Flux Of Negative Ions Research Articles

Dielectric barrier discharge as bipolar ion source for aerosol charging: Application to filtration, thin film, and aerosol size measurement

We have investigated the potentialities of dielectric barrier discharges (DBD) for aerosol electro-processing with a special focus on ion production and aerosol charging for deposition (filtration and materials) and aerosol measurement tools. The advantages and drawbacks of aerosol injection downstream or upstream of the discharge are compared according to the application. The main parameters that control ion production and transport are investigated based on post-discharge ion current measurements. Post-DBD aerosol chargers are compared for submicron particle size measurements based on aerosol bipolar charging and electrical mobility analysis. Finally, the principle of counterflow extraction of bipolar ion from an atmospheric pressure DBD is established from upstream measurements of positive and negative ion fluxes in air, without neither ozone (<0.1 ppmv) nor nanoparticles.

Mathematical modeling of injection of negative ion flux into the wall plasma

Mathematical modeling of injection of negative ion flux into the wall plasma

Measurement of negative ion fluxes during DC reactive magnetron sputtering of Ti in Ar/O2 atmosphere using a magnetic-filtering probe

The flux of negative oxygen ions in a dc reactive magnetron discharge has been measured using a specially developed magnetic-filtering probe. The gridded probe which operates as a retarding field analyzer has a magnetic filter attached to the front entrance to suppress plasma electrons entering the device. As a result, the fluxes of unmagnetized negative ions with energies of higher than ~10 eV can arrive to an ion collector and contribute to the detected probe current. During the oxide mode of Ti–Ar/O2 magnetron sputtering discharge, the measured negative ion flux is considered to be contributed by the sputtered O− ions generated at the target surface and the plasma O− ions created in the discharge volume. However, in specific discharge conditions, the sputtered ions are likely to dominate in the measured flux signal. As a result, the yield of the sputtered O− ions and associated energy flux can be examined. The magnetic-filtering probe could potentially be employed as a real-time diagnostic tool to monitor the sputtered O− flux as well as the oxide state of the target surface during reactive magnetron sputtering processes.

Mode transition in an oxygen low-pressure, very high frequency (162 MHz), multi-tile electrode capacitively coupled plasma

An experimental investigation of electrode voltage/discharge current, plasma density, including negative ions and ion flux, and ion energy distributions (IEDs) is performed in a low-pressure oxygen discharge excited by a multi-tile electrode, very high frequency (162 MHz) capacitively coupled plasma system. The results show a mode transition vs RF power. An inflection point is observed in the measured electrode voltage and current near to the mode transition. The negative ion density inferred from the measured electron density and ion flux using resonance hairpin probe and planar probe, respectively, shows an initial increase and then decrease after mode transition. The IED shows a symmetric narrow distribution and the mean energy first increases up to the transition point and then decreases with further increase in RF power. A change in the current coupling mechanism and variation in the discharge impedance due to the presence of negative ions are responsible for the observed mode transition.

Effect of trace SF6 on negative corona characteristics in SF6/N2 gas mixtures under DC voltages

Corona discharge will affect the performance of insulating gas, threatening the safe and reliable operation of gas-insulated power equipment. In this paper, we investigate the negative corona characteristics in SF6/N2 gas mixtures with trace SF6 [φ(SF6) ≤ 1%] under DC voltages. With decreasing SF6 content, two types of the discharge–stage transition process as a function of mean current are observed. One process is from the Townsend stage, through the pulse stage, to the breakdown stage; the other one presents a normal-glow stage between the pulse stage and the breakdown stage. The threshold of SF6 content which can change the discharge–stage transition process in SF6/N2 gas mixtures is between 0.1% and 0.15%. The lower limit of positive-ion flux in the normal-glow stage is 6–7 orders of magnitude higher than the upper limit of positive-ion flux in the Townsend stage. The variation in pulse characteristic parameters, including pulse–time parameters and pulse amplitude, with increasing SF6 content is investigated. The pulse–time parameters show a trend of decreasing first and then saturation with increasing SF6 content, while the pulse amplitude shows a slightly increasing trend. Moreover, the “return stroke” phenomenon of negative corona in SF6/N2 gas mixtures is observed through discharge light emission images. The “return stroke” phenomenon does not necessarily occur in the normal-glow stage, resulting from competition between the position of the negative-ion cloud and the negative-ion flux.

Kinetic sheath in presence of multiple positive ions, negative ions, and particle wall emission

The region between a Maxwellian plasma source and a floating or current-carrying surface is described by a static, one-dimensional collisionless kinetic sheath model. In the plasma source, electrons, negative ions, and several positive ion species with different temperatures can be included. The surface (wall) can emit electrons and/or negative ions. When the flux of surface-emitted negative ions and/or electrons reaches a critical value, the sheath becomes space-charge saturated, which leads to the formation of a virtual cathode in front of the emitting wall and sets the maximum current density that can be transported from the surface to the plasma. The analytical results are benchmarked against a particle-in-cell code.

Electronegative plasma diagnostic by laser photo-detachment combined with negatively biased Langmuir probe

We propose a new technique for diagnosing negative ion properties using Langmuir probe assisted pulsed laser photo-detachment. While the classical technique uses a laser pulse to convert negative ions into electron-atom pairs and a positively biased Langmuir probe tracking the change of electron saturation current, the proposed method uses a negatively biased Langmuir probe to track the temporal evolution of positive ion current. The negative bias aims to avoid the parasitic electron current inherent to probe tip surface ablation. In this work, we show through analytical and numerical approaches that, by knowing electron temperature and performing photo-detachment at two different laser wavelengths, it is possible to deduce plasma electronegativity (ratio of negative ion to electron densities) α, and anisothermicity (ratio of electron to negative ion temperatures) γ−. We present an analytical model that links the change in the collected positive ion current to plasma electronegativity and anisothermicity. Particle-In-Cell simulation is used as a numerical experiment covering a wide range of α and γ− to test the new analysis technique. The new technique is sensitive to α in the range 0.5 &amp;lt; α &amp;lt; 10 and yields γ− for large α, where negative ion flux affects the probe sheath behavior, typically α &amp;gt; 1.

Electrostatic properties and characterization of two-layer paper sheets

The aim of the research is to determine the electrostatic properties of two-layer paper sheets composed of laboratory filter paper and polyethylene. The volume resistivity and conductivity in a filter paper sheet, polyethylene film and a two-layer sheet consisting of them were studied. The experiment was done using two techniques. The investigated samples were exposed to static electric field and the surface and volume conductivity were measured in accordance with ASTM D257 standard. The same samples were also exposed to positive and negative air ion flux that allows a periodical deposition of a dosed amount of charge in order to measure electrostatic properties of the samples, i.e. to measure maximum surface voltage, sheet capacitance, surface voltage decay half time, volume resistivity, et cetera. A study of a two-layer sample consisting of laboratory filter paper and polyethylene film superimposed on the conductive surface in one position when the polyethylene film is on top and in other when the sample is in turned over position shows that the electrostatic properties of the top layer become dominant. The obtained results appear to be useful for more precise understanding of the phenomena occurring in multilayer sheets and to find a way to improve the multilayer sheets features.

Magnetized retarding field energy analyzer measuring the particle flux and ion energy distribution of both positive and negative ions.

This paper presents the development of a magnetized retarding field energy analyzer (MRFEA) used for positive and negative ion analysis. The two-stage analyzer combines a magnetic electron barrier and an electrostatic ion energy barrier allowing both positive and negative ions to be analyzed without the influence of electrons (co-extracted or created downstream). An optimal design of the MRFEA for ion-ion beams has been achieved by a comparative study of three different MRFEA configurations, and from this, scaling laws of an optimal magnetic field strength and topology have been deduced. The optimal design consists of a uniform magnetic field barrier created in a rectangular channel and an electrostatic barrier consisting of a single grid and a collector placed behind the magnetic field. The magnetic barrier alone provides an electron suppression ratio inside the analyzer of up to 6000, while keeping the ion energy resolution below 5 eV. The effective ion transparency combining the magnetic and electrostatic sections of the MRFEA is measured as a function of the ion energy. It is found that the ion transparency of the magnetic barrier increases almost linearly with increasing ion energy in the low-energy range (below 200 eV) and saturates at high ion energies. The ion transparency of the electrostatic section is almost constant and close to the optical transparency of the entrance grid. We show here that the MRFEA can provide both accurate ion flux and ion energy distribution measurements in various experimental setups with ion beams or plasmas run at low pressure and with ion energies above 10 eV.

Electron-less negative ion extraction from ion-ion plasmas

This paper presents experimental results showing that continuous negative ion extraction, without co-extracted electrons, is possible from highly electronegative SF6 ion-ion plasma at low gas pressure (1 mTorr). The ratio between the negative ion and electron densities is more than 3000 in the vicinity of the two-grid extraction and acceleration system. The measurements are conducted by both magnetized and non-magnetized energy analyzers attached to the external grid. With these two analyzers, we show that the extracted negative ion flux is almost electron-free and has the same magnitude as the positive ion flux extracted and accelerated when the grids are biased oppositely. The results presented here can be used for validation of numerical and analytical models of ion extraction from ion-ion plasma.

Bacterial Counts and Oxidative Properties of Chicken Breast Inoculated with Salmonella Typhimurium Exposed to Gaseous Ozone

AbstractPoultry meats are highly perishable due to bacterial contamination; thus, elimination of bacterial contaminants is a challenge for food safety industry. This study was designed to evaluate the bacterial counts and oxidative properties of chicken breast contaminated with Salmonella Typhimurium (ST) exposed to gaseous ozone during refrigerated storage. Slices of chicken breast were inoculated with ST by immersing them in peptone water containing 1.0 × 107 cfu of ST per milliliter peptone water. Samples were placed in a container with normal air (no ozone) or in a container equipped with an ozone generator that produce a continuous flux of ozone (10 × 10−6 kg O3/m3/h) and negative ions. Gaseous ozone exposure significantly reduced ST and the total aerobic and anaerobic bacterial counts during storage compared with the nontreatment group (P &lt; 0.05). Gaseous ozone exposure significantly reduced CIE L* and increased CIE b* surface color values after 3 and 2 days of storage, respectively (P &lt; 0.05). Gaseous ozone exposure significantly inhibited catalase and glutathione peroxidase activity after 3 days (P &lt; 0.05). A higher TBARS (2‐thiobarbituric acid reactive substances) value (P &lt; 0.05) was found in chicken breast subjected to ozone exposure after 3 days of storage. In conclusion, gaseous ozone exposure reduced the bacterial counts in chicken breast from the beginning of the study and affected the oxidative properties on the last day of the study.Practical ApplicationsThis study shows that gaseous ozone exposure can be used as an antimicrobial agent for refrigerated meat. This information will be useful in the development of an effective ozone generator inside a refrigerator. Further studies regarding the appropriate ozone concentrations are needed.

Pulsed Cl2/Ar inductively coupled plasma processing: 0D model versus experiments

Comparisons between measurements and spatially-averaged (0D) simulations of low-pressure Ar and Cl2 pulsed-plasmas in an industrial inductively coupled reactor are reported. Our analysis focuses on the impact of the pulsing parameters (frequency f, duty cycle dc) on the chemical reactivity of the plasma and on the ion fluxes to the walls. Charged particle densities and ion fluxes are highly modulated when the plasma is pulsed at 1 kHz < f < 20 kHz. In rare gas Ar plasmas, the ion flux rise time is short (50 μs), therefore the dc has almost no influence on the ion flux value during the pulse. By contrast, in molecular electronegative Cl2 plasmas, both the value and rise/decay time of the ion flux during the on and off-periods depend strongly on the dc. This is because in Cl2 both the plasma chemistry and electronegativity depend on the dc. During the off-period, the electron density drops much faster than the negative ion density, leading to a large increase in plasma electronegativity. A minimum afterglow time (75 µs) is required for an ion-ion plasma to form and for the sheath to collapse, exposing the walls and wafers to a negative ion flux. The positive ion flux is 3 to 10 times smaller in Cl2 than in Ar for the same operating conditions. In contrast with charged species, the radical (Cl) kinetics are slow and thus the radical density is hardly modulated for f > 1 kHz. However, the dc strongly influences the Cl2/Cl density ratio and is an excellent knob for controlling the plasma chemical reactivity: the higher the dc the higher the Cl density. The trends and quantities in the 0D simulation are in close agreement with experiments. This proves the capacity of global models to reproduce the fundamental features of pulsed plasmas in simple chemistries and to assist the development of pulsed processes.

Momentum transfer driven textural changes of CeO2 thin films

The influence of the target erosion depth on the film texture was investigated during DC reactive magnetron sputter deposition of CeO2 thin films. Three fluxes towards the substrate surface (the relative negative oxygen ion flux, the material flux, and the energy flux) were measured and related to the ongoing erosion of a cerium target. As the deposition rate increased for more eroded targets, both the energy flux and the negative ion flux decreased. Cerium oxide thin films that were deposited at different target erosion states, exhibited a change in preferential crystalline orientation from [200] to [111]. This textural change cannot be explained in terms of the energy per arriving atom concept. Instead, it is shown that the momentum of the high energetic negative ions is an essential condition to clarify the witnessed trends.

Nonlinear ion flux caused by a periodic ion-acoustic wave in plasma with two-temperature electrons

The propagation of periodic ion-acoustic waves in plasma with two-temperature electrons and cold ions is analyzed. The equations for the wave potential are derived in the first- and second-orders of the perturbation theory, and their nonsecular periodic solutions are obtained. The average nonlinear ion flux is determined, and its properties are studied as functions of the ratios between the densities and temperatures of the cold and hot electron components. The conditions are analyzed under which the ion flux is co- or counter-directed to the wave propagation direction. For the case in which, depending on the plasma parameters, the ion flux at a given wave amplitude can be either positive or negative, the domains of existence of positive and negative ion fluxes in the “temperature ratio-density ratio” plane are determined.

Experimental validation of the dual positive and negative ion beam acceleration in the plasma propulsion with electronegative gases thruster

The PEGASES (Plasma Propulsion with Electronegative Gases) thruster is a gridded ion thruster, where both positive and negative ions are accelerated to generate thrust. In this way, additional downstream neutralization by electrons is redundant. To achieve this, the thruster accelerates alternately positive and negative ions from an ion-ion plasma where the electron density is three orders of magnitude lower than the ion densities. This paper presents a first experimental study of the alternate acceleration in PEGASES, where SF6 is used as the working gas. Various electrostatic probes are used to investigate the source plasma potential and the energy, composition, and current of the extracted beams. We show here that the plasma potential control in such system is key parameter defining success of ion extraction and is sensitive to both parasitic electron current paths in the source region and deposition of sulphur containing dielectric films on the grids. In addition, large oscillations in the ion-ion plasma potential are found in the negative ion extraction phase. The oscillation occurs when the primary plasma approaches the grounded parts of the main core via sub-millimetres technological inputs. By controlling and suppressing the various undesired effects, we achieve perfect ion-ion plasma potential control with stable oscillation-free operation in the range of the available acceleration voltages (±350 V). The measured positive and negative ion currents in the beam are about 10 mA for each component at RF power of 100 W and non-optimized extraction system. Two different energy analyzers with and without magnetic electron suppression system are used to measure and compare the negative and positive ion and electron fluxes formed by the thruster. It is found that at alternate ion-ion extraction the positive and negative ion energy peaks are similar in areas and symmetrical in position with +/− ion energy corresponding to the amplitude of the applied acceleration voltage.

The correlation between the radial distribution of high-energetic ions and the structural as well as electrical properties of magnetron sputtered ZnO:Al films

The origin of the pronounced radial distributions of structural and electrical properties of magnetron sputtered ZnO:Al films has been investigated. The film properties were correlated with the radially resolved ion-distribution functions. While the positive ions exhibit low energies and a radial distribution with a maximum intensity opposite the center of the target, the negative ions can have energies up to several hundred eV, depending on the target potential, with a radial distribution with two maxima opposite the erosion tracks. The most prominent positive ion is that of the working gas (Ar+), while the highest flux of the negative ions is measured for negative oxygen O−. The radial distribution of the flux of the high-energetic negative ions can clearly be related to the radial variations of the structural (c-axis lattice parameter, crystallite size) and electronic (resistivity) properties for sputtering from the planar target, which points to the decisive role of the high-energetic negative oxygen ions for the film quality. The relation between the negative ion bombardment and the structural as well as electronic properties can be explained by a qualitative model recently developed by us. The same target has also been investigated in the eroded state. In this case, the limited acceptance angle of the mass spectrometer leads to a misinterpretation of the radial distribution of the flux of the high-energetic negative ions. This effect can be explained by a simulation, based on the assumption that the high-energetic negative ions are mainly accelerated in the cathode (target) sheath perpendicular to the uneven substrate surface.

Controlled deposition of sulphur-containing semiconductor and dielectric nano-structured films on metals in SF6 ion-ion plasma

In the present paper, the deposition processes and formation of films in SF6 ion-ion plasma, with positive and negative ion flows accelerated to the surface, are investigated. The PEGASES (acronym for Plasma Propulsion with Electronegative GASES) source is used as an ion-ion plasma source capable of generating almost ideal ion-ion plasma with negative ion to electron density ratio more than 2500. It is shown that film deposition in SF6 ion-ion plasma is very sensitive to the polarity of the incoming ions. The effect is observed for Cu, W, and Pt materials. The films formed on Cu electrodes during negative and positive ion assisted deposition were analyzed. Scanning electron microscope analysis has shown that both positive and negative ion fluxes influence the copper surface and leads to film formation, but with different structures of the surface: the low-energy positive ion bombardment causes the formation of a nano-pored film transparent for ions, while the negative ion bombardment leads to a continuous smooth insulating film. The transversal size of the pores in the porous film varies in the range 50–500 nm, and further analysis of the film has shown that the film forms a diode together with the substrate preventing positive charge drain, and positive ions are neutralized by passing through the nano-pores. The film obtained with the negative ion bombardment has an insulating surface, but probably with a multi-layer structure: destroying the top surface layer allows to measure similar “diode” IV-characteristics as for the nano-pored film case. Basing on results, practical conclusions for the probes and electrodes cleaning in ion-ion SF6 plasmas have been made. Different applications are proposed for the discovered features of the controlled deposition from ion-ion plasmas, from Li-sulphur rechargeable batteries manufacturing and nanofluidics issues to the applications for microelectronics, including low-k materials formation.

Comparison of ion energies and fluxes at the substrate during magnetron sputtering of ZnO : Al for dc and rf discharges

Energy spectra of positive and negative ions have been measured in magnetron sputtering of a ZnO : Al target. The discharges have been operated in dc or rf, the latter with a frequency of 13.56 or 27.12 MHz, at the same geometry, argon pressure, and discharge power. While the average energy of the positive ions against a grounded surface increases with increasing frequency due to increased plasma potential, the average energy of the negative ions strongly decreases due to a target voltage decrease. The flux of negative ions simultaneously decreases, whereas that of the positive ions slightly increases. Combining these quantities to the energy flux onto floating substrates shows a drastic decrease of the energy flux with increasing frequency for high-energetic negative ion bombardment of the films while the low-energetic positive ion impingement is weakly increased. From the point of the energetic bombardment rf discharges are hence favoured to obtain high-quality films. The energy distributions of negative ions have a sharp peak in the dc mode but broad distributions in the rf discharges. Additionally, the latter are characterized by an overlaid peak structure that is explained by the oscillation of target and plasma potential.

In situ analyses on negative ions in the indium-gallium-zinc oxide sputtering process

The origin of negative ions in the dc magnetron sputtering process using a ceramic indium-gallium-zinc oxide target has been investigated by in situ analyses. The observed negative ions are mainly O− with energies corresponding to the target voltage, which originates from the target and barely from the reactive gas (O2). Dissociation of ZnO−, GaO−, ZnO2−, and GaO2− radicals also contributes to the total negative ion flux. Furthermore, we find that some sputtering parameters, such as the type of sputtering gas (Ar or Kr), sputtering power, total gas pressure, and magnetic field strength at the target surface, can be used to control the energy distribution of the O− ion flux.

Observations of ionic species produced in an atmospheric pressure pulse-modulated RF plasma needle

Pulse-modulated generation of atmospheric pressure plasma promises lower operating temperatures and enhanced energy efficiencies compared with those generated using continuous excitation. In this study, the effect of pulse modulation on the ionic content of a radio-frequency helium plasma needle was explored using molecular beam mass spectrometry (MBMS). In all cases reported the pulse repetition frequency was held constant at 6 kHz while the duty cycle and distance between the plasma needle and MBMS were varied from 30% to 90% and 4 mm to 10 mm, respectively. Decreasing the duty cycle was seen to enhance the negative ion flux to the MBMS and yields a clear shift to higher masses for positive ion species. Time-resolved ion intensity measurements (5 µs resolution) demonstrated that negative ions were created almost exclusively in the pulse off-time while positive ions were generated mainly in the on-time. The ions detected were also seen to change as the distance between the sampling orifice and tip of the plasma needle increased, highlighting the effect of different ion mobilities in the flowing gas stream and the presence of an extended cooler afterglow region. The results suggest that there is a possibility of tailoring the ionic plasma chemistry through pulsed modulation, increasing the versatility of the already widely studied plasma needle device.