Measurement of sheath thickness at a floating potential

Abstract

Sheath thickness is important for plasma diagnostics such as the widely-used Langmuir probe and for plasma etching in semiconductor manufacturing. To understand the electrodynamic properties and a variety of chemical and physical phenomena in processing discharges, many computational plasma models have been presented in the previous literatures. Because the sheath thicknesses are only a few times λDs and the ion mean free path in pre-sheath region could be up to hundreds of λDs in low pressure, high density plasmas, computation of an exact solution is very time consuming. In previous works, sheath models have been proposed and tried to probe the physical mechanism of the sheath. Nevertheless, the sheath thickness should in general be determined by experiments. In this paper, we present a method, a combination of a floating harmonics method and a wave cutoff method, for measuring the plasma sheath thickness at a floating potential. The measured ion current include sheath area. Therefore, if we know unknown factor, such as plasma density, electron temperature, and ion current, the sheath thickness can be obtained. The measured sheath thickness was compared with collisionless sheath theories at the floating potential. The difference of the sheath thicknesses are within the 10% range. The sheath thicknesses obtained by the experimental are reliable.