Experimental quantification of ion flux reduction at the sheath edge due to ion–neutral collisions in low temperature plasmas

Abstract

Reduction of the ion flux at the sheath edge due to ion–neutral collisions in low temperature DC plasmas is experimentally quantified for low to intermediate neutral gas pressures (<102 mTorr). The reduction factor is defined as a ratio of the ion flux at the sheath edge in a collisional plasma to that in a collisionless limit in this work. Its quantification as a function of the collisionality with a Langmuir probe has been hindered since the measured data contain two intermingled effects, namely, the flux reduction and the sheath expansion, which are difficult to isolate one from the other. The sheath expansion effect with and without the flux reduction effect are analyzed, and by comparing the two, the reduction factor as a function of the collisionality has been estimated with Langmuir probe data from approximately 1000 systematic scans of the plasma conditions. Neutral gas pressures ranging from 0.2–30.0 mTorr for Ar and 1.0–65.0 mTorr for He discharges are generated in a multidipole chamber with hot filaments. The estimated reduction factors are found to agree with the results from the particle-in-cell simulations for He discharges [Beving et al., Plasma Sources Sci. Technol. 31, 084009 (2022)]. The estimated reduction factors for Ar discharges are larger than those for He discharges, and the dependence of the reduction factor on species is discussed. Reduction of the ion flux at the sheath edge at intermediate gas pressures highlights the importance of taking into account ion–neutral collisions in many plasma applications.