Primary Electrons Research Articles

The Influence of Primary Electrons on the Mechanism of Plasma Generation

It is shown that at high percentage content of primary electrons, plasma is generated due to the retention of ions in a potential well created by electrons and not due to the accumulation of secondary electrons in the region of a potential hump formed by ions.

SEY and low-energy SEY of conductive surfaces

The study of Secondary Electron Yield (SEY) is widely performed to address important properties of materials to be used in a very wide spectrum of applications. It is, therefore, extremely important to understand the SEY dependence on material type, surface contaminants, structural quality and surface damage. We review here our recent studies of such items performed by looking at some representative conductive materials as noble metals and carbon based surfaces. Polycrystalline Ag, Au and Cu samples have been studied as introduced in the ultra-high vacuum chamber (therefore with an significant surface contamination) and after having been cleaned by ion sputtering. The comparison between the curves confirms that the SEY behavior is strongly influenced by the chemical state of the metal surfaces. We demonstrate the ability of SEY to determine work function values with high accuracy if the experimental system allows using very slow primary electrons. We also investigated, for the Cu sample, the effect on SEY of minimal amount of contaminants in the sub-monolayer regime showing that SEY is highly sensitive to the presence of adsorbates even at such very low coverages, specially for low energy primary electrons. In the case of C surfaces we summarize here the effect that the structural ordering of the C lattice has on the macroscopic SEY properties of ultrathin C layers. In particular we followed the SEY evolution during the thermal graphitization of thin amorphous carbon layers and during the amorphization of highly oriented pyrolytic graphite by means of Ar+ bombardment. In the first case the SEY decrease observed with the progressive conversion of sp3 hybrids into six-fold aromatic domains was related to the electronic structure of the C-films close to the Fermi level. We found that a moderate structural quality of the C layer, corresponding to aromatic clusters of limited size, is sufficient to obtain a SEY as low as ∼1. For the bombarded graphite, the strong lattice damage remains limited to the near surface layer, where the high density of defects reduces the transport of incoming and secondary electrons. Then, the SEY curves resulted differently modified in the low and high primary energy regions, but their maximal values remained favorably low. Our findings demonstrate that SEY, besides being an indispensable mean to qualify technical materials in many technological fields, can be also used as a flexible and advantageous diagnostics to probe surfaces and interfaces.

Study of Dust Acoustic Wave Propagation in a Lorentzian Dusty Plasma in Presence of Secondary Electron Emission

In this paper, we have investigated the effect of secondary electron emission on the propagation characteristics of linear dust acoustic waves in dusty plasmas consisting of suprathermal inertialess primary electrons, secondary electrons, Boltzmann distributed ions, and negatively charged inertial dust grains. The suprathermal electrons are assumed to follow kappa velocity distribution. We have derived the linear dispersion relation and shown that real and imaginary frequencies of the waves are influenced by the strength of secondary electron emission as well as suprathermal electron population.

The function of the CFTR Cl- channel in the exocrine/endocrine pancreas under pathological conditions

We present simulations for electron transmission through a platinum macrocapillary (diameter d = 3.3 mm, length l = 48 mm) using classical transport theory. Both elastic and inelastic scattering events of primary electrons colliding with the inner wall of the capillary are taken into account. We also model the generation and transport of secondary electrons inside the material. We find excellent agreement of our simulated electron-energy spectra with recent experimental data for 200 eV primary electrons.

Estimating Step Heights from Top-Down SEM Images.

Scanning electron microscopy (SEM) is one of the most common inspection methods in the semiconductor industry and in research labs. To extract the height of structures using SEM images, various techniques have been used, such as tilting a sample, or modifying the SEM tool with extra sources and/or detectors. However, none of these techniques focused on extraction of height information directly from top-down images. In this work, using Monte Carlo simulations, we studied the relation between step height and the emission of secondary electrons (SEs) resulting from exposure with primary electrons at different energies. It is found that part of the SE signal, when scanning over a step edge, is determined by the step height rather than the geometry of the step edge. We present a way to quantify this, arriving at a method to determine the height of structures from top-down SEM images. The method is demonstrated on three different samples using two different SEM tools, and atomic force microscopy is used to measure the step height of the samples. The results obtained are in qualitative agreement with the results from the Monte Carlo simulations.

Formation of carbon-based nanotubular structures by in situ electron irradiation

Structural modification throughout the bulk of materials can be triggered by irradiation, depending on the incident energy. Electron beams are used to irradiate samples for several purposes, from sample analysis to structural changes. In Transmission Electron Microscopy (TEM) a beam of highly energetic electrons is used to analyze samples. During irradiation, interactions may occur between primary electrons (TEM electrons) and bulk sample. Accordingly, they can cause transitory or permanent modifications on the structure of a specimen. In this study, the formation of carbon nanotube-like structures has been obtained in situ during electron irradiation of polymeric nanoparticles (PNPs) in a Transmission Electron Microscopy. The main component of our PNPs is a copolymer based on methacrylic acid (PMA) and ethyl acrylate (PA) [1:1] polymers. This composition appears to be important in the nanotube-structures formation, since PMA degrades very rapidly when exposed to irradiation through scission schemes, while PA undergoes cross-linking. These structural modifications, from amorphous carbonaceous structure to crystalline ones, happen rapidly, on the order of ∼20 s. This mainly due to the high incident current density (∼108 A cm−2) transported by the electron radiation stemming from the microscope.

Damage-induced voltage alteration (DIVA) contrast in SEM images of ion-irradiated semiconductors

The ion-irradiation damage effects in semiconductors were directly visualized by means of scanning electron microscopy at low beam acceleration voltages (low-kV SEM). The Al0.55Ga0.45As (p-type and n-type) epitaxial layers grown over GaAs substrates were irradiated with energetic He+ ions with fluencies ranging from 8e12 to 8e13 cm−2 and studied in cross-sectional view after cleavage. Secondary electron images collected at low energy (0.5 – 1 keV) of primary electrons show strong contrast related to the local differences in the material resistivity resulting from the ion-induced damage. The main aim of the paper is to present, for the first time, the interpretation of the mechanism of SEM image contrast formation, which has been referred to as Damage-Induced Voltage Alteration (DIVA) contrast and is based on the charging effect in SEM. The unrivalled advantage of this imaging technique is the possibility of immediate two-dimensional visualization of damage effects without complex sample preparation.

Cosmic ray transport and radiative processes in nuclei of starburst galaxies

AbstractThe high rate of star formation and supernova explosions of starburst galaxies make them interesting sources of high-energy radiation. Depending on the level of turbulence present in their interstellar medium, the bulk of cosmic rays produced inside starburst galaxies may lose most of their energy before escaping, thereby making these sources behave as calorimeters, at least up to some maximum energy. Contrary to previous studies, here we investigate in detail the conditions under which cosmic ray confinement may be effective for electrons and nuclei and we study the implications of cosmic ray confinement in terms of multifrequency emission from starburst nuclei and production of high-energy neutrinos. The general predictions are then specialized to three cases of active starbursts, namely, M82, NGC 253, and Arp220. Both primary and secondary electrons, as well as electron–positron pairs produced by gamma-ray absorption inside starburst galaxies are taken into account. Electrons and positrons produced as secondary products of hadronic interactions are found to be responsible for most of the emission of leptonic origin. In particular, synchrotron emission of very high energy secondary electrons produces an extended emission of hard X-rays that represents a very interesting signature of hadronic process in starburst galaxies, potentially accessible to current and future observations in the X-ray band. A careful understanding of both the production and absorption of gamma-rays in starburst galaxies is instrumental to the assessment of the role of these astrophysical sources as sources of high-energy astrophysical neutrinos.

Low Energy Precipitating Electrons in the Diffuse Aurorae

AbstractAuroral electron precipitation is often classified into three categories: diffuse, monoenergetic, and broadband. However, some precipitating electrons do not fit any of the three categories, which are assumed to be “diffuse” electrons for convenience. Many of these nonconforming electrons exhibit high fluxes at low energies. Most studies assume that the precipitating electrons originate from the magnetosphere. However, this primary precipitation is only the first step in the formation of the electron precipitation. The primary precipitation creates secondary electrons and both electron populations bounce many times between the conjugate points. Thus, the full electron precipitation consists of not only the primary but the Multiply Reflected Primary and Secondary (MRPS) electrons. The MRPS electrons are modeled with Superthermal Electron Transport model. The model‐data comparisons suggest that the low energy electrons in the nonconforming precipitating electrons can be attributed to MRPS electrons.

Assessment of the heating of conductive targets with an electron beam. Results of computational experiment

The problem of heat distribution in conducting (metal and semiconductor) materials irradiated by sharply focused low-energy electron beams in the absence of heat exchange between the target and the external medium was considered with the methods of mathematical modelling. The simulation is based on solving a multidimensional stationary heat equation using the Green’s function. A model is based on a separate description of the contributions to the energy of absorbed and backscattered electrons dissipated in the target. It is applicable to a wide class of solids and the energy range of primary electrons. The results of calculations using the model under consideration of heating a target with an electron probe are given for various metallic and semiconductor materials. The obtained results can be used in the planning of experimental studies of conducting materials.

Origin of cosmic ray electrons and positrons

With experimental results of AMS on the spectra of cosmic ray (CR) $e^{-}$, $e^{+}$, $e^{-}+e^{+}$ and positron fraction, as well as new measurements of CR $e^{-}+e^{+}$ flux by HESS, one can better understand the CR lepton ($e^{-}$ and $e^{+}$) spectra and the puzzling electron-positron excess above $\sim$10 GeV. In this article, spectra of CR $e^{-}$ and $e^{+}$ are fitted with a physically motivated simple model, and their injection spectra are obtained with a one-dimensional propagation model including the diffusion and energy loss processes. Our results show that the electron-positron excess can be attributed to uniformly distributed sources that continuously inject into the galactic disk electron-positron with a power-law spectrum cutting off near 1 TeV and a triple power-law model is needed to fit the primary CR electron spectrum. The lower energy spectral break can be attributed to propagation effects giving rise to a broken power-law injection spectrum of primary CR electrons with a spectral hardening above $\sim$40 GeV.

Impact of secondary electron emission noise in SEM.

In semiconductor-device inspection using scanning electron microscopes (SEMs), the irradiation dose of the electron beam becomes lower because of increasing needs for higher throughput and lower damage to the samples. Therefore, it is necessary to form images using fewer primary electrons, making noise reduction of SEM images one of the main challenges. We have modeled the imaging process of SEMs, which consists of the generation of primary, secondary and tertiary electrons (PEs, SEs and TEs, respectively), and detection. Furthermore, a method to accurately evaluate the fluctuation in the number of SEs and TEs are proposed. We found that SEM-image noise can be minimized by directly detecting SEs generated in the sample, in which case the fluctuation in the number of SEs determines the image quality. The variance number of SEs emitted from a 500-eV PE irradiation onto a Si wafer is 1.9 times as large as the value derived assuming a Poisson process. A Monte-Carlo simulation result was used to explain the experimental results and predict that PE energy less than 1 keV suppresses the fluctuation in the number of SEs, and consequently, the SEM-image noise level. These findings provide a method for determining imaging conditions that improve the throughput of SEMs.

Влияние первичных электронов на механизм генерации плазмы

Влияние первичных электронов на механизм генерации плазмы

Effect of a magnetic cusp configuration on the ion species ratio in a NBI ion source

In arc discharge plasma, ion species ratio depends on plasma density and ion confinement time. To increase an atomic ion species ratio in an NBI (neutral beam injector) ion source, various ion sources have been designed with a higher ratio of plasma volume to effective ion loss area for increasing ion confinement time. In such designs, most NBI ion sources have a large arc chamber with strong cusp magnetic field. KSTAR NBI ion source also has a large arc chamber with a strong azimuthal line cusp configuration but its atomic ion (H+) beam fraction is small because its normal operation power levels of the arc discharges are comparatively lower than those of other NBI ion sources. In particular, the H2+ ion beam fraction is relatively large at about 40%. This means that there are many primary electrons in the beam extraction region. Through several Ele-orbit code simulations, it was found that some magnetic cusp configurations are effective in preventing primary electrons from nearing the plasma grid facing a plasma and having a high voltage potential. To reduce the molecular ion fraction in KSTAR NBI ion source and consequently increase its atomic ion fraction, experiments regarding the application of several new magnetic cusp configurations to the arc chamber were performed. Experimental results show that the new magnetic cusp configuration can increase the atomic ion fraction to approximately 80%, despite the low arc operation power at KSTAR NBI ion source.

Thermalization dynamics of primary and secondary electrons in metals

Collisional relaxation of high-energy primary electrons (PEs) and generation of secondary electrons (SEs) in a metal are simulated self-consistently by the quantum Boltzmann equation taking into account dynamic screening. An efficient scheme to evaluate the multi-dimensional integral in the collision term is introduced, which enables one to compute time evolutions of nonequilibrium energy distribution functions over an energy range up to a few keV until the whole system reaches thermal equilibrium. Numerical results reveal that rapid energy loss of PEs by plasmon excitation occurs in an early stage, followed by a cascade excitation of SEs near the Fermi level. The overall energy distribution eventually converges to the equilibrium Fermi distribution of partially degenerate electron gas, corroborating the quantum-mechanical H-theorem; time evolutions of the corresponding nonequilibrium entropies are also simulated. The electron temperature and total number of SEs in the final state are consistent with thermodynamic values predicted from the initial energy density of PEs. The number of SEs excited during an early stage (initial 10 fs) turns out smaller for higher PE energies; this feature would bear relevance to an estimation of transient radiation damage in a metallic sample irradiated by an intense femtosecond x-ray laser pulse.

Time-dependent characteristics of secondary electron emission

The recent development of the time-resolving capability for scanning electron microscopy (SEM) enables it to be a real 4D space-time imaging technique, which is extremely suitable for investigating the ultrafast dynamic processes concerned with secondary electron emission (SEE). This paper attempts to investigate the dynamic SEE process with the aid of a Monte Carlo method; the understanding of the mechanism will surely benefit the construction and application of various kinds of time-resolved SEMs. Our simulation modeling is based on the use of the Mott cross section and a dielectric function approach for the respective description of the electron elastic and inelastic scattering. One secondary electron is assumed to be produced in an inelastic scattering event, and the owned kinetic energy enables it to transport and produce other secondary electrons, forming the cascade production process. From the simulation, not only the time delay of SEE from the incidence instant of primary electrons but also the time dependences of the involved physical quantities, including the energy-, depth-, direction-, emission site-, and production site-distributions can be theoretically derived. The calculations provide useful knowledge on the time dependence of SEE from the theoretical perspective for the applications to the available time-resolved SEMs.

Electron energy-loss spectroscopy (EELS) with a cold-field emission scanning electron microscope at low accelerating voltage in transmission mode

A commercial electron energy-loss spectrometer (EELS) attached to a high-resolution cold-field emission scanning electron microscope in transmission mode (STEM) is evaluated and its potential for characterizing materials science thin specimens at low accelerating voltage is reviewed. Despite the increased beam radiation damage at SEM voltages on sensitive compounds, we describe some potential applications which benefit from lowering the primary electrons voltage on less-sensitive specimens. We report bandgap measurements on several dielectrics which were facilitated by the lack of Cherenkov radiation losses at 30 kV. The possibility of volume plasmon imaging to probe local composition changes in complex materials was demonstrated using energy-filtered STEM, either via spectrum imaging or elemental mapping using the “three-windows” method. As plasmonic materials are increasing used for energy, electronics or biomedical applications, the ability of reliably evaluate their properties at low accelerating voltage in a SEM is very appealing and is demonstrated. The energy resolution of the spectrometer, taken as the full width at half maximum of the zero-loss peak, was routinely measured at around 0.55 eV and it is demonstrated that t/λ ratios up to 1.5 allowed practical EEL spectroscopy at 30 kV.

Axial Ring-Cusp Hybrid (ARCH) plasma discharge: an approach to highly efficient miniature-scale ion sources

The miniaturization of conventional direct-current ion sources is predominantly restricted by efficiency limitations associated with the increased surface area-to-volume ratio of smaller-scale discharge chambers—reducing the effective confinement length of the high-energy ‘primary’ electrons that is necessary for efficient plasma generation. The Axial Ring-Cusp Hybrid (ARCH) plasma discharge addresses this scaling limitation by using a new approach that combines magnetic and electrostatic confinement to decouple the primary and plasma electrons loss mechanisms. Simulated ion thruster performance measurements show that the ARCH discharge may be capable of achieving a discharge loss and a propellant mass utilization of 175 eV/ion and 0.87, respectively. These estimates are supported by full internal maps of the plasma properties, including the electron energy distribution function, inside the discharge chamber. The measurements show highly effective confinement of the primary electrons, high average plasma electron temperatures of ∼5 eV, and low plasma sheath potential relative to the anode—attributes generally found only in efficient conventional-scale discharges with good overall plasma confinement. As such, the new ARCH discharge design approach may allow miniature ion thrusters to achieve the performance and efficiency levels similar to those of highly efficient conventional ion thrusters.

Recent advances in bubble-assisted Liquid Hole-Multipliers in liquid xenon

We report on recent advances in the operation of bubble-assisted Liquid Hole-Multipliers (LHM). By confining a vapor bubble under or adjacent to a perforated electrode immersed in liquid xenon, we could record both radiation-induced ionization electrons and primary scintillation photons in the noble liquid. Four types of LHM electrodes were investigated: a THGEM, standard double-conical GEM, 50 μm-thick single-conical GEM (SC-GEM) and 125 μm-thick SC-GEM—all coated with CsI photocathodes. The 125 μm-thick SC-GEM provided the highest electroluminescence (EL) yields, up to ∼ 400 photons per electron over 4π , with an RMS pulse-height resolution reaching 5.5% for events comprising ∼ 7000 primary electrons. Applying a high transfer field across the bubble, the EL yield was further increased by a factor of ∼ 5. The feasibility of a vertical-mode LHM, with the bubble confined between two vertical electrodes, and the operation of a two-stage LHM configuration were demonstrated for the first time. We combine electrostatic simulations with observed signals to draw conclusions regarding the location of the liquid-gas interface and suggest an explanation for the observed differences in EL yield between the investigated electrodes.

Synchrotron emission in molecular cloud cores: the SKA view

Understanding the role of magnetic fields in star-forming regions is of fundamental importance. In the near future, the exceptional sensitivity of the Square Kilometre Array (SKA) will offer a unique opportunity to evaluate the magnetic field strength in molecular clouds and cloud cores through synchrotron emission observations. The most recent Voyager 1 data, together with Galactic synchrotron emission and Alpha Magnetic Spectrometer data, constrain the flux of interstellar cosmic-ray electrons between ∼3 MeV and ∼832 GeV, in particular in the energy range relevant for synchrotron emission in molecular cloud cores at SKA frequencies. Synchrotron radiation is entirely due to primary cosmic-ray electrons, the relativistic flux of secondary leptons being completely negligible. We explore the capability of SKA in detecting synchrotron emission in two starless molecular cloud cores in the southern hemisphere, B68 and FeSt 1-457, and we find that it will be possible to reach signal-to-noise ratios of the order of 2 − 23 at the lowest frequencies observable by SKA (60 − 218 MHz) with one hour of integration.