Analysis of the Transmission Spectrum of the Ceramic-Shielded Cutoff Probe in Low-Pressure Plasma

Electron density is a fundamental parameter in plasma physics and plasma processing technologies. Among the various diagnostic methods for measuring electron density, the microwave cutoff probe is recognized as a precise method. Moreover, to facilitate measurements in harsh plasma and processing plasma conditions, a ceramic-shielded cutoff probe (CSC) has also been developed. However, research on the influence of various ceramic shield structures and the material of the probe body on the transmission characteristics of CSCs has not been conducted. This study investigates the influence of ceramic shield characteristics, including thickness and geometry, on the transmission spectrum and electron density measurements obtained using CSCs in high-density or processing plasmas. Electromagnetic simulations and circuit modeling showed that if the ceramic shield is more than 1.6 times thicker than the coaxial cable in the cutoff probe, parasitic capacitance from the ceramic material can cause up to a 3% shift in the cutoff frequency. Also, frequency shifts can arise from the shield's shape and its coupling method with the CSC body, and the simulation results were validated through experiments. These findings provide valuable insights for accurate plasma diagnostics using CSCs.