EEPF measured from optical emission spectroscopy (OES) and conventional method based on single Langmuir probe. The method enhances DR of measured EEPF about 35 ∼ 40 dB in comparison with the EEPF from conventional method, especially at two energy regions near zero electron energy and high energy. There are two requirements for proposed method. The distance between two probes is small enough to maintain that ΔV p due to the difference of measurement position is smaller than ΔV p due to R int where ΔV p is the difference of V p between two probes. Also signal-to-noise ratio of larger collecting area probe should be larger than 55 dB to ensure the performance of Savitzky-Golay method in low energy regime.