Byung-Keun Na

An analysis on transmission microwave frequency spectrum of cut- off probe

We investigated the formation mechanism of transmission microwave frequency (TMF) spectrum of cut-off probe using a simple circuit model to elucidate the physics behind the TMF spectrum. The result showed that the overall shape of the TMF spectrum of cut-off probe (N – shape spectrum) is well reproduced with our proposed circuit model and can be understood as the combined result of two different resonances caused by the elements between two probe tips (a sheath, a plasma, and a vacuum which is filled by the plasma). Furthermore, based on this simple modeling, a more precise method to find the plasma frequency by taking account with the e-n collision frequency and the pressure limitation of the cut-off probe application is established.

Wave transmission characteristics of a wave-cutoff probe in weakly ionized plasmas

The physical properties of a wave transmission spectrum were analyzed using a wave-cutoff probe and a three-dimensional wave simulation in weakly ionized plasma. The experimental wave transmission was compared to the calculated wave transmission. The new path of wave transmission below the wave-cutoff frequency was found, and the wave transmission properties depending on the chamber geometry above the wave-cutoff frequency were also analyzed. Through these wave transmission results, the causes of an electromagnetic propagation phenomenon below the wave-cutoff frequency were examined. Moreover, the impacts of a cavity mode within the plasma chamber on the features of wave transmission were identified.

Computational characterization of cutoff probe system for the measurement of electron density

The wave cutoff probe, a precise measurement method for measuring the electron density, was recently proposed. To characterize the cutoff probe system, in this paper, the microwave simulations of a cutoff probe system were performed at various configurations of the cutoff probe system. The influence of the cutoff probe spectrum stemming from numerous parametric elements such as the probe tip length, probe tip distance, probe tip plane orientation, chamber volume/geometry, and coaxial cable length is presented and discussed. This article is expected to provide qualitative and quantitative insight into cutoff probe systems and its optimization process.

Pressure limitation of electron density measurement using a wave-cutoff method in weakly ionized plasmas

Wave-cutoff method using microwave provides capabilities for diagnostics of various processing plasmas, and can give the precise absolute electron densities1. In this study, pressure limitation of electron density measurement using a wave-cutoff method is presented. As gas pressure increases, the wave-cutoff signal disappears. The disappearance of signal happens when the electron-neutral collision frequency is over the plasma frequency. At that time, the electron motion cannot catch up to the movement of the electromagnetic wave, and the electromagnetic waves begin to penetrate into plasma. In result, the wave-cutoff signal disappears.

Measurement and analysis of electron-neutral collision frequency in the calibrated cutoff probe

As collisions between electrons and neutral particles constitute one of the most representative physical phenomena in weakly ionized plasma, the electron-neutral (e-n) collision frequency is a very important plasma parameter as regards understanding the physics of this material. In this paper, we measured the e-n collision frequency in the plasma using a calibrated cutoff-probe. A highly accurate reactance spectrum of the plasma/cutoff-probe system, which is expected based on previous cutoff-probe circuit simulations [Kim et al., Appl. Phys. Lett. 99, 131502 (2011)], is obtained using the calibrated cutoff-probe method, and the e-n collision frequency is calculated based on the cutoff-probe circuit model together with the high-frequency conductance model. The measured e-n collision frequency (by the calibrated cutoff-probe method) is compared and analyzed with that obtained using a Langmuir probe, with the latter being calculated from the measured electron-energy distribution functions, in wide range of gas pressure.

Measurement of electron density with the phase-resolved cut-off probe method

The phase resolved cut-off probe method, a precise measurement method for the electron density, was recently proposed [J. H. Kwon et al., Appl. Phys. Lett. 96, 081502 (2010)]. This paper presents the measurements of electron density using the method under various experimental conditions (different pressures, powers, chamber volumes, and discharge sources). The result shows that the method is not only in good agreement with the previous method using wave transmittance under various experimental conditions but it is also able to find the cut-off point clearly even under difficult conditions such as high pressure (∼ 1 Torr), high discharge power, and small plasma volume. The details of the experimental setup, the operating mechanism of the probe method, and the data processing procedure (algorithm) are also addressed. Furthermore, the reliability of the measurement method is investigated by using an electromagnetic field simulation with cold plasma model (CST-Drude model, Computer Simulation Technology).

Measurement of electron density using reactance cutoff probe

This paper proposes a new measurement method of electron density using the reactance spectrum of the plasma in the cutoff probe system instead of the transmission spectrum. The highly accurate reactance spectrum of the plasma-cutoff probe system, as expected from previous circuit simulations [Kim et al., Appl. Phys. Lett. 99, 131502 (2011)], was measured using the full two-port error correction and automatic port extension methods of the network analyzer. The electron density can be obtained from the analysis of the measured reactance spectrum, based on circuit modeling. According to the circuit simulation results, the reactance cutoff probe can measure the electron density more precisely than the previous cutoff probe at low densities or at higher pressure. The obtained results for the electron density are presented and discussed for a wide range of experimental conditions, and this method is compared with previous methods (a cutoff probe using the transmission spectrum and a single Langmuir probe).

Investigation of self-oscillation using particle balance model

Self-oscillation obtained using a DC-only power supply under specific anode voltage conditions is investigated in a cylindrical system with thermal electrons using tungsten filaments. Analysis of the obtained oscillation profiles reveals that the experimental data are consistent with a model derived from the particle balance model. The self-oscillation period characteristics with respect to the pressure and gas species are also analyzed. As the physics and particle motion of self-oscillation near the plasma transition region are analyzed from different perspectives, this paper may advance the study of this phenomenon.

Cutoff probe using Fourier analysis for electron density measurement

This paper proposes a new method for cutoff probe using a nanosecond impulse generator and an oscilloscope, instead of a network analyzer. The nanosecond impulse generator supplies a radiating signal of broadband frequency spectrum simultaneously without frequency sweeping, while frequency sweeping method is used by a network analyzer in a previous method. The transmission spectrum (S21) was obtained through a Fourier analysis of the transmitted impulse signal detected by the oscilloscope and was used to measure the electron density. The results showed that the transmission frequency spectrum and the electron density obtained with a new method are very close to those obtained with a previous method using a network analyzer. And also, only 15 ns long signal was necessary for spectrum reconstruction. These results were also compared to the Langmuir probes measurements with satisfactory results. This method is expected to provide not only fast measurement of absolute electron density, but also function in other diagnostic situations where a network analyzer would be used (a hairpin probe and an impedance probe) by replacing the network analyzer with a nanosecond impulse generator and an oscilloscope.

Fast measurement of a pulsed plasma using a Fourier cutoff probe

A new method for time-resolved measurement of pulsed plasmas is suggested for reducing the measurement time. A short impulse has a broadband spectrum, and it can be used to make a spectrum in a short time. The use of a cutoff probe with a Fourier analysis (Fourier Cutoff Probe, FCP) provides the absolute electron densities with high speed. The measurement results from the FCP show good agreement with Langmuir probes measurement results. However, it took only 1 minute 45 seconds using the FCP to make the temporal profile of electron densities in a pulsed plasma, versus 46 minutes for the Langmuir probe. The FCPs measurement was about 26 times faster than that by the Langmuir probe. This method will provide researchers a faster and convenient diagnostic method for pulsed plasmas.