Measurements Of Ion Current Density Research Articles

In situ measurement of electron emission yield at Si and SiO2 surfaces exposed to Ar/CF4 plasmas

Abstract Plasma simulations require accurate yield data to predict the electron flux that is emitted when plasma-exposed surfaces are bombarded by energetic particles. One can measure yields directly using particle beams, but it is impractical to create a separate beam of each particle produced by typical plasmas. In contrast, measurements made in situ, during plasma exposure, provide useful values for the total emitted flux and effective yield produced by all incident particles. Here, in situ measurements were made at thermally oxidized and bare silicon wafers placed on the radio-frequency (rf) biased electrode of an inductively coupled plasma system. The rf current and voltage across the sheath at the wafer were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are input into a numerical sheath model, which allows the emitted electron current to be distinguished from other currents. The effective yield, i.e. the ratio of the total emitted electron flux to the incident ion flux, was determined at incident ion energies from 40 eV to 1.4 keV, for Si and SiO2 surfaces in Ar, CF4, and Ar/CF4 mixtures at 1.33 Pa (10 mTorr). Yields for Ar plasmas are compared with previous work. For SiO2 surfaces in Ar/CF4 mixtures and pure CF4, the yield is dominated by ion kinetic emission, which is the same for all mixtures, and, presumably, for all ions. For Si surfaces in Ar/CF4 and CF4, the yield at high energies can be explained in part by fragmentation of molecular ions, and the yield from Ar+ can be distinguished from the other ionic species. Analytic fits of the yields are provided for use in plasma simulations.

Ion current density on the substrate during short-pulse HiPIMS

A probe method for measuring the ion current density and theoretical calculations of the dynamics of neutral and charged plasma particles using the ionization region model (IRM) is used to study short and ultra-short pulse high-power impulse magnetron sputtering (HiPIMS). This paper studies reasons for the increase in the average ion current density on the substrate at shorter pulses, when the average discharge power does not change. HiPIMS pulses are applied to the copper target at constant values of average discharge power (1000 W) and peak current (150 А), respectively, while the pulse time of the discharge voltage ranges from 4 to 50 µs. A power supply with low output inductance is designed to generate ultra-short pulses. It is shown that shorter discharge pulses lead to a multiple growth (from 2 to 7 mA cm−2) in the average ion current density on the substrate and a growth in the peak intensity of Ar+, Cu+ and Cu2+ recorded by optical emission spectroscopy. A theoretical model of this effect is based on the spatially averaged IRM, which considers afterglow effects. According to theoretical calculations, the increase in the average ion current density on the substrate is determined by the plasma dissipation in the ionized region after the pulse ends. Also, a decrease in the copper deposition rate from 180 to 60 nm min−1 with decreasing pulse time from 40 to 4 µs is explored. A comparison of experimental data with those obtained earlier shows that the suggested dependences of the ion current density and deposition rate on the HiPIMS pulse time are typical for discharge systems with different cathode materials and configurations, i.e., for single- and dual-magnetron systems. This indicates a common nature of the phenomena observed and additionally confirms the results obtained.

Measurement system for ion beam profiles using fixed parallel wires and Faraday cup array

The radial profiles of ion current density are measured using a combination of two techniques—fixed parallel wires and a Faraday cup array—in a ring cusp ion source. These measurements are carried out for Ar+ ion beam current in the range of 20–90 mA and energy in the range of 850–1650 eV. Current density profiles near the extraction plane of the ion source are obtained by Abel inversion of the profiles of current measured using the parallel wires. In addition, the Faraday cup array provides radial profiles of ion current density at two different locations along the beam axis. A numerical model is developed and utilized for estimating the beam divergence and focal length from the profiles of ion current density measured using the two techniques. A fixed parallel wire array is established as an effective method for monitoring the ion beam profiles close to the ion source's extraction plane when the ion beam is utilized for other applications. The performance of the two measurement techniques is compared by estimating the ion beam current from the measurement of ion current density with the beam current measured from the power supply.

Characterization of a Multicusp Ion Source With Two-Grid Extraction System for Studying Extraction and Transport of Ion Beam

A low-energy multipole line cusp ion source with two-grid extraction system has been developed and characterized for studying the role of the charge exchange processes on ion beam extraction, transport, and neutralization. The ion source is operated for extraction of Ar <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion beam of current 50–240 mA at energies 850–1650 eV. The characterization of the ion beam is carried out at various axial positions along a 200–815-mm beam path with a background pressure of 4– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> Torr. An 11-channel Faraday cup array is used to measure the radial profiles of ion current density and angular beam divergence at these positions. The beam divergences are found to be in the range of 6°–15° for beam perveance of 0.50– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.75\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> AV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3/2</sup> . The attenuation of ion beam current along the beam path is observed due to the charge exchange neutralization of the energetic ions in the presence of high background pressure. Since ion current density along the beam path can be reduced by both the charge exchange processes and beam divergence, the accounting of beam particles, i.e., energetic ions and neutrals, at larger distances is difficult by the measurements of ion current density using the Faraday cup alone. The flux of energetic particles (energetic ions and fast neutrals) is therefore obtained by a thrust measurements on a beam target fixed at a distance of 800 mm from the ion source. The neutralization of the ion beam obtained by the emission of electrons from a hot filament neutralizer is also investigated in the presence of the charge exchange processes. Space-charge neutralization is provided initially by the secondary electrons emitted from the vessel walls or by the charge exchange processes even without the neutralizer operating at the desired temperature. However, the current neutralization is achieved only by the electrons emitted from the hot filament neutralizer operating in a space-charge-limited mode.

In situ measurement of electron emission yield at silicon dioxide surfaces exposed to argon plasmas

Plasma simulations require accurate yield data to predict the electron flux that is emitted when plasma-exposed surfaces are bombarded by energetic particles. One can measure yields directly using particle beams, but it is impractical to create a separate beam of each particle produced by typical plasmas. In contrast, measurements made in situ, during plasma exposure, provide useful values for the total emitted flux produced by all incident particles. Here, in situ measurements were performed in a radio-frequency (rf) biased, inductively coupled plasma (icp) system in 0.67 Pa and 1.33 Pa (5 mTorr and 10 mTorr) of argon gas. The rf current and voltage across the sheath adjacent to the rf-biased electrode were measured, along with Langmuir probe measurements of ion current density and electron temperature. The measurements are input into a numerical sheath model, which allows the emitted electron flux to be distinguished from other current mechanisms. The effective yield, i.e., the ratio of the total emitted electron flux to the incident ion flux, is also determined, as a function of incident ion energy. Results for the effective yield of a sputter-deposited SiO2 film are reported and compared with previous work. The measured effective yield can be considered the yield from Ar+ kinetic emission, which, in these experiments, dominates other emission processes. From additional literature data, emission yields for other processes—incident photons, Ar metastables, fast Ar atoms, and Ar+ potential emission—are recommended and are shown to be consistent.

Study on influencing factors of ion current density measurement in corona discharge of HVDC transmission lines

The ground level ion current density produced by corona discharge in high voltage direct current (HVDC) transmission lines can reflect the operation status of the lines, but the distorted electric field at the edge of the Wilson plate seriously affects the measurement results of ion current density. In order to measure the ground level ion current density accurately and directly, a new reduced scale wire-plate experimental device in which the Wilson plate is flush with the grounding plate is designed. The influence of protective annulus width and the height of the Wilson plate from the grounding plate on ground level ion current density are studied. In addition, the differences between the micro-current galvanometer method and the sampling resistance method in the measurement of ion current density are compared. Finally, the ground level ion current density distributions of unipolar and bipolar HVDC transmission lines are measured. The results show that the edge effect of the Wilson plate can be neglected when the width of protective annulus is less than 2 mm, and the ion current density is nonlinear with the height of the Wilson plate from the grounding plate. Moreover, the internal resistance of the digital voltmeter seriously affects the measurement results and it is necessary to correct the results. Finally, at the same applied voltage, the ground level ion current density in the negative conductor region is higher than that in the positive conductor region.

Simultaneous DC measurements of ion current density and electron temperature using a tunnel probe

The tunnel probe is a concave Langmuir probe designed to operate in strongly magnetized plasma. Due to its shape, the tunnel probe is immune to sheath expansion effects and thus provides absolutely calibrated measurements of the parallel ion current density. A two-dimensional, self-consistent kinetic model is employed to model the flow of charges within the cavity of the tunnel probe. The calculation predicts that the distribution of the ion flux onto the inner conductors depends on the electric field inside the tunnel, which in turn depends on the electron temperature. Therefore, if the tunnel is divided into two negatively biased collectors, it is possible to use the simulation results to determine the electron temperature from the measured ion current ratio. This means that a DC-biased tunnel probe can be used to provide fast, simultaneous measurements of the parallel ion current density and the electron temperature without collecting a single electron. Measurements in the CASTOR and Tore Supra tokamaks agree well with the numerical simulations.

Langmuir probe and optical emission spectroscopy studies in magnetron sputtering plasmas for Al-doped ZnO film deposition

This work reports investigation of the Al-doped ZnO (AZO) film deposition process, at different working pressures, in a conventional magnetron sputtering system. The primary goal of this study is to investigate the plasma formation and deposition process using various diagnostic tools, by utilizing low-temperature deposition process. In addition, this paper also presents a systematic Langmuir probe (LP) analysis procedure to determine the maximum information about plasma parameters. For the present study, we have extensively used LP method to characterize the deposition process for the control of plasma parameters. Along with the LP method, we have also used optical emission spectroscopy diagnostic to examine the favorable deposition condition for the fabrication of conductive AZO film. Utilizing diagnostics, this also reports measurements of ion current density, substrate temperature, and deposition rates to fabricate low resistivity films of ∼3 mΩ cm.

Ion current density measurements in a copper vacuum arc with different refractory anode thicknesses

The time-dependent ion current density was measured in a hot refractory anode vacuum arc (HRAVA) sustained between a consumed water-cooled cylindrical Cu cathode and non-consumed cylindrical W anodes with thickness d = 5, 10, 15, 20, or 30 mm separated by an h = 10 mm gap. Arc currents of I = 130, 150, 175, and 200 A were applied for a period of 90 s. Ion current density Ji extracted from the plasma was measured using a probe located at varying distances from the electrode axis. The active surface of the probe was oriented to be either perpendicular or parallel to the radially expanding plasma to measure the directed or random component of Ji, respectively. Ji started at arc ignition and grew slowly, passed through a peak, and reached a final steady state level. This level increased with arc current and decreased with probe distance. The time to reach the steady state decreased when d was decreased from 30 to 5 mm, for I = 200 A from 48 to 12 s and for I = 150 A from 69 to 20 s, and weakly depended on probe orientation. Ji to the perpendicular probe significantly exceeded that for the parallel orientation.

Retarding field energy analyser ion current calibration and transmission

Accurate measurement of ion current density and ion energy distributions (IEDs) is often critical for plasma processes in both industrial and research settings. Retarding field energy analysers (RFEAs) have been used to measure IEDs because they are considered accurate, relatively simple and cost effective. However, their usage for critical measurement of ion current density is less common due to difficulties in estimating the proportion of incident ion current reaching the current collector through the RFEA retarding grids. In this paper an RFEA has been calibrated to measure ion current density from an ion beam at pressures ranging from 0.5 to 50.0 mTorr. A unique method is presented where the currents generated at each of the retarding grids and the RFEA upper face are measured separately, allowing the reduction in ion current to be monitored and accounted for at each stage of ion transit to the collector. From these I–V measurements a physical model is described. Subsequently, a mathematical description is extracted which includes parameters to account for grid transmissions, upper face secondary electron emission and collisionality. Pressure-dependent calibration factors can be calculated from least mean square best fits of the collector current to the model allowing quantitative measurement of ion current density.

Effects of Internally Mounted Cathodes on Hall Thruster Plume Properties

The effects of cathode position on the operation and plume properties of an 8-kW Hall thruster are discussed. Thruster operation was investigated at operating conditions ranging from 200 to 500 V of discharge voltage, 10-40 A of discharge current, and 2-8 kW of discharge power, with a cathode positioned either in the traditional externally mounted configuration outside the outer magnetic pole piece or in an internally mounted configuration central to the inner magnetic core. With the external cathode, substantial emission in the visible spectrum that follows magnetic field lines surrounds the exterior pole pieces of the thruster. With the internal cathode, the emission is largely absent while the cathode plume is compressed and elongated in the axial direction by the strong axial magnetic field on the thruster centerline. Discharge current oscillation and ion species fraction measurements were found to be similar for the cathode locations, whereas the operation with the internal cathode was found to favor an improved coupling of the cathode plume with the thruster discharge. Ion current density measurements show that with respect to externally mounted designs, internally mounted cathodes reduce plume divergence and increase the symmetry of the near-field plume. The impacts of internally mounted cathodes on thruster physics and spacecraft integration activities are assessed.

Numerical Simulation of Hall Thruster Plasma Plumes in Space

Hall thrusters are an important form of electric propulsion that are being developed and implemented to replace chemical systems for many on-orbit propulsion tasks on communications and exploration spacecraft. A concern in the use of these devices is the possible damage their plasma plumes may cause to the host spacecraft. Computer models of Hall thruster plumes play an important role in integration of these devices onto spacecraft as the space environment is not easily reproduced in ground testing facilities. In this paper, an advanced hybrid particle-fluid model is applied to model the plumes of the Hall thrusters used on the Russian Express and European SMART-1 spacecraft. The results from the model are compared directly with measurements of ion current density and ion energy distributions taken in space by these spacecraft

Effect of Backpressure on Ion Current Density Measurements in Hall Thruster Plumes

The effects of facility backpressure and localized electric fields on the measured ion current densities of Hall thrusters are investigated. Langmuir probe measurements are taken in the near-field plasma surrounding a nude Faraday probe, which is located 1 m from the exit plane of the University of Michigan/U.S. Air Force Research Laboratory P5 Hall thruster. The thruster is operated at an anode flow rate of 5.30 mg/s, at backpressures of 1.5 × 10 −3 Pa (1.1 × 10 −5 torr) and 4.8 × × 10 −4 Pa (3.6 × × 10 −6 torr), corrected for xenon. The effect of the facility backpressure is clearly seen in the wings of the plume. A combination of charge-exchange collisions and vacuum chamber gas ingestion into the thruster is believed to be the cause of this phenomenon. The Langmuir probe results indicate that the electric fields near the nude Faraday probe are functions of facility backpressure and the angle from the thruster centerline. The plasma potential measured within 20 mm of the probe varied by no more than 3V .Thus, the electric fields near the nude Faraday probe are not large enough to explain the increased collection of charge-exchange ions at elevated facility background pressures and large angles from the thruster centerline.

Far field modeling of the plasma plume of a Hall thruster

Hall thrusters are an attractive form of electric propulsion that are being developed and implemented to replace chemical systems for many in orbit propulsion tasks on communications satellites. One concern in the use of these devices is the possible damage their plumes may cause to the host spacecraft. Computer models of Hall thruster plumes play an important role in integration of these devices onto spacecraft as the space environment is not easily reproduced in ground testing facilities. In this article, a hybrid particle-fluid model of a Hall thruster plume is applied to model the SPT-100 thrusters used on the Russian Express satellites. The emphasis of the article is on making assessment of the model through direct comparison with measurements of ion current density and ion energy distributions taken on board Express spacecraft. A model for simulating atom–ion collisions is described. The sensitivity of the plume simulation results to various aspects of the physical modeling is investigated. The plume model is able to predict many of the most important characteristics of the measured data.

Parameter space map of an electron cyclotron resonance plasma in a compact chamber

Ion current density measurements were made in an electron cyclotron resonance (ECR) plasma reactor for both argon and oxygen discharges. Spatial changes in the ion current density were also recorded across the reactor diameter for changes in pressure and power. These measurements revealed a minimum in the ion current density on the reactor axis. This observation has been explained as a consequence of the shape of the ECR region, which, in turn, is dependent on the mode of coupling. Current density measurements were made as a function of reactor pressure and microwave power for two different axial locations in the system. A Langmuir probe was also used at these two locations to measure the electron temperature as a function of these process conditions. It was observed that the ion current density and/or plasma density measured downstream from the ECR zone, increased significantly in the low-pressure/high-microwave power region. Results from this region of the operating parameter space have not previously been reported. Further existing models do not predict this observed increase in plasma density or ion current density. It has been proposed that a rarefication of the gas in the ECR region, as a result of gas heating, has acted to increase the outward diffusion of electrons from the ECR zone and, thus, has increased the ambipolar diffusion of ions to the downstream location. This proposal has been partially validated by experimental results in which the ion energy was measured as a function of reactor pressure and gas flow rate. The shape of the oxygen parameter space map differs significantly from that for Ar. The principal reasons for these changes are a number of different inelastic electron scattering mechanisms which effect the transport electrons out of the ECR zone and through ambipolar diffusion also the transport of ions. The second factor is the production of negative ionic species which varies with reactor pressure and, thus, T/sub e/.

Cathodic-arc plasma density, radial profile, and total plasma inventory as measured by microwave interferometer and Faraday cup

The total amount of plasma, peak plasma density, and plasma density radial profile are derived from a series of measurements of ion current density (with a Faraday cup) and integrated electron line density (with a microwave interferometer) for a cathodic arc derived plasma. Comparisons are made between cathode material (erbium and titanium), arc current, background gas pressure, and the presence or absence of a series magnetic solenoid around the coaxial anode and cathode.

Influence of low-energy bombardment of an RF magnetron sputtering discharge on texture formation and stress in ZnO films

In an oxygen planar RF magnetron sputtering discharge, the time-averaged flux and energy of positive ions drifting out of the plasma and striking the substrate surface have been determined as a function of RF discharge power over a range of 100 to 1000 W, and as a function of chamber pressure from 0.2 to 6 Pa by measurement of ion-current density and time-averaged plasma sheath potential at the substrate. These data were related to the resulting crystal structure of the deposited ZnO films which had been studied in detail using well-known methods of X-ray diffraction. The impact energy of the positive ions bombarding the growing film varies from some 10 eV to close 50 eV depending on magnetron RF discharge power and oxygen pressure, respectively. The incident ion flux was found to be below 1× 10 15 cm −2 s −1 up to 1 × 10 16 cm −2 s −1, a value of the same order of magnitude as that for the condensing rate of sputtered ZnO species. The structural results obtained show that both the ion energy and the ion flux in the range mentioned above cause significant changes in the degree of crystallinity, preferred orientation and texture sharpness of the deposited ZnO films. Furthermore, positive ion bombardment during film growth has been found to alter the ZnO unit cell dimension up to 2% relative to the equilibrium bulk or powder value which is responsible for the formation of strong compressive residual stress of up to several GPa within the ZnO film. Following these results, one of the criterions for preparing highly c-axis oriented ZnO films with columnar grain structure is to decrease both the energy and the flux of the positive ion bombardment without decreasing the deposition rate of ZnO species. At a such slight-bombardment RF magnetron deposition the compressive residual stress of the ZnO film can be reduced towards zero.

磁気中性点による散乱効果を用いたイオン源の基本特性

Two types of ion sources with a single magnetron injection gun-type-thermionic cathode were fabricated and tested. The cathode was positioned on the extended line of the cylindrical axis. These sources have magnetic multipole linecusps with different directions. One of them had azimuthally oriented magnetic field, and the other had axially oriented one. The measurement of ion current density and ion energy distribution revealed that the beam profile became a hollow-beam whose intensity increases in proportion to discharge current, and that the ion source with azially oriented magnetic field produces ion beams 450 times higher than those with azimuthally oriented magnetic field. Single extraction grid-ion source is also compared in extracted beam profile with the two grid-ion source.

Measurement of Ion Current Density at Ground Level in the Vicinity of High Voltage DC Transmission Lines

Measurement of Ion Current Density at Ground Level in the Vicinity of High Voltage DC Transmission Lines

Corona Studies for Biploar HVDC Transmission at Voltages Between ±600 kV and ±1200 kV Part 1: Long-Term Bipolar Line Studies

The corona performance characteristics of conductor bundles for use on transmission lines operating in the range of ±600 kV to ±1200 kV, obtained as part of a comprehensive corona study for transmission systems in this voltage range, are presented in this paper. The results presented in this paper complement those already published1 for a 4 x 1.6 conductor bundle chosen for a nominal voltage of ±750 kV. In addition to presenting the corona performance characteristics of 6 x 1.6 and 8 x 1.8 conductor bundles, corresponding to nominal voltages of ±900 kV and ±1050 kV respectively, the paper also presents a general analysis of some of the results obtained for all the three bundles tested. The study includes a detailed investigation of the important corona performance parameters, namely radio interference (RI), audible noise (AN) and corona losses (CL). Some measurements of ion current density and electric field at ground level are also presented.