Secondary Electron Generation Research Articles

Novel low-temperature and high-flux hydrogen plasma source for extreme-ultraviolet lithography applications

Inside extreme-ultraviolet (EUV) lithography machines, a hydrogen plasma is generated by ionization of the background gas by EUV photons. This plasma is essential for preventing carbon build-up on the optics, but it might affect functional performance and the lifetime of other elements inside the machine. The interaction of scanner materials and components with hydrogen plasma is investigated in controlled experiments using laboratory (off-line) setups, where the properties of EUV-generated plasmas are mimicked. Here, we present a novel experimental setup at TNO, where a low-temperature hydrogen plasma is generated by means of electron-impact ionization using a high-current, high-pressure electron beam (e-beam) gun. We show that the produced ion flux, peak ion energies, and radical-to-ion ratio are similar to that of the EUV-generated plasma. Since the e-gun has the option of operating the e-gun in the pulsed mode, it is possible to reproduce the time-dependent behavior of the scanner plasma as well. Moreover, as shown by Luo et al. [RSC Adv. 10, 8385 (2020)], electrons that impinge on surfaces mimic EUV photons in the generation of secondary electrons, which often drive radiation-induced processes (e.g., surface oxidation, reduction, and growth of carbon). We conclude that e-beam generated hydrogen plasma is a very promising technology for cost-effective lifetime testing of materials and optics, as compared to setups with EUV sources.

Characterization of a radially emitting spray jet in a cylindrical inertial electrostatic confinement plasma source based on particle in cell simulation

This paper investigates the outflow mechanism of a cylindrical inertial electrostatic confinement plasma source using Particle in Cell plasma simulations. Various phenomena such as ion-wall interactions, impact ionization, excitation, and secondary electron generation are considered. Together with a fine time and cell resolution to reveal relevant phenomena. The results indicate a high electron and ion density in front of the cathode at the widest grid opening, which suggests the formation of a cascade-like charge carrier emission, commonly referred to as the “spray jet”. Initially, a virtual anode is formed within the cathode grid and is shifted toward the grid opening as time elapsed. Ions are accelerated in the plasma sheath around the cathode and collide with the cathode grids or oscillate through the grid opening. Electrons are accelerated and ionize the background gas at the plasma sheath edge. While the electron flow is disordered within the cathode it forms a cascade-like quasi-neutral plasma at the outlet of the plasma source. Through the formation of a virtual anode the discharge becomes self-sustained. With this simulation results the working principle of the inertial electrostatic confinement plasma source can be derived.

Mie Resonant Metal Oxide Nanospheres for Broadband Photocatalytic Enhancements.

Metal oxides are widely used in heterogeneous catalysis as supports to disperse catalytically active nanoparticles, isolated atomic sites, or even as catalysts themselves. Herein, we present a method to produce optically active metal oxide supports that exhibit size-dependent Mie resonances based on TiO2 nanospheres with tunable size, crystalline phase composition, and optical properties. Mie resonant TiO2 nanospheres were used as supports to disperse Au, Pt, and Pd nanoparticles. We have found up to a 50-fold enhancement of the electric field at the metal oxide/metal interface corresponding to wavelength-dependent multipolar resonances in the TiO2 structure. Using Au/TiO2 as a prototypical photocatalyst, we demonstrate broadband rate enhancements between 400 and 800 nm during CO oxidation, with a noticeable increase below 500 nm. This increased reactivity at higher photon energies is due to improved photon utilization and interband absorption in the gold that results in greater secondary electron generation through electron-electron scattering processes, thus leading to higher rates in conjunction with the Mie scattering TiO2 support. This study not only highlights the potential of Mie resonant TiO2 in broadband photocatalytic enhancements but also for developing various Mie resonant metal oxide supports, such as ZnO or Cu2O, which can improve photocatalytic performance for a number of critical reactions. As the chemical and energy industries move toward conversion technologies driven by renewable energy sources, the strategy of designing optical resonances into oxide supports that are already widely used could enable a straightforward adaptation of photochemical processing based on traditional heterogeneous catalysts.

Breakdown modes in nanosecond pulsed micro-discharges at atmospheric pressure

Nanosecond pulse micro-discharges at atmospheric pressure have garnered attention because of their unique physics and numerous applications. In this study, we employed a one-dimensional particle-in-cell/Monte Carlo collision model coupled with an external circuit, using an unequal weight algorithm to investigate the breakdown processes in micro-discharges driven by pulses with voltage ranging from 1 kV to 50 kV at atmospheric pressure. The results demonstrate that nanosecond pulse-driven microplasma discharges exhibit different breakdown modes under various pulse voltage amplitudes. We present the discharge characteristics of two modes: ‘no-breakdown’ when the breakdown does not occur, and ‘runaway breakdown mode’ and ‘normal breakdown mode’ when the breakdown does happen. In the runaway breakdown mode, the presence of runaway electrons leads to a phenomenon in which the electron density drops close to zero during the pulse application phase. Within this mode, three submodes are observed: local mode, transition mode, and gap mode, which arise from different secondary electron generation scenarios. As the pulse voltage amplitude increases, a normal breakdown mode emerges, characterized by the electron density not dropping close to zero during the pulse application phase. Similarly, three sub-modes akin to those in the runaway breakdown mode exist in this mode, also determined by secondary electrons. In these modes, we find that electron loss during the pulse application phase is dominated by boundary absorption, whereas during the afterglow phase, it is dominated by recombination. Ion losses are primarily governed by recombination. These findings contribute to a better understanding of the discharge mechanisms during the breakdown process.

Nonlocal dynamics of secondary electrons in capacitively coupled radio frequency discharges

In capacitively coupled radio frequency discharges, the interaction of the plasma and the surface boundaries is linked to a variety of highly relevant phenomena for technological processes. One possible plasma-surface interaction is the generation of secondary electrons (SEs), which significantly influence the discharge when accelerated in the sheath electric field. However, SEs, in particular electron-induced SEs (δ-electrons), are frequently neglected in theory and simulations. Due to the relatively high threshold energy for the effective generation of δ-electrons at surfaces, their dynamics are closely connected and entangled with the dynamics of the ion-induced SEs (γ-electrons). Thus, a fundamental understanding of the electron dynamics has to be achieved on a nanosecond timescale, and the effects of the different electron groups have to be segregated. This work utilizes particle-in-cell/Monte Carlo collisions simulations of a symmetric discharge in the low-pressure regime (p = 1 Pa) with the inclusion of realistic electron-surface interactions for silicon dioxide. A diagnostic framework is introduced that segregates the electrons into three groups (‘bulk-electrons’, ‘γ-electrons’, and ‘δ-electrons’) in order to analyze and discuss their dynamics. A variation of the electrode gap size is then presented as a control tool to alter the dynamics of the discharge significantly. It is demonstrated that this control results in two different regimes of low and high plasma density, respectively. The fundamental electron dynamics of both regimes are explained, which requires a complete analysis starting at global parameters (e.g. densities) down to single electron trajectories.

Photoemission spectroscopy on photoresist materials: A protocol for analysis of radiation sensitive materials

The continued downscaling, following the so-called Moore's law, has motivated the development and use of extreme ultraviolet (EUV) lithography scanners with specialized photoresists. Since the quality and precision of the transferred circuit pattern are determined by the EUV-induced chemical changes in the photoresist, having a deep understanding of these chemical changes is of pivotal importance. For this purpose, several spectroscopic and material characterization techniques have already been employed. Among them, photoemission can be essential as it not only allows direct probing of chemical bonds in a quantitative way but also provides useful information regarding the generation and distribution of primary and secondary electrons. However, since high energy photons are being employed for characterization of a photosensitive material, modification of the sample during the measurement is possible, and this must be considered when investigating the chemical changes in the photoresist before and after exposure to EUV light. In this paper, we investigate the chemical changes occurring during the photoemission measurements of an unexposed, model chemically amplified resist based on the well-known environmentally stable chemically amplified photoresist as a function of a number of measurement parameters, using both an x ray (AlKα, 1486.6 eV) and a UV source (HeII, 40.8 eV). We will show that these chemical changes can be simulated through theoretical modeling of the photoemission spectra. Based on these results, we propose a measurement protocol allowing to minimize or eliminate this modification which will help to guide (or enable) photoemission measurements on radiation-sensitive materials (e.g., photoresists).

Effect of iron thicknesses on spin transport in a Fe/Au bilayer system

This paper is concerned with a theoretical analysis of the behavior of optically excited spin currents in bilayer and multilayer systems of ferromagnetic and normal metals. As the propagation, control, and manipulation of the spin currents created in ferromagnets by femtosecond optical pulses is of particular interest, we examine the influence of different thicknesses of the constituent layers for the case of electrons excited several electronvolts above the Fermi level. Using a Monte-Carlo simulation framework for such highly excited electrons, we first examine the spatiotemporal characteristics of the spin current density driven in a Fe layer, where the absorption profile of the light pulse plays an important role. Further, we examine how the combination of light absorption profile, spin-dependent transmission probabilities, and iron layer thickness affects spin current density in a Fe/Au bilayer system. For high-energy electrons studied here, the interface and secondary electron generation have a small influence on spin transport in the bilayer system. However, we find that spin injection from one layer to another is most effective within a certain range of iron layer thicknesses.

Dosimetric effects of different hip prosthesis materials during pelvic radiotherapy using high energy photons

This study investigates the effects of different hip prosthesis materials on the secondary neutron and electron contamination produced by high energy, i.e. 18 MV, photon beams during pelvic irradiation. Three different materials, including Titanium-alloy (Ti-alloy), Stainless Steel (SST), and Cobalt–Chromium–Molybdenum-alloy (Co–Cr–Mo), were evaluated in a Monte Carlo (MC) study. The Varian 2100 C/D linac head and an ICRP reference adult male voxel phantom with a realistic hip prosthesis were simulated using the MCNPX (ver. 2.6) MC code. As expected, results showed that secondary electron and neutron generation mainly depends on the shape and material type of the prosthesis. The mean dose increase factor (DIF) for electron contamination is 1.72, 1.53, and 1.35 for Co–Cr–Mo, SST, and Ti-alloy, respectively, at the tissue-prosthesis interface. Electron contamination also depends on the prosthesis thickness and it is increased by increasing the thickness. The maximum DIF for neutrons occurs at the center of the prosthesis stem and its mean DIF is 2.91, 2.49, and 2.34 underneath Co–Cr–Mo, SST, and Ti-alloy prostheses, respectively. The amount of neutron DIF is slightly reduced above prostheses. Furthermore, the results indicate that the most significant change in the electron dose distribution is observed at the tissue-prosthesis interface. The neutron dose equivalent is increased from 76% to 200% in the presence of the different materials of the prosthesis. Hip prostheses made of Ti-alloy can significantly decrease the dose due to neutron and electron contamination, compared to the other materials considered in the simulations.

Different ionization mechanisms in pulsed micro-DBD’s in argon at different pressures

In this research we analyse different plasma wave propagation mechanism of microcavity discharge in pure argon at two different pressures. Experimental results of a pulsed micro-DBD with 2 and 50 kPa argon, 180 μm gap, at room temperature, show that two distinct pressure-dependent propagation modes exist. In the low pressure regime, the discharge propagates perpendicular to the applied electric field forming distinct channels, but many vertically-oriented filaments distributed throughout the domain at high pressure discharge. And the discharge duration time in high pressure is around 5 times shorter than that in low pressure. A 2D particle-in-cell (PIC-MCC) model with chemical reactions, photoemission, and secondary electron generation, is established to investigate the formation mechanism of the two propagation modes. Models of the initial ionization processes show that there are two different breakdown mechanisms for these two pressures, where secondary emission of electrons from the dielectric is dominated either by ion impact or by photon impact. The investigation is of great significance for further reveal of the principle of microplasmas discharge.

Carbon-oxygen surface formation enhances secondary electron yield in Cu, Ag and Au

First-principles calculations coupled with Monte Carlo simulations are used to probe the role of a surface CO monolayer formation on secondary electron emission (SEE) from Cu, Ag, and Au (110) materials. It is shown that formation of such a layer increases the secondary electron emission in all systems. Analysis of calculated total density of states (TDOS) in Cu, Ag, and Au, and partial density of states (PDOS) of C and O confirm the formation of a covalent type bonding between C and O atoms. It is shown that such a bond modifies the TDOS and extended it to lower energies, which is then responsible for an increase in the probability density of secondary electron generation. Furthermore, a reduction in inelastic mean free path is predicted for all systems. Our predicted results for the secondary electron yield (SEY) compare very favorably with experimental data in all three materials, and exhibit increases in SEY. This is seen to occur despite increases in the work function for Cu, Ag, and Au. The present analysis can be extended to other absorbates and gas atoms at the surface, and such analyses will be present elsewhere.

Localized and cascading secondary electron generation as causes of stochastic defects in extreme ultraviolet projection lithography (Erratum)

The <i>Journal of Micro/Nanopatterning, Materials, and Metrology</i> (JM3) publishes peer-reviewed papers on the core enabling technologies that address the patterning needs of the electronics industry.

Tuning the Performance of Negative Tone Electron Beam Resists for the Next Generation Lithography

AbstractA new class of electron bean negative tone resist materials has been developed based on heterometallic rings. The initial resist performance demonstrates a resolution of 15 nm half‐pitch but at the expense of a low sensitivity. To improve sensitivity a 3D Monte Carlo simulation is used that utilizes a secondary and Auger electron generation model. The simulation suggests that the sensitivity can be dramatically improved while maintaining high resolution by incorporating appropriate chemical functionality around the metal–organic core. The new resists designs based on the simulation have the increased sensitivity expected and illustrate the value of the simulation approach.

Touchless Potential Sensing of Differentially Charged Spacecraft Using Secondary Electrons

The secondary electron method has been recently proposed to touchlessly sense the electrostatic potential of non-cooperative objects in geosynchronous equatorial orbits and deep space. This process relies on the detection of secondaries generated at the target surface that is actively irradiated by an electron beam. Although the concept has already been demonstrated with basic geometries, the electric field around complex bodies leads to a highly inhomogeneous distribution of secondary electrons that determines the observability of the system. This paper employs vacuum chamber experiments and particle tracing simulations to investigate the secondary electron flux generated over a spacecraft-like electrode assembly. The differential charging scenario, in which the assembly is charged to multiple potentials, is also studied. A computationally efficient three-dimensional particle tracing framework that couples the electron beam propagation and secondary electron generation processes is introduced and validated, showing its utility as a diagnostic tool. The system geometry, potential field, and electron beam steering configure the observability space of the target, which is limited to well-defined regions where the potentials are measured with high accuracy. The analysis provides theoretical and technical insight into the development of future electron-based touchless potential sensing technologies.

Spatial resolution in secondary-electron microscopy.

We first review the significance of resolution and contrast in electron microscopy and the effect of the electron optics on these two quantities. We then outline the physics of the generation of secondary electrons (SEs) and their transport and emission from the surface of a specimen. Contrast and resolution are discussed for different kinds of SE imaging in scanning electron microscope (SEM) and scanning-transmission microscope instruments, with some emphasis on the observation of individual atoms and atomic columns in a thin specimen. The possibility of achieving atomic resolution from a bulk specimen at SEM energies is also considered.

Monte Carlo simulation of ultrafast nonequilibrium spin and charge transport in iron

Spin transport and spin dynamics after femtosecond laser pulse irradiation of iron (Fe) are studied using a kinetic Monte Carlo model. This model simulates spin dependent dynamics by taking into account two interaction processes during nonequilibrium: elastic electron–lattice scattering, where only the direction of the excited electrons changes, and inelastic electron–electron scattering processes, where secondary electrons are generated. An analysis of the spin dependent particle kinetics inside the material shows that a smaller elastic scattering time leads to a larger spatial spread of electrons in the material, whereas generation of secondary electrons extends the time span for superdiffusive transport and increases the spin current density.

Analysis of secondary emission mechanism in electron avalanches propagating in cylindrical nanoruptures in liquid water

Recently, a bouncing-like mechanism for electron multiplication inside long nano-ruptures during the early stages of nanosecond discharge in liquid water has been proposed in (Bonaventura 2021 Plasma Sources Sci. Technol. 30 065023). This mechanism leads to the formation of electron avalanches within nano-ruptures caused by strong electrostrictive forces. The avalanche propagation is a self-sustaining process: the electrons emitted from the water surface to the cavity support the propagation of the avalanche and the avalanche itself is a source of the parent electrons impinging on the surface of the nano-rupture and causing secondary emission. We analyze the process of the electron secondary emission directly from the simulation results of the electron avalanche propagation. This allow us to perform an in situ study of the secondary emission and related physical processes. We present the results of an extensive parametric study performed using the state-of-the-art simulation toolkit Geant4-DNA for modeling electron-liquid water interactions. It is shown that the typical lifetime of an electron in an avalanche is about 0.1 to 0.2 picoseconds and that the electron experiences about 4 bounces before ending up in liquid water. In addition, it is shown that the secondary electrons are formed in a layer adjacent to the nano-rupture surface that is only a few nanometres thin. The secondary electron velocity distribution at the moment of the electron birth, the velocity space of electrons (re-)emitted from the water, and the velocity space of electrons at the moment of their impact to the cavity surface are analyzed in detail. Electron bouncing and secondary electron generation efficiency are quantified using the secondary emission coefficient, the secondary emission efficiency, and the effective energy consumed to produce new electrons.

Galaxies lacking dark matter produced by close encounters in a cosmological simulation

Electrons accelerated on Earth by a rich variety of wave scattering or stochastic processes generate hard non-thermal X-ray bremsstrahlung up to >~ 1 MeV and power Earth's various types of aurorae. Although Jupiter's magnetic field is an order of magnitude larger than Earth's, space-based telescopes have previously detected X-rays only up to ~7 keV. On the basis of theoretical models of the Jovian auroral X-ray production, X-ray emission in the ~2-7 keV band has been interpreted as thermal (arising from electrons characterized by a Maxwell-Boltzmann distribution) bremsstrahlung. Here we report the observation of hard X-rays in the 8-20 keV band from the Jovian aurorae, obtained with the NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope 0.60+/-0.22 - a spectral signature of non-thermal, hard X-ray bremsstrahlung. We determine the electron flux and spectral shape in the keV to MeV energy range using coeval in situ measurements by the Juno spacecraft's JADE and JEDI instruments. Jovian electron spectra of the form we observe have previously been interpreted to arise in stochastic acceleration, rather than coherent acceleration by electric fields. We reproduce the X-ray spectral shape and approximate flux observed by NuSTAR, and explain the non-detection of hard X-rays by Ulysses, by simulating the non-thermal population of electrons undergoing precipitating electron energy loss, secondary electron generation and bremsstrahlung emission in a model Jovian atmosphere. The results highlight the similarities between the processes generating hard X-ray auroras on Earth and Jupiter, which may be occurring on Saturn, too.

Observation and origin of non-thermal hard X-rays from Jupiter

Electrons accelerated on Earth by a rich variety of wave-scattering or stochastic processes1,2 generate hard, non-thermal X-ray bremsstrahlung up to ~1 MeV (refs. 3,4) and power Earth’s various types of aurorae. Although Jupiter’s magnetic field is an order of magnitude larger than Earth’s, space-based telescopes have previously detected X-rays only up to ~7 keV (ref. 5). On the basis of theoretical models of the Jovian auroral X-ray production6,7,8, X-ray emission in the ~2–7 keV band has been interpreted as thermal (arising from electrons characterized by a Maxwell–Boltzmann distribution) bremsstrahlung5,9. Here we report the observation of hard X-rays in the 8–20 keV band from the Jovian aurorae, obtained with the NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope of 0.60 ± 0.22—a spectral signature of non-thermal, hard X-ray bremsstrahlung. We determine the electron flux and spectral shape in the kiloelectronvolt to megaelectronvolt energy range using coeval in situ measurements taken by the Juno spacecraft’s JADE and JEDI instruments. Jovian electron spectra of the form we observe have previously been interpreted as arising in stochastic acceleration, rather than coherent acceleration by electric fields10. We reproduce the X-ray spectral shape and approximate flux observed by NuSTAR, and explain the non-detection of hard X-rays by Ulysses11, by simulating the non-thermal population of electrons undergoing precipitating electron energy loss, secondary electron generation and bremsstrahlung emission in a model Jovian atmosphere. The results highlight the similarities between the processes generating hard X-ray aurorae on Earth and Jupiter, which may be occurring on Saturn, too.

Theoretical aspects of direct conversion of radio-chemical energy in electric by radiation-stimulated SiC*/Si heterostructure

The authors consider their own CVD technology for the SiC growing on a Si substrate in order to create a beta converter. Since the beta converter contains a heavy C-14 atom, the finished beta converter works as an ”inner sun”, and the structure has specific mark * in the name: SiC*/Si. Authors focus on the problems of the theoretical description of: 1) the growth of the SiC*/Si film (with C-14 atoms inside) and the position of the p-n junction in the doping process; 2) method of a placement radioisotopes into a semiconductor material; 3) physical properties of radioisotopes; 4) defects formation; 5) generation of secondary electrons in the region of the p-n junction.

Simulation Study of Air Effects on SGEMP Based on Swarm Model

A system-generated electromagnetic pulse (SGEMP) occurs when the system is exposed to an X-ray or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> -ray beam. And the presence of air will greatly alter the fields of SGEMP. In this article, 1-D simulations are performed with a newly developed relativistic swarm model to investigate the air effects on SGEMP. For comparison, the equilibrium model and the non-relativistic swarm model are also used in simulations. The simulation results are divided into different pressure regions for discussion, that is, the high-pressure region (I and II), the transition region, and the intermediate pressure region. In the high-pressure region, the air plasma will dramatically reduce the electric field generated by the primary current. And the electric field increases with decreasing air density in high-pressure region I, due to the decreases of the ionization source of primary electrons interacting with air molecules. Then the electric field decreases with decreasing air density in high-pressure region II, mainly because of cascading ionization begins to dominate the generation of secondary electrons. In the transition region, the electric field is reduced to a minimal level, as the cascading rate saturates with the increase of electron energy. As air density continues to decrease, the electric field rises again and begins to oscillate in the intermediate region, where the relativistic effect becomes important. This investigation will be helpful in understanding future results of both experiment and simulation.