K. Matyash

Radio-frequency discharges in oxygen: I. Particle-based modelling

In this series of three papers we present results from a combined experimental and theoretical, particle-based study to quantitatively describe capacitively coupled radio-frequency discharges in oxygen. The particle-in-cell Monte Carlo model on which the theoretical description is based is described in this paper. It treats space charge fields and transport processes on an equal footing with the most important plasma–chemical reactions. For given external voltage and pressure, the model determines the electric potential within the discharge and the distribution functions for electrons, negatively charged atomic oxygen and positively charged molecular oxygen. Previously used scattering and reaction cross section data are critically assessed and in some cases modified. To validate our model, we compare the densities in the bulk of the discharge with experimental data and find good agreement, indicating that essential aspects of an oxygen discharge are captured.

Observation of Ω mode electron heating in dusty argon radio frequency discharges

The time-resolved emission of argon atoms in a dusty plasma has been measured with phase-resolved optical emission spectroscopy using an intensified charge-coupled device camera. For that purpose, three-dimensional dust clouds have been confined in a capacitively coupled rf argon discharge with the help of thermophoretic levitation. While electrons are exclusively heated by the expanding sheath (α mode) in the dust-free case, electron heating takes place in the entire plasma bulk when the discharge volume is filled with dust particles. Such a behavior is known as Ω mode, first observed in electronegative plasmas. Furthermore, particle-in-cell simulations have been carried out, which reproduce the trends of the experimental findings. These simulations support previous numerical models showing that the enhanced atomic emission in the plasma can be attributed to a bulk electric field, which is mainly caused by the reduced electrical conductivity due to electron depletion.

Rotating dust ring in an RF discharge coupled with a dc-magnetron sputter source. Experiment and simulation

During an experiment involving coating of dust grains trapped in an RF discharge using a sputtering dc-magnetron source, a rotating dust ring was observed and investigated. After the magnetron was switched on, the dust cloud levitating above the RF electrode formed a ring rotating as a rigid body. Langmuir probe diagnostics were used for the measurement of plasma density and potential. It was discovered that the coupling of the dc-magnetron source to the RF discharge causes steep radial gradients in electron density and plasma potential. The rotation of the dust ring is attributed to the azimuthal component of the ion drag force, which appears due to the azimuthal drift of the ions caused by crossed radial electric and axial magnetic fields. In order to get more insight into the mechanism of dust ring rotation, a Particle-in-Cell simulation of a rotating dust cloud was performed. The results of the experiment and simulation are presented and discussed.

Computation of charge and ion drag force on multiple static spherical dust grains immersed in rf discharges

Charging of multiple spherical dust grains located in presheath and sheath regions of an rf discharge has been studied using a three-dimensional particle-particle-particle-mesh (P3M) code. First, dust charge, potential, and ion drag force on two dust particles for various interparticle separations are computed. It is found that for dust separations larger than the shielding length the dust parameters for the two dust particles match with the single particle values. As the dust separation is equal to or less than the shielding length, the transverse component of ion force increases which is due to dynamic shielding effect caused by neighboring dust particle. However, dust charge, potential, and ion drag are found not to be affected considerably. Further, dust charge and potential on an arrangement of nine dust particles are computed. The dust charge and potential do not differ much from the single particle values for the presheath. However the dust charges of multiple dust particles in the sheath are much ...

Computation of dust charge and potential on a static spherical dust grain immersed in rf discharges

Dust charge and potential on static spherical dust grains located in an argon rf discharge under typical laboratory experiment conditions have been computed using a three-dimensional particle-particle-particle-mesh code. Elastic and inelastic collisions have been included in the current model to obtain realistic rf discharge plasma conditions. Dust charge, potential, and potential distribution around the dust have been computed for various sizes of dust placed at different locations in the rf discharge. The dust charge is found to be smaller than the values from the simple orbit motion limited model due to ion-neutral collisions. Further, the dust potential has been found to be increasing with dust size. Moreover, the shielding length of the dust has been found between electron and ion Debye lengths.

Radio-frequency discharges in oxygen: III. Comparison of modelling and experiment

We present results of 1d3v particle-in-cell Monte Carlo collisions simulations of a capacitive RF discharge in oxygen. Several direct comparisons between experiment and modelling are presented. The calculated ion energy distributions show good agreement with the experimentally measured ones for different discharge parameters. A plausible explanation of a double emissive layer near the powered electrode recently discovered in experiments is suggested. Heavy particle dissociative excitation collisions seem to be responsible for the formation of a second emissive layer close to the electrode. Introducing this process into the simulation a rather good agreement of the simulated axial emission profile with the experimentally observed one can be achieved. This delivers an estimate for the cross section of this collision. (Some figures in this article are in colour only in the electronic version)

Modeling of hydrocarbon species in ECR methane plasmas

ECR methane laboratory plasmas are modeled using both a simple zero-dimensional particle balance model and a fully kinetic model. The kinetic model consists of a two-dimensional in space, three-dimensional in velocity space particle-in-cell model with Monte-Carlo collisions in which electrons, ions and neutrals are treated as particles, moving in self-consistent electric and external magnetic fields. The model results are discussed and compared with experimental data.

Particle-In-Cell simulation of laser photodetachment in capacitively coupled radio frequency oxygen discharges

Particle-In-Cell simulations with Monte Carlo collision of capacitively coupled radio frequency oxygen discharges are used to study the appearance and characteristics of two experimentally observed electronegative modes, the high electronegative mode for low peak-to-peak voltage, and the low electronegative mode for high peak-to-peak voltage. For the high electronegative mode, the simulated laser photodetachment signal agrees very well with the experiment. The simulation identifies the dominant transport processes for high electronegativities: electrons flow fast out of the perturbed region, where the laser pulse generates laser detachment of negative ions. Negative ions are not streaming inward, but are produced within this region by dissociative attachment after the laser pulse.

The effect of boundaries on the ion acoustic beam-plasma instability in experiment and simulation

The ion acoustic beam-plasma instability is known to excite strong solitary waves near the Earths bow shock. Using a double plasma experiment, tightly coupled with a 1-dimensional particle-in-cell simulation, the results presented here show that this instability is critically sensitive to the experimental conditions. Boundary effects, which do not have any counterpart in space or in most simulations, unavoidably excite parasitic instabilities. Potential fluctuations from these instabilities lead to an increase of the beam temperature which reduces the growth rate such that non-linear effects leading to solitary waves are less likely to be observed. Furthermore, the increased temperature modifies the range of beam velocities for which an ion acoustic beam plasma instability is observed.

Simulation of ion energy distributions in Ar/CH4 rf discharges with ion extraction

Capacitive radio-frequency discharges in argon–methane gas mixture are used for hydrocarbon deposition on substrates. Ion energy distributions (IEDs) are important for the physics in such discharges. One diagnostics for obtaining IEDs is the energy resolved mass spectrometry. A one-dimensional particle-in-cell code has been used to give better insight into the principle of operation of the ion extraction to interpret more exactly the measured IEDs at the powered electrode. The effects of the ion transfer optics and the modulation effects of the potential between the aperture and ion extractor lens have been studied. A better match between simulation and experiment is achieved by introducing an effective drift length for the ion optics. However, problems remain for reactive species like hydrocarbons indicating more complex plasma reactions within the mass spectrometer.