Average Ion Flux Research Articles

Growth and characterization of W thin films with controlled Ne and Ar contents deposited by bipolar HiPIMS

This work is focused on controlling and optimizing the inclusion of neon (Ne) and argon (Ar) impurities in tungsten (W) reference coatings. A tungsten target has been sputtered in monopolar and bipolar High Power Impulse Magnetron Sputtering (HiPIMS) operation modes, in Ar-Ne gas mixture. It has been found that the process of noble gas retention in W thin layers is strongly dependent on both ion energy and average flux to the substrate. The ion energy can be accurately controlled by the amplitude of the positive voltage pulse while the average ion flux can be controlled by the duration of the negative pulse. Measurements of the plasma potential in front of the substrate and the ion saturation current at the substrate position allow explaining the mechanism of ion acceleration and the control of ion energy and flux. The effect of the pulsing configuration on the microstructure of the co-deposited films has been investigated by X-ray Diffractometry, Scanning Electron Microscopy and Atomic Force Microscopy, while the amount of noble gas enclosed in the coatings has been measured by Non-Rutherford Backscattering Spectrometry. Ne and Ar amount in the W co-deposited layers increases up to 3.0 at.% each as the amplitude of the positive pulse increases to + 300 V or the negative pulse duration decreases to 3 µs. The highest film crystallinity, with W(1 1 0) preferential growth, has been obtained for a positive pulse voltage of + 200 V and a negative pulse duration of 5 µs. The thickness of the deposited layers ranges between 350 nm in bipolar HiPIMS with a positive pulse voltage of + 300 V and 550 nm in monopolar HiPIMS.

Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation

The charged particle dynamics in low-pressure oxygen plasmas excited by odd harmonic dual frequency waveforms (low frequency of 13.56 MHz and high frequency of 40.68 MHz) are investigated using a one-dimensional numerical simulation in regimes of both low and high electronegativity. In the low electronegativity regime, the time and space averaged electron and negative ion densities are approximately equal and plasma sustainment is dominated by ionisation at the sheath expansion for all combinations of low and high frequency and the phase shift between them. In the high electronegativity regime, the negative ion density is a factor of 15–20 greater than the low electronegativity cases. In these cases, plasma sustainment is dominated by ionisation inside the bulk plasma and at the collapsing sheath edge when the contribution of the high frequency to the overall voltage waveform is low. As the high frequency component contribution to the waveform increases, sheath expansion ionisation begins to dominate. It is found that the control of the average voltage drop across the plasma sheath and the average ion flux to the powered electrode are similar in both regimes of electronegativity, despite the differing electron dynamics using the considered dual frequency approach. This offers potential for similar control of ion dynamics under a range of process conditions, independent of the electronegativity. This is in contrast to ion control offered by electrically asymmetric waveforms where the relationship between the ion flux and ion bombardment energy is dependent upon the electronegativity.

First joint in situ and global observations of the medium‐energy oxygen and hydrogen in the inner magnetosphere

AbstractWe present the first simultaneous observations of the in situ ions and global Energetic Neutral Atom (ENA) images of the composition‐separated, medium‐energy (~1–50 keV) particle populations of the inner magnetosphere. The ENA emissions are mapped into L shell/magnetic local time space based on the exospheric density along the line of sight (LOS). The ENA measurement can then be scaled to determine an average ion flux along a given LOS. The in situ ion flux tends to be larger than the scaled ENAs at the same local time. This indicates that the ion population is more concentrated in the Van Allen Probes orbital plane than distributed along the Two Wide‐angle Imaging Neutral‐atom Spectrometers LOS. For the large storm of 14 November 2012, we observe that the concentration of O (in situ ions and ENAs) increases during the storm's main phase with a relatively larger increase than H. The ratio of the O+/H+ can be measured both from the in situ observations and from the ENA images. During the main phase, this O+/H+ increase is initially seen near midnight, but when the storm reaches its peak value the O+/H+ ratio increases across all local times, with the largest at dusk and dawn.

Semiconducting WO3 thin films prepared by pulsed reactive magnetron sputtering

WO3 crystalline semiconductor thin films for water-splitting applications were prepared by pulsed unbalanced reactive magnetron sputtering with W target and Ar + O2 gas mixture. Postdeposition annealing at temperature of 450 °C was applied to the WO3 samples to improve their crystallinity and semiconductor properties. Various pulsing modes were tested in deposition experiments with different pulsing frequencies, discharge power applied in pulse, and average applied power. To determine the influence of the plasma parameters on the deposition process, the pulsed and average ion flux density on the substrate were measured using an ion probe. The WO3 films had monoclinic crystalline structure after the annealing process. Different crystallite orientations were found for different modes of discharge pulsing. Preferential orientation of the (200) plane parallel to the substrate surface was identified for higher frequency of discharge pulsing with lower substrate pulsed ion flux but higher average substrate ion flux. The WO3 films with this type of texture had the highest photocurrents in photoelectrochemical (PEC) measurements.

A spatially resolved retarding field energy analyzer design suitable for uniformity analysis across the surface of a semiconductor wafer.

A novel retarding field energy analyzer design capable of measuring the spatial uniformity of the ion energy and ion flux across the surface of a semiconductor wafer is presented. The design consists of 13 individual, compact-sized, analyzers, all of which are multiplexed and controlled by a single acquisition unit. The analyzers were tested to have less than 2% variability from unit to unit due to tight manufacturing tolerances. The main sensor assembly consists of a 300 mm disk to mimic a semiconductor wafer and the plasma sampling orifices of each sensor are flush with disk surface. This device is placed directly on top of the rf biased electrode, at the wafer location, in an industrial capacitively coupled plasma reactor without the need for any modification to the electrode structure. The ion energy distribution, average ion energy, and average ion flux were measured at the 13 locations over the surface of the powered electrode to determine the degree of spatial nonuniformity. The ion energy and ion flux are shown to vary by approximately 20% and 5%, respectively, across the surface of the electrode for the range of conditions investigated in this study.

Deuterium retention in graphite exposed to high flux plasma at high temperatures

Deuterium trapping in graphite under high flux plasma exposure at high temperatures was studied. A target of MPG-8 fine grain graphite was exposed to plasma beam in LENTA device at zero potential. The average ion flux was of about 1021D/m2s with a maximum in the center. The temperature was 1473K in the center and 873K outside the beam spot. After irradiation, the target was sectioned along and across the surface and studied by SEM, NRA, and TDS. Deuterium profiles measured by NRA showed a peak close to the surface and long tails in the bulk. TDS of samples taken for the front and back sides of the target were very similar. It is suggested, that D retention at the plasma irradiation is mainly controlled by trapping from the gas phase, locally created in the graphite pores by plasma ions injection and subsequent flow of gas to the back side through internal porosity.

Baseline high‐latitude ionospheric parameters and field‐aligned ion motion

Establishing baseline values for ionospheric parameters allows for the identification of special events and comparison of events that occur at different magnetic local times, seasons, or solar cycle phases. Variations in average electron densities, ion and electron temperatures, and ion velocities between 100 and 800 km altitude are determined from Svalbard and Sondrestrom radar data. Despite their similar geomagnetic latitudes, Svalbard and Sondrestrom are found to measure significantly different populations. The average ion flux and ion, electron and plasma pressures can be calculated from the measured quantities. The primary location for upward moving ions is the dayside noon region, where the cusp can be expected. The role of plasma pressure is discussed.

Simulation of plasma-based ion implantation of a sawtooth target

Plasma-based ion implantation of a two-dimensional target consisting of a periodic array of symmetric, 90°, triangular sawteeth is studied using a particle-in-cell plasma simulation. The average ion flux, ion impact angle, ion impact energy and the energy-resolved dose are computed. When the sheath width is large compared to the tooth size, the sheath edge becomes planar, the ion flux is nearly uniform and ions impact the surface of the target at 45°. The incident dose is greatest near the convex corner, and least in the concave corner.

Measurements of plasma in the magnetospheric tail lobes

The plasma spectrometer SKS on the Prognoz 8 satellite was able to measure the very dilute plasma in the tail lobe at distances 15 – 20 Earth radii about 50% of the time. The energy of the ions, being in the range of several hundred electron volts, decreases as the satellite moves from the high-latitude magnetopause to the plasma sheet. The velocity spread of the ions is narrow in the direction of motion, corresponding to a temperature of several eV. The temperature in the transverse direction is 2 to 3 times higher. Comparison with magnetospheric convection models suggests that these ions originate at low altitudes in the region near the cusp and are separated in the magnetospheric velocity spectrometer. Observed velocity variations along the spacecraft trajectory suggest the existence of a characteristic velocity profile and a characteristic temporal/spatial scale. Comparison of simultaneous E q measurements with mass-selective detector measurements of protons and alpha particles suggests that lobe plasma at these distances consists primarily of oxygen ions. The average ion flux density is about 3×10 5 cm −2 s −1, giving estimated total flux over the lobe about 10 25 s −1, comparable to cusp/cleft source.

A two-dimensional fluid model for an argon rf discharge

A fluid model for an argon rf discharge in a cylindrical discharge chamber is presented. The model contains the particle balances for electrons and ions and the electron energy balance. A nonzero autobias voltage is obtained by imposing the condition that the time-averaged current toward the powered and grounded electrode is zero. Particle densities and ionization profiles peak strongly in front of the smaller, powered electrode. There electric fields are stronger and the electron current density is higher, resulting in more ohmic heating and therefore higher ionization rates. The radial uniformity of the plasma in front of the powered electrode gives a homogeneous ion flux toward this electrode. The asymmetric character of the profiles of the cylindrical geometry is in clear contrast with the essentially one-dimensional infinite parallel-plate geometry, which is fully symmetric with respect to the center of the discharge and has a zero dc autobias voltage. A comparison with results of a one-dimensional model shows that the average ion density, the average ion flux, and the average ionization rate in the cylindrical reactor are comparable to those in a parallel-plate reactor. The numerical treatment of the time evolution of the transport equations and Poisson’s equation needs an implicit method to avoid numerical instabilities. The resulting system of discretized equations is solved by a multigrid technique. The spatial discretization uses the Sharfetter–Gummel scheme.

A turbulent flow theory of electrodialysis

A hydrodynamic theory is presented of desalination by electrodialysis in a parallel channel system with unobstructed fully developed turbulent flow. A set of turbulent transport equations is derived for the mean salt concentration and electric field. Closure of the set is brought about by introducing a turbulent diffusion coefficient whose behavior is assumed known from empirical mass transfer considerations, and by making two different but phenomenologically reasonable assumptions about that part of the average ion flux which is due to the correlation between the concentration and electric field fluctuations. Estimates show both assumptions to lead to sensibly the same final results. From the equations and boundary conditions it is shown that there are five parameters which govern the system performance. Four are the same as for the corresponding laminar case, while the fifth measures the thickness of the concentration diffusion sublayer, the value of which is taken to be given by semi-empirical turbulent mass transfer results for high Schmidt numbers. The thickness of this diffusion layer is found to be the critical factor governing the salt removal rate. An integral method of solution is used to obtain the distributions of salt concentration and current density. A numerical quadrature is required for the complete solution. Analytic solutions are obtained in the unpolarized and fully polarized limits. Because of the very abrupt transition between these two limits, taken together the limiting analytic solutions are adequate to describe the system performance over the entire range of operating variables. For small fractional salt removal a universal correlation for the average current density embodying the five similarity parameters of the system is suggested based on the limiting solutions. Comparison of the theory is made with the experimental results of Cowan and Brown on average limiting current density and the agreement is found to be satisfactory.