Dual-role plasma absorption probe to study the effects of sheath thickness on the measurement of electron density

A sensitive plasma absorption probe (PAP) is reported for measuring electron density in processing plasmas. The sheath formed around the probe tip is important for the resonance of surface waves. For determining the absolute electron density from the absorption frequency of the sensitive PAP, a proper value of sheath thickness relative to the Debye length is required to be assigned in the data processing. In this paper, a dual-role PAP has been proposed to study the effects of sheath thickness on the measurement of electron density. It is used as a Langmuir probe and a sensitive PAP simultaneously. Based on these two functions, the sheath thickness is calibrated before the measurement of electron density. The calibrated value is assigned in the data processing to replace the fitting coefficient used in the previous work. Therefore, the measurement error caused by an inaccurately assigned sheath thickness can be minimized effectively. Because of the bi-functional characteristic, the dual-role PAP is an independent diagnostic tool.