Bulk Plasma Region Research Articles

Direct ignition and S-curve transition by in situ nano-second pulsed discharge in methane/oxygen/helium counterflow flame

A well-defined plasma assisted combustion system with novel in situ discharge in a counterflow diffusion flame was developed to study the direct coupling kinetic effect of non-equilibrium plasma on flame ignition and extinction. A uniform discharge was generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of CH4/O2/He diffusion flames were investigated by measuring excited OH∗ and OH PLIF, at constant strain rates and O2 mole fraction on the oxidizer side while changing the fuel mole fraction. It was found that ignition and extinction occurred with an abrupt change of OH∗ emission intensity at lower O2 mole fraction, indicating the existence of the conventional ignition-extinction S-curve. However, at a higher O2 mole fraction, it was found that the in situ discharge could significantly modify the characteristics of ignition and extinction and create a new monotonic and fully stretched ignition S-curve. The transition from the conventional S-curves to a new stretched ignition curve indicated clearly that the active species generated by the plasma could change the chemical kinetic pathways of fuel oxidation at low temperature, thus resulting in the transition of flame stabilization mechanism from extinction-controlled to ignition-controlled regimes. The temperature and OH radical distributions were measured experimentally by the Rayleigh scattering technique and PLIF technique, respectively, and were compared with modeling. The results showed that the local maximum temperature in the reaction zone, where the ignition occurred, could be as low as 900K. The chemical kinetic model for the plasma–flame interaction has been developed based on the assumption of constant electric field strength in the bulk plasma region. The reaction pathways analysis further revealed that atomic oxygen generated by the discharge was critical to controlling the radical production and promoting the chain branching effect in the reaction zone for low temperature ignition enhancement.

Effect of the target power density on high-power impulse magnetron sputtering of copper

We present a model analysis of high-power impulse magnetron sputtering of copper. We use a non-stationary global model based on the particle and energy conservation equations in two zones (the high density plasma ring above the target racetrack and the bulk plasma region), which makes it possible to calculate time evolutions of the averaged process gas and target material neutral and ion densities, as well as the fluxes of these particles to the target and substrate during a pulse period. We study the effect of the increasing target power density under conditions corresponding to a real experimental system. The calculated target current waveforms show a long steady state and are in good agreement with the experimental results. For an increasing target power density, an analysis of the particle densities shows a gradual transition to a metal dominated discharge plasma with an increasing degree of ionization of the depositing flux. The average fraction of target material ions in the total ion flux onto the substrate is more than 90% for average target power densities higher than 500 W cm−2 in a pulse. The average ionized fraction of target material atoms in the flux onto the substrate reaches 80% for a maximum average target power density of 3 kW cm−2 in a pulse.

The recovery of glow-plasma structure in atmospheric radio frequency microplasmas at very small gaps

In atmospheric radio frequency discharges at 13.56 MHz, with the electrode gap reduced, the sheath region eventually occupies a main portion of the electrode spacing and the bulk plasma region is significantly compressed. The computational results in this letter based on a one-dimensional fluid model show that by increasing the excitation frequency over 13.56 MHz, the traditional glow-plasma structure could gradually recover even at very small sizes with a well defined quasineutral plasma region, and the electron density is improved but the electric fields in sheath region are reduced. This study indicates that the excitation frequency can be used to modulate the discharge structure and then tailor the plasma-surface interaction in atmospheric microplasmas.

Internal current measurements in high power impulse magnetron sputtering

The transport of charged particles in a high power impulse magnetron sputtering (HiPIMS) discharge is of considerable interest when optimizing this promising deposition technique with respect to deposition rate and control of the ion acceleration. In this study the internal current densities Jφ (azimuthal direction) and Jz (axial direction) have therefore been spatially and temporally resolved in the bulk plasma region above a cylindrical magnetron using Rogowski coils. From the measurements a phenomenological model has been constructed describing the evolution of the current density in this pulsed plasma. The core of the model is based on three different types of current systems, which characterize the operating transport mechanisms, such as current transport along and across magnetic field lines. There is a gradual change between these current systems during the initiation, build-up and steady state of a HiPIMS plasma. Furthermore, the data also show that there are spatial and temporal variations of the key transport parameter Jφ/Jz, governing the cross-B resistivity and also the energy of the charged particles. The previously reported faster-than-Bohm cross-B electron transport is verified here, but not for all locations. These results on the plasma dynamics are essential input when modeling the axial electric field, governing the back-attraction of ionized sputtered material, and might furthermore provide a link between the different resistivities reported in HiPIMS, pulsed-DC, and DC magnetron discharges.

Non-equilibrium atmospheric pressure radio-frequency glow-like discharges

At atmospheric pressure uniform radio-frequency (RF) glow-like discharges can be generated in helium in gap spacings down to 0.1mm. In larger gaps an α discharge is present, which exhibits a decreasing bulk plasma region with decreasing gap spacing. At gap spacings <0.5mm the discharge changes to a sheath-only structure. At gap spacings >0.4mm two regimes can be observed, a normal regime with partial coverage and an abnormal regime with full coverage of the electrodes. In all cases impedance measurements have been performed and compared with equivalent circuit models. There is a good agreement for the α discharges in the normal and abnormal regime, but still a lack of knowledge in the case of the sheath-only structure.

One-Dimensional Fluid Model for Dust Particles in Dual-Frequency Capacitively Coupled Silane Discharges

A self-consistent fluid model, which incorporates density and flux balances of electrons, ions, neutrals and nanoparticles, electron energy balance, and Poisson's equation, is employed to investigate the capacitively coupled silane discharge modulated by dual-frequency electric sources. In this discharge process, nanoparticles are formed by a successive chemical reactions of anion with silane. The density distributions of the precursors in the dust particle formation are put forward, and the charging, transport and growth of nanoparticles are simulated. In this work, we focus our main attention on the influences of the high-frequency and low-frequency voltage on nanoparticle densities, nanoparticle charge distributions in both the bulk plasma and sheath region.

Time-resolved measurements of the E-to-H mode transition in electronegative pulse-modulated inductively coupled plasmas

Time-resolved measurements of electronegative pulse-modulated inductively coupled plasmas (ICPs) were carried out using various measurement techniques. In order to explain the experimentally observed results, it is proposed that the structure of an Ar∕CF4 plasma during an afterglow dynamically changes, passing through three stages when the period of the afterglow is long enough: (1) The first stage is the initial afterglow where the electron temperature suddenly decreases due to inelastic collision with CF4. Electron density decreases and the density of fluorine negative ions increases by electron attachment, but the sheath potential still exists and the negative ions are confined in the bulk plasma region. Since charge neutrality should be maintained, the density of positive ions is almost constant during this stage. (2) The second stage is the intermediate afterglow where the plasma consists mainly of negative and positive ions but sheath potentials remain, reducing the negative ion flux from the plasma. The sheath potential gradually disappears and the densities of all the charged particles (electrons and positive and negative ions) decrease because of increased loss to the wall surface. (3) The third stage is where the sheath structure has disappeared completely, and the plasma consists of mainly positive and negative ions and losses are dominated by ambipolar diffusion. The presence of these stages during the afterglow is very important in understanding the behavior of pulsed ICPs with E-to-H mode transitions. The state of an afterglow plasma at the moment power is reapplied determines the discharge characteristics of the pulsed ICPs (e.g., the appearance of an E mode, the duration of the E mode, stability of the plasma).

Evolution of Discharge Structure in Capacitive Radio-Frequency Atmospheric Microplasmas

Conventional radio-frequency (rf) nonthermal atmospheric plasmas are generated in a millimeter gap. In this Letter, we present a self-consistent numerical study of rf atmospheric microplasmas in a submillimeter gap comparable to their sheath thickness. It is shown that the narrow electrode gap deforms the discharge structure, ultimately removing the bulk-plasma region and disabling electron trapping. Significantly, these properties permit rf atmospheric microplasmas to operate at very high current densities thus simultaneously achieving higher stability and greater chemical reactivity.

Enhanced magnetic ionization in hydrogen reflex discharge plasma source

The effect of enhanced magnetic ionization on the external and internal parameters of a high-density, low pressure reflex plasma source operating in hydrogen is studied. The Langmuir probe method and Druyvesteyn procedure coupled with suitable software are used to measure the internal parameters. The bulk plasma region is free of an electric field and presents a high degree of uniformity. The electron energy distribution function is bi-Maxwellian with a dip∕shoulder structure around 5.5eV, independent of external parameters and radial position. Due to the enhanced hollow cathode effect by the magnetic trapping of electrons, the electron density ne is as high as 1018m−3, and the electron temperature Te is as low as a few tens of an electron volt, for dissipated energy of tens of Watts. The bulk plasma density scales with the dissipated power.

On Rapid Computation of Time Periodic Steady State in Simulation of Capacitively Coupled RF Plasma

This paper proposes a methodology reducing the number of RF cycles necessary to reach time periodic steady-state solutions for efficient simulations of a three-moment plasma model. Our methodology employs a feedback control approach in conjunction with an implicit time integration scheme. Feedback gains are rigorously estimated by taking into account plasma responses during simulation. Through the one-dimensional simulation of a parallel plate capacitively coupled RF argon plasma, the proposed feedback control approach demonstrates its capability to dramatically reduce the number of RF cycles required to reach the time periodic steady state, resulting in several factors of speedup in simulation time. The simulation result showed that electrons emitted in a temporary cathodic sheath could acquire supersonic velocities and shocks could form near the corresponding sheath boundary as they enter a bulk plasma region.

Computational study of capacitively coupled high-pressure glow discharges in helium

The structure of a capacitively coupled high-pressure glow (HPG) discharge in high-purity helium is investigated using a detailed one-dimensional modeling approach. Impurity effects are modeled using trace amounts of nitrogen gas in helium. Average electron temperatures and densities for the HPG discharge are similar to their low-pressure counterpart. Helium-dimer ions dominate the discharge structure for sufficiently high-current densities, but model impurity nitrogen ions are found to be dominant for low-discharge currents. Helium dimer metastable atoms are found to be the dominant metastable species in the discharge. The high collisionality of the HPG plasma results in significant discharge potential drop across the bulk plasma region, electron Joule heating in the bulk plasma, and electron elastic collisional losses. High collisionality also results in very low ion-impact energies of order 1 eV at the electrode surfaces.

Structure of direct-current microdischarge plasmas in helium

A one-dimensional, self-consistent, continuum model is used to elucidate plasma phenomena in a parallel-plate dc microdischarge with a 250 μm gap at a pressure of 250 Torr. The microdischarge is found to have a bulk plasma region and a cathode sheath region with sizes that are comparable. Depending on the discharge current densities, peak electron densities of order up to 1014 cm−3 are predicted. Electron temperature of several eV are predicted within the cathode sheath while temperatures between 2 and 3 eV are observed in the bulk plasma. Gas temperatures of the order of 1000 K are predicted, emphasizing the importance of gas heating phenomena in dc microdischarges.

Experimental observation of sheath and magnetic presheath over an oblique metallic plate in the presence of a magnetic field

Experimental observation on sheath and magnetic presheath over an oblique metallic plate in the presence of a magnetic field has been performed. The plate has been deployed in the plasma environment, making some finite angle with the externally applied magnetic field. Studies have been carried out for various angles, magnetic field gradients and plate-biasing voltages as well. The analyses reveal that the magnetic presheath thickness increases while the angle is varied from minimum to maximum; whereas the reverse happens in case of the sheath thickness. Furthermore, measurements of the sheath widths with increasing magnetic field strengths and the plate biasing voltages have been done which imply that sheath width enhances with increasing both the parameters. The electron temperature in the bulk plasma region is also evaluated for all the plasma conditions so as to find out its influence on the characteristic behavior of the sheath formation.

Kinetic modeling of a one-dimensional, bounded plasma in the ambipolar regime

In this paper we present a self-consistent kinetic simulation of a diffusion dominated bulk plasma region. Collisions have been modeled by a velocity-dependent Krook collision operator. The technique is capable of handling large systems—the results presented here are for systems 100λmfp in extent—yet retains the details of the edge physics present. The distribution functions for the trapped and the transiting orbits and their moments are obtained. The density and potential profiles inside the bulk shows overall agreement with ambipolar predictions. The kinetic equivalent of DA is obtained and compared to the fluid prediction. The validity of the code and the observed deviations from fluid treatments are discussed.

The Factors Affecting the Particle Distributions Inside the Silane PCVD Reactor for Semiconductor Processing

The distributions of particles inside the silane plasma chemical vapor deposition (PCVD) reactor were theoretically investigated by analyzing the transport phenomena of particles for various plasma conditions. We included the effects of fluid convection, particle diffusion, and external forces (ion drag, electro static, and gravitational forces) onto the particles to analyze the movements of particles inside the plasma reactor. Initially, we assumed that the particles are uniformly distributed inside the plasma reactor and showed how these particles move and how they are distributed for various plasma conditions. The dominant force for the particle movement is the electrostatic force in the sheath region and the ion drag force in the bulk plasma region. Both the electrostatic and ion drag forces are towards the sheath boundaries and most of the particles are concentrated in the regions near the sheath boundaries by the balance of both forces, but the particle concentrations in the sheath region and in the...

Theoretical Analysis of a Radio Frequency Excited Slab CO2 Laser

Theoretical analysis of a radio frequency (RF) excited slab CO2 laser was carried out, considering not only the bulk plasma region but also the plasma boundary layer. To evaluate laser characteristics, electron density and electron temperature were estimated. The result of computation indicates that electron density and electron temperature would be higher in the boundary layer than in the bulk plasma region. Using such conditions, small-signal gain was computed to be approximately 1.5%/cm in the bulk plasma region and 0.5%/cm in the boundary layer. Thus, the gain in the boundary layer is lower than that in the bulk plasma region, but the boundary layer can sufficiently act as a gain medium as well as the bulk plasma region.

Dynamics of a coplanar-electrode plasma display panel. II. Cell optimization

Plasma display panels (PDPs) are a leading technology for large-area flat panel displays. As a result, there is significant interest in improving their efficiency, luminosity, and lifetime. In this article, results from a two-dimensional model are used to investigate the consequences of operating conditions, gas mixture, cell dimensions, and material properties on the visible light generation capacity (luminosity and efficiency) of a coplanar-electrode PDP cell sustained in He/Ne/Xe gas mixtures. Of the species that dominantly lead to the generation of visible light (Xe*, Xe**, and Xe2*), Xe2* makes the largest contribution for our conditions since its UV radiation is optically thin and Xe2* is efficiently generated from the long-lived xenon metastable. Significant improvements could be made in PDP light generation efficiency by choosing operating conditions that favor production of Xe2*, such as increasing gas pressure to enhance the three-body collision processes that generate Xe2*. Gas mixtures with more Ne (or less He) were found to produce more visible light at higher efficiency since electron transport in Ne is less collisional than He and Xe2* is produced more efficiently in three body collisions with Ne. PDP light emission characteristics are sensitive to the spacing between the dielectrics and there is an optimum spacing where both total visible light output and efficiency are high. It was found that PDP cells do not generate visible light efficiently during the postavalanche discharge phase due to low values of E/N (electric field/total gas density) in the bulk plasma region. Slight improvements can be made in light generation efficiency be choosing conditions for which PDP cell spends less time in the discharge phase in each cycle.

Monte Carlo Simulation of Normal and Abnormal Glow Discharge Plasmas Using the Limited Weight Probability Method

A self-consistent Monte Carlo modelling technique has been developed to study normal and abnormal glow discharge plasmas. To simulate nonequilibrium particles, a limited weight probability method is introduced and a fine subslab system is used. These two methods are applied to a DC Ar-like gas discharge simulation. The simulations are performed for conditions corresponding to the experimental voltage and current sets of normal and abnormal glow disharges. The characteristic spatial profiles of plasmas for normal and abnormal glow discharges with high nonequilibrium electron energy distributions are obtained. The increase in the current and the voltage from the normal glow leads to the following: (1) the density peak of the ions rises in the cathode region, (2) the density peak of electrons rises and catches up with that of ions and the peak position occurs closer to the cathode simultaneously; instead of a small increase of plasma density in the bulk plasma region, (3) reversal field strength next to the cathode fall increases and (4) the two groups of the enregy distribution separates into three groups at the cathode fall edge.

Dominant plasma species for TiN film formation by plasma CVD

An experimental study has been performed to identify the dominant plasma species which contribute to the formation of TiN films during CVD plasma deposition. To this effect, we measured the TiN film growth rate as a function of selected process variables and determined as well the radiation emission intensity of the major plasma species and their scaling with respect to the same variables, in the bulk plasma region and in the vicinity of the substrate. On these bases we calculated the corresponding flux rates to the substrate surface for the most abundant neutral species containing titanium and nitrogen, as well as their scaling as a function of the selected process variables. The results obtained suggest that Ti atoms and metastables are the major species contributing to the supply of activated titanium and nitrogen to the substrate surface. In addition, a scaling correlation has been established between the concentration of these species in the bulk plasma and the film growth rate which supports this result.

Modeling of the Evolutions of Negative Ions in Silane Plasma Chemical Vapor Deposition for Various Process Conditions

The evolutions of negative ions in the silane plasma chemical vapor deposition (PCVD) reactor for semiconductor processing were analyzed, considering the effects of chemical reactions, fluid convection, diffusion and electrical migration. The 23 plasma chemical reactions and 18 chemical species which might be important for the evolutions of negative ions were considered in this analysis. The governing equations were solved numerically. Most of the negative ions are located in the bulk plasma region and the concentrations of negative ions are almost zero in the sheath regions, as they are expelled from the sheath regions to the bulk plasma by electrostatic repulsion. The concentrations of negative ions at the downstream sheath boundary were higher than those at the upstream sheath boundary due to the effect of convection. As the reactor pressure increases, the concentrations of neutrals and positive ions increase, but the concentrations of negative ions decrease, based on the reaction chemistry proposed in this study. As the electric field strength increases, the negative ion concentrations increase, because more negative ions are contained in the bulk region by the stronger electrostatic repulsive forces.