Triple-frequency floating probe method for plasma parameters applicable to processing plasmas

Abstract

Measuring plasma parameters, such as plasma density and electron temperature, is crucial for precise plasma process control, as these parameters directly influence chemical reactions and ion energy in the processing chamber, ultimately affecting the quality and consistency of plasma processing results. However, in commercial reactors, byproducts from processing steps often deposit on probes immersed in the plasma, distorting measurement accuracy. In this study, we present an improved floating harmonic method (FHM) designed to accurately measure ion density and electron temperature in conditions where a dielectric layer is deposited on the probe. Under such conditions, when an AC voltage is applied to the probe, a voltage drop occurs not only across the sheath between the probe surface and the plasma but also through the deposited dielectric layer. Our method compensates for this voltage drop by directly measuring the voltage across the dielectric layer. By applying three different voltages with varying frequencies to the probe, we accurately determine both sheath and dielectric layer impedances from the measured currents. Plasma density and electron temperature are measured via the FHM while accounting for the voltage drop caused by the deposited film. Furthermore, the dielectric thickness is calculated from the impedance of the deposited film. Our method offers robust and accurate measurements, providing a reliable solution for plasma diagnostics in challenging environments.