RF compensation of single Langmuir probe in low density helicon plasma

Abstract

Interpretations of Single Langmuir probe measurements in electrode-less radio frequency (RF) plasmas are noteworthy tricky and require adequate compensation of RF. Conventional RF compensation technique is limited only at high density (>1017m−3) RF plasmas. RF compensation of single Langmuir probe at low density RF plasmas (∼1016m−3) is presented in this paper. In RF driven plasmas, where the RF voltage is high (∼50V) and density is in the range (∼1016m−3), the primary RF compensation condition (Zck>>Zsh) is very difficult to fulfill, because of high sheath impedance (Zsh) at 13.56MHz and the construction limitation of a self-resonant tiny chock (Zck) with very high impedance. Introducing a large auxiliary electrode (Ax), (Ax>>>Ap), close to the small Langmuir probe (Ap) tip, connected in parallel with probe via a coupling capacitor (Ccp), significantly reduces the effective sheath impedance (Zsh) and allows probe bias to follow the RF oscillation. Dimensional requirements of the auxiliary electrode and the role of suitable coupling capacitor are discussed in this paper. Observations show proper compensation leads to estimation of more positive floating potentials and lower electron temperatures compared to uncompensated probe. The electron energy probability function (EEPF) is also obtained by double differentiating the collected current with respect to the applied bias voltage using an active analog circuit.