Ion Reflection Research Articles

Effects of reflection of electrons and negative ions on magnetized electronegative and collisional plasma sheath

The effects of the reflection of electrons and negative ions in magnetized electronegative and collisional plasma sheath on the Bohm criterion and the sheath structure are numerically investigated. The Bohm criterion expression of the sheath with considering the reflection of electrons and negative ions is derived theoretically. The lower limit of ion Mach number versus parameters and the distribution curve of charged particle density in sheath are obtained by numerical simulation when Boltzmannian model and reflection model are applied to electrons and negative ions. The results show that the upper limit of ion Mach number is identical to that of Boltzmannian model, but their lower limit expressions are different. The lower limit of ion Mach number in the reflection model is also related to the wall potential, and with the increase of the wall potential, ion Mach number first increases and then remains unchanged after reaching the same value as that from Boltzmannian model, and the speeds of their reaching the maximum values are different due to the difference in sheath edge negative ion concentration and temperature. In both Boltzmannian and the reflection model, the lower limit of the ion Mach number decreases with the concentration of the negative ion at the sheath edge increasing and the negative ion temperature decreasing, but the maximum value is smaller in the reflection model. The lower limit of ion Mach number for each of the two models increases with sheath edge electric field increasing, but increases faster and the final value is larger in Boltzmannian model. The lower limit of ion Mach number for each of the two models decreases with the increase of collision parameter or magnetic field angle, but decreases faster in Boltzmannian model with the increase of collision parameter or magnetic field angle. The lower limits of ion Mach number in the two models tend to be the same with the increase of magnetic field angle. When the wall potential is small, the reflection of electrons and negative ions has a great influence on the sheath structure. When the wall potential is large, the reflection of electrons and negative ions have little effect on the density distribution of charged particles in the sheath.

Effect of U-shape trench etching process on electrical properties of GaN vertical trench metal-oxide-semiconductor field-effect transistor

As reported by several market analysts, GaN-based power devices show great potential applications in the low and medium voltage range ( < 900 V). For high voltage ( > 1200 V), including ship transportation and power grid, the future applications of GaN highly depend on the development of vertical devices based on GaN substrates. Several vertical devices have been reported, such as current aperture vertical electron transistors (CAVETs), U-shape trench metal-oxide-semiconductor field-effect transistors (UMOSFETs), and fin power transistors. And the UMOSFETs show potential advantages due to greater simplicity in material epitaxy and fabrication process. In the fabrication of UMOSFETs, the U-shape trench dry etching is the most critical process. The GaN sidewalls after dry etching directly affect the interface state characteristics in the MOS structure and the channel electron transport. In this work, etching optimization including etching radio-frequency (RF) power and etching mask is investigated and process-dependent electrical characteristics of GaN UMOSFETs are also studied. The appropriate decrease of RF power ensuring the steep sidewalls can effectively improve the channel electron mobility from 35.7 cm<sup>2</sup>/(V·s) to 48.1 cm<sup>2</sup>/(V·s) and consequently increase the ON-state current and reduce the ON-state resistance. Larger etching damage to the p-GaN sidewall caused by higher RF power leads the scattering effects to increase and the mobility of the channel carriers to decrease. The interface state density at the channel can be extracted by the subthreshold swing. The interface state density decreases to 1.90 × 10<sup>12</sup> cm<sup>–</sup><sup>2</sup>·eV<sup>–1</sup> when the RF power is regulated to 50 W, which is only half of the interface state density when RF power is 135 W. Similar breakdown voltages (350－380 V) are measured for these devices with varying RF power, which are governed by gate early breakdown. Positive valence band offset is formed in the SiO<sub>2</sub>/GaN MOS structure and the early breakdown occurs due to the holes accumulating at the SiO<sub>2</sub>/GaN interface. The etching uniformity at the bottom of U-shape trench can be improved by using the SiO<sub>2</sub> hard masks instead of photoresist masks. Sub-trenches at both ends of the trench bottom are observed in the device with photoresist masks, leading the carrier scattering to increase and ON-state current to decrease. Besides, the interface state density decreases from 3.42 × 10<sup>12</sup> cm<sup>–2</sup>·eV<sup>–1</sup> to 2.46 × 10<sup>12</sup> cm<sup>–2</sup>·eV<sup>–1</sup> with a SiO<sub>2</sub> hard mask layer used. Compared with 1.6 μm photoresist mask, the thinner SiO<sub>2</sub> mask with a thickness of 500 nm has a small sidewall area, which weakens the high-energy ion reflection in the inductively coupled plasma system. Consequently, the over-etching at the bottom ends of the trench is improved significantly and therefore the fabricated GaN UMOSFET has higher channel mobility and a lower interface state density.

Fluorescence detection as a new diagnostics tool for electrostatic ion beam traps

In the development towards the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS), an optical detection region for the observation of fluorescent light is added to an electrostatic ion beam trap (EIBT). In addition to its use for highly sensitive collinear laser spectroscopy, this fluorescence detection is introduced as a diagnostics tool for the study of the ion dynamics inside an EIBT. First measurements of collision-induced fluorescence in an EIBT demonstrate the technique’s diagnostics power by tracking the evolution of an ion bunch’s temporal width over its storage time inside the ion trap. Thereby, the time-focus point of the ion bunch can be determined and the influence of space-charge effects in the EIBT can be investigated. Good qualitative agreement is obtained between the measured trend of temporal widths and the simulations of the ions’ trajectories in the trap. Particularly, the observation of self-bunching on the ion-bunch structure for many simultaneously stored ions is reproduced.

Kinematic Collisionless Relaxation of Ions in Supercritical Shocks

We show that kinematic collisionless relaxation in the macroscopic electric and magnetic fields plays the main role in the formation of the downstream ion distributions in a super-critical shock with ion reflection present. It is done by comparison of a theoretically predicted magnetic profile with the magnetic profile of a shocks observed by MMS mission. It is shown that pressure balance remains the main constraint for the shock stability.

Formation mechanisms of etched feature profiles during Si etching in Cl2/O2 plasmas

Feature profiles of poly-Si etched in Cl2/O2 plasmas have been analyzed through a mechanistic comparison between experiments and simulations. The emphasis was placed on a comprehensive understanding of the formation mechanisms for profile anomalies of tapering, microtrenching, and footing (or corner rounding near the feature bottom). Experiments were conducted in a commercial etching reactor with ultra-high-frequency plasmas by varying O2 percentage, wafer stage temperature, rf bias power, and feed gas pressure. Simulations of the feature profile evolution were done by using a semiempirical, atomic-scale cellular model based on the Monte Carlo method that we have developed. The experiments indicated that sidewall profiles become more tapered with increasing O2 addition to Cl2 plasmas, while microtrenching and footing are pronounced in pure Cl2 plasma, being suppressed with increasing O2. A comparison with the simulations indicated that the tapered profiles are caused by the deposition of etch products/by-products on feature sidewalls from the plasma, being enhanced with increasing oxygen flux (due to synergistic effects between deposition of products/by-products and surface oxidation) and being reduced with increasing ion energy and neutral reactant flux. On the other hand, the footing is attributed to the redeposition of etch products on sidewalls from the feature bottom being etched, being reduced with increasing oxygen flux, ion energy, and neutral reactant flux. Microtrenching is caused by the ion reflection from feature sidewalls on incidence, being reduced with increasing oxygen flux (partly due to surface oxidation of the feature bottom) and being enhanced and then reduced with increasing ion energy and neutral reactant flux. The tapering, footing, and microtrenching were found to be closely related to each other: the footing near the feature bottom fades away under conditions of increased tapering of sidewalls, and the microtrenching is affected significantly by the degree of footing as well as the taper angle of the sidewalls.

High‐Resolution Measurements of the Cross‐Shock Potential, Ion Reflection, and Electron Heating at an Interplanetary Shock by MMS

AbstractThe Magnetospheric Multiscale (MMS) spacecraft obtained unprecedented high‐time resolution multipoint particle and field measurements of an interplanetary shock event on 8 January 2018. The spacecraft encountered the supercritical forward shock of a forward/reverse shock pair in the pristine solar wind upstream of the bow shock near the subsolar point as they neared apogee at ~25 RE. The high‐time resolution measurements from the four spacecraft, separated by only ~20 km, allowed direct measurement of particle distributions revealing evidence of electron heating and near specularly reflected ions. The cross‐shock potential is calculated directly from 3‐D electric field measurements. This is the first reported direct high temporal resolution (<1 s) observation at an interplanetary shock of near specularly reflected ions. Calculation of the cross‐shock potential yields a potential jump significant enough to reflect at least some of the protons from the incident solar wind beam. The cross‐shock potential calculated here is consistent with previous estimations based on particle measurements and numerical/analytical simulations. The ambipolar contribution to the cross‐shock potential calculated from the four‐spacecraft divergence of the electron pressure tensor is somewhat higher than that inferred form the Liouville‐mapped electron energy gain across the shock. Furthermore, the high‐time‐resolution 3‐D electric field measurements reported here reveal small‐scale nonlinear structures embedded in the shock layer that contribute to the nonmonotonic shock transition.

Simulations of a proof-of-principle experiment for collinear laser spectroscopy within a multi-reflection time-of-flight device

For nearly four decades Collinear Laser Spectroscopy (CLS) has been employed to determine ground-state properties of short-lived radionuclides. To extend its reach to the most exotic radionuclides with very low production yields, the novel Multi Ion Reflection Apparatus for CLS (MIRACLS) is currently under development at ISOLDE/CERN. In this setup, 30-keV ion bunches will be trapped between two electrostatic mirrors of a multi-reflection time-of-flight (MR-ToF) device such that the laser beam will probe the ions during each revolution. Thus, the observation time will be extended and the experimental sensitivity will be increased significantly while maintaining the high resolution of conventional CLS. A proof-of-principle experiment is currently being performed to demonstrate the potential of CLS within a low-energy MR-ToF device. Its first experimental results benchmark the validity of ion-optical simulations from the CLS perspective, which will also be applied to MIRACLS’ 30-keV apparatus.

First steps in the development of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy

Collinear laser spectroscopy (CLS) has been combined with the multi-reflection time-of-flight (MR-ToF) technique. To this end, a photodetection system has been implemented at the drift region of a MR-ToF apparatus and a laser beam has been sent along the path of the ions that are stored between the two ion-optical mirrors. The main goal of the present proof-of-principle (PoP) experiments, is the confirmation of the expected increase in sensitivity compared to conventional fluorescence-based CLS due to the repeated probing of the trapped ion bunches. The novel method will be used for the precise measurement of nuclear ground- and isomeric-state properties of exotic nuclei with low production yields at radioactive ion-beam facilities. A significant sensitivity improvement of CLS is expected, depending on the half-life and mass of the nuclide of interest. The status of the PoP setup and future improvements are discussed.

Calculation of Ion Reflection from Solids: Computer Simulation and Theory

A computer program is developed to study ion reflection from solid surfaces. The program uses the cutoff Coulomb potential, the binary collision approximation, and the model of local inelastic energy losses. For a fixed ion-target combination, the reflection coefficient is calculated as a function of the ion energy and incidence angle. A reflection theory for the scattering cross section which depends on the transferred energy in accordance with the power law is constructed. Good agreement of the simulation results with the experiment and the theory is obtained.

Reflection of the strahl within the foot of the Earth's bow shock

Abstract. The reflection of a fraction of the solar wind at the bow shock to some extent defines the physical properties of what is known as the foreshock, the region where the interplanetary magnetic field has a direct connection to the bow shock. Both ion and electron reflection have been observed and together form a significant source of free energy that is responsible for many of the instabilities observed in this region. In this paper we concentrate on the reflection of electrons at the shock and report two significant findings: the first is that the strahl, the field-aligned component of the electron solar wind distribution, appears to be fully reflected at the bow shock; the second finding is that the reflection is observed to occur in the foot of the shock and not in the shock ramp. This latter observation implies that mirroring in these examples is not the primary determinant of the electron reflection process.

High-flux high-energy ion beam production from stable collisionless shock acceleration by intense petawatt-picosecond laser pulses

A scheme to achieve stable collisionless shock acceleration (CSA) of ions from a near-critical plasma by intense petawatt-picosecond laser pulses is proposed, where the plasma is confined in a high-Z solid tube. The application of the tube, on the one hand, restrains the plasma from transverse thermal expansion, helping to sustain sufficient density steepening required for shock formation and maintenance; on the other hand, due to the induced sheath field along its wall, pinches hot electrons for recirculation near laser axis, aiding to reach efficient plasma heating that is crucial to have a strong shock velocity for ion reflection. Consequently, stable ion CSA can be maintained for picosecond time scales, resulting in production of high-flux high-energy ion beams. Two-dimensional PIC simulations show that proton beams with narrow energy spread between 50 and 80 MeV and high flux with particle number about 1012 are produced by a laser pulse at intensity 8.8 × 1019 W cm−2 and duration 1 ps. By extending the pulse duration to 3 ps, over 100 MeV high-flux proton beams are obtained.

Synthesis of praseodymium trioxide doped lead-boro-tellurite glasses and their optical and physical properties

This paper presents the influence of TeO2 on the optical and physical properties of rare earth ion Pr3+ doped 75PbO - (24-x) B2O3 - xTeO2 - 1Pr2O3 glass matrix. Using the absorption coefficient measurements, optical band gap energies of the studied glasses are estimated. Oxide ion polarizability, refractive index, optical basicity, reflection loss and transmission coefficient are discussed and reported for the investigated glasses. Physical parameters like the optical dielectric constant, polaron radius, interatomic separation, molar refraction and the metallization criterion are also studied through molar density and molar volume. The FTIR analysis reveals the shift of the band appearing at 1029 to 870 cm−1 because of the high concentration of PbO (75 mol% in the present glass) in the matrix. The Pb-O-B linkage is considered to be the important in the network structure of these glasses. For the present glasses metallization parameter values are <1. The polarizability values are high but decrease with increasing value of TeO2 concentration. This may be due to densification of restricted or localized states caused by the creation of dangling bonds.

The increasing of H−current from Penning ion source with electrically biased metal hydride cathode

The possibility of the current increasing of negative hydrogen ions H− extracted in the longitudinal direction from gas-feed-free Penning type ion source with metal hydride cathode by the electric cathodes bias has been investigated. The separation of negative ions from the extracted flux of charged particles was performed by an electromagnetic filter, which consists of a magnetic coil for electron diverting and a grid under positive potential for positive ion reflection. The optimum for the effective extraction of H− ions has been determined. The H− ion current of 10 μA was obtained at an input power of 6 W.

Shock ripples observed by the MMS spacecraft: ion reflection and dispersive properties

Shock ripples are ion-inertial-scale waves propagating within the front region of magnetized quasi-perpendicular collisionless shocks. The ripples are thought to influence particle dynamics and acceleration at shocks. With the four magnetospheric multiscale (MMS) spacecraft, it is for the first time possible to fully resolve the small scale ripples in space. We use observations of one slow crossing of the Earth’s non-stationary bow shock by MMS. From multi-spacecraft measurements we show that the non-stationarity is due to ripples propagating along the shock surface. We find that the ripples are near linearly polarized waves propagating in the coplanarity plane with a phase speed equal to the local Alfvén speed and have a wavelength close to 5 times the upstream ion inertial length. The dispersive properties of the ripples resemble those of Alfvén ion cyclotron waves in linear theory. Taking advantage of the slow crossing by the four MMS spacecraft, we map the shock-reflected ions as a function of ripple phase and distance from the shock. We find that ions are preferentially reflected in regions of the wave with magnetic field stronger than the average overshoot field, while in the regions of lower magnetic field, ions penetrate the shock to the downstream region.

Origin of plasma-induced surface roughening and ripple formation during plasma etching: The crucial role of ion reflection

Plasma-induced surface roughening and ripple formation has been studied based on Monte Carlo simulations of plasma-surface interactions and feature profile evolution during Si etching in Cl-based plasmas, with emphasis being placed on the role and effects of ion reflection from microstructural feature surfaces on incidence. The simulation model included the effects of Cl+ ion reflection (and/or its penetration into substrates) through calculating the momentum and energy conservation in successive two-body elastic collisions with substrate Si atoms every ion incidence. The “reflection coefficient ri” was then further introduced in the model (0 ≤ ri ≤ 1), representing the fraction of ions incident on surfaces with the reflection/penetration calculation scheme turned on. The coefficient ri is, in a sense, a measure of the reflection probability for impacts of an ion species onto Si surfaces relative to that for Cl+ impacts. Simulations for ion incidence angles of θi = 0°, 45°, and 75° onto substrate surfaces with incident energies in the range Ei = 20−500 eV showed that as ri is slightly decreased from unity, the roughness decreases substantially, and the ripple formation fades away: the roughness remains at the low level of stochastic roughening during etching for decreased ri ≤ ri* ≈ 0.95−0.75 (the critical ri* tends to be lower at higher Ei and θi) with no ripple structures at off-normal θi. This elucidates that the ion reflection is indispensable in surface roughening and rippling during plasma etching, and their degree relies significantly on the reflectivity of ions. Simulations further showed that at intermediate off-normal θi = 45°, the ripple wavelength increases significantly with decreasing ri, while the increase in amplitude is relatively less significant; thus, sawtooth-like ripple profiles pronounced for ri = 1 tend to be collapsed with decreasing ri. These effects of reduced ion reflection on plasma-induced surface roughening and ripple formation are discussed in terms of effectively enhanced smoothing due to neutral reactants, which competes with the roughening and rippling caused by ion bombardment.

Roughness Evolution and Charging in Plasma-Based Surface Engineering of Polymeric Substrates: The Effects of Ion Reflection and Secondary Electron Emission.

The interaction of plasma with polymeric substrates generates both roughness and charging on the surface of the substrates. This work, toward the comprehension and, finally, the control of plasma-induced surface roughness, delves into the intertwined effects of surface charging, ion reflection, and secondary electron-electron emission (SEEE) on roughness evolution during plasma etching of polymeric substrates. For this purpose, a modeling framework consisting of a surface charging module, a surface etching model, and a profile evolution module is utilized. The case study is etching of a poly(methyl methacrylate) (PMMA) substrate by argon plasma. Starting from an initial surface profile with microscale roughness, the results show that the surface charging contributes to a faster elimination of the roughness compared to the case without charging, especially when ion reflection is taken into account. Ion reflection sustains roughness; without ion reflection, roughness is eliminated. Either with or without ion reflection, the effect of SEEE on the evolution of the rms roughness over etching time is marginal. The mutual interaction of the roughness and the charging potential is revealed through the correlation of the charging potential with a parameter combining rms roughness and skewness of the surface profile. A practical implication of the current study is that the elimination or the reduction of surface charging will result in greater surface roughness of polymeric, and generally dielectric, substrates.

Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

AbstractA dipolarization front (DF) is known as the leading edge of an earthward high‐speed flow with a sharp enhancement in the northward magnetic field (Bz). Analysis of an event observed by Cluster shows that the behavior of oxygen ions (O+) around the DF is very different from protons (H+). After the crossing of the DF, the O+ density decreases more gradually than H+. The distance between the density minimum of O+ and the DF layer is ~4 times longer than that of H+, which is close to their gyroradii ratio with the same energy. A flux dropout is observed in the O+ energy spectrum, whose energy dependence indicates that ions with higher energies can reach locations farther tailward of the DF. Similar variations are also seen in studies of 22 events in which a common pattern of ion properties is obtained by performing a superposed epoch analysis. Finally, using backward tracing test‐particle simulations, we reproduce the characteristics of the flux dropout and verify that the time dependence of the dropout is highly correlated with the gyromotion of different energy O+ behind the DF. All these results provide a further understanding of ion dynamics associated with DFs and suggest that the observed O+ ions are reflected within a half gyromotion in the central plasma sheet.

Probing the solid-liquid transition of thin propanol and butanol films through interactions with LiI

This report describes how LiI mixes with thin propanol and butanol films at cryogenic temperatures using time-of-flight secondary ion mass spectrometry and reflection absorption infrared spectroscopy. The glass transition temperature, Tg, is assignable as the uptake onset of LiI additives in the thin film interior. The uptake rate of LiI in the deeply supercooled region is small for 1-propanol and 2-butanol, but concentrated solutions are formed on a LiI film at temperatures higher than 1.2–1.3 Tg. The supercooled 2-propanol and 1-butanol are short-lived; their crystallites form complexes at the interface with LiI because of premelting.

Anomalous Coefficient of Ion Reflection from a Crystal under Surface-Channeling Conditions

A significant difference in behavior between the coefficients of deuterium-atom reflection upon scattering at crystal and amorphous tungsten is shown. The region where the reflection coefficient is close to 100% is determined by scattering at the first and second crystal layers and depends on the orientation. The effect of ion focusing on second-layer atoms and also various orientation effects are observed. It is shown that, at small grazing angles, the dependence of the reflection coefficient on the energy of transverse particle motion is universal. The dependences on the azimuthal exit angle ϕ exhibit the rainbow scattering effect caused by particle motion in the surface semichannel, while the dependences on the polar angle demonstrate the effect of angular distribution narrowing (ion focusing).

Response of carbon and tungsten surfaces to hydrogen plasma of different temperatures

The basic understanding of the interaction between energetic hydrogen plasma with carbon and tungsten based surfaces is crucially important for analyzing plasma-wall interaction in divertors of fusion devices and other plasma applications. The ion reflection coefficient, ion absorption coefficient, total ion charge density, and ion density distribution have been studied using a kinetic trajectory simulation model at different ion temperatures. It has been observed that the ion reflection coefficient and the ion absorption coefficient of the incident particles depend on kinetic energy: higher energy ions are less likely to be reflected as they have enough energy to bury themselves within the solid.