Shin-Jae You

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.

Plasma density measurements by phase resolved cutoff

A method for precise measurement of absolute electron density in the plasma using plasma frequency is proposed. The phase difference between two antennas (radiating and detecting antennas) and its slow time modulation was used, instead of the transmittance used in previous cutoff probe, and a remarkable result is observed. This method can determine the absolute electron density precisely: It is not only in a good agreement with the previous method very closely but also is able to find the cutoff point clearly even in difficult conditions where there are ambiguities to determine the cutoff point with previous method.

A self-consistent global model of solenoidal-type inductively coupled plasma discharges including the effects of radio-frequency bias power

We developed a self-consistent global simulator of solenoidal-type inductively coupled plasma discharges and observed the effect of the radio-frequency (rf) bias power on the plasma density and the electron temperature. We numerically solved a set of spatially averaged fluid equations for charged particles, neutrals, and radicals. Absorbed power by electrons is determined by using an analytic electron heating model including the anomalous skin effect. To analyze the effects of rf bias power on the plasma properties, our model also combines the electron heating and global transport modules with an rf sheath module in a self-consistent manner. The simulation results are compared with numerical results by using the commercial software package cfd-ace + (ESI group) and experimental measurements by using a wave cutoff probe and a single Langmuir probe.

Electron and ion kinetics in magnetized capacitively coupled plasma source

One-dimensional particle-in-cell Monte Carlo collision simulations of magnetized argon plasmas in an asymmetric capacitively coupled plasma reactor are presented. At low pressure (10mTorr), electron kinetics are strongly affected by the magnetic field and transitions from nonlocal to local kinetic property occur with increasing magnetic field which are reflected in spatially resolved calculations of the electron-energy probability function. For high-energy electrons, the transition takes place when the energy-relaxation length is smaller than the system length. For low-energy electrons, however, the transition occurs when the electron-diffusion time scale in the energy space is shorter than the spatial-diffusion time scale in coordinate space. These observations are in agreement with experimental data and theoretical calculations deduced from the Boltzmann equation. The ion energy distribution function (IEDF) on the driven electrode changes from the ion-neutral collisional type to the ion-neutral collisio...

Anomalous evolution of Ar metastable density with electron density in high density Ar discharge

Recently, an anomalous evolution of argon metastable density with plasma discharge power (electron density) was reported [A. M. Daltrini, S. A. Moshkalev, T. J. Morgan, R. B. Piejak, and W. G. Graham, Appl. Phys. Lett. 92, 061504 (2008)]. Although the importance of the metastable atom and its density has been reported in a lot of literature, however, a basic physics behind the anomalous evolution of metastable density has not been clearly understood yet. In this study, we investigated a simple global model to elucidate the underlying physics of the anomalous evolution of argon metastable density with the electron density. On the basis of the proposed simple model, we reproduced the anomalous evolution of the metastable density and disclosed the detailed physics for the anomalous result. Drastic changes of dominant mechanisms for the population and depopulation processes of Ar metastable atoms with electron density, which take place even in relatively low electron density regime, is the clue to understand the result.

Gap length effect on electron energy distribution in capacitive radio frequency discharges

A study on the dependence of electron energy distribution function (EEDF) on discharge gap size in capacitive rf discharges was conducted. The evolution of the EEDF over a gap size range from 2.5to7cm in 65mTorr Ar discharges was investigated both experimentally and theoretically. The measured EEDFs exhibited typical bi-Maxwellian forms with low energy electron groups. A significant depletion in the low energy portion of the bi-Maxwellian was found with decreasing gap size. The results show that electron heating by bulk electric fields, which is the main heating process of the low-energy electrons, is greatly enhanced as the gap size decreases, resulting in the abrupt change of the EEDF. The calculated EEDFs based on nonlocal kinetic theory are in good agreement with the experiments.

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.

Sheath width effect on the determination of plasma frequency in the cutoff probe

To analyze the sheath width effect on the determination of the plasma frequency in the cutoff probe, two simulations (a simplified circuit simulation and an E/M simulation) were conducted. Both two simulations showed a consistent result: the cutoff frequency ωc, which is selected as a frequency of minimum peak in a transmission microwave frequency (TMF) spectrum, deviates from the plasma frequency ωp while increasing the sheath width on the two probe tips. The physics behind the movement of cutoff frequency in the TMF spectrum deviating from the plasma frequency was analyzed and discussed in this paper. This study can provide not only the basis for the precise measurement of the cutoff probe but also the discernible boundary conditions for the cutoff probe application.

Evolution of electron temperature in low pressure magnetized capacitive plasma

The evolution of electron temperature in a low pressure magnetized capacitive discharge was investigated under the collisionless electron heating regime. The results showed that while the electron temperature increases monotonously with the magnetic field in previous study [Turner et al., Phys. Rev. Lett. 76, 2069 (1996)], the electron temperature in our experiment exhibited nonmonotonic evolution behavior with the magnetic field. This nonmonotonic evolution of the electron temperature with the magnetic field was shown to be a combined effect of suppressing electron resonance heating and enhancing collisional heating while increasing the magnetic field.

Measurements of Electron Energy Probability Functions in inductively coupled plasma with laser Thomson scattering

Electron energy probability functions (EEPFs) having a fine resolution of electron energy were measured in low-pressure inductively coupled plasma with laser Thomson scattering method (LTS) at various plasma conditions (rf powers and gas pressures) and compared with the EEPFs measured by a single Langmuir probe (SLP) at the same experimental conditions. The result of LTS showed that the measured electron density normally increases with the rf power and the gas pressure, and the electron temperature decreased with the rf power and the gas pressure. This result have a good agreement not only with the previous reports qualitatively but also with our SLP measurement result quantitatively [15]. The reasons for the discrepancy between two methods in absolute value might be expected to these facts as following: the probe perturbation effect from the probe holder volume, RF noise, and not sufficient signal level of laser Thomson scattering.