Masako Shindo

Measurements of the negative ion density in SF6/Ar plasma using a plane electrostatic probe

A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.

Ion acoustic waves in one- and two-negative ion species plasmas

Ion acoustic waves in multi-ion plasmas including two negative ion species are investigated both numerically and experimentally. Numerically, the kinetic dispersion relation in two-negative ion plasmas is investigated. There are three modes of the ion acoustic waves in two-negative ion plasmas. In an Ar+–F−–SF6− plasma, only one of the three modes is dominant, regardless of the values of the electron and the ion temperatures. In a Xe+–F−–SF6− plasma, on the other hand, two modes can be important for a certain range of the electron–ion temperature ratio. The results also imply the possibility of the coexistence of the fast mode and the slow mode in one-negative ion plasmas. Experimentally, ion acoustic waves are observed in an Ar+–F−–SF6− plasma and are found to show a mode transition that agrees with the theoretical prediction for one of the three ion acoustic modes.

Estimate of the Negative-Ion Density in O2/Ar ECR Plasma Utilizing Ion Acoustic Waves

An attempt to measure the negative ion density in an O 2 /Ar electron cyclotron resonance (ECR) plasma is made by means of the propagation of ion acoustic waves (IAWs) (fast mode). IAWs are launched along magnetic fields using a wire antenna to which positive pulse voltages are applied. The IAWs are detected as a fluctuation of the ion saturation current flowing into a plane Langmuir probe. The change of the phase velocity of IAWs is observed when the gas mixture ratio is varied. It is found that the density ratio of negative ions to positive ions, α, is less than 0.3, and has an optimum value at the gas mixture ratio of 15%.

Propagation characteristics of ion acoustic waves in an Ar/SF6 plasma

There has been a great interest in wave phenomena in a negative ion plasma. Usually sulfur hexafluoride (SF6) gas is used to produce a negative ion plasma, because SF6 gas has a large electron attachment cross-section for electron energies less than 1 eV. Furthermore, the negative ion concentration, r, can be controlled by partial pressure of the gas. However, the introduction of SF6 gas produces several ion species in discharge plasmas. In fact, there are many discussions on ion species in discharge plasmas. In the most of the experiment on the ion acoustic wave in a negative ion plasma, only F− or only SF6 was assumed as the negative ion species. 1, 2) On the other hand, in many experimental studies on the ion acoustic soliton in Ar/SF6 plasmas it was assumed that the light components, that is, F− contributes more effectively to the ion acoustic wave, 4) and the results of these experiments were consistent with this assumption. Cooney et al., however, obtained the soliton velocity that shifted gradually from the theoretical value for Ar and F− to that for Ar and SF6 as r increased. 5) Although they mentioned that this was perhaps due to an increase in the relative concentration of SF6 to F − as r increased, there was no evidence for the phenomenon. On the other hand, it was recently observed by the mass spectrometry that the relative concentration of SF6 to F− increases with increasing r in the double plasma (DP) device. Thus, in order to confirm whether the variations in the relative concentrations of ion species affect the propagation characteristics of the ion acoustic wave or not, we detected dominant ion species in an Ar/SF6 plasma with a quadrupole mass spectrometer (QMS) and measured the phase velocity of the ion acoustic wave for different values of r. While much work on the ion acoustic wave in two-positive ion plasmas have been done, 8) the study on the ion acoustic wave in two-negative ion plasmas hardly has been carried out. The purpose of this paper is to observe the dependence of the ion acoustic wave on ion species, especially negative ion species. The experiments were carried out using a multidipole DP device. The diameter and the length of the device

Measurements of negative ion density in O2/Ar electron cyclotron resonance plasma

An attempt to measure the negative ion density in an O 2 /Ar electron cyclotron resonance (ECR) plasma is made with two methods. The first one is to estimate the density ratio of negative ions to positive ions, a, from the reduction rate of the ion and electron saturation current obtained with the Langmuir probe. The second one is to calculate a from the measured phase velocity of the fast mode of ion acoustic waves (IAWs). The IAWs are launched along magnetic fields using a wire-antenna where positive pulse voltages are applied, and are detected as a fluctuation of ion saturation current. As a result, a < 0.3 is found. It is also found that a obtained with both methods agree qualitatively. Furthermore, the spatial distribution and the gas mixture ratio dependence of a are investigated. It is found that a has the optimum value at the gas mixture ratio of 15%, and that a near the wall is larger than that in the center region.

Parameters measurement of ECR C4F8/Ar plasma

Abstract The parameters of an electron cyclotron resonance (ECR) C 4 F 8 /Ar plasma were measured with a heated Langmuir probe and an 8 mm microwave interferometer. As the ratio of C 4 F 8 gas to Ar gas was increased, the electron density measured with the microwave interferometer decreased, which suggests that a large quantity of negative ions exist in the ECR C 4 F 8 plasma. An attempt to estimate the negative ion density was then made with the heated Langmuir probe. It was found that the ratio of negative ions to positive ions is from 30 to 80% in the ECR C 4 F 8 plasma.

Determination of negative-ion density in an electron cyclotron resonance C4F8 plasma

Abstract The parameters of an electron cyclotron resonance (ECR) C 4 F 8 /Ar plasma were measured as a function of gas mixture rate and incident microwave power at different radial positions using both a heated Langmuir probe and a microwave interferometer. As the ratio of C 4 F 8 gas to argon gas was increased, the electron density measured with the microwave interferometer decreased, which suggests that a large quantity of negative ions exist in the ECR C 4 F 8 /Ar plasma. An attempt to estimate the negative-ion density was made with the Langmuir probe. It was found that the ratio of negative ions to positive ions is from 60 to 80% in the ECR C 4 F 8 plasma.

Estimate of the negative ion density in reactive gas plasmas

The negative ion density in a SF6/Ar double plasma is estimated. Here, the density ratio of negative ions to positive ions α is evaluated from the reduction rates of the ion and electron saturation current. Furthermore, the negative ion density obtained with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) when α<0.6, where the positive and negative ion mass are obtained from the spectrum analysis with a quadruple mass spectrometer (QMS) system. The negative ion density in fluorocarbon ECR plasma is also estimated by means of the Langmuir probe method. It is found that α in fluorocarbon ECR plasma is less than 0.5.

Measurements of negative ion density in fluorocarbon ECR plasma

Abstract The parameters of an electron-cyclotron resonance (ECR) C 4 F 8 /Ar gas plasma were measured using both a heated Langmuir probe and an 8-mm microwave interferometer. An attempt to estimate the negative ion density was made with three methods in the case where C 4 F 8 and Ar gas mixture rate was less than 25%; the first method is based on the analysis of the probe characteristics, and the others on the charge neutrality condition. It was found that the ratio of the negative ion density to the positive ion density was 20–50%, depending on the assumed positive ion mass. Furthermore, the self-bias effect in C 4 F 8 /Ar ECR plasma was discussed.

Development of a hybrid PIG-ECR ion source

AbstractA combined ion source with 20 cm beam diameter, utilizing both a cold-cathode Philips or Penning ionization gauge (PIG )discharge and an electron cyclotron resonance (ECR ) discharge, has been built and tested. The ion source has been designed tostudy what effects, if any, might be induced by superimposing microwave power at the ECR condition onto a cold-cathode PIGion source at low pressures (F10 Torr y4 ). The first experiments with argon gas indicate that the ECR coupling at low pressuresresults in: (a) transformation of the PIG discharge mode with a dominant anode fall into the hybrid discharge mode with adominant cathode fall; (b) stabilization of the PIG discharge; and (c) enhanced ion beam output due to increased ionizationinside the ion source. The ion beam current is 150 mA (with a beam current density at its axis of 0.35 mA ycm 2 ), at an anodevoltage of 750 V, a microwave power of 200 W and a gas pressure of 0.05 mTorr. The ion source design and the results ofpreliminary experiments are presented. A stable low-pressure operation mode of the ion source might be promising for broad ionbeam production. 2003 Elsevier Science B.V. All rights reserved.