Shigeru Yonemura

Electron Energy Distributions in Inductively Coupled Plasma of Argon

The effects of gas pressure, electron density and coil current on the electron energy distribution functions (EEDFs) in inductively coupled discharges of argon are studied numerically. The EEDF for low gas pressure is close to the Maxwellian, whereas the EEDF for high gas pressure is lower than the Maxwellian in the high energy tail due to inelastic collisions. Since the energy loss due to inelastic collisions is compensated by the energy deposition due to the induced electric field, the EEDF near the quartz wall becomes close to the Maxwellian. However, this EEDF is not the real Maxwellian because the velocity distribution of electrons near the wall is anisotropic in velocity space and hence in nonequilibrium. We proposed the factor ξ which represents the magnitude of the effect of Coulomb collisions. The factor is the ratio of electron–electron collision frequency to electron-atom inelastic collision frequency. The effect of Coulomb collisions on the EEDF is negligibly small for ξ< 0.01. The distribution function for the azimuthal component of electron velocity varies significantly as the radial position changes, whereas the distribution functions for the other components do not vary so much.

Self-consistent particle-in-cell/Monte Carlo simulation of RF magnetron discharges of oxygen/argon mixture: effects of partial pressure ratio

The characteristics of RF planar magnetron discharges of O/sub 2//Ar mixture are clarified using the particle-in-cell/Monte Carlo method. The simulation is carried out for axisymmetrical magnetic fields. The spatial and temporal behavior of magnetron discharge is examined in detail. The O/sup -/ ions are trapped in the plasma bulk by the time-averaged potential. The O/sup -/ density is governed by a balance of generation and loss of O/sup -/. The positive ions Ar/sup +/, O/sub 2//sup +/, and O/sup +/ are distributed in such a way that the charge neutrality is kept in the plasma bulk. Since the peak point of O/sup -/ density does not coincide with that of electron density, the spatial distributions of Ar/sup +/, O/sub 2//sup +/, and O/sup +/ have two peaks. The number densities of Ar/sup +/ and O/sup -/ ions drastically increase when oxygen is added to the discharge gas Ar, and then saturated. In the case when the mole fraction of oxygen increases, the self-bias voltage on the target decreases. This phenomenon is investigated in detail and a clear physical explanation is presented.

Electron Energy Distributions in Inductively Coupled Plasma: Comparison of Chlorine Discharge with Argon Discharge

The characteristics of the electron energy distribution functions (EEDFs) in inductively-coupled discharges of chlorine and argon gases are studied numerically. The EEDFs in atomic gases such as Ar and Cl show the three-temperature distribution with depletion in the high-energy region and enhancement in the low-energy region. By contrast, the EEDF for Cl2 shows the bi-Maxwellian distribution close to the Maxwellian one. The reason for this difference is explained by the difference of electronic inelastic collision cross sections between atomic and molecular gases. The EEDFs obtained in the present simulation agree with the earlier experimental results qualitatively.

Molecular dynamics study of the effects of translational energy and incident angle on dissociation probability of hydrogen/deuterium molecules on Pt(111)

The dissociation probabilities of H2 and D2 molecules on a Pt(111) surface with thermal motion were analyzed using the molecular dynamics (MD) method. The potential constructed using the embedded atom method was used as the interaction potential between a gas molecule and the surface. The effects of changing the translational energy and incident polar angle of D2 molecules impinging on a Pt(111) surface were analyzed using MD simulations. The effect of initial orientation, incident azimuthal angle, rotational energy of gas molecules, and the impinging points on the surface were averaged by setting the initial values in a random manner. When the molecules approach normal to the surface, the dissociation probability increases with the initial translational energy. At larger incident angles, the probability becomes smaller. The impinging processes were categorized in terms of reaching the chemisorption layer by analyzing the repulsion forces from the surface. The effective translational energies for impingem...

A numerical study for transport phenomena of nanoscale gas flow in porous media

Gas flow in porous media occurs in various engineering devices such as catalytic converters and fuel cells. In order to improve the performance of such devices, it is important to understand transport phenomena in porous media. In porous media with pores as small as a molecular mean free path, molecular motions need to be directly considered instead of treating gas flow as a continuum, and effects of complicated channels need to be taken into account. Therefore, such gas flow was analyzed by using the direct simulation Monte Carlo (DSMC) method, which is the stochastic solution of the Boltzmann equation. Numerical simulations of gas flow driven by pressure gradient without surface reaction were performed to clarify transport phenomena in porous media imitated by arranging nanoscale solid particles randomly. The effects of pressure gradient, diameter of particles and porosity on gas flow rates and permeability of porous media were investigated.

Particle Modeling of Ionization and Three-Body Recombination in Fully Ionized Plasmas

In optical field ionized (OFI) plasmas, the electron energy distribution is highly deviated from equilibrium distribution; ionization and recombination in such plasmas should be treated using the particle model since the usual concept of a rate constant is not applicable in nonequilibrium. A particle model is proposed for the simulation of ionization and three-body recombination in fully ionized plasmas of Li3+. The nonequilibrium rate constant is defined and calculated using the cross section for reactive collision, which is derived from the equilibrium rate constant using the inverse Laplace transform. The use of the model, together with the Coulomb collision simulation, makes it possible to examine the relaxation of the electron energy distribution in nonequilibrium systems.

Numerical Analysis of Gas Flow in Porous Media with Surface Reaction

Gas flow with surface reaction in porous media appears in various regions of engineering. In porous media with holes as small as a molecular mean free path, Kn of gas flow in the narrow channel is on the order of unity. Therefore, the direct simulation Monte Carlo (DSMC) method is suitable to solve transport phenomena in such kind of porous media. We perform 2D DSMC simulations of such a flow. The shape of narrow channel in porous media is complicated. To reduce complexity, we propose the simplification for porous structures by cubes and polyhedra. Results for the simplification by polyhedra agree well with the result obtained in the case without simplification. Results for the simplification by cubes also show good agreement with the result without simplification when surface reaction probability is modified.

Simulation study of SiH4 microdischarges in a narrow channel

Using the Particle-in-Cell/Monte Carlo (PIC/MC) method, the behavior of SiH4 plasma discharge in a narrow channel was studied. Simulations were carried out at a gas pressure of 5 Torr for an electrode distance of 20 mm and a narrow channel with an inner diameter of 2 mm. Most SiH2+ ions were found to be changed into SiH3+ ions due to SiH2+-SiH4 collisional charge exchange. The dominant charged species in the discharge were electrons and SiH3+ ions. The inner surface of the narrow channel greatly affected SiH4 plasma behavior due to an accumulation of charged particles. An intense ionization rate was found for the first time in the sheath on the inner surface of the narrow channel. The largest electron and SiH3+ ion densities appeared at the edge of the plasma bulk abutting the sheath on the inner surface of the narrow channel.

Effect of Micro Gas Flow on Low Friction Properties of Diamond Coating with Partly Polished Surface

Nakamori et al. found experimentally that the friction between a partly polished diamond coating and a metal surface was drastically reduced to zero as relative speed was increased. It seems that diamond coating took off the counter surface because sliding became noiseless. This phenomenon shows the possibility of innovative air lubrication system. In the present work, we make the floating mechanism clear by using the direct simulation Monte Carlo (DSMC) method. It is found that the phenomenon is peculiar to the high Knudsen number flows.

Increase of Ar/sup +/ ion density in argon RF magnetron discharges caused by the addition of a small amount of oxygen

Particle-in-cell/Monte Carlo (PIC/MC) simulations were performed for radio frequency (RF) planar magnetron discharges of pure argon gas and an O/sub 2/-Ar gas mixture. The number densities of Ar/sup +/ and O/sup -/ ions drastically increased when oxygen was added to the Ar discharge gas. Moreover, the spatial distributions of positive ions, Ar/sup +/, O/sub 2//sup +/, and O/sup +/, had two peaks in the O/sub 2//Ar mixture discharge. The positive ions were distributed in such a way that the charge neutrality was kept in the plasma bulk, indicating that the behavior of Ar/sup +/ ions is influenced by the existence of O/sup -/ions. The mechanism of this phenomenon is clearly explained by visualizing results of the simulation.