**Computational Characterization of Microwave Planar Cuto**ff **Probes for Non-Invasive Electron Density Measurement in Low-Temperature Plasma: Ring- and Bar-Type Cuto**ff **Probes**

**Si Jun Kim 1,2, Jang Jae Lee 2, Young Seok Lee 2, Hee Jung Yeom 3, Hyo Chang Lee 3, Jung-Hyung Kim <sup>3</sup> and Shin Jae You 2,4,\***


Received: 14 August 2020; Accepted: 28 September 2020; Published: 12 October 2020

**Abstract:** The microwave planar cutoff probe, recently proposed by Kim et al. is designed to measure the cutoff frequency in a transmission (S21) spectrum. For real-time electron density measurement in plasma processing, three different types have been demonstrated: point-type, ring-type (RCP), and bar-type (BCP) planar cutoff probes. While Yeom et al. has shown that the RCP and BCP are more suitable than the point-type probe for process monitoring, the basic characteristics of the ring- and bar-type probes have yet to be investigated. The current work includes a computational characterization of a RCP and BCP with various geometrical parameters, as well as a plasma parameter, through a commercial three-dimensional electromagnetic simulation. The parameters of interest include antenna size, antenna distance, dielectric thickness of the transmission line, and input electron density. Simulation results showed that the RCP has several resonance frequencies originating from standing-wave resonance in the S21 spectrum that the BCP does not. Moreover, the S21 signal level increased with antenna size and dielectric thickness but decreased with antenna distance. Among the investigated parameters, antenna distance was found to be the most important parameter to improve the accuracy of both RCP and BCP.

**Keywords:** plasma diagnostics; electron density measurement; planar microwave cutoff probe; bar-type cutoff probe; ring-type cutoff probe; computational characterization
