A harmonic method for measuring electron temperature and ion density using an asymmetric double probe

Abstract

The harmonic method using a symmetric double probe was developed for measuring electron temperature and ion density Oh et al (2012 Meas. Sci. Technol. 23 085001). When an alternating voltage is applied to the symmetric double probe where the two areas of the collector for current collection are equal, the fundamental frequency current and third harmonic currents are generated. The electron temperature and ion density are obtained by measuring the fundamental frequency current and the third harmonic current. However, it is observed that the third harmonic current can rapidly decrease to the level of base noise when the ratio of the applied voltage to the electron temperature decreases. Therefore, it is necessary to increase the harmonic currents generated to improve measurement accuracy for electron temperature and ion density. In this paper, a harmonic method using an asymmetric double probe with different collection areas is proposed to measure electron temperature and ion density. By using the double probe with different collector area, the fundamental frequency current and the second harmonic current are generated. In the proposed method, the electron temperature and ion density are obtained by measuring the fundamental frequency current and the second harmonic current. It is found that the accuracy of the electron temperature can be improved by measuring the second harmonic rather than measuring the third harmonic current. For quantitative comparison, the electron temperature and ion density obtained by the proposed method were compared with the electron temperature and electron density obtained by the measurement electron energy probability function, which showed good agreement between them in argon plasma at various conditions. In addition, it was experimentally verified that the electron temperature can be accurately measured even when the chamber is electrically insulated, and a dielectric layer is deposited on the collectors of the double probe, such as in the plasma process.