Low Primary Electron Energy Research Articles

Study on line-scan profile for a trapezoid line under varying sample temperatures through Monte-Carlo simulation.

This study investigates the influence of the sample inherent temperature on the line-scan profile for a silicon trapezoid line with different sidewall angles by Monte-Carlo simulation. This study demonstrates that the profile varies with temperature, particularly focusing on the 'shoulder', which becomes more pronounced with larger sidewall angles. The contrast of the secondary electron profile increases at low primary electron energy but decreases at relatively high PE energy as the temperature rises. The trend of the backscattering electron profile is similar but less noticeable. The underlying mechanism is discussed in detail. This study has potential to provide valuable insights into thermometry in nanostructures using SEMs.

Electron Energy-Loss Spectroscopy Method for Thin-Film Thickness Calculations with a Low Incident Energy Electron Beam

In this study, the thickness of a thin film (tc) at a low primary electron energy of less than or equal to 10 keV was calculated using electron energy-loss spectroscopy. This method uses the ratio of the intensity of the transmitted background spectrum to the intensity of the transmission electrons with zero-loss energy (elastic) in the presence of an accurate average inelastic free path length (λ). The Monte Carlo model was used to simulate the interaction between the electron beam and the tested thin films. The total background of the transmitted electrons is considered to be the electron transmitting the film with an energy above 50 eV to eliminate the effect of the secondary electrons. The method was used at low primary electron energy to measure the thickness (t) of C, Si, Cr, Cu, Ag, and Au films below 12 nm. For the C and Si films, the accuracy of the thickness calculation increased as the energy of the primary electrons and thickness of the film increased. However, for heavy elements, the accuracy of the film thickness calculations increased as the primary electron energy increased and the film thickness decreased. High accuracy (with 2% uncertainty) in the measurement of C and Si thin films was observed at large thicknesses and 10 keV, where tλ≈1. However, in the case of heavy-element films, the highest accuracy (with an uncertainty below 8%) was found for thin thicknesses and 10 keV, where tλ≤0.29. The present results show that an accurate film thickness measurement can be obtained at primary electron energy equal to or less than 10 keV and a ratio of tλ≤2. This method demonstrates the potential of low-loss electron energy-loss spectroscopy in transmission electron microscopy as a fast and straightforward method for determining the thin-film thickness of the material under investigation at low primary electron energies.

Monte Carlo Simulation of Free-standing Thin Films under Low Energy Electron Bombardment: Electron Inelastic Mean Free Path (IMFP) Determination Using Elastic Peak of the Transmitted Electrons

Abstract: The electron energy spectra of transmitted scattered electrons from free-standing films are simulated using a Monte Carlo computational approach. Elastic scattering is simulated using Mott cross-sections and inelastic scattering via discrete processes determined from dielectric function data. This allows one to simulate the secondary electrons as well as the loss peaks near the elastic (zero-loss) peak. The current study suggested a directed approach for determining the electron inelastic mean free path (IMFP) of materials at low primary electron energies. The IMFP of the reference material is not necessary for the suggested technique. The suggested technique uses the ratio between the transmitted elastic peak intensity and the background intensity of backscattered electrons. Free-standing films of Si, Cu, and Au were studied with thicknesses varying from 2 to 12 nm. Primary electron energies of 1, 3, and 5 keV were applied. The results appeared very good, with the percentage error range being between 5% and 25%. We also investigated the proportion of the first and second plasmon peak intensities to the elastic peak intensity. We believe that the latter could provide a directed method of measuring the IMFP of materials. Keywords: Inelastic mean free path, Monte Carlo simulation, Geant4, Free-standing film, Zero-loss peak.

Theoretical Simulation of Backscattering Electron Coefficient for SixGe1-x/Si Heterostructure as a Function of Primary Electron Beam Energy and Ge Concentration

Abstract: This study aims to investigate the backscattering electron coefficient for SixGe1-x/Si heterostructure sample as a function of primary electron beam energy (0.25-20 keV) and Ge concentration in the alloy. The results obtained have several characteristics that are as follows: the first one is that the intensity of the backscattered signal above the alloy is mainly related to the average atomic number of the SixGe1-x alloy. The second feature is that the backscattering electron coefficient line scan shows a constant value above each layer at low primary electron energies below 5 keV. However, at 5 keV and above, a peak and a dip appeared on the line scan above Si-Ge alloy and Si, respectively, close to the interfacing line. Furthermore, the shape and height of peak and dip broadening depend on the primary electron energy and incidence position with respect to the interfacing line. The last feature is that the spatial resolution of the backscattered signal at the interfacing line is improving by decreasing the primary electron energy (below 5 keV) and the shared element (Si) concentration. On the other hand, a poor compositional contrast has been shown at low primary electron energy below 5 keV. For energies above 5 keV, the spatial resolution becomes weak. These results can be explained by the behavior of the incident electrons inside the solid (interaction volume), especially at a distance close to the interfacing line and their chance to backscatter out of the sample. In general, a good compositional contrast with a high spatial resolution can be achieved at primary electron energy equal to 1 keV. Keywords: Monte Carlo model, Backscattering electron coefficient, Si-Ge/Si, Elastic scattering, Spatial resolution, Compositional contrast.

Measurements of the quantitative analytical depth resolution at evaporated aluminium- and silver-layers with the FEG-EPMA JEOL JXA-8530F

The aim of this study is to extend our general approach to the spatial resolution in the quantitative element analysis that can be achieved with a FEG-EPMA by systematic measurements of the depth resolution. The test specimens are Al and Ag surface layers with various thicknesses on top of non-fluorescent substrates. By choosing an appropriate X-ray line and by varying the primary electron energy, E0 , we determined the minimum value of E0 for which the depth of the signal source volume is identical to the layer thickness. This value is defined as the depth resolution. With respect to the excitation energy of the characteristic X-rays, the depth resolution for the quantitative element analysis is found to be below 150 nm for Al (Kα at 3 kV) as well as for Ag (Lα at 5 kV).The prediction of the depth resolution at low primary electron energy E0 by means of calculations of the ionisation range of electrons or by Monte Carlo (MC) simulations is quite inaccurate because both predict better values compared to the measurements. Since we regard test specimens without disturbing secondary fluorescence it is obvious that both calculations and simulation predict too small signal source volumes for a so far unknown reason. The same discrepancy was observed for the determination of the lateral quantitative resolutions in previous studies. Therefore systematic measurement errors can be excluded. The measurements presented in this paper are very helpful for further improvements of calculations and MC-simulations at low primary electron energies E0 because specimens without secondary fluorescence are considered only.

Beam Broadening Measured in Transmission Mode at Low Electron Energies in a Scanning Electron Microscope

The broadening of the electron beam in the sample has to be considered when performing scanning transmission electron microscopy (STEM) at low primary electron energies. This work presents direct measurements of the beam broadening in a range of materials. The experimental results are compared with the theoretical model by Gauvin and Rudinsky that uses the concept of anomalous diffusion to obtain an analytical equation for the beam broadening.

Ti-doped MgO thin film by magnetron sputtering for cesium frequency standard

The main component of magnetically selected cesium frequency standard is electron multiplier. Its working principle is to magnify cesium beam signal by emitting secondary electron through surface of material. Magnesium oxide (MgO) has been adopted due to its excellent capability to emit secondary electron. There are many ways to prepare MgO thin film, including evaporation, pulsed laser deposition and MOCVD. It has been studied that doping can be used to enhance its emission capability and decrease its working potential of the thin film. In this paper, Ti-doped MgO thin films were deposited by magnetron sputtering method. Titanium metal and MgO ceramic were co-sputtered in oxygen containing argon ambient with DC and RF power source, respectively. XRD results show that the deposited film has the texture of (100) and (110) orientation. The ratio of Ti/Mg were consistent in XPS and EDS. SEM and AFM show that the film is polycrystalline. Electron multiplier device was assembled, and the secondary electron emission coefficient of the Ti-doped MgO film is greater than that of the undoped one, especially 36% higher at low primary electron energy of 200 eV which is meaningful to cesium atomic frequency standard.

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.

Electron beam broadening in electron-transparent samples at low electron energies.

Scanning transmission electron microscopy (STEM) at low primary electron energies has received increasing attention in recent years because knock-on damage can be avoided and high contrast for weakly scattering materials is obtained. However, the broadening of the electron beam in the sample is pronounced at low electron energies, which degrades resolution and limits the maximum specimen thickness. In this work, we have studied electron beam broadening in materials with atomic numbers Z between 10 and 32 (MgO, Si, SrTiO3 , Ge) and thicknesses up to 900 nm. Beam broadening is directly measured using a multisegmented STEM detector installed in a scanning electron microscope at electron energies between 15 and 30 keV. For experimental reasons, the electron beam diameter is defined to contain only 68% of the total intensity instead of the commonly used 90% of the total beam intensity. The measured beam diameters can be well described with calculated ones based on a recently published model by Gauvin and Rudinsky. Using the concept of anomalous diffusion the Hurst exponent H is introduced that varies between 0.5 and 1 for different scattering regimes depending on t/Λel with the specimen thickness t and the elastic mean free path length Λel . The calculations also depend on the fraction of the beam intensity that defines the electron beam diameter. A Hurst exponent H of 1 is characteristic for the ballistic scattering regime with t/Λel → 0 and can be excluded for the experimental conditions of our study with 6 ≦ t/Λel ≦ 30. We deduced H = 0.75 from measured beam diameters which is larger than H = 0.5 that is expected under diffusion conditions. The deviation towards larger H values can be rationalised by our definition of electron diameter that contains only 68% of the total beam intensity and requires therefore larger sample thicknesses before the diffusion regime is reached. Our results clearly deviate from previous analytical approaches to describe beam broadening (Goldstein etal., Reed, Williams etal., Kohl and Reimer). Measured beam diameters are compared with simulated ones, which are obtained by solving the electron transport equation. This approach is advantageous compared to the commonly used Monte Carlo simulations because it is an exact solution of the electron transport equation and requires less computer time. Simulated beam diameter agree well with the experimental data and yield H = 0.80. LAY DESCRIPTION: In scanning transmission electron microscopy (STEM), a focused electron beam is scanned over an electron-transparent sample and an image is formed by detecting the intensity of the transmitted electrons by a STEM detector. STEM resolution is ultimately limited by the electron beam diameter and can be better than 0.1nm for the best microscopes. However, the electron-beam diameter increases with increasing specimen thickness because electrons are scattered by the interaction of the specimen material and electrons. Electron scattering leads to a change of the electron propagation direction and reduces focusing of the electron beam. The associated electron-beam broadening degrades the lateral resolution of STEM and generally limits the maximum specimen thickness that can be imaged with good resolution. STEM is up to now mainly performed at high electron energies of 80 keV and above. Lower electron energies are beneficial for the study of weakly scattering and radiation-sensitive materials but electron beam broadening becomes more pronounced with decreasing electron energies. Knowledge of beam broadening is therefore particularly important for the interpretation of STEM images that are taken with low-energy electrons. In this work we have studied electron-beam broadening in different materials with thicknesses up to 900nm at low electron energies between 15 and 30keV. Beam broadening is directly measured with a newly developed technique. We compare measured beam diameters with different models on beam broadening from literature and find that only a recently published model is well suited to describe the experimental results under our experimental conditions. In addition, beam broadening is simulated by modelling electron propagation in the specimen. The simulation results agree well with the measured beam diameters.

ULV-SEM-EDX analysis of fine precipitates in Cr-Mo steel using windowless silicon-drift detector

Elemental analysis technique using a latest windowless silicon-drift detector (SDD) and ultra-low voltage scanning electron microscope (ULV-SEM) have been applied to a Cr-Mo added steel containing several types of carbides and aluminum nitride. Low primary electron energy of around 1 keV reduces interaction volume and then provides remarkably high spatial resolution for x-ray analysis. We obtain elemental mappings of the precipitates with almost the same spatial resolution as ULV-SEM imaging.

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.

A comparison on absorption coefficients for secondary electron emission obtained from two different formulas

In this work, the absorption coefficients for secondary electron emission, α and β, that appeared respectively in the two different formulas, \(\delta (E_p ) = k\int_0^\infty {\left( {\frac{{dE}} {{dz}}} \right)E_p \exp ( - \alpha z)dz} \) and \(\delta (E_p ) = k\int_0^\infty {\left( {\frac{{dE}} {{dz}}} \right)E_p \exp ( - \alpha z)dz} \), were derived with a standard deviation rate analysis method based on a Monte Carlo simulated secondary electron yield, δ(Ep). Both the energy dissipation in depth for primary electrons, \(\left( {dE/dz} \right)E_p \), and the depth distribution for the number of secondary electrons including cascade electrons, n(z, Ep), were obtained by the same Monte Carlo method, in which the discrete inelastic scattering model was employed. The calculation results for Cu and Mg show that the n(z, Ep)-curve differs significantly from the \(\left( {dE/dz} \right)E_p \)-curve, and thus as well as a from b, for varied incidence angles (0°–80°) and low-energy primary electrons (up to 3 keV). The absorption coefficient β-values derived from a realistic depth distribution of cascade secondary electrons, n(z, Ep), then describe more accurately the nature of attenuation behavior of secondary electrons than a-values that associated with the approximate formula.

DC surface flashover characteristics of polyimide in vacuum under electron beam irradiation

In order to avoid the surface discharge of sensitive components on spacecraft, it is very important to understand the mechanism of surface flashover characteristics of the insulator materials, such as polyimide (PI). This paper studied the influence of electron beam irradiation on the negative polarity dc surface flashover characteristics of PI plate specimen in vacuum. Results showed that the flashover voltage under 0.5-40 keV electron beam irradiation is enhanced compared to none irradiation situation. The flashover voltage increased at first then decreased with the increase of the electron beam energy, reaching maximum at 20 keV. And under the constant electron beam energy irradiation, the voltage decreased with the increase of the beam density. The experiments suggest that the negative space charge of cathode triple junction (CTJ), radiation-induced conductivity (RIC) and radiation degradation are important reasons for the rise of flashover voltage under various radiation electron beam energy. Besides, the low energy primary electrons play critical roles in the reduction of surface flashover voltage under different beam density.

A Monte Carlo study of reflection electron energy loss spectroscopy spectrum of a carbon contaminated surface

It has been experimentally found that the carbon surface contamination influences strongly the spectrum signals in reflection electron energy loss spectroscopy (REELS) especially at low primary electron energy. However, there is still little theoretical work dealing with the carbon contamination effect in REELS. Such a work is required to predict REELS spectrum for layered structural sample, providing an understanding of the experimental phenomena observed. In this study, we present a numerical calculation result on the spatially varying differential inelastic mean free path for a sample made of a carbon contamination layer of varied thickness on a SrTiO3 substrate. A Monte Carlo simulation model for electron interaction with a layered structural sample is built by combining this inelastic scattering cross-section with the Mott's cross-section for electron elastic scattering. The simulation results have clearly shown that the contribution of the electron energy loss from carbon surface contamination increases with decreasing primary energy due to increased individual scattering processes along trajectory parts carbon contamination layer. Comparison of the simulated spectra for different thicknesses of the carbon contamination layer and for different primary electron energies with experimental spectra clearly identifies that the carbon contamination in the measured sample was in the form of discontinuous islands other than the uniform film.

Graphene defect formation by extreme ultraviolet generated photoelectrons

We have studied the effect of photoelectrons on defect formation in graphene during extreme ultraviolet (EUV) irradiation. Assuming the major role of these low energy electrons, we have mimicked the process by using low energy primary electrons. Graphene is irradiated by an electron beam with energy lower than 80 eV. After e-beam irradiation, it is found that the D peak, I(D), appears in the Raman spectrum, indicating defect formation in graphene. The evolution of I(D)/I(G) follows the amorphization trajectory with increasing irradiation dose, indicating that graphene goes through a transformation from microcrystalline to nanocrystalline and then further to amorphous carbon. Further, irradiation of graphene with increased water partial pressure does not significantly change the Raman spectra, which suggests that, in the extremely low energy range, e-beam induced chemical reactions between residual water and graphene are not the dominant mechanism driving defect formation in graphene. Single layer graphene, partially suspended over holes was irradiated with EUV radiation. By comparing with the Raman results from e-beam irradiation, it is concluded that the photoelectrons, especially those from the valence band, contribute to defect formation in graphene during irradiation.

SEM‐EDS with low primary electron energy as a tool of surface analysis

Energy dispersive X‐ray spectroscopy (EDS) has been applied to thin Cr films on Fe and Fe–Ni alloys with the primary electron energy (EP) of scanning electron microscopy (SEM) as low as 1.5 keV. The detection depth of SEM‐EDS with an EP of 1.5 keV is estimated to be less than 20 nm for the L emission of Fe in Cr. Linear relationships were obtained between Fe–Ni alloy compositions determined by chemical analysis and those derived by a quantitative calculation from the Fe‐L and Ni‐L peaks measured with the EP from 1.5 to 15 keV. SEM‐EDS can be a quantitative analysis technique with high spatial resolution for material surfaces when combined with low EP's. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Direct Detection of Electron Backscatter Diffraction Patterns

We report the first use of direct detection for recording electron backscatter diffraction patterns. We demonstrate the following advantages of direct detection: the resolution in the patterns is such that higher order features are visible; patterns can be recorded at beam energies below those at which conventional detectors usefully operate; high precision in cross-correlation based pattern shift measurements needed for high resolution electron backscatter diffraction strain mapping can be obtained. We also show that the physics underlying direct detection is sufficiently well understood at low primary electron energies such that simulated patterns can be generated to verify our experimental data.

Extraction of topographic and material contrasts on surfaces from SEM images obtained by energy filtering detection with low-energy primary electrons

Secondary electron microscope (SEM) images have been obtained for practical materials using low primary electron energies and an in-lens type annular detector with changing negative bias voltage supplied to a grid placed in front of the detector. The kinetic-energy distribution of the detected electrons was evaluated by the gradient of the bias-energy dependence of the brightness of the images. This is divided into mainly two parts at about 500 V, high and low brightness in the low- and high-energy regions, respectively and shows difference among the surface regions having different composition and topography. The combination of the negative grid bias and the pixel-by-pixel image subtraction provides the band-pass filtered images and extracts the material and topographic information of the specimen surfaces.

Evidence for strong secondary electron emission in the tokamak scrape-off layer

A novel method to measure net secondary electron emission yield due to primary electron irradiation from a tokamak scrape-off layer plasma is described. High emission coefficients close to unity are obtained independent of the electron temperature. Enhanced elastic backscattering of low-energy primary electrons, already identified as an important phenomenon in many other fields of research, is likely to explain the high yields.

Earth magnetic field effects on the cosmic electron flux as background for Cherenkov Telescopes at low energies

Cosmic ray electrons and positrons constitute an important component of the background for imaging atmospheric Cherenkov Telescope Systems with very low energy thresholds. As the primary energy of electrons and positrons decreases, their contribution to the background trigger rate dominates over protons, at least in terms of differential rates against actual energies. After event reconstruction, this contribution might become comparable to the proton background at energies of the order of few GeV. It is well known that the flux of low energy charged particles is suppressed by the Earth’s magnetic field. This effect strongly depends on the geographical location, the direction of incidence of the charged particle and its mass. Therefore, the geomagnetic field can contribute to diminish the rate of the electrons and positrons detected by a given array of Cherenkov Telescopes.In this work we study the propagation of low energy primary electrons in the Earth’s magnetic field by using the backtracking technique. We use a more realistic geomagnetic field model than the one used in previous calculations. We consider some sites relevant for new generations of imaging atmospheric Cherenkov Telescopes. We also study in detail the case of 5@5, a proposed low energy Cherenkov Telescope array.