Sung-Ryul Huh

Global model analysis of negative ion generation in low-pressure inductively coupled hydrogen plasmas with bi-Maxwellian electron energy distributions

A global model was developed to investigate the densities of negative ions and the other species in a low-pressure inductively coupled hydrogen plasma with a bi-Maxwellian electron energy distribution. Compared to a Maxwellian plasma, bi-Maxwellian plasmas have higher populations of low-energy electrons and highly vibrationally excited hydrogen molecules that are generated efficiently by high-energy electrons. This leads to a higher reaction rate of the dissociative electron attachment responsible for negative ion production. The model indicated that the bi-Maxwellian electron energy distribution at low pressures is favorable for the creation of negative ions. In addition, the electron temperature, electron density, and negative ion density calculated using the model were compared with the experimental data. In the low-pressure regime, the model results of the bi-Maxwellian electron energy distributions agreed well quantitatively with the experimental measurements, unlike those of the assumed Maxwellian electron energy distributions that had discrepancies.

How to determine the relative ion concentrations of multiple-ion-species plasmas generated in the multi-dipole filament source

This paper introduces how to determine concentrations of ion species in a mixed gas plasma that are not linearly proportional to their neutral partial pressures. A particle balance model was developed to predict the relative ion concentrations in multiple-ion-species plasmas, considering their ionization rates and loss fluxes to the wall. Analysis is carried out especially with Ar/Xe and Ar/He multi-dipole plasmas in which the neutral gases are directly ionized by the mono-energetic primary electrons. The experimental data of ion concentrations were obtained using the ion acoustic wave measurement method of the concentration of two ion species. The comparison reveals that the ion concentration ratio is linearly proportional to the ratio of the ionization cross sections and the ion loss velocity between two gas species. Especially, the model prediction is improved with using the two-ion-species sheath model (recently reported by Baalrud and Hegna) for obtaining the ion loss velocity at the sheath boundary.

Characterization of Two–Radio-Frequency–Driven Dual Antenna Negative Hydrogen Ion Sources

Abstract The characteristics of a two–radio-frequency (RF)–driven dual antenna inductively coupled hydrogen plasma is investigated for the development of a high efficient RF negative hydrogen ion source driver. The two-RF-driven dual antenna system consists of a 2 MHz–driven solenoidal antenna wound around a cylindrical chamber and a 13.56 MHz–driven planar antenna placed on top of it. Compared to the conventional single frequency antenna inductively coupled plasmas, the two-RF-driven dual antenna inductively coupled plasma reveals two distinctive features, i.e., an increase in the power transfer efficiency and the bi-Maxwellization of the electron energy distribution function due to the collisionless heating. These characteristics allow the two-RF-driven dual antenna inductively coupled plasma to accomplish enhanced generation of negative ions and their precursors with a high RF efficiency.

Improvement of dynamic range of electron energy probability function from two asymmetrical collecting area probe data filtered by Savitzky-Golay and Blackman window methods

The electron energy probability function (EEPF) measured by Langmuir probe is required to be reasonable in low energy regime and have large dynamic range (DR) in high energy regime to investigate the kinetics of low pressure plasma. However the internal resistance (Rint) in bias circuit of probe and the adaption of digital smoothing filter to increase DR destruct these requirements by distorting the EEPF in low energy regime. Rint is sum of the resistances due to the chamber wall sheath and surface of chamber wall. The existence of Rint gives distortion of measured EEPF in low energy regime by overestimating measured probe voltage. Adapting digital smoothing filter gives additional distortion of EEPF in low energy regime since it flattens the peak shape near zero electron energy. A new method is proposed to acquire EEPF which has reasonable value in low energy regime and large DR in high energy regime. The overestimated probe voltage is corrected by removing the effect of Rint which is determined from two sets of plasma potential (Vp) and electron saturation current (Ipe*). The Savitzky-Golay and Blackman window filters are adapted to the I-V characteristics of larger collecting area probe, which has larger signal-to-noise ratio. The two digital smoothing filters are optimized to maximize the strengths of each filter by considering the property of EEPF in low and high energy regime. The verification and capability evaluation of the proposed method are performed by comparing the EEPF measured from optical emission spectroscopy (OES) and conventional method based on single Langmuir probe. The method enhances DR of measured EEPF about 35 ~ 40 dB in comparison with the EEPF from conventional method, especially at two energy regions near zero electron energy and high energy. There are two requirements for proposed method. The distance between two probes is small enough to maintain that ΔVp due to the difference of measurement position is smaller than ΔVp due to Rint where ΔVp is the difference of Vp between two probes. Also signal-to-noise ratio of larger collecting area probe should be larger than 55 dB to ensure the performance of Savitzky-Golay method in low energy regime.

Determination of electron energy distribution function shape for non-Maxwellian plasmas using floating harmonics method

A new floating harmonics method is developed to determine the electron energy distribution shape in the non-Maxwellian plasmas. The slope and curvature of the electron energy probability function (EEPF) at the floating potential can be obtained by measuring amplitude ratios among the first three sideband harmonics of the probe electron current. Together with the generalized EEPF formula, the EEPF shapes of the non-Maxwellian plasmas are able to be determined from the slope and curvature. Here, the new method is experimentally verified and its results are compared with EEPF measurements using the Langmuir probe (LP) method. The effective electron temperature and the EEPF shape parameter obtained by the method give good agreements with those of the LP measurements.

Laser-Assisted Hμ Spectroscopy for Measurement of Negative Ion Density in a Hydrogen Plasma

Abstract A novel laser-assisted Hμ spectroscopy is proposed to measure negative ion density in a hydrogen plasma. The laser-induced photodetachment of negative ions leads to a decrease in Hμ intensity due to blocking of the mutual neutralization channel associated with generation of H (n=3) atoms. The relationship between the reduced Hμ intensity and the negative ion density is investigated experimentally and analytically. It is observed that the reduced Hμ intensity follows the trend in the negative ion density as a function of pressure, indicating that this spectroscopy holds promise for determining the negative ion density. In addition, a departure from linearity between the reduced Hμ intensity and the negative ion density is also analyzed because it can affect the quantitative determination of the negative ion density in the laser-assisted Hμ spectroscopy. The departure is found to be attributed to the change in the mutual neutralization reaction rates depending on plasma conditions.

Standing wave effect on plasma distribution in an inductively coupled plasma source with a short antenna

The effect of a standing wave on plasma density is observed in an inductively coupled plasma (ICP) with a single turn antenna whose circumferential length is much shorter than the wavelength of the driving frequency. The experiment is performed in an argon plasma operated at 5 mTorr with 13.56 MHz power applied to the single turn antenna with diameter of 300 mm. Measured plasma density near the ground termination of the antenna is higher than near the power input, which degrades the uniformity in the ICP source. It is analysed with the standing wave effect of the current and voltage on the antenna, which is carried out with a transmission line model based on the transformer circuit model of the ICP. The wavelength and the distribution of the inductively and capacitively coupled powers along the antenna are obtained from the model with the measured impedance of the antenna. The expected density distribution agrees well with the measured one, revealing that the wavelength is shortened and the nonuniformity of the plasma density becomes severe with increase of the input power.