Retarding Field Energy Analyzer Research Articles

Peculiarities of measuring ion energy distribution in plasma with a retarding field analyzer.

Presented are the results from tailoring the retarding field energy analyzer to measure the degree of charge compensation and regular patterns in the separations of ions of different mass, with the multicomponent ion flow spreading out in the plasma-optical mass separator model.

Design and analysis of retarding field energy analyzer with controllable focusing voltage

In order to solve the problems of poor insulation, low resolution and sparking etc., a retarding field energy analyzer is upgraded, simulated and analyzed to test and analyze the electron beam parameters accurately, which derive from the guns of traveling wave tubes and beams after beam-wave interaction, providing a reference for the design, performance improvement and development of the traveling wave tubes. The retarding energy analyzer with a controllable focusing voltage insulates high-voltage cylinder electrically from retarding mesh and applies a controllable small voltage between them. The second electrostatic lens accelerates and focuses the diverging beam near the area of the mesh, to weaken too much expansion in the region of the weak part that the first lens functions, due to space-particle effects and beam emittance, etc. and optimize the slope of the particle trajectory and resolution. Meanwhile, the whole structure tends to miniaturizition and lightweight. A computer code called CST is used to simulate the particle distribution in particle interfaces and the whole electric field distribution, and Matlab code helps data processing and exports the key parameters. Besides, the paper focuses on analyzing the optimization and improvement of the performance reflected by the resolution and Full Width at Half Maximum, etc. The result shows that this energy analyzer is competent to apply to the experiments of electron beams.

Experimental analysis of a new retarding field energy analyzer

In this paper, a new compact retarding field energy analyzer (RFEA) is designed for diagnosing electron beams of a K-band space travelling-wave tube (TWT). This analyzer has an aperture plate to sample electron beams and a cylindrical electrode to overcome the defocusing effects. The front end of the analyzer constructed as a multistage depression collector (MDC) structure is intended to shape the field to prevent electrons from being accelerated to escape. The direct-current (DC) beams of the K-band space TWTs with the removing MDC can be investigated on the beam measurement system. The current density distribution of DC beams is determined by the analyzer, while the anode voltage and helix voltage of the TWTs are 7000V and 6850V, respectively. The current curve’s slope effect due to the reflection of secondary electrons on the copper collector of the analyzer is discussed. The experimental analysis shows this RFEA has a good energy resolution to satisfy the requirement of beam measurement.

A comparison of ion beam measurements by retarding field energy analyzer and laser induced fluorescence in helicon plasma devices

Both Laser-Induced Fluorescence (LIF) and Retarding Field Energy Analyzers (RFEA) have been applied to the investigation of beams formed in inductively coupled helicon plasmas. While the LIF technique provides a direct measurement of the velocity distribution in the plasma, the RFEA measures ion flux as a function of a retarding potential. In this paper, we present a method to compare the two techniques, by converting the LIF velocity distribution to an equivalent of a RFEA measurement. We applied this method to compare new LIF and RFEA measurements in two different experiments; the Hot Helicon Experiment (HELIX) - Large Experiment on Instabilities and Anisotropies (LEIA) at West Virginia University and Njord at University of Tromsø. We find good agreement between beam energies of the two methods. In agreement with earlier observations, the RFEA is found to measure ion beams with densities too low for the LIF to resolve. In addition, we present measurements of the axial development of the ion beam in both experiments. Beam densities drop exponentially with distance from the source, both in LIF and RFEA measurements. The effective quenching cross section from LIF in LEIA is found to be σb,*=4×10−19 m2, and the effective beam collisional cross sections by RFEA in Njord to be σb=1.7×10−18 m2.

Plasma property of inductively coupled discharge and substrate bias co-assisted very-high-frequency magnetron sputtering

Very-high-frequency (VHF) magnetron sputtering is an important method to deposit the polycrystalline films at low temperature. To increase the plasma density, ion flux and to control the ion energy, the inductively coupled plasma (ICP) and substrate bias co-assisted VHF magnetron sputtering was developed. The plasma properties of this system were measured by a Langmuir probe and a retarding field energy analyzer. In the VHF magnetron sputtering, the ICP discharge can increase the plasma density effectively but has a small influence on the ion energy and ion flux; the substrate bias can increase the plasma density and ion flux more effectively but result in the divergence of ion energy. When the ICP discharge and substrate bias are simultaneously applied, the divergence of ion energy can be suppressed, while the high plasma density (2.6×1017m−3) and ion flux (4.3×1020m−2⋅s−1) can be remained. Therefore, the ICP and substrate bias co-assisted VHF magnetron sputtering is a possible way to deposit the polycrystalline films at low temperature with a higher growth rate.

可控聚焦型减速场能量分析器的设计分析

可控聚焦型减速场能量分析器的设计分析

Collision-induced dissociation of Nb x O y + (x = 1, 2, y = 2–12) clusters: crossed molecular beams and collision cell studies

Oxygen-rich niobium oxide clusters are formed by mixing laser-produced Nb plasma with pure oxygen, and their stability is investigated by mass spectrometry and collision-induced dissociation. We use an experimental configuration recently developed by our group, where the cluster ions beam is crossed with a secondary beam of noble gas atoms, and the fragments are rejected by a retarding field energy analyzer. In this way, the relative collision cross sections of Nb x O + (x = 1, 2, y = 2–12) clusters have been measured and information about their fragmentation channels has been obtained.

Coincident ion acceleration and electron extraction for space propulsion using the self-bias formed on a set of RF biased grids bounding a plasma source

We propose an alternative method to accelerate ions in classical gridded ion thrusters and ion sources such that co-extracted electrons from the source may provide beam space charge neutralization. In this way there is no need for an additional electron neutralizer. The method consists of applying RF voltage to a two-grid acceleration system via a blocking capacitor. Due to the unequal effective area of the two grids in contact with the plasma, a dc self-bias is formed, rectifying the applied RF voltage. As a result, ions are continuously accelerated within the grid system while electrons are emitted in brief instants within the RF period when the RF space charge sheath collapses. This paper presents the first experimental results and a proof-of-principle. Experiments are carried out using the Neptune thruster prototype which is a gridded Inductively Coupled Plasma (ICP) source operated at 4 MHz, attached to a larger beam propagation chamber. The RF power supply is used both for the ICP discharge (plasma generation) and powering the acceleration grids via a capacitor for ion acceleration and electron extraction without any dc power supplies. The ion and electron energies, particle flux and densities are measured using retarding field energy analyzers (RFEA), Langmuir probes and a large beam target. The system operates in Argon and N2. The dc self-bias is found to be generated within the gridded extraction system in all the range of operating conditions. Broad quasi-neutral ion-electron beams are measured in the downstream chamber with energies up to 400 eV. The beams from the RF acceleration method are compared with classical dc acceleration with an additional external electron neutralizer. It is found that the two acceleration techniques provide similar performance, but the ion energy distribution function from RF acceleration is broader, while the floating potential of the beam is lower than for the dc accelerated beam.

Scrape-off layer ion temperature measurements at the divertor target during type III ELMs in MAST measured by RFEA

Edge-localised modes (ELMs) can carry significant fractions of their energy as far as main chamber plasma-facing components in divertor tokamaks. Since in future devices (e.g. ITER, DEMO) these energies could cause issues for material lifetime and impurity production, the energy and temperature of ions in ELMs needs to be investigated. In MAST, novel divertor measurements of Ti during ELMs have been made using the divertor retarding field energy analyser (RFEA) probe. These measurements have shown instantaneous ion energy distributions corresponding to an effective Ti at 5cm from the strike point at the target that can be as high as 60eV and that this decreases with time after the ELM start. This is consistent with the hottest, fastest ions arriving at the target first by parallel transport, followed by the lower end of the ion energy distribution. This analysis will form a basis for future data analysis of fast swept measurements of ion distributions in ELMs.

Ion ejection from a permanent-magnet mini-helicon thruster

A small helicon source, 5 cm in diameter and 5 cm long, using a permanent magnet (PM) to create the DC magnetic field B, is investigated for its possible use as an ion spacecraft thruster. Such ambipolar thrusters do not require a separate electron source for neutralization. The discharge is placed in the far-field of the annular PM, where B is fairly uniform. The plasma is ejected into a large chamber, where the ion energy distribution is measured with a retarding-field energy analyzer. The resulting specific impulse is lower than that of Hall thrusters but can easily be increased to relevant values by applying to the endplate of the discharge a small voltage relative to spacecraft ground.

Movable multi-probes for plasma boundary measurement in Sino-UNIted Spherical Tokamak.

A novel movable multi-probes is developed to get local magnetic and electrostatic profiles on Sino-UNIted Spherical Tokamak (SUNIST). This multi-probes combines a four-tips Langmuir probe, a magnetic coil, and a retarding field energy analyzer (RFEA). It can be used to simultaneously measure the poloidal magnetic field Bp, electric field Er, electron temperature Te, electron density ne, and ion temperature Ti. Its small overall size (20 × 20 × 38 mm(3)) enables the movable multi-probes to measure the magnetic and electrostatic profiles in high spatial resolution, with negligible impact to plasma in SUNIST. This paper presents the design of the movable multi-probes, in particular, details of RFEA for reliable ion energy measurements. Preliminary experimental results of the movable multi-probes are given as well.

Plasma–surface interaction during low pressure microcrystalline silicon thin film growth

The role of plasma–surface interactions during microcrystalline silicon thin film growth has been studied under low power–low pressure conditions, and a correlation between the plasma–surface interactions and material properties has been provided. The atomic hydrogen flux, inferred by optical emission spectroscopy measurements, has been investigated. The hydrogen-to-silicon growth flux resulted in a ratio of 25 : 140 for the amorphous-to-mixed phase transition, whereas it increased to 40 : 170 for the mixed phase-to-microcrystalline phase transition. The ion contribution to the plasma–surface interaction has also been investigated. For this purpose, a capacitive probe has been implemented locally for direct measurement of the ion flux. For the amorphous-to-microcrystalline phase transition the ion-to-silicon growth flux ratio has been determined to increase from 0.15 to 1, two orders of magnitude smaller than the atomic hydrogen-to-silicon growth flux ratio. The ion energy has been measured with a retarding field energy analyser, which allowed determination of the ion-energy distribution as a function of the silane flow rate. Average ion energies of 15–20 eV have been found for a decreasing silane flow rate, thereby indicating the limited energy transfer to the surface. The main effect caused by the ion arrival at the surface is a locally induced thermal spike enhancing the radical surface diffusion. No prominent detrimental effect, i.e. Si surface or bulk displacement requiring energies greater than 18 eV and 40 eV respectively, or sputtering occurring above 50 eV, has been observed.

Control of ions energy distribution in dual-frequency magnetron sputtering discharges

The ion energy distributions (IEDs) in the dual-frequency magnetron sputtering discharges were investigated by retarding field energy analyzer. Increasing power ratio of 2 MHz to 13.56 (27.12 or 60) MHz led to the evolution of IEDs from a uni-modal distribution towards a uni-modal distribution with high-energy peak shoulder and a bi-modal distribution. While increasing power ratio of 13.56 MHz to 27.12 MHz and 27.12 MHz to 60 MHz, led to the increase of peak energy. The evolution of IEDs shape and the increase of peak energy are due to the change of ions responding to the average field of high-frequency period towards the instantaneous sheath potential of low-frequency period.

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.

Effect of driving frequency on plasma property in radio frequency and very high frequency magnetron sputtering discharges

Ion energy distributions (IEDs), electron energy distributions (EEDs) and other plasma parameters of magnetron sputtering discharges driven by 13.56, 27.12 and 60 MHz sources were investigated by a retarding field energy analyzer and Langmuir probe measurements. An increase in driving frequency leads to an increase in ion energy and the evolution of IEDs from a uni-modal distribution at the 13.56 MHz discharge toward a bi-modal distribution at 27.12 MHz, and a multi-modal distribution at the 60 MHz discharge. For IEDs near the target surface, this evolution is related to the ion acceleration and the charge transfer collisions between Ar atoms and Ar+ ions in the presheath, while for IEDs at the substrate, the evolution depends on the ratio of the ion transit time across the sheath to the radio frequency period. The increase in driving frequency also leads to the evolution of EED function from a Maxwellian type at the 13.56 MHz discharge toward a bi-Maxwellian type at the 27.12 MHz discharge and a Druyvesteyn-like type at the 60 MHz discharge due to the change in the generation and loss mechanisms of electrons. In addition, increasing the driving frequency can lead to a higher electron temperature and a lower electron density. Therefore, the driving frequency becomes an effective tool to control the plasma properties of magnetron sputtering discharges.

End-boundary sheath potential, electron and ion energy distribution in the low-pressure non-ambipolar electron plasma

The end-boundary floating-surface sheath potential, electron and ion energy distribution functions (EEDf, IEDf) in the low-pressure non-ambipolar electron plasma (NEP) are investigated. The NEP is heated by an electron beam extracted from an inductively coupled electron-source plasma (ICP) through a dielectric injector by an accelerator located inside the NEP. This plasma's EEDf has a Maxwellian bulk followed by a broad energy continuum connecting to the most energetic group with energies around the beam energy. The NEP pressure is 1–3 mTorr of N2 and the ICP pressure is 5–15 mTorr of Ar. The accelerator is biased positively from 80 to 600 V and the ICP power range is 200–300 W. The NEP EEDf and IEDf are determined using a retarding field energy analyser. The EEDf and IEDf are measured at various NEP pressures, ICP pressures and powers as a function of accelerator voltage. The accelerator current and sheath potential are also measured. The IEDf reveals mono-energetic ions with adjustable energy and it is proportionally controlled by the sheath potential. The NEP end-boundary floating surface is bombarded by a mono-energetic, space-charge-neutral plasma beam. When the injected energetic electron beam is adequately damped by the NEP, the sheath potential is linearly controlled at almost a 1 : 1 ratio by the accelerator voltage. If the NEP parameters cannot damp the electron beam sufficiently, leaving an excess amount of electron-beam power deposited on the floating surface, the sheath potential will collapse and become unresponsive to the accelerator voltage.

Collisional effect on the time evolution of ion energy distributions outside the sheath during the afterglow of pulsed inductively coupled plasmas

The time evolution of ion energy distributions (IEDs) during the afterglow is measured in various inert gas plasmas generated by an inductively coupled plasma source. The afterglow regimes are prepared by periodically pulsing the plasmas, with a period of 1000 µs. The time-resolved retarding field energy analyzer method and Langmuir probe method are used to measure the detailed time evolution of IEDs at 5 µs intervals to investigate precisely what happens in the vicinity of the rf-off moment. The measured results show that double-peaked IEDs appear at the early afterglow. By analyzing the force equation of the ions in a time-varying plasma potential environment, we found that this double-peaked structure originates from the delayed arrival of ions under our discharge conditions. This effect is expected to disappear under high-pressure conditions where ions lose their energy due to the momentum loss caused by collisions between background particles, and the time evolution of IEDs during the afterglow changes its form to a typically expected pattern, a single-peaked structure. The findings in this report can be utilized to understand pulsed plasma characteristics in the semiconductor fabrication area where pulsed plasmas are now widely used in the dry etching process.

Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition

The contribution of ions to the growth of microcrystalline silicon thin films has been investigated in the well-known high-pressure-depletion (HPD) regime by coupling thin-film analysis with plasma studies. The ion flux, measured by means of a capacitive probe, has been studied in two regimes, i.e., the amorphous-to-microcrystalline transition regime and a low-to-high power regime; the latter regime had been investigated to evaluate the impact of the plasma power on the ion flux in collisional plasmas. The ion flux was found not to change considerably under the conditions where the deposited material undergoes a transition from the amorphous to the microcrystalline silicon phase; for solar-grade material, an ion-to-Si deposition flux of ∼0.30 has been determined. As an upper-estimation of the ion energy, a mean ion energy of ∼19 eV has been measured under low-pressure conditions (<1 mbar) by means of a retarding field energy analyzer. Combining this upper-estimate with an ion per deposited Si atom ratio of ∼0.30, it is concluded that less than 6 eV is available per deposited Si atom. The addition of a small amount of SiH4 to an H2 plasma resulted in an increase of the ion flux by about 30% for higher power values, whereas the electron density, deduced from optical emission spectroscopy analysis, decreased. The electron temperature, also deduced from optical emission spectroscopy analysis, reveals a slight decrease with power. Although the dominant ion in the HPD regime is SiH3+, i.e., a change from H3+ in pure hydrogen HPD conditions, the measured larger ion loss can be explained by assuming steeper electron density profiles. These results, therefore, confirm the results reported so far: the ion-to-Si deposition flux is relatively large but has neither influence on the microcrystalline silicon film properties nor on the phase transition. Possible explanations are the reported high atomic hydrogen to deposition flux ratio, mitigating the detrimental effects of an excessive ion flux.

Investigation of reactive HiPIMS + MF sputtering of TiO2 crystalline thin films

A hybrid high-power impulse magnetron sputtering (HiPIMS)+mid-frequency (MF) magnetron sputtering system was used for the deposition of crystalline TiO2 thin films at a low substrate temperature. Ion velocity distribution functions (IVDFs) were measured in a pulsed magnetron discharge at substrate positions depending on the type of plasma excitation and on the working gas mixtures. Several different pulsed discharge configurations were used: (i) HiPIMS, (ii) pulsed bipolar MF with frequency 350kHz and (iii) both HiPIMS+MF connected in parallel to the magnetron cathode. The timing of HiPIMS excitation was set to period time T=10ms with “ON” time TON=100μs. Ti targets were sputtered in three different types of atmospheres: (i) inert pure Ar, (ii) a reactive mixture of Ar+O2 and (iii) a reactive mixture of Ar+O2+N2 with a constant gas pressure p=1Pa. All IVDFs were measured using a time-resolved retarding field energy analyzer (RFEA) located in the substrate position. TiO2 thin films were deposited under identical experimental conditions on silica (SiO2) glass substrate, glass with an indium tin oxide (ITO) electrode and polycarbonate polymer foil. The thin film properties are discussed with respect to the measured plasma parameters. It is shown that the combination of HiPIMS+MF excitation in a reactive atmosphere effectively reduces the delay between the edge of cathode voltage and current onset. The highest ion energy was reached in the case of an inert Ar atmosphere due to the highest ratio of fast Ti+ ions in the overall ion flux. All TiO2 thin films deposited in reactive atmospheres formed pure rutile phase regardless of the excitation mode; however, those films deposited by a HiPIMS+MF excitation mode exhibited the greatest ability to produce a photocurrent. Furthermore, HiPIMS+MF was the best setting for the deposition of crystalline TiO2 on polycarbonate foil because of the low heating flux on the substrate and suitable plasma parameters leading to the formation of the rutile phase.

Scrape-off layer ion temperature measurements at the divertor target in MAST by retarding field energy analyser

Knowledge of the ion temperature (Ti) is of key importance for determining heat fluxes to the divertor and plasma facing components, however data regarding this is limited compared to electron temperature (Te) data. Ti measurements at the divertor target, between edge-localised modes (inter-ELM) H-mode, have been made using a novel retarding field energy analyser (RFEA).