High Energy Primary Electrons Research Articles

Etching of Si3N4 induced by electron beam plasma from hollow cathode plasma in a downstream reactive environment

An etching system based on the interaction of electrons extracted from a direct current hollow cathode (HC) Ar plasma and injected toward an Si3N4 covered silicon substrate located in the downstream reactive environment created by an additional remote CF4/O2 plasma source was developed and evaluated. By controlling the properties of the injected beam electrons, this approach allows to deliver energy to a surface functionalized by exposure to reactive species and initiate surface etching. The energy of the primary beam electrons is controlled by the acceleration voltage relative to the HC discharge. Ar atoms flow from the high-pressure HC discharge into the low pressure downstream reactive environment in the process chamber. For an acceleration voltage greater than the ionization potential of Ar and/or process gas species, the energetic primary beam electrons produce a secondary plasma in the process chamber and can also cause additional dissociation. The authors have characterized the properties of the secondary plasma and also surface etching of Si3N4 as a function of process parameters, including acceleration voltage (0–80 V), discharge current of the HC discharge (1–2 A), pressure (3.5–20 mTorr), source to substrate distance (1.5–5 cm), and feed gas composition (20% and 80% O2 in CF4/O2). The electron energy probability function measured with a Langmuir probe about 2.5 cm below the extraction ring suggests several major groups of electrons for this situation, including high energy primary beam electrons with an energy that varies as the acceleration voltage is changed and low-energy electrons produced by beam electron-induced ionization of the Ar gas in the process chamber. When a remote CF4/O2 plasma is additionally coupled to the process chamber, Si3N4 surfaces can be functionalized, and by varying the energy of the beam electrons, Si3N4 etching can be induced by electron-neutral synergy effect with plasma-surface interaction. For conditions without beam electron injection, the remote plasma etching rate of Si3N4 depends strongly on the O2 concentration in the CF4/O2 processing gas mixture and can be suppressed for O2-rich process conditions by the formation of an SiONF passivation layer on the Si3N4 surface. The combination of the HC electron beam (HCEB) source with the remote plasma source makes it possible to induce Si3N4 etching for O2-rich remote plasma conditions where remote plasma by itself produces negligible Si3N4 etching. The electron enhanced etching of Si3N4 depends strongly on the O2/CF4 mixing ratio reflecting changing arrival rates of O and F species at the surface. Optical emission spectroscopy was used to estimate the ratio of gas phase F and O densities and found to be controlled by the gas mixing ratio and independent of HCEB operating conditions. At this time, the detailed sequence of events operative in the etching mechanism is unclear. While the increase of the electron energy is ultimately responsible for initiating surface etching, presently, the authors cannot rule out a role of ions from the simultaneously produced secondary plasma in plasma-surface interaction mechanisms.

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.

Relationship Between Secondary-Electron Yield and Structure of Polycrystalline Diamond Prepared Under Different Methane Concentrations

To understand the mechanism of electron transport and escape to vacuum of polycrystalline chemical-vapor-deposited diamond films prepared under different methane (CH4) concentrations, the secondary electron yield (SEY) δ as a function of primary electron (PE) energy Ep has been investigated. The δ–Ep curves exhibited different features for films synthesized under different CH4 concentrations, and the highest SEY was obtained when the CH4 concentration was 2%. A physical model was used to compute the key parameters of escape depth λs and surface factor f0·As. The results indicated that λs is closely related to the crystal quality of the diamond film, with diamond of high quality having larger λs, while the surface factor f0·As is mainly determined by the surface morphology, which is associated with the surface roughness of the film. Using the above model, the SEY as a function of varying λs and f0·As was also calculated; the results suggested that f0·As plays a key role in determining SEY for low-energy PEs, especially for energies < 500 eV, while the SEY is affected by both λs and f0·As for high-energy PEs.

Miniature ion thruster ring-cusp discharge performance and behavior

Miniature ion thrusters are an attractive option for a wide range of space missions due to their low power levels and high specific impulse. Thrusters using ring-cusp plasma discharges promise the highest performance, but are still limited by the challenges of efficiently maintaining a plasma discharge at such small scales (typically 1–3 cm diameter). This effort significantly advances the understanding of miniature-scale plasma discharges by comparing the performance and xenon plasma confinement behavior for 3-ring, 4-ring, and 5-ring cusp by using the 3 cm Miniature Xenon Ion thruster as a modifiable platform. By measuring and comparing the plasma and electron energy distribution maps throughout the discharge, we find that miniature ring-cusp plasma behavior is dominated by the high magnetic fields from the cusps; this can lead to high loss rates of high-energy primary electrons to the anode walls. However, the primary electron confinement was shown to considerably improve by imposing an axial magnetic field or by using cathode terminating cusps, which led to increases in the discharge efficiency of up to 50%. Even though these design modifications still present some challenges, they show promise to bypassing what were previously seen as inherent limitations to ring-cusp discharge efficiency at miniature scales.

Spatial dependent diffusion of cosmic rays and the excess of primary electrons derived from high precision measurements by AMS-02**Supported by Natural Sciences Foundation of China (11135010)

The precise spectra of Cosmic Ray (CR) electrons and positrons have been published by the measurement of AMS-02. It is reasonable to regard the difference between the electron and positron spectra (ΔΦ = Φe− −Φe+) as being dominated by primary electrons. The resulting electron spectrum shows no sign of spectral softening above 20 GeV, which is in contrast with the prediction of the standard model of CR propagation. In this work, we generalize the analytic one-dimensional two-halo model of diffusion to a three-dimensional realistic calculation by implementing spatial variant diffusion coefficients in the DRAGON package. As a result, we can reproduce the spectral hardening of protons observed by several experiments, and predict an excess of high energy primary electrons which agrees with the measurement reasonably well. Unlike the break spectrum obtained for protons, the model calculation predicts a smooth electron excess and thus slightly over-predicts the flux from tens of GeV to 100 GeV. To understand this issue, further experimental and theoretical studies are necessary.

Secondary Electron Emission from Cylindrical Particles

The secondary electron emission (SEE) caused by high energy electrons, incident obliquely on a cylindrical particle has been derived by considering appropriate paths traversed by the primary electron in the particle into account. Expressions for the SEE efficiency corresponding to mono-energetic as well as Maxwellian energy distribution of primary electrons have been derived. Further the dependence of this efficiency on the penetration depth (xm), angle of incidence (λ), particle radius (a) and the mean energy (kTe) of the incident primary electrons has been analyzed and graphically illustrated. The analysis reflects that the SEE is dominant for small cylindrical particles and high energy primary electrons. The yield is more pronounced in the case of spherical particles than that in the case of cylindrical particles of the same radius. The study is of relevance to space and laboratory dusty plasmas where cylindrical particles are frequent.

Enhancing secondary yield of a diamond amplifier using a nitrogen layer

A thin nitrogen-doped layer less than 4% of the total thickness in diamond thin film amplifier is shown to reduce losses of generated secondaries to the back contact, generated by a high energy primary electron beam compared to a thin film without the substitutional nitrogen layer modification. Simulation indicates that the losses due to absorption of diffusing electrons by the back contact may be considerably reduced by a factor of 2 (depending on field across the film, width of the nitrogen layer, and boron doping level), thereby mitigating undesirable effects associated with trace amounts of boron.

Controlling the physicochemical state of carbon on graphene using focused electron-beam-induced deposition.

Focused electron-beam-induced deposition (FEBID) is a promising nanolithography technique using "direct-write" patterning by carbon line and dot deposits on graphene. Understanding interactions between deposited carbon molecules and graphene enables highly localized modification of graphene properties, which is foundational to the FEBID utility as a nanopatterning tool. In this study, we demonstrate a unique possibility to induce dramatically different adsorption states of FEBID-produced carbon deposits on graphene, through density functional theory calculations and complementary Raman experiments. Specifically, an amorphous carbon deposit formed by direct irradiation of high energy primary electrons exhibits unusually strong interactions with graphene via covalent bonding, whereas the FEBID carbon formed due to low-energy secondary electrons is only weakly interacting with graphene via physisorption. These observations not only are of fundamental importance to basic physical chemistry of FEBID carbon-graphene interactions but also enable the use of selective laser-assisted postdeposition ablation to effectively remove the parasitically deposited, physisorbed carbon films for improving FEBID patterning resolution.

Two contributions to the ratio of the mean secondary electron generation of backscattered electrons to primary electrons at high electron energy

Based on the main physical processes of secondary electron emission, experimental results and the characteristics of backscattered electrons (BE), the formula was derived for describing the ratio (βangle) of the number of secondary electrons excited by the larger average angle of emission BE to the number of secondary electrons excited by the primary electrons of normal incidence. This ratio was compared to the similar ratio β obtained in the case of high energy primary electrons. According to the derived formula for βangleand the two reasons why β &gt; 1, the formula describing the ratio βenergyof β to βangle, reflecting the effect that the mean energy of the BE WAV p0is smaller than the energy of the primary electrons at the surface, was derived. βangleand βenergycomputed using the experimental results and the deduced formulae for βangleand βenergywere analyzed. It is concluded that βangleis not dependent on atomic number z, and that βenergydecreases slowly with z. On the basis of the two reasons why β &gt; 1, the definitions of β and βenergyand the number of secondary electrons released per primary electron, the formula for βE-energy(the estimated βenergy) was deduced. The βE-energycomputed using WAV p0, energy exponent and the formula for βE-energyis in a good agreement with βenergycomputed using the experimental results and the deduced formula for βenergy. Finally, it is concluded that the deduced formulae for βangleand βenergycan be used to estimate βangleand βenergy, and that the factor that WAV p0increases slowly with atomic number z leads to the results that βenergydecreases slowly with z and β decreases slowly with z.

A flange on electron spectromicroscope with spherical deflector analyzer—simultaneous imaging of reciprocal and real spaces

An instrumental realization of the idea for the electron emission spectromicroscope based on the newly developed imaging energy filter called α-SDA (Spherical Deflector Analyzer) is reported. Its compact design enables the realization of the flange-on spectromicroscope concept. It is equipped with two independent energy selective imaging channels: one for real and another for reciprocal space visualization. These images can be acquired quasi-simultaneousely by means of the software based on the switching on and off potentials of the energy filter. An electron gun located inside the immersion objective lens allows a new kind of sample illumination by high energy primary electrons and thus, opens a new application field for electron spectromicroscopy under laboratory conditions.

Effects of fibre-form nanostructures on particle emissions from a tungsten surface in plasmas

The effects of fibre-form nanostructure of a tungsten surface on both electron emission and sputtering in helium/argon plasmas are represented. Generally, a nano-fibre forest, the so-called ‘fuzz’, made of tungsten with helium gas inside is found to have the tendency of suppressing the particle emission substantially. The electron emission comes from the impact of high-energy primary electrons. In addition, a deeply biased tungsten target, which inhibits the influx of even energetic primary electrons, seems to produce an electron emission, and it may be suppressed on the way to nanostructure formation on the surface of the W target. Such an emission process is discussed here. The sputtering yield of the He-damaged tungsten surface with the fibre-form nanostructure depends on the surface morphology while the sputtering itself changes the surface morphology, so that the time evolutions of sputtering yield from the W surface with an originally well-developed nanostructure are found to show a minimum in sputtering yield, which is about a half for the fresh nanostructured tungsten and roughly one-fifth of the yield for the original flat normal tungsten surface. The surface morphology at that time is, for the first time, made clear with field emission scanning electron microscopy observation. The physical mechanism for the appearance of such a minimum in sputtering yield is discussed.

Performance of the low pressure MWPCs for fission fragments under a high background

Two couples of low pressure multi-wire proportional chambers (MWPC) were located in the target chamber to detect fission fragments in a hypernuclei producing experiment at Thomas Jefferson National Laboratory (Jlab). In the experiment, a continuous wave (CW) electron beam was applied to form hypernuclei by electromagnetic interaction. In the target chamber, the high energy (1.853 GeV) and high intensity (500 nA) primary electron beam caused a high particle background, which influenced the detection of the fission fragments. This report described the design of the MWPCs and studied the fission-fragment detecting performance of them under such a high background. The efficiency of the MWPCs was given with the help of a high resolution kaon spectrometer. At the same time, the background particles were discussed with a Monte Carlo code based on GEANT4.

10.1007/s11451-008-1031-2

The processes accompanying the formation of ytterbium films on the Si(111) surface at room temperature are investigated by the contact potential difference method, Auger electron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. It is shown that the grown metal films are uniform in thickness and that Si atoms virtually do not dissolve in the films. The atoms of the silicon substrate can diffuse in limited amounts into the Yb metal film only when the surface is bombarded by high-energy primary electron beams employed in Auger electron spectroscopy. The results obtained permit the conclusion that the previously observed oscillations of the work function in Yb-Si(111) thin-film structures cannot originate from dissolution of silicon atoms in the ytterbium film.

Specific features of the formation of ytterbium films on the Si(111) surface at room temperature

The processes accompanying the formation of ytterbium films on the Si(111) surface at room temperature are investigated by the contact potential difference method, Auger electron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. It is shown that the grown metal films are uniform in thickness and that Si atoms virtually do not dissolve in the films. The atoms of the silicon substrate can diffuse in limited amounts into the Yb metal film only when the surface is bombarded by high-energy primary electron beams employed in Auger electron spectroscopy. The results obtained permit the conclusion that the previously observed oscillations of the work function in Yb-Si(111) thin-film structures cannot originate from dissolution of silicon atoms in the ytterbium film.

Resolving electrons from protons in ATIC

The Advanced Thin Ionization Calorimeter (ATIC) experiment is designed for high energy cosmic ray ion detection. The possibility to identify high energy primary cosmic ray electrons in the presence of the ‘background’ of cosmic ray protons has been studied by simulating nuclear-electromagnetic cascade showers using the FLUKA Monte Carlo simulation code. The ATIC design, consisting of a graphite target and an energy detection device, a totally active calorimeter built up of 2.5 cm × 2.5 cm × 25.0 cm BGO scintillator bars, gives sufficient information to distinguish electrons from protons. While identifying about 80% of electrons as such, only about 2 in 10,000 protons (@ 150 GeV) will mimic electrons. In September of 1999 ATIC was exposed to high-energy electron and proton beams at the CERN H2 beam line, and this data confirmed the electron detection capabilities of ATIC. From 2000-12-28 to 2001-01-13 ATIC was flown as a long duration balloon test flight from McMurdo, Antarctica, recording over 360 h of data and allowing electron separation to be confirmed in the flight data. In addition, ATIC electron detection capabilities can be checked by atmospheric gamma-ray observations.

Transport issues in focused electron beam chemical vapor deposition

In this contribution we critically review heat and mass transfer issues in the Focused Electron Beam Chemical Vapor Deposition (FEB-CVD). In general, the transport of both the precursor molecules and the primary/secondary electrons facilitate nanostructure deposition. Depending on the operating pressure either continuous advection–diffusion mass conservation equation or the kinetic Boltzmann Transport Equation (BTE) describes the transport of precursor molecules to the substrate surface. At the surface, some of the precursor molecules are adsorbed, spatially re-distributed by surface diffusion, and, finally, a fraction of the adsorbed molecules become converted into a solid deposit. This occurs upon interaction with back-scattered primary electrons and secondary electrons, yielded by the substrate and deposit upon impingement of the high-energy primary electron beam. The interactions of the primary electrons with the substrate and nanoscale-confined deposit possibly induce significant localized heating. Such energy transfer process is complex, involves non-classical heat conduction, and may greatly influence the deposition process. The pertinent question is then what controls the FEB-CVD process, i.e., both time dependent growth of the nanostructure and its shape evolution? The answer to this question can be obtained via complimentary theoretical and experimental studies, discussed here with the main focus on transport phenomena underlying FEB-CVD.

Optimization of the performance of CVD diamond electron multipliers

Aiming at the production of image-enhancing devices (electron multipliers) to amplify the backscattered electron signal in scanning electron microscopy, a study of the stationary secondary electron emission from CVD diamond films, irradiated by a high-energy primary electron beam, was performed in order to deepen the understanding of the specific material characteristics that appear to control the electronic emission. An extensive analysis of the secondary electron emission yield δ, as a function of the primary electron beam energy, was performed on polycrystalline CVD samples of different thickness. The emission yield was observed to increase up to a maximum value corresponding to a film thickness of about 15 μm and to decrease at larger thickness values. The experimental results are analyzed according to a theoretical model previously developed [P. Ascarelli, E. Cappelli, F. Pinzari, M.C. Rossi, S. Salvatori, P.G. Merli, A. Migliori, J. Appl. Phys. 89 (2001) 689] and extended within this work. We realized that the most important material parameter controlling the secondary electron emission yield as a function of the primary electron energy is the mean escape depth λ S of the secondary electrons. The obtained λ S values were found to be adequate to explain the film emission performance dependence on its thickness d. Moreover, using the atomic force microscopy technique and developing an algorithm based on a three-points method to identify the mean orientation of crystal facets in space, we observed that the λ S( d) (and consequently the secondary electron emission performance) evolution during the growth process may be largely explained by a variation of the film surface mean orientation and a concomitant variation of the film quality.

A new surface analysis by soft X-ray excited by low energy electron beam

A novel spectroscopy, low energy electron excited soft X-ray spectroscopy (LEEXS) has been demonstrated in surface analysis of Si(100) crystal surfaces. The adoption of glancing angle as the incident angle as well as take-off angle makes it possible to highly detect the X-ray signal from surface top layers. In addition to the surface sensitivity, the use of the low-energy primary electron (600 eV to 2.6 keV) enables a (nondestructive) detection of light elements on the surface with its high sensitivity that has ever been impossible for similar X-ray spectroscopy with high-energy primary electrons. [DOI: 10.1380/ejssnt.2006.443]

Current-voltage relationship in the auroral particle acceleration region

Abstract. The current-voltage relationship in the auroral particle acceleration region has been studied statistically by the Akebono (EXOS-D) satellite in terms of the charge carriers of the upward field-aligned current. The Akebono satellite often observed field-aligned currents which were significantly larger than the model value predicted by Knight (1973). We compared the upward field-aligned current estimated by three different methods, and found that low-energy electrons often play an important role as additional current carriers, together with the high-energy primary electrons which are expected from Knight's relation. Such additional currents have been observed especially at high and middle altitudes of the particle acceleration region. Some particular features of electron distribution functions, such as "cylindrical distribution functions" and "electron conics", have often been observed coinciding with the additional currents. They indicated time variability of the particle acceleration region. Therefore, we have concluded that the low-energy electrons within the "forbidden" region of electron phase space in the stationary model often contribute to charge carriers of the current because of the rapid time variability of the particle acceleration region. "Cylindrical distribution functions" are expected to be found below the time-varying potential difference. We statistically examined the locations of "cylindrical distribution function", and found that their altitudes are related to the location where the additional currents have been observed. This result is consistent with the idea that the low-energy electrons can also carry significant current when the acceleration region changes in time.

Imaging the Probe Skirt in the Environmental SEM

Abstract In low vacuum scanning electron microscopes, the primary beam is partially scattered by the gas present in the specimen chamber. The development of these microscopes, in particular the so-called ‘Environmental’ SEM, was initiated when it was realized that this scattering does not necessarily compromise the imaging capabilities of the instrument. Indeed, some modern commercial instruments are capable of better than 2 nanometer resolution at gas pressures of several torr. The accepted explanation for this is as follows: The mean-free-path of the high energy primary electrons is several millimeters in one torr of water vapour (for example). Because the actual pathlength of electrons travelling through the gas is only a few millimeters, most of them do not scatter at all. Those that do scatter are supposedly distributed over a relatively large area. Thus, the probe features a high-intensity central region surrounded by a slowly decaying low-intensity skirt. High resolution imaging is possible because the signal-to-(skirt)background ratio is high.