Grid Ion Source Research Articles

Performance characterization of the μ10 electron-cyclotron-resonance ion thruster using alternative propellants: krypton vs. xenon

A first krypton vs. xenon experimental comparison of the Hayabusa2 and DESTINY+ μ10 Electron Cyclotron Resonance (ECR) gridded ion sources, and their ECR neutralizer, is presented. When operating on krypton, the ion sources deliver similar current in the same propellant consumption range and provide equivalent thrust levels. Nevertheless, the system experiences about 30% of thruster efficiency loss as the neutralizer propellant demand for comparable current levels more than doubles.

Comparison of pulse-modulated and continuous operation modes of a radio-frequency inductive ion source

The paper describes an experimental study of the characteristics of a pulse-modulated radio-frequency (RF) discharge sustained at low pressures, typical of the operating modes of RF gridded ion sources. The motivation for the study is the question of whether the RF pulse-modulated mode can increase the efficiency of the ion source. The ion current values extracted from an RF inductive ion source operating in continuous and pulse-modulated modes were compared. The experimental data were also compared with the parameter calculations based on a 0D numerical model of the discharge. The measurements showed that the pulse-modulated operation mode of the RF ion source had a noticeable advantage when the power of the RF generator was 140 W or lower. However, as the generator power increased, the advantage was lost because the pulse-modulated operation mode, having a higher RF power instant value, entered the region of existence sooner than the continuous mode, where the ion production cost begins to grow with RF power.

Effects of secondary γ-electrons from accelerator grid under ion impingement in gridded ion sources

Thus far, effects of secondary γ-electrons emitted from accelerator grids (AGs) of gridded ion sources on ionization in discharge chambers have not been studied. The presence and induced processes of such secondary electrons in a microwave electron cyclotron resonance gridded ion source are confirmed by the consistent explanations of: (1) the observed jump of ion beam current (I b) in case of a low-density plasma appearing at the chamber’s radial center due to the microwave skin effect; (2) the evolution of glow images recorded from the end-view of the ion source during the jump of I b; (3) the over-large jump step of I b with increasing microwave power; (4) the pattern appearing on the temperature sticker exposed to the discharge operated in the regime where the arrayed energetic-electron beamlets are injected into the discharge chamber; (5) the measured step-increment in the voltage drop across the screen grid (SG) sheath. A positive feedback loop composed of involved processes is established to elucidate the underlying mechanism. Energetic γ-electrons from the AG and warm δ-electrons from the opposite antenna do not produce direct excitation and ionization, but they enhance the electrical confinement of cold electrons by elevating the voltage drop across the sheaths at the antenna and SG, thus leading to the jump of I b. The energetic γ-electrons-based model can be also modified to explain abnormal results observed in the other gridded ion sources. Energetic γ-electrons from AGs should be taken into account in understanding gridded ion sources.

Plasma plume expansion with pulsed electron neutralization

Electrons neutralizing the ion beam from a gridded ion source are typically provided by an external cathode. This cathode emits a continuous current that ensures quasi-neutrality of the plume, and current balance of the ion source. A new type of neutralization scheme has recently been identified in the context of radio-frequency (RF) biased ion sources, where instead of a continuous electron current, the plume is neutralized by electron pulses emitted from the same plasma source as the ion beam itself. In contrast to conventional gridded ion sources, experiments have shown that pulsed neutralization produces hot electrons with a strongly anisotropic energy distribution in the plume. By making use of a two-dimensional particle-in-cell (PIC) simulation, we analyze the pulsed neutralization and plasma expansion to understand the fundamental plume physics in these systems, and perform a direct comparison with the expansion observed in typical DC systems. Electron trapping in the near-field plume region is found to be critical for ensuring quasi-neutrality, and the plume potential is observed to be higher than the downstream acceleration grid potential to prevent excessive electron backstreaming into the plasma source. This potential difference results in the formation of high-energy electron beams that generate collective plume oscillations with frequencies above the applied RF frequency. A detailed parametric study is performed to investigate the influence of the pulse frequency, emission current, and capacitance between the source and outer surrounding boundaries. In particular, the pulse frequency and emission current have a significant effect on the resulting plume potential, and the effectiveness of the resulting ion beam neutralization.

Numerical study of the collisionless interaction between positive and negative ion beams

We study, through two dimensional Particle-In-Cell simulations, the expansion of an ion-ion beam in vacuum. This beam is generated by a continuous extraction of positive and negative ions from two adjacent gridded ion sources. The grid systems are biased to extract and accelerate, continuously, positive and negative ions from two distinct ion sources. The ion sources are positioned such that their grid systems form an angle θ. In this work, we study two configurations, θ = π and θ = π/2. The proposed device constitutes an alternative approach to the usual positive ion beam neutralized by electrons. This work aims, on the one hand, to demonstrate that the neutralization of a continuously extracted ion beam space charge might be achieved by the use of oppositely charged ions. On the other hand, this work investigates the physical properties of the generated ion-ion beam. Our results show that, for θ = π, potential barriers form in the close vicinity of the acceleration grids. These potential barriers oppose to extraction and induce an ion backflow. This backflow increases with the increase in extracted ion current density. It represents ∼60% to ∼80% of extracted ions for an extracted ion current density ranging from 1 to 10 A/m2. Moreover, for θ = π/2, the potential barriers are located downstream the grids, typically at one source diameter. For this configuration, we found that the backflow is drastically reduced to about 25% of the extracted ions.

Fractal-like structures formed in the interelectrode gap during grid ion-source electrode sputtering

Carbon-carbon electrode sputtering in the accelerating negatively charged grid of an ion source is investigated. It is ascertained that complicated fractal-like structures of sputtered material atoms are formed during the sputtering process. Possible conditions and causes underlying their formation and probable models of growth thereof caused by similar physical processes are discussed.

Interpretation of optical emission in a strongly inhomogeneous PIAD environment

A variety of methods exists for the production of high quality optical coatings. These are for instance magnetron or ion beam sputtering, or thermal evaporation assisted by ion or plasma ion beam sources. The choice of a particular technique is due to the specific demands for tailoring the thin film properties such as homogeneity, refractive index, absorption, mechanical stress, porosity (optical shift), etc.. To allow for economic production of complex multilayer designs, the issue of reproducibility at the highest deposition rate possible has to be faced.The plasma ion assisted deposition (PIAD) has been invented to avoid contamination of the process environment present when gridded ion sources are employed. One example for this approach is the Advanced Plasma Source (APS) which holds a considerable market share in the field of optical coatings. The APS is a hot cathode (LaB6) DC glow discharge with an auxiliary magnetic field, typically operated with argon. A high density (ne ~ 1012 cm-3) and high temperature (Te ~ 20eV) plasma is generated in the source region (V ~ 0.7l) and expands to the chamber (V ~ 103 l) which is held at high vacuum (p ~ 2 • 10-2Pa). The expansion induces a strong drop of the plasma potential Vp which results in an acceleration of the ions towards the substrates. The setup is outlined in figure 1. By varying the discharge voltage (VD=50..150 V), the magnetic field (Bmax=10..40mT) and the gas flux (GAr=2..20 sccm) different characterstics of the plasma ion beam are obtained, where typical ion energies are Ei=50..150 eV. As is described in the references, various probe techniques were adopted to elucidate the mechanisms of plasma beam formation in this particular PIAD setup. In an earlier work the approach of optical emission spectroscopy (OES) has been pursued. This paper contains a description of the diagnostic installation allowing for tomographic reconstruction of the local optical emission near the source exit. With a simple corona model ne and Te could be estimated for an Ar/He mixture. The interpretation of emission close to the APS is hampered by the lack of detailed knowledge of neutral density and temperature in this region.In this extended abstract the preparation of a more elaborate approach using collisional radiative modelling is sketched. The new aspect is the consideration of a global electron energy probability function (EEPF) based on the nonlocal approximation. This concept is useful for reducing the complexity of electron kinetics by coordinate transformation from the 6D phase-space to a 1D total energy space. The analysis of OES data is not merely ment as a proof of principle. The final goal is to interpret the OES data in terms of electron parameters, as a foundation of a control scheme for the APS using non-invasive optical diagnostics. Although global parameters, such as voltages and currents can be maintained accurately, drifts in the plasma parameters which are not measured routinely in the industrial application, may lead to limitations in the reproducibility of the optical coatings. The main reason is suspected to be the alteration of the electrodes of the APS during the PIAD process.

A stationary plasma thruster for modification of polymer and ceramic surfaces

A stationary plasma thruster (SPT)-type source with high plasma density is introduced and its physical principle and structure are briefly summarized. As general characteristic performance of the gridless SPT plasma source, ion current density, its angular distributions and distribution of ions over energies for three working gases of Ar, N2 and O2 was examined and also compared with that of a conventional Kaufman-type grid ion source. In order to investigate the possible application of SPT for the surface modification, polymer and ceramic surfaces are irradiated by the low energy and high fluence ion beam generated from SPT. The wetting angle of polyimide is greatly reduced from 78° to 2–4° by the irradiation of reactive O2+ and N2O+ ions with an average energy of 180eV. Furthermore when α-Al2O3 (0001) is irradiated by N2+ ion of SPT source at various ion fluences, the formation of new AlON and AlN chemical bonding are identified by X-ray photoelectron spectroscopy. From the results, it is revealed that the SPT can be used as an effective ion source for the enhancement of surface energy of polymer and for the formation of functional groups on ceramic surface.

A modified broad beam ion source for low-energy hydrogen implantation

A modified broad beam ion source for low-energy hydrogen ion implantation of semiconductors is described. Based on a Kaufman type ion source two different solutions are presented: (a) an ion source with an extraction system consisting of two molybdenum grids with a low gas flow conductance reworked for hydrogen operation, and (b) a ten-grid mass separating ion beam system which enables the mass selection of H+, H2+, and H3+. The ion energy could be set in the range of 200–500 eV with a current density reaching from 1 to 100 μA/cm2. It is shown that at higher pressure the main ion created in the ion source is H3+ due to ion-molecule processes, whereas at lower pressure only H2+ and H+ are produced. Special consideration is given to the ion beam analysis of the two grid ion source operating in the 10−3 mbar range allowing to explain the different peak structures by the potential distribution across the ion source and different charge transfer processes. In addition, the analysis reveals neutral and ionized ...

Ultrahigh vacuum chemically assisted ion beam etching system with a three grid ion source

A chemically assisted ion beam etching system has been developed which performs high quality, highly anisotropic etching of AlGaAs/GaAs at relatively low ion energies (200 eV). The use of three grid ion optics in a Kaufman ion source allows etching at low energies with reasonable rates without loss of profile verticality. All ultrahigh vacuum etching chamber construction with a high throughput turbomolecular pump and high vacuum loadlock provide routine high quality etching of AlGaAs without the use of etch chamber cryo panels or cryo pumps. Low ion energy and a clean vacuum environment permit the use of a single level, non hard baked photoresist mask. Benefits include high pattern resolution and fidelity, low mask erosion rate, good dimensional control, and easy, complete mask stripping as well as reduced substrate damage.

Wafer scale processing of InGaAsP/InP lasers

Chemically assisted ion beam etching (CAIBE) and electron cyclotron resonance (ECR) plasma deposition have been used to etch and coat 1.3 μm InGaAsP/InP heterostructure lasers in full wafer form. Ar/Cl2 CAIBE, using an ECR dual grid ion source, was used to etch 4 μm deep vertical (90°±0.5°), smooth facets at rates up to 1.3 μm/min. An integrated back facet monitor was simultaneously fabricated in the same heterostructure. High-reflectivity Si/SiO2 optical coatings were deposited on the etched facets by low-temperature (<120 °C) ECR plasma deposition and selectively patterned by liftoff. Full wafer testing of the processed devices showed good uniformity (±3%) with laser threshold currents of 25 mA and a slope efficiency of 0.23 W/A at 25 °C and 0.11 W/A at 85 °C. Back facet monitor efficiency was 0.4 A/W over the whole temperature range.

Total pressure measurement down to 10−12 Pa without electron stimulated desorption ion errors

In order to measure a total pressure as low as 10−12 Pa, the avoidance of electron stimulated desorption (ESD) ion errors is more important than the x-ray limit, because even if we can contrive to reduce ESD ions in a gauge, we do not know what fraction the ESD ion error makes in the measured pressure value. This is the largest defect in the reliability of the measurements. Therefore, the development of the ion spectroscopy gauge which avoids ESD ion errors, was recalled with a historical review of the limiting processes in a hot-cathode gauge. The most advanced ion spectroscopy gauge was constructed with a fine platinum alloy spherical grid ion source, immersed in a copper block with stainless steel flanges. Gas phase ions were effectively separated from ESD ions by the actions of electron space charge in the grid and a hemispherical 180° ion energy analyzer. The x-ray limit of the advanced gauge was approximately 2.5×10−13 Pa, and the outgassing rate of 2×10−9 Pa ℓ/s (about 1/100 that of an ordinary nude Bayard–Alpert gauge) was achieved.

Ion spectroscopy gauge: Total pressure measurements down to 10−12 Pa with discrimination against electron-stimulated-desorption ions

The hot-cathode magnetron gauge by Lafferty (which presently holds the possibility of making the lowest-pressure measurements, 10−15 Pa) and the improved Helmer gauge by Benvenuti and Hauer (which carried out the lowest-pressure measurements to date, 2.5×10−12 Pa) have been examined regarding the measurement limit due to electron-stimulated-desorption (ESD) ions. On the basis of this examination, a new hot-cathode total pressure gauge, called an ion spectroscopy gauge, which substantially avoids errors caused by ESD ions down to 10−12 Pa, has been developed. Discrimination against ESD ions is accomplished by combining three technical elements: (i) a spherical grid ion source; (ii) a spherical 180° ion energy analyzer; and (iii) burying the ion source in an aluminum alloy flange. The ion source plays an important role in the generation of a large kinetic energy difference between gas-phase ions and ESD ions, by using electron space charge. The 180° analyzer, which has a high energy resolution, promotes ion focusing and tends to reduce the x-ray limit. The burying technique reduces the outgassing rate of the hot-cathode gauge. Performance measurements of the new gauge have been carried out in a new aluminum extreme high vacuum system. The effective separation of gas-phase ions from ESD ions for two variants of the gauge, one with a molybdenum grid and one with a platinum grid, was demonstrated using oxygen dosing. The performance of the platinum-grid gauge was confirmed down to 2×10−11 Pa, essentially free from errors caused by ESD ions. The x-ray limits and sensitivities of the new gauges were: below 2×10−12 Pa with a sensitivity of 4.5×10−4 A/Pa for the molybdenum grid and 5.6×10−12 Pa with a sensitivity of 1.8×10−4 A/Pa for the platinum grid.

ＥＳＤイオン誤差を除去したＸ線限界１０｀−１２´Ｐａのイオン分光ゲージ

A new hot-cathode ionization pressure gauge, called an ion spectroscopy gauge, which avoids errors due to electron-stimulated desorption (ESD) ions down to 10-12 Pa has been developed. The elimination of ESD ion errors is accomplished by combining three technical elements, a spherical grid ion-source, a spherical 180° energy analyzer, and reduction of desorption from a hot-cathode gauge. The X-ray limit of the new gauge is below 1.3×10-12 Pa with a sensitivity of 3×10-4 (A/Pa). Performance measurements of the new gauge have been carried out in a new aluminum XHV system and confirmed down to 2×10-11 Pa.

As-grown epitaxial YBaCuO films prepared by low energy oxygen ion assisting ion beam sputtering

The as-grown epitaxial YBaCuO films on (100) and (110)SrTiO 3 substrates were prepared by ion beam sputtering method with low accelerating oxygen ion assist. Oxygen ions were produced by the Kaufman type single grid ion source with the filament protection to oxygen. The orientation of epitaxial films on(100)SrTiO 3 substrate were varied by both the assisting ion current density and substrate temperature; the c-axis of film tended to be normal at higher substrate temperature and lower oxygen ion current, while the c-axis of the films tended to lie on the substrate at lower substrate temperature and higher oxygen ion current. The same trend was observed on a (110)SrTiO 3 substrate; the 〈110〉 orientation formed at lower substrate temperature and higher oxygen ion current, in contrast 〈103〉 formed at higher substrate temperature and lower oxygen ion current.

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.

Ｋａｕｆｍａｎ型シングルグリッドイオン源の低エネルギー特性

In this present work the electrical properties of Kaufman type fine mesh single grid ion source were investigated for low energy ion irradiation during deposition of the semiconductor or superconductor films which were sensitive to defects due to high energy ion bomberdment. A 5.8 cm diameter ion source with a 1.7 cm diameter extraction grid of tungsten mesh (100 mesh/inch) was fablicated and tested. Maximum argon ion current density of 0.97 mA/cm2 over the energy range from 40 to 150 eV was obtained. The ion energy and the ion current density independently varied with the acceleration voltage and the discharge current respectively.