Reproducibility of the cutoff probe for the measurement of electron density

et al. 2022

Sensors

Although the recently developed cutoff probe is a promising tool to precisely infer plasma electron density by measuring the cutoff frequency (fcutoff) in the S21 spectrum, it is currently only applicable to low-pressure plasma diagnostics below several torr. To improve the cutoff probe, this paper proposes a novel method to measure the crossing frequency (fcross), which is applicable to high-pressure plasma diagnostics where the conventional fcutoff method does not operate. Here, fcross is the frequency where the S21 spectra in vacuum and plasma conditions cross each other. This paper demonstrates the fcross method through three-dimensional electromagnetic wave simulation as well as experiments in a capacitively coupled plasma source. Results demonstrate that the method operates well at high pressure (several tens of torr) as well as low pressure. In addition, through circuit model analysis, a method to estimate electron density from fcross is discussed. It is believed that the proposed method expands the operating range of the cutoff probe and thus contributes to its further development.

) given by

where j is a complex number,

is the vacuum dielectric constant,

is the microwave frequency,

(=8980

) is the plasma oscillation frequency,

is the electron density in units of cm

, and

is the electron–neutral collision frequency.…”

Section: Simulation Demonstrationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Crossing Frequency Method Applicable to Intermediate Pressure Plasma Diagnostics Using the Cutoff Probe

Kim¹,

et al. 2022

Sensors

“…The cutoff probe was connected to vector network analyzer 2 (S3601B 100 kHz–8.5 GHz, SALUKI Inc., Taipei, Taiwan) via two SMA cables. The cutoff probe can precisely measure the electron density by measuring the cutoff frequency in S

; details can be found elsewhere [ 9 , 46 , 47 , 48 , 49 ]. In the next section, operation of the MOLE probe at the center of the chamber is first demonstrated compared to the cutoff probe.…”

Section: Experimental Demonstrationmentioning

confidence: 99%

“…The SWR frequency is proportional to the rf power, and as the pressure increases, the Q-factor of the peak abruptly decreases. As shown in Figure 8 d–g, the cutoff frequency (

) in S

, which is marked with an arrow at each rf power condition, is proportional to the rf power [ 47 , 48 , 49 ]. This implies that the electron density is also proportional to the rf power.…”

Section: Experimental Demonstrationmentioning

confidence: 99%

Development of the Measurement of Lateral Electron Density (MOLE) Probe Applicable to Low-Pressure Plasma Diagnostics

Kim

You

et al. 2022

Sensors

Self Cite

As the importance of measuring electron density has become more significant in the material fabrication industry, various related plasma monitoring tools have been introduced. In this paper, the development of a microwave probe, called the measurement of lateral electron density (MOLE) probe, is reported. The basic properties of the MOLE probe are analyzed via three-dimensional electromagnetic wave simulation, with simulation results showing that the probe estimates electron density by measuring the surface wave resonance frequency from the reflection microwave frequency spectrum (S11). Furthermore, an experimental demonstration on a chamber wall measuring lateral electron density is conducted by comparing the developed probe with the cutoff probe, a precise electron density measurement tool. Based on both simulation and experiment results, the MOLE probe is shown to be a useful instrument to monitor lateral electron density.

“…Among the various types of microwave probes that have been proposed, the CP has proved a reliable [21], reproducible [22], and precise instrument to measure electron density [10], sheath width [23], and electron-neutral collision frequency [24]. Besides, various types of the CP itself have been developed for particular applications, such as the reactance CP [12], phase-resolved CP [25], Fourier CP [26], and ring-type CP [27].…”

Section: Introductionmentioning

confidence: 99%

Effect of an inhomogeneous electron density profile on the transmission microwave frequency spectrum of the cutoff probe

Kim

Plasma Sources Sci. Technol.

et al. 2020

Self Cite

The cutoff probe (CP), which precisely measures electron density from a microwave transmission (S 21) spectrum, has been successfully developed through physical models under the assumption of homogeneous plasma between the probe tips. In practice, however, the chamber and sheath structure, flow effects, and the probe insertion itself produce plasma inhomogeneity between the probe tips, which necessitates the study of any related effects on the CP measurement results. This paper investigated the effect of inhomogeneous plasma on the S 21 of the CP through a three-dimensional E/M wave simulation with various density gradient directions. The simulations found, notably, double cutoff peaks and a cutoff frequency shift in the S 21 that depend on the gradient direction. These results are analyzed via a simple circuit model of the CP, and their origins are elucidated at the end of this paper.