Akihiro Kono

Vacuum ultraviolet absorption spectroscopy employing a microdiacharge hollow-cathode lamp for absolute density measurements of hydrogen atoms in reactive plasmas

We have developed a measurement technique for absolute H-atom densities in process plasmas using vacuum ultraviolet absorption spectroscopy employing a high-pressure microdischarge hollow-cathode lamp (MHCL) as a Lyman α (Lα, 121.6 nm) emission light source. Characterization of the Lα emission-line profile could be simplified by using a high-pressure discharge at about 1 atm. The effect of self-absorption in the MHCL was reduced to an insignificant level by decreasing the H2 partial pressure. The contribution of the collisional broadening to the Lα emission profile was estimated from the saturation characteristics of the absorption intensity when the optical thickness of the plasma was varied. The technique was applied to the measurement of the absolute H-atom density in an inductively coupled H2 plasma.

Development of vacuum ultraviolet absorption spectroscopy technique employing nitrogen molecule microdischarge hollow cathode lamp for absolute density measurements of nitrogen atoms in process plasmas

We have developed a vacuum ultraviolet absorption spectroscopy (VUVAS) technique employing a high-pressure nitrogen molecule (N2) microdischarge hollow cathode lamp (N2 MHCL) as a light source of the atomic nitrogen (N) resonance lines for measuring absolute N densities in process plasmas. The estimations of self-absorption and the emission line profiles of the N2 MHCL, which are necessary for absolute N density determination, were carried out. The measurement of absolute N densities have been demonstrated for an inductively coupled N2 plasma using the VUVAS system employing the N2 MHCL.

Absolute concentration and loss kinetics of hydrogen atom in methane and hydrogen plasmas

A measurement technique of the absolute concentration of hydrogen (H) atoms in methane (CH4) and/or hydrogen molecule (H2) plasmas has been established. The H-atom concentration was evaluated by vacuum ultraviolet absorption spectroscopy (VUVAS) using a high-pressure H2 microdischarge hollow cathode lamp (H2-MHCL) as the Lyman α (Lα 121.6 nm) light emission source. A measurement technique of the background absorption caused by species other than H atoms at the Lα line was developed by using the VUVAS technique with the MHCL employing nitrogen molecules (N2-MHCL). The lines around Lα used for the background absorption measurements are 2p23s 4P5/2–2p3 4S3/20 at 119.955 nm, 2p23s 4P3/2–2p3 4S3/20 at 120.022 nm, and 2p23s 4P1/2–2p3 4S3/20 at 120.071 nm of the N atom. By using the VUVAS technique with the MHCLs and subtracting the background absorption from the absorption of H atoms at Lα, we have achieved the measurement of the H-atom concentration in an inductively coupled plasma operated in CH4 and/or H2. T...

Formation of an oscillatory potential structure at the plasma boundary in electronegative plasmas

Self-consistent potential structures in front of a negatively biased electrode immersed in electronegative plasmas are studied numerically in spherical symmetry using a collisionless plasma model with cold positive ions. It is shown that for a large electron/negative-ion temperature ratio, a large electrode size, and a certain range of negative-ion/electron density ratio, an oscillatory potential structure is formed; due to the oscillatory potential structure, in some conditions the positive ion current to the electrode exhibits discontinuity and hysteresis with varying potential applied to the electrode. An oscillatory potential structure with deep potential wells is expected to be significantly modified if collisions, however infrequent, exist, since collisions produce ions which are trapped in the potential wells; how these trapped ions affect the potential structure is an unsolved problem, requiring an elaborate kinetic treatment.

Laser Thomson scattering for low-temperature plasmas

In recent years, low-temperature discharge plasmas with electron temperatures around a few eV have been actively studied and used for fundamental discharge physics research and industrial process applications. Here the electron density and temperature (and sometimes also the electron energy distribution function) are the most fundamental parameters that dictate the fates of these discharge plasmas and this information is of utmost importance. Laser Thomson scattering, which was developed for high-temperature plasmas in the early 1960s, has gained widespread use in the low-temperature plasma community since the late 1970s and has now matured as an established method of plasma diagnostics. Scattering diagnostic techniques for high-temperature plasmas have had to be modified to accommodate particular situations and constraints, such as laser perturbation of plasmas, low electron densities and the presence of material surfaces near to the plasmas. In this review, starting from a basic description of the technique, we outline some of the most salient results, which would not have been obtained without it, in discharges ranging from high-pressure to low-pressure gases, and near to material surfaces. Also, the signal-to-noise ratio and future potential of the method are discussed.

Laser-Induced-Fluorescence Detection of SiH2 Radicals in a Radio-Frequency Silane Plasma

Silylene radicals (SiH2) in a 40-W 40-mTorr RF (13.56 MHz) SiH4/Ar plasma were detected by use of a laser-induced-fluorescence (LIF) technique. The observed SiH2 density increased with increasing Ar partial pressure. The absolute SiH2 density, estimated from the comparison of the LIF intensity with the intensity of Rayleigh scattering caused by N2 molecules, is in the range of 109-1010 cm-3.

Reaction rate constant of Si atoms with SiH4 molecules in a RF silane plasma

Ultraviolet absorption spectroscopy using a ring dye laser and a hollow cathode lamp was applied to measurement of Si(3p2, 1D2) and Si(3p2, 3P2) atom density decay in the afterglow of a radio frequency SiH4-Ar plasma. The dependence of the Si density decay rate on SiH4 and Ar partial pressure and also on the radio frequency input power was investigated, from which Si(3p2,1D2)-SiH4 reaction rate constant was determined to be (7.4+or-0.4)*10-10 cm3 molecule-1 s-1, the Si(3p2, 3P2)-SiH4 reaction rate constant (3.5+or-1.0)*10-10 cm3 molecule-1 s-1 and the diffusion coefficient for Si(3p2, 1D2) in Ar (at 320 K) (4.0+or-0.8)*104 cm2 Pa s-1.

Production of CW High-Density Non Equilibrium Plasma in the Atmosphere Using Microgap Discharge Excited by Microwave

A new technique for cw production of high-pressure, high-density, non-equilibrium plasma is presented. Using microwave excitation at 2.45 GHz, a stable atmospheric glow discharge was sustained between two knife-edge electrodes, having a length of 10 mm and facing each other across a ~100 µm microgap. Laser Thomson scattering diagnostics indicates that the plasma density in the microgap is as high as 1.6×1015 cm-3 at a microwave power of 100 W. The optical emission of the N2 second positive band indicates that the gas temperature in the microgap is 1800 K, much lower than the electron temperature.

Complex sheath formation around a spherical electrode in electronegative plasmas: a comparison between a fluid model and a particle simulation

The structure of the plasma-wall boundary around a negatively biased spherical electrode immersed in low-pressured electronegative plasma was studied numerically using a fluid model as well as a particle-in-cell Monte Carlo (PIC-MC) simulation. The range of plasma parameters, such as negative-ion/electron density ratio and electron/ion temperature ratios, in which the boundary region involves both positive and negative space charges, is derived using the fluid model with warm positive ions and with and without positive-ion collisionality. Contrary to the recent results for planar discharge (Franklin R N and Snell J 2000 J. Phys. D: Appl. Phys. 33 1990), with the cold-positive-ion assumption relaxed, the fluid model does predict a complex sheath structure with non-monotonic potential, even for positive-ion temperatures as high as the electron temperature if the negative ion temperature is sufficiently low. A comparison between the fluid model and PIC-MC simulation was made for a case where the fluid model predicts potential oscillations. In the collisionless limit, the PIC-MC results were almost identical with the fluid-model results. However, with weak collisionality included, the results from the two approaches differed drastically. While the fluid-model prediction remained essentially unchanged, the PIC-MC simulation showed a temporally oscillating non-monotonic potential, the oscillation apparently periodically releasing the ions trapped in the potential well. This indicates that when the fluid model predicts spatial potential oscillation, instability can be induced in the real physical potential.

Negative ions in processing plasmas and their effect on the plasma structure

Three fundamental issues: (1) how many negative ions exist in processing plasmas, (2) how negative ions affect the spatial distribution of charged particles, and (3) how negative ions affect the structure of the sheath, are discussed on the basis of knowledge attained in the past decade. Photodetachment techniques for measuring negative ion density are reviewed and results of measurements for low density as well as high-density plasmas containing halogenated gases and oxygen are discussed from a view point of how the negative ion/electron density ratio varies as a function of electron/feedstock gas density ratio. The effect of negative ions on the spatial distributions of charged particles is illustrated by solving plasma balance equations based on an ambipolar diffusion model for various electron attachment rate coefficients and electron densities; some comparisons of the results with other model studies are made. The structure of the sheath in electronegative plasmas recently clarified by a number of workers is illustrated and discussed by giving typical fluid model solutions in the spherical symmetry.