Monte Carlo Collision Simulation Research Articles

Ion flux formation in asymmetric 2 MHz capacitive discharge

From two-dimensional particle-in-cell–Monte Carlo collisions simulations, the effect of chamber geometry on the ion flux is studied in a low gas pressure capacitive 2 MHz discharge. It is shown that the optimization of the reactor geometry significantly enhances the ion flux to the small electrode at a given applied voltage. The uniformity of the ion flux and the ion energy distribution at the electrode are studied for the applied voltage amplitude ranging from 2 to 4 kV. The dependence of sheath voltages from the ratio of driven and grounded electrode areas is discussed.

Large amplitude of electrostatic waves associated with magnetic field in divertor plasmas

The characteristics of divertor plasmas are investigated dynamically using a one-dimension-in-space and three-dimension-in-velocity particle-in-cell Monte Carlo collision simulation technique, and it is found that a strong magnetic field with a large incident angle of the magnetic field on the divertor plate gives rise to a hybrid electrostatic wave, which is made of waves with distinctive frequencies. These waves are then identified as being related, respectively, to the lower hybrid wave mode and electron Bernstein wave modes. The wave associated with the lower hybrid wave mode dominates the potential fluctuation, modulates the incident energy flux onto the divertor plate in a large magnitude and causes the wave-like variation of impurity production from the carbon-based divertor plate.

Plasma density enhancement in atmospheric-pressure dielectric-barrier discharges by high-voltage nanosecond pulse in the pulse-on period: a PIC simulation

A particle-in-cell (PIC) plus Monte Carlo collision simulation is employed to investigate how a sustainable atmospheric pressure single dielectric-barrier discharge responds to a high-voltage nanosecond pulse (HVNP) further applied to the metal electrode. The results show that the HVNP can significantly increase the plasma density in the pulse-on period. The ion-induced secondary electrons can give rise to avalanche ionization in the positive sheath, which widens the discharge region and enhances the plasma density drastically. However, the plasma density stops increasing as the applied pulse lasts over certain time; therefore, lengthening the pulse duration alone cannot improve the discharge efficiency further. Physical reasons for these phenomena are then discussed.

Influence of N2 gas pressure on the chemical bonds of amorphous carbon nitride films

A study on the dependence of the microstructure of CNx films on N2 gas pressure was conducted in an RF magnetron sputtering system. A significant change in microstructure was observed as gas pressure decreased: the concentration of sp2 C=N and hydrogenated bonds decreased, while that of sp3 C–N and sp C≡N bonds increased. This implies that low pressure favours the synthesis of CNx films of good mechanical properties. A further investigation using a Langmuir probe and Monte Carlo collision simulations revealed that the energy of ions bombarding the film increased significantly with decreasing gas pressure. This can lead to changes in hydrogen content and chemical bonds in the growing CNx films.

Control of ion energy distribution in low-pressure and triple-frequency capacitive discharge

One-dimensional particle-in-cell Monte Carlo collision (PIC-MCC) simulations of low-pressure (10 mTorr) argon plasmas sustained by a triple-frequency (1, 30 and 120 MHz) source in symmetrical current-driven and voltage-driven capacitively coupled plasma reactors are carried out. We concluded that the effective current, the effective voltage and the effective frequency are helpful in explaining the physics of triple-frequency capacitively coupled plasma sources (CCPs) alike single-frequency CCPs. The rf discharge parameters such as the ion energy distribution function (IEDF), the sheath length, the plasma potential and the powers dissipated by electrons and ions can be expressed as the effective frequency and the effective current density (or effective voltage). The analytical model of the IEDF for triple-frequency CCPs in the high-frequency regime is developed. The analytical calculations of the IEDF in the high-frequency regime through the effective frequency visualized in this paper are compared with the simulation results of the IEDF calculated from the 1D PIC-MCC model. The ion energy width and the average ion energy of the IEDF are controlled by the effective frequency, which is expressed as a function of the current density (or voltage) and frequency ratios of the triple-frequency source. The evolution of the effective frequency with the current density or voltage ratio of three frequency sources is different depending on the mode of operating source, which is either voltage or current. The effective frequency in voltage-driven CCPs is 2–10 times higher than that of current-driven CCPs at the same ratio of current density and voltage. As a result, the current-driven CCPs is more desirable than the voltage-driven CCPs from the aspect of independent control of ion flux and ion bombardment energy because the ion energy width increases with decreasing effective frequency.

Characteristics of nanosecond pulse needle-to-plane discharges at high pressure: a particle-in-cell Monte Carlo collision simulation

A model of one dimensional in position and three dimensional in velocity space self-consistent particle in cell with Monte Carlo collision technique was employed to simulate the argon discharge between the needle and plane electrodes at high pressure, in which a nanosecond rectangular pulse was applied to the needle electrode. The work focused on the investigation of the spatiotemporal evolution of the discharge versus the needle tip size and working gas pressure. The simulation results showed that the discharge occurred mainly in the region near the needle tip at atmospheric pressure, and that the small radius of the needle tip led to easy discharge. Reducing the gas pressure gave rise to a transition from a corona discharge to a glowlike discharge along the needle-to-plane direction. The microscopic mechanism for the transition can arguably be attributed to the peak of high-energy electrons occurring before the breakdown; the magnitude of the number of these electrons determined whether the breakdown can take place.

The particle-in-cell simulation and Monte Carlo collision simulation of the interaction between electrons and microwave in electron cyclotron resonance discharge

The theoretical and computational model is presented to study the interaction between electrons and microwave in electron cyclotron resonance discharge. The interaction between charged particles and microwave is described by particle-in-cell method, the collisions and the interaction between charged particles and boundaries of system are described by Monte Carlo collision method. The quasi-three-dimensional simulation code is original. And the spatio-temporal evolvements of electron energy and microwave field in ECR discharge is diagnosed.

Effect of gas pressure on ion flux in 2MHz capacitive discharge

We have studied the ion energy and angular distribution functions in a 2MHz capacitive discharge using combined particle in cell Monte Carlo collision simulations. We found that secondary electrons produced by ion bombardment from the electrodes make important contribution to the ionization at higher gas pressure. Calculations showed that the ion flux on the electrode is very sensitive to the plasma parameters. However, the ion angular distribution function weakly responds to a change of the gas pressure.

Calculation of Electron Swarm Parameters of SF6/N2 in a Uniform Field Using an Improved Monte Carlo Collision Simulation Method

The electron swarm parameters of SF6/N2 are calculated in the present study using an improved Monte Carlo collision simulation method (MCS). And some improved sampling techniques are also adopted. The simulation results show that the improved simulation method can provide more accurate results.

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.

Electron and Ion Kinetics in a DC Microplasma at Atmospheric Pressure

The results of a particle-in-cell Monte Carlo collision (PIC-MCC) simulation of a direct current (DC) helium microplasma that operates at atmospheric pressure are presented. Electron and ion kinetic information that is not available from previous fluid studies is reported. Despite the high collisionality at atmospheric pressure, electrons are found to be in nonequilibrium. Similar to large-scale low-pressure dc discharges, the electron energy probability function (EEPF) in the bulk plasma presents three temperatures near the cathode, and it evolves into a bi-Maxwellian distribution as electrons approach the anode. The bi-Maxwellian character of the EEPF in the elastic energy region is not accounted for in fluid models, and as a result, PIC-MCC simulations predict a lower electron temperature than fluid models. The mean energy of ions that are impinging on the cathode is found to be significantly lower than in low-pressure discharges due to the large collisionality of the sheaths.

Electron and ion kinetics in magnetized capacitively coupled plasma source

One-dimensional particle-in-cell Monte Carlo collision simulations of magnetized argon plasmas in an asymmetric capacitively coupled plasma reactor are presented. At low pressure (10mTorr), electron kinetics are strongly affected by the magnetic field and transitions from nonlocal to local kinetic property occur with increasing magnetic field which are reflected in spatially resolved calculations of the electron-energy probability function. For high-energy electrons, the transition takes place when the energy-relaxation length is smaller than the system length. For low-energy electrons, however, the transition occurs when the electron-diffusion time scale in the energy space is shorter than the spatial-diffusion time scale in coordinate space. These observations are in agreement with experimental data and theoretical calculations deduced from the Boltzmann equation. The ion energy distribution function (IEDF) on the driven electrode changes from the ion-neutral collisional type to the ion-neutral collisionless type with increasing magnetic field strength. The maximum ion energy in the IEDF decreases and the angular spread in the ion angle distribution function slightly increases with increasing magnetic field strength. These changes are explained in terms of the ratio of the ion-transit time to rf frequency, the sheath length, and the mean potential difference between the driven electrode and the plasma. At high pressure (218mTorr), electron-neutral collisions disrupt electron gyromotion so that the effects of the magnetic field on electron and ion kinetics are greatly reduced.

Low-pressure plasma generation inside slender tubes

Low pressure ( < 50 mTorr ) argon discharges inside tubes of a few millimeters in diameter have been studied by means of one- and two-dimensional particle-in-cell Monte Carlo collision simulations. Magnetically confined DC and microwave discharges have been sustained in a coaxial configuration. For DC discharges, the magnetic field needs to be strong enough to confine secondary electrons emitted from the cathode, i.e. the amplitude of the cycloidal motion described by secondary electrons has to be smaller than the discharge gap. For microwave excited discharges, the power absorption profile depends on the magnitude of the magnetic field. Power absorption beyond the sheath boundary in the plasma bulk and better confinement for magnetic fields below the ECR condition lead to maximum densities at ω c / ω r f ∼ 0.5 .

Electron and ion kinetics in a micro hollow cathode discharge

Electron and ion kinetics in a micro hollow cathode discharge are investigated by means of two-dimensional axisymmetric particle-in-cell Monte Carlo collision simulations. Argon discharges at 10 and 300 Torr are studied for various driving currents. Electron and ion energy probability functions (IEPF) are shown at various times and locations to study the spatio-temporal behaviour of the discharge. The electron energy probability function (EEPF) evolves from the Druyvesteyn type in the early stages of the discharge into a two (or three) temperature distribution when steady state is reached. In steady state, secondary electrons accelerated across the cathode fall populate the high energy tail of the EEPF while the low energy region is populated by trapped electrons. The IEPF evolves from a Maxwellian in the negative glow (bulk) to a two temperature distribution on the cathode surface. The overpopulation of low energy ions near the cathode surface is attributed to a larger collision cross section for low energy ions and ionization within the cathode fall.

Secondary electron emission coefficients in plasma display panels as determined by particle and fluid simulations

The secondary electron emission coefficient (γse) of ions and excited species in a plasma display panel (PDP) was studied by means of 2-dimensional fluid and 1-dimensional particle-in-cell Monte Carlo collision simulations. Relations between the driving voltage and the luminous efficiency observed in experiments are reproduced at low and high Xe concentrations. Agreement between experiments and simulations, however, requires careful selection of the γse of ions and excited species. The trend of the efficiency as a function of the driving voltage is particularly sensitive to the γse of Xe excited species. For the conditions typically encountered in PDP cells (pd = 1–10 Torr cm), the dependence of the γse on the energy of impinging ions can be neglected in discharges of pure gases. This is a consequence of multiple charge exchange collisions in the cathode region. In Xe–Ne mixtures, however, the ion energy distribution function on the cathode depends on the mixture ratio. Typically more energetic ions can reach the cathode in gas mixtures and the γse is enhanced.

Particle-in-cell Monte Carlo and fluid simulations of argon-oxygen plasma: Comparisons with experiments and validations

Particle-in-cell Monte Carlo collision (PIC-MCC) and fluid simulations of argon-oxygen plasmas in capacitively and inductively coupled plasma reactors are presented. Potential profiles and electron/ion kinetic information such as electron/ion energy distributions and temperatures are compared with experimental data as well as with other analytical and numerical results. One-dimensional PIC-MCC simulations compare favorably with experimental data obtained in capacitively coupled reactors over a wide range of pressure and power. Two-dimensional fluid simulations of capacitive discharges differs from the results of PIC-MCC simulations as nonlocal effects play an important role in these discharges. Fluid simulations as nonlocal inductively coupled plasmas, however, agree favorably with experimental observations.

Two-dimensional, three-velocity particle-in-cell with Monte Carlo collisions simulations of LANSCE’s H− source

A two-dimensional, three-velocity particle-in-cell with Monte Carlo collisions (MCC) gun code is under development at LANSCE to study surface conversion H− ion source physics from a first-principles approach. The code is electromagnetostatic and is based on a sine transform/tridiagonal matrix Poisson solver implemented in axisymmetric geometry. A filament discharge is simulated by the injection of seed electrons at a biased filament surface. These electrons drive the discharge through a collection of (presently 22) reactions implemented as a MCC package. The simulation region is rectangular with internal electrodes of arbitrary shape. Given the nature of the code, no ad hoc models are necessary to model sheath physics, diffusion across magnetic-field lines, ion extraction, or other plasma phenomena.

The mechanism of striation formation in plasma display panels

Despite the high pressure employed in plasma display panels, the energy balance of low-energy electrons is found to be dominated by inelastic collisions, and the resulting nonlocal electron kinetics plays a key role in the striation formation. Surface charge accumulation on the anode dielectric, however, is also needed for striations to form. It is the combined effect of surface charges and nonlocal electron kinetics that results in the striation formation in plasma display panel cells. Two-dimensional fluid simulations, which assume local electron kinetics, and two-dimensional particle-in-cell Monte Carlo collision simulations with a bare conducting anode show that striations do not form if either the nonlocal electron kinetics or the surface charge accumulation is not considered.

Ion extraction and collection studied by parallel electrode method on considerin g sputtering loss

One-dimensional ion extraction and collection is studied by parallel electrode method using particle in cell_Monte Carlo collision (PIC-MCC) simulation. The e nergy and angle distribution of extracted ions is recorded and the sputtering lo ss is calculated. The results show that the extraction time and collision loss a re decreased by increasing extraction voltage, but the sputtering loss increases and collection ratio decreases. The extraction time is decreased and the collec tion ratio is increased by increasing electron temperature, while the extraction time is increased and the collection ratio is decreased by increasing initializ ed plasma density;and the collision loss is decreased and the collection ratio i s increased, if the target isotope concentration is higher. Improving the parall el electrode method leads to decreasing sputtering loss and increasing collectio n ratio greatly.

Mechanism for Sustainment of High Density in Surface Wave Plasma

Through particle-in-cell Monte Carlo Collision simulations, it has been predicted that high-energy electrons produced by electron plasma waves, which are resonantly excited at approximately the cutoff density by an evanescent E-field of a surface wave parallel to the density gradient, may sustain high-density plasma. The effective collision frequency at a resonant point is in fairly close agreement with the prediction of Aliev et al. [Plasma Source Sci. Technol. 1 (1992) 126].