Effect Of Secondary Electrons Research Articles

Evaluation of Major Factors Affecting Spatial Resolution of Gamma-Rays Camera

Gamma-rays camera is mainly used in image diagnostics of intense pulse radiation sources. The spatial resolution of the camera was measured on a 60Co gamma-rays source with edge method. The spatial resolution MTF (modulation transfer function) at the 10% intensity was about 2 lp/mm and the maximal single-noise ratio (SNR) of the camera was found to be about 5:1. In addition, the spatial resolution of the camera was measured with pulse X-rays with 0.3 MeV in average energy and exclusion of effects of secondary electrons from consideration. Accordingly, the spatial resolution MTF at the 10% intensity was about 5lp/mm, verifying effects of secondary electrons induced by 1.25MeV gamma-rays in the scintillator upon the spatial resolution. Based on our analysis, dispersion sizes of secondary electrons in the scintillator are about 0.4 mm∼0.6 mm. Comparatively, as indicated by detailed analysis of spatial resolutions of the MCP image intensifier and CCD devices, both have little effect on the camera's spatial resolution and could be well neglected.

Secondary electron effect on electron beam induced charging of SiO2 particle analyzed by electron holography.

Charging of a SiO2 particle induced by electron illumination was investigated by changing the illuminated area of the particle and its support film through control of the position of the mask plate inserted in a transmission electron microscope illumination system. The electric fields around the charged SiO2 particle were analyzed using electron holography. The amount of charge was evaluated quantitatively by comparing the reconstructed phase images with the simulated phase images. When the support film was not covered against the incident electron beam, secondary electrons emitted from the conductive support film were attracted to the charged particle, resulting in particle discharge. In contrast, when the support film was completely covered, secondary electrons were not emitted from the film, so that the particle remained positively charged.

The development and applications of <italic>in situ</italic> liquid chamber TEM technologies

Transmission electron microscopy (TEM), with the high spatial and temporal resolution, is widely used in nano material characterizations. Due to the vacuum compatibility issue, traditional TEM is mostly used for solid and dry samples. The progress of nano science involves a wide range of liquid environments, and the development of in situ liquid chamber TEM technologies has made it possible for real time dynamic analysis on samples in liquid environments, with nanometer scale imaging resolutions, and greatly expanded the application ranges of TEM. This field has now brought much attention internationally, and is currently growing with a very rapid speed. In this paper, we review the progress about in situ liquid chamber TEM that our group has participated in and/or carried out in the recent years. It includes the development of the in situ liquid chamber TEM experiment apparatus, and its applications in the fields of nano-material preparations and characterizations, electrochemical reaction processes, new energy materials and devices, and life sciences. At last, the future development trend of in situ liquid chamber TEM is prospected. Firstly, some in situ liquid chamber TEM experiment apparatuses have been developed. These in situ chambers are home-made, based on metal cages and O-ring seals, which allow silicon nitride window substrates to be quickly assembled together, with liquid samples sealed inside. Besides simple imaging, electrochemical and heating-cooling functions have been incorporated into the setups, allowing for more advanced analyses. Using these in situ TEM apparatus, several nano material and reaction systems have been monitored. A sliver particle aqueous solution sample has been characterized. The particles were observed to be about 5–18 nm, attached to the Si3N4 windows, and could not be able to move freely in the liquid. Fine structures within single particles could be clearly seen. Bubbles commonly exist in an in situ liquid chamber, which are important for effectively reducing the local liquid layer thickness and enhancing the imaging resolution. In Au particle aqueous solution sample, particle and particle cluster motions have been further observed. The results show that particles in a cluster gradually fused together with time. For Au/AgCl composite nanocube sample, more dramatic electron beam effect has been seen, and the nanocubes were transformed quickly into hollow structures. By means of a liquid phase electron beam induced deposition (LP-EBID) approach, Si and SiC x nano structures have been made with in situ liquid chamber TEM. Nano dots, nano lines and three-dimensional nano structures can be controllably made by adjusting the electron beam exposure conditions. Secondary electron effects are attributed to the nano structural growth mechanism. Choosing Ni/NiCl2 system, both electrochemical deposition and etching of the nickel nano films have been observed by applying electric biases of different polarities to the nickel electrodes in the in situ chamber. Instead of extensional growth on existing crystals, it is more commonly observed that new nickel nano particles grow out in front of the existing film first and then merge into the film. During electrochemical etching, holes form in the Ni films due to Rayleigh instability. The structures of E. coli cells in aqueous solutions have been studied. Most of the cells remaine intact under the electron beam irradiation, Pili motions have been observed in the TEM imaging. Fluorescent imaging data confirm that the cells can keep alive after long time electron bean irradiation during the experiment. In situ liquid chamber TEM has opened up a new way for conducting research in nano science, new energy materials and devices, life sciences, involving liquid environments. In the future, it is expected that advanced functions of the in situ liquid chamber TEM will be further developed, and the application scope will be further expanded.

Single event upset characteristics and physical mechanism for nanometric SOI SRAM induced by space energetic ions

Based on Monte-Carlo method, the characteristics and physical mechanisms for deposited-energy spectra in sensitive volume (SV), single event upset cross sections, and on-orbit error rates in 65-32 nm silicon-on-insulator static random access memory (SOI SRAM) devices induced by space energetic ions are investigated. Space ions on geostationary earth orbit exhibit a flux peak at an energy point of about 200 MeV/n. In consequence, the single event response of nanometric SOI SRAMs under 200 MeV/n heavy ions is studied in detail. The results show that 200 MeV/n space ions exhibit the large straggling of deposited-energy in the device SV with thickness ranging from 60 nm to 40 nm, which causes the single event upsets to occur in the sub-LETmth region. The device SV can only partially collect the electron-hole pairs in the single ion track with a wide distribution of secondary electrons. As a result, the maximum and average deposited-energy in the SV decrease by 25% and 33.3%, respectively. Further, the single event upset probability decreases and the on-orbit error rate decreases by about 80%. With the downscaling of feature size, the per-bit saturated cross sections and on-orbit error rates of nanometric SOI SRAM devices decrease dramatically. The phenomenon of constant-increasing single event upset cross section with higher ion linear energy transfer (LET) is not observed, owing to the fact that (a) the density of electron-hole pairs in the track of 200 MeV/n space ion is relatively low and (b) the SOI device has thin sensitive volume, which results in the fact that the secondary-electron effect cannot upset nearby sensitive cells. Besides, it is found that the direct-ionization process of trapped protons leads to an increase of on-orbit error rate of 65 nm SOI SRAM by one to two orders of magnitude.

Ionization of molecular hydrogen and stripping of oxygen atoms and ions in collisions of Oq++H2 ([formula omitted]): Data for secondary electron production from ion precipitation at Jupiter

Energetic oxygen and sulfur ion precipitation into the atmosphere of Jupiter is thought to produce an X-ray aurora as well as to contribute to ionization, heating, and dissociation of the molecules of the atmosphere. At high energy, stripping of electrons from these ions by atmospheric gas molecules results in the production of high charge states throughout a portion of this passage through the atmosphere. Therefore, to enable modeling of the effects of secondary electrons produced by this ion precipitation, from either the solar wind or magnetospheric sources such as the Galilean moons, a large range of ionization and stripping data is calculated and tabulated here that otherwise is not available. The present data are for the abundant precipitating species, oxygen, colliding with the dominant upper atmosphere gas, molecular hydrogen, and cover the principal reaction channels leading to secondary electron production (single and double ionization, transfer ionization, and double capture followed by autoionization, and single and double stripping of electrons from the projectile). Since the ions possess initial energies at the upper atmosphere in the keV to MeV range, and are then slowed as they pass through the atmosphere, results are calculated for 1–2000 keV/u Oq++H2 (q=0–8). In addition to the total cross sections for ionization and stripping processes, models require the distribution in energy and angle of the ejected electrons, so cross sections differential in these parameters are also calculated. The data may be used to model the energy deposited by ion precipitation in Jupiter’s atmosphere and thereby contribute to the elucidation of the ionosphere–atmosphere coupling.

Computational Investigation of the Dissociative Recombination of Adenine, Guanine, Thymine, and Cytosine

Dissociative recombination (DR) is a process during which a molecular cation captures an electron and subsequently dissociates into neutral particles. DR is one of the very efficient processes of molecular cation annihilation and of neutral particles production in dilute environment. The DR of small molecules such as H2+ and H3+ were thoroughly investigated during the past three decades. Recently, the cations of interest have been expanded from small inorganic molecules to macromolecules (Phys. Chem. Chem. Phys., 2010,12, 11670-11673). On the other side, DR in dense environment had been neglected, due perhaps to the challenge that its experimental as well as theoretical investigation represent. However, there are so many important applications of DR in non-dilute environment. For example, DNA could lose an electron due to an ionizing radiation, which, at the same time, generated lots of low-energy electrons. The effects of secondary electrons on molecules of biological interests are currently a hot topic. It is now accepted that the attachment of slow electron on a neutral DNA may induce a strand break via dissociative electron attachments (see Radiat. Res.157, 227, 2002). Because these secondary electrons are slow, they could also be easily captured by the DNA+ which dissociative recombination may lead to a single strand and double strand breaks. As a consequence, the DNA chain break would later result in a mutation amongst the nucleotide bases. We studied the DR of temporary DNA+ using the modern electronic structure program ORCA. Given the periodic structure of DNA, we were doing quantum calculations on Guanine, Adenine, Cytosine, and Thymine only. Our results indicate the existence of autoionizing states within the various nucleotide bases and support the possibility of DR to being a channel of DNA strand break.

The role of low-energy electrons in focused electron beam induced deposition: four case studies of representative precursors.

Focused electron beam induced deposition (FEBID) is a single-step, direct-write nanofabrication technique capable of writing three-dimensional metal-containing nanoscale structures on surfaces using electron-induced reactions of organometallic precursors. Currently FEBID is, however, limited in resolution due to deposition outside the area of the primary electron beam and in metal purity due to incomplete precursor decomposition. Both limitations are likely in part caused by reactions of precursor molecules with low-energy (<100 eV) secondary electrons generated by interactions of the primary beam with the substrate. These low-energy electrons are abundant both inside and outside the area of the primary electron beam and are associated with reactions causing incomplete ligand dissociation from FEBID precursors. As it is not possible to directly study the effects of secondary electrons in situ in FEBID, other means must be used to elucidate their role. In this context, gas phase studies can obtain well-resolved information on low-energy electron-induced reactions with FEBID precursors by studying isolated molecules interacting with single electrons of well-defined energy. In contrast, ultra-high vacuum surface studies on adsorbed precursor molecules can provide information on surface speciation and identify species desorbing from a substrate during electron irradiation under conditions more representative of FEBID. Comparing gas phase and surface science studies allows for insight into the primary deposition mechanisms for individual precursors; ideally, this information can be used to design future FEBID precursors and optimize deposition conditions. In this review, we give a summary of different low-energy electron-induced fragmentation processes that can be initiated by the secondary electrons generated in FEBID, specifically, dissociative electron attachment, dissociative ionization, neutral dissociation, and dipolar dissociation, emphasizing the different nature and energy dependence of each process. We then explore the value of studying these processes through comparative gas phase and surface studies for four commonly-used FEBID precursors: MeCpPtMe3, Pt(PF3)4, Co(CO)3NO, and W(CO)6. Through these case studies, it is evident that this combination of studies can provide valuable insight into potential mechanisms governing deposit formation in FEBID. Although further experiments and new approaches are needed, these studies are an important stepping-stone toward better understanding the fundamental physics behind the deposition process and establishing design criteria for optimized FEBID precursors.

Dust in Plasma II. Effects of Secondary Electrons: Ionization and Surface Emission

AbstractIn this second paper, the effect of secondary electrons on the charge and potential of a dust particle immersed in plasma has been studied. The processes of electron‐induced ionization and those of photo‐electron and secondary electron emission from the particle surface as a function of primary electron temperature have been taken into account. Starting from temperatures as low as 6 eV in an Ar plasma, ionization produces an extra ion flux to the dust surface comparable to that of the ion charge exchange effect. For what concerns the surface emission, results show that a transition from negative to positive dust charge/potential takes place, and that the transition regime is characterized by a non‐monotonic behavior of the electric potential around the particle. In the case of photoelectric emission, the dust charge and potential are monotonic decreasing functions of the electron temperature, while in the case of emission induced by primary electrons a minimum charge/potential is reached before they grow towards positive values. In no case multiple dust charge states have been observed due to the presence of the potential well attached to the particle surface. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-precision determination of ¹³C-¹⁸O bonds in CO₂ using multicollector peak hopping.

Rapid development of the CO2 'clumped-isotope' temperature proxy (Δ47 ) has involved both resource- and labor-intensive analytical methods. We report strategies for measuring Δ47 on a conventional mass spectrometer, with the usual CO2 set of three Faraday collectors while reducing the time devoted to standardization with temperature-equilibrated CO2. We measured Δ47 in CO2 on a mass spectrometer using only three Faraday cups by 'multicollector peak hopping', directing (46)CO2(+) and (47)CO2(+) ion beams into the Faraday cups normally used for m/z 44 and 45. Δ47 was calculated from the previously measured δ(13) C and δ(18) O values and we then measured (47)CO2(+)/(46)CO2(+). We invoke a protocol for correcting ion-beam intensities for secondary electrons to address common problems with compositional non-linearity and both long- and short-term measurement stability. Our method eliminates the need for six Faraday cups for measuring Δ47, a possible advantage for many laboratories. In addition, by using a correction based on removing the effects of secondary electrons, we remove the Δ47 dependence on δ(47)CO2. As a result, our methods are robust against mass spectrometer operating conditions. Δ47 can be precisely measured on mass spectrometers currently employed for measuring δ(13) C and δ(18) O values without significant resource investment. Accounting for ion scatter during measurements of CO2 results in improved external precision and long-term stability, while significantly increasing the machine time available for sample analysis.

Heating mechanism in direct current superposed single-frequency and dual-frequency capacitively coupled plasmas

In this work particle-in-cell/Monte Carlo collision simulations are performed to study the heating mechanism and plasma characteristics in direct current (dc) superposed radio-frequency (RF) capacitively coupled plasmas, operated both in single-frequency (SF) and dual-frequency (DF) regimes. An RF (60/2 MHz) source is applied on the bottom electrode to sustain the discharge, and a dc source is fixed on the top electrode. The heating mechanism appears to be very different in dc superposed SF and DF discharges. When only a single source of 60 MHz is applied, the plasma bulk region is reduced by the dc source, thus the ionization rate and hence the electron density decrease with rising dc voltage. However, when a DF source of 60 and 2 MHz is applied, the electron density can increase upon addition of a dc voltage, depending on the gap length and applied dc voltage. This is explained from the spatiotemporal ionization rates in the DF discharge. In fact, a completely different behavior is observed for the ionization rate in the two half-periods of the LF source. In the first LF half-period, the situation resembles the dc superposed SF discharge, and the reduced plasma bulk region due to the negative dc bias results in a very small effective discharge area and a low ionization rate. On the other hand, in the second half-period, the negative dc bias is to some extent counteracted by the LF voltage, and the sheath close to the dc electrode becomes particularly thin. Consequently, the amplitude of the high-frequency sheath oscillations at the top electrode is largely enhanced, while the LF sheath at the bottom electrode is in its expanding phase and can thus well confine the high-energy electrons. Therefore, the ionization rate increases considerably in this second LF half-period. Furthermore, in addition to the comparison between SF and DF discharges and the effect of gap length and dc voltage, the effect of secondary electrons is examined.

The effect of secondary electrons on emission

The effect of secondary electrons on emission is studied by modelling the electrons behaviours in multi-layers, including electron injection, transportation, multiplication, and emission. The dielectric constant model and carrier mobility model are presented to describe the voltage distribution in multi-layers for the non-current injection and current injection respectively. After injection, the electrons are accelerated in SiO2, where they collide with the electrons, generating secondary electrons, consequently contributing to emission. A multiplying factor M is introduced to describe the secondary electrons multiplication in certain electrical field strength. The prediction was further proved by comparing two groups of devices with and without the accelerating layer: ITO/MEH-PPV/SiO2/Al and ITO/MEH-PPV/BCP/Al. The current avalanche observed in current–illumination experiment is a proof of the existence and contribution of secondary electrons.

Evaluation of Major Factors Affecting Spatial Resolution of Gamma-Rays Camera

The spatial resolution of the gamma-rays camera was measured on a60Co gamma-rays source with edge method. The gamma-rays camera is consisting with rays-fluorescence convertor, optical imaging system, MCP image intensifier, CCD camera, electronic control system and other devices, and is mainly used in the image diagnostics of the intense pulse radiation sources [1]. Due to the relatively big quantum detective efficiency (DQE) and quantum gain of the gamma-rays, etc., the experimental data were processed by averaging multiple images and fitting curves. According to the experimental results, the spatial resolution MTF (modulation transfer function) at the 10% intensity was about 2lp/mm. Meanwhile, because of the relatively big dispersion effects of the fluorescence transmissions in the scintillator and the optical imaging system, the maximal single-noise ratio (SNR) of the camera was found to be about 5:1. In addition, the spatial resolution of the camera was measured with pulse X-rays with 0.3MeV in average energy and exclusion of the effects of secondary electrons from consideration. Accordingly, the spatial resolution MTF at the 10% intensity was about 5lp/mm. This could be an additional evidence to verify the effects of secondary electrons induced by the 1.25MeV gamma-rays in the scintillator upon the spatial resolution. Based on our analysis, the dispersion sizes of the secondary electrons in the scintillator are about 0.4mm-0.6mm. Comparatively, as indicated by the detailed analysis of the spatial resolutions of the MCP image intensifier and CCD devices, both of them have little effect on the spatial resolution of the gamma-rays camera that could be well neglected.

Influence of secondary electrons in high-energy electron beam lithography

The effect of secondary electrons in electron beam lithography is investigated based on a technique called point exposure distribution measurements. This technique involves printing a series of dots at different exposure doses using a high contrast electron beam resist. The experimental results indicate that the secondary electrons are the most likely cause for exposure events. The secondary electrons generated due to forward scattering also limit the highest achievable resolution. An analytical model is developed here based on the physics governing secondary electron generation and transport. The model is able to predict the dependence of dose on the observed diameter to within a reasonable accuracy. The experimental results were verified with the model at both 100 and 50 keV.

Fundamental investigations of capacitive radio frequency plasmas: simulations and experiments

Capacitive radio frequency (RF) discharge plasmas have been serving hi-tech industry (e.g. chip and solar cell manufacturing, realization of biocompatible surfaces) for several years. Nonetheless, their complex modes of operation are not fully understood and represent topics of high interest. The understanding of these phenomena is aided by modern diagnostic techniques and computer simulations. From the industrial point of view the control of ion properties is of particular interest; possibilities of independent control of the ion flux and the ion energy have been utilized via excitation of the discharges with multiple frequencies. ‘Classical’ dual-frequency (DF) discharges (where two significantly different driving frequencies are used), as well as discharges driven by a base frequency and its higher harmonic(s) have been analyzed thoroughly. It has been recognized that the second solution results in an electrically induced asymmetry (electrical asymmetry effect), which provides the basis for the control of the mean ion energy. This paper reviews recent advances on studies of the different electron heating mechanisms, on the possibilities of the separate control of ion energy and ion flux in DF discharges, on the effects of secondary electrons, as well as on the non-linear behavior (self-generated resonant current oscillations) of capacitive RF plasmas. The work is based on a synergistic approach of theoretical modeling, experiments and kinetic simulations based on the particle-in-cell approach.

Characteristics of a Sheath with Secondary Electron Emission in the Double Walls of a Hall Thruster

In order to investigate the effects of secondary electrons, which are emitted from the wall, on the performance of a thruster, a one-dimensional fluid model of the plasma sheath in double walls is applied to study the characteristics of a magnetized sheath. The effects of secondary electron emission (SEE) coefficients and trapping coefficients, as well as magnetic field, on the structure of the plasma sheath are investigated. The results show that sheath potential and wall potential rise with the increment of SEE coefficient and trapping coefficient which results in a reduced sheath thickness. In addition, magnetic field strength will influence the sheath potential distributions.

Prototype of the particle fluence measurement device

A method of measuring particle fluence and ion beam current in experiments of ion implantation is presented. The design and technical details of a realized prototype device are discussed. Special care is devoted to the effect of secondary electrons and its elimination. An electrostatic solution with two electrodes for the suppression of secondary electrons has been realized. Our approach provides redundancy and allows lower bias voltages to be used for the suppression of electrons. The presented results illustrate the efficiency of the proposed solution.

플라즈마 삽입전극의 전류에 미치는 밀도 구배의 영향

Department of Nuclear Engineering, Seoul National University, Seoul 151-724, Korea(Received January 3, 2011; Revised February 3, 2011; Accepted May 5, 2011)Abstract: The conducting current of non-uniform plasma immersed electrode consists of ion current and secondary electron emission current caused by the impinging ion current. The ion current is determined by the ion dose passing through the sheath in front of electrode and the ion distribution in front of the electrode plays an important role in the secondary electron emission. The investigation of the distributed plasma and secondary electron effect on electrode ion current was carried out as the stainless steel electrode plugged with quartz tube was immersed in the inductively coupled Ar plasma using the antenna powered by 1 kw and the density profile was measured. After that, the negative voltage was applied by 1 kV∼6 kV to measure the conduction current for the analysis of ion current. Keywords: Secondary electron emission coefficient, Conducting current, Ion current density, Plasma density distribution, Non-uniform plasma

Secondary electrons in dual-frequency capacitive radio frequency discharges

Two fundamentally different types of dual-frequency (DF) capacitively coupled radio frequency discharges can be used for plasma processing applications to realize separate control of the ion mean energy, ⟨Ei⟩, and the ion flux, Γi, at the substrate surface: (i) classical discharges operated at substantially different frequencies, where the low- and high-frequency voltage amplitudes, ϕlf and ϕhf, are used to control ⟨Ei⟩ and Γi, respectively; (ii) electrically asymmetric (EA) discharges operated at a fundamental frequency and its second harmonic with fixed, but adjustable phase shift between the driving frequencies, θ. In EA discharges the voltage amplitudes are used to control Γi and θ is used to control ⟨Ei⟩. Here, we report our systematic simulation studies of the effect of secondary electrons on the ionization dynamics and the quality of this separate control in both discharge types in argon at different gas pressures. We focus on the effect of the control parameter for ⟨Ei⟩ on Γi for different secondary yields, γ. We find a dramatic effect of tuning ϕlf in classical DF discharges, which is caused by a transition from α- to γ-mode induced by changing ϕlf. In EA discharges we find that no such mode transition is induced by changing θ within the parameter range studied here and, consequently, Γi remains nearly constant as a function of θ. Thus, despite some limitations at high values of γ the quality of the separate control of ion energy and flux is generally better in EA discharges compared with classical DF discharges.

The effect of secondary electrons on the separate control of ion energy and flux in dual-frequency capacitively coupled radio frequency discharges

Dual-frequency capacitive discharges are used to separately control the mean ion energy, ε¯ion, and flux, Γion, at the electrodes. We study the effect of secondary electrons on this separate control in argon discharges driven at 2+27 MHz at different pressures using Particle in Cell simulations. For secondary yield γ≈0, Γion decreases as a function of the low frequency voltage amplitude due to the frequency coupling, while it increases at high γ due to the effective multiplication of secondary electrons inside the sheaths. Therefore, separate control is strongly limited. ε¯ion increases with γ, which might allow an in situ determination of γ-coefficients.

Electron-induced interaction of selected hydrocarbons withTiO2 surfaces: the relevance to extreme ultraviolet lithography

The aim of this work is to characterize desorption inducedby electronic transition processes that affect the reflectivity ofTiO2-capped multilayer mirrors used in extreme ultraviolet (EUV) lithography. A low energyelectron beam is employed to mimic excitations initiated by EUV radiation. Temperatureprogrammed desorption, x-ray photoelectron spectroscopy, and low energy ionscattering are used to analyze the surface reactions. Carbon film growth on theTiO2(011) crystalline surface is measured during 10–100 eV electron bombardment inbenzene or methyl methacrylate vapor over a wide range of pressures andtemperatures near 300 K. Low energy secondary electrons excited by EUVphotons contribute substantially to the carbon accumulation on cleanTiO2 cap layers. Forbenzene on clean TiO2, secondary electron effects dominate in the initial stages of carbon accumulation, whereas for C-coveredTiO2, direct excitations appear to dominate. We report on the adsorption energy, thesteady-state coverage of the molecules on the surface and the cross sections forelectron-stimulated dissociation: all key parameters for understanding and modeling theprocesses relating to the EUV lithography mirrors.