Evaluation of the effective cross‐sections for recombination and trapping in the case of pure spinel

In this paper, we have investigated the evolution of the secondary electron emission in the case of pure spinel during electron irradiation, achieved in a scanning electron microscope at room temperature, which is derived from the measurement of the induced and the secondary electron currents. It was observed from the experimental results, that there are two regimes during the charging process: a plateau followed by a linear variation, which are better identified by plotting the logarithm of the secondary electron emission yield lnσ as function of the total surface density of trapped charges in the material Q_T. For positive charging, E₀ = 1.1 and 5 keV, the slope of the linear part, whose value is of about 10^?10 cm² charge^?1, is independent of the primary electron energy. It is interpreted as a microscopic cross section for electron–hole recombination. For negative charging of pure spinel, E₀ = 15 and 30 keV, the slope is associated with an electron trapping cross section close to 10^?14 cm² charge^?1, which can be assigned to the microscopic cross section for electron trapping. This trapping cross section is four orders of magnitude lower than the recombination one.     

Control of charging in low-voltage SEM

Charging of the specimen under electron beam irradiation is a common problem in scanning electron microscopy (SEM). It results in unstable imaging conditions and a loss in resolution due to defocus of the beam. In addition, it can cause permanent changes in some specimens from translocation of mobile ions under the influence of the induced electrostatic field. To minimize charging and its associated problems, the incident beam energy must be carefully chosen to be the value E₂ at which a dynamic charge balance is obtained. This article presents data on E₂ values for a variety of materials and demonstrates how E₂ is affected by the choice of angle of beam incidence.     

Charging in scanning electron microscopy "from inside and outside"

This paper is an attempt to analyse most of the complicated mechanisms involved in charging and discharging of insulators investigated by scanning electron microscopy (SEM). Fundamental concepts on the secondary electron emission (SEE) yield from insulators combined with electrostatics arguments permit to reconsider, first, the widespread opinion following which charging is minimised when the incident beam energy E0 is chosen to be equal to the critical energy E(o)2, where the nominal total yield delta(o) + eta(o) = 1. For bare insulators submitted to a defocused irradiation, it is suggested here that the critical energy under permanent irradiation EC2 corresponds to a range of primary electrons, R, and nearly equals the maximum escape depth of the secondary electrons, r. This suggestion is supported by a comparison between published data of the SEE yield delta(o) of insulators (short pulse experiments) and experimental results obtained from a permanent irradiation for EC2. New SEE effects are also predicted at the early beginning of irradiation when finely focused probes are used. Practical considerations are also developed, with specific attention given to the role of a contamination layer where a negative charging may occur at any beam energy. The role of the various time constants involved in charging and discharging is also investigated, with special attention given to the dielectric time constant, which explains the dose rate-dependent effects on the effective landing energy in the steady state. Numerical applications permit to give orders of magnitude of various effects, and several other practical consequences are deduced and illustrated. Some new mechanisms for the contrast reversal during irradiation or with the change of the primary electron (PE) energy are also suggested.     

Elastic peak electron spectroscopy

Elastic peak electron spectroscopy (EPES) determines the elastic peak intensity N (E_p) in absolute units (percentage). The experimental curve N (E_p) is affected by the type of electron spectrometer used (CMA, RFA, etc.) and represents the probability P_e (E, Z) of elastic electron reflection detected by the angular window of the spectrometer. P_e (E, Z) = N_Aσ_eff (E, Z) Δ is the product of the inelastic mean free path Δ, the density N_A of atoms and the effective elastic scattering cross-section σ_eff (E, Z). Elementary processes of elastic scattering are treated with a simplified model based on a single scattering process. A more refined analysis will be to use the Monte-Carlo approach. σ_eff is given by the differential elastic scattering cross-section determined by the atomic number Z, the energy E_p, and the input and scattering angles. Results of σ_eff (E, Z) are presented based on tabulated differential cross-section data published in the literature. Angular effects are discussed. A number of EPES applications are described for AES, plasmon-, ionization-, and low energy loss spectroscopies and SEM. EPES has been successful in the experimental determination of the inelastic mean free path Δ for a number of elements and compounds.     

Contrast in the transmission mode of a low-voltage scanning electron microscope

The contrast thicknesses (x_k) of thin carbon and platinum films have been measured in the transmission mode of a low-voltage scanning electron microscope for apertures of 40 and 100 mrad and electron energies (E) between 1 and 30 keV. The measured values overlap with those previously measured for E (≥ 17keV) in a transmission electron microscope. Differences in the decrease of x_k with decreasing E between carbon and platinum agree with Wentzel-Kramer-Brillouin calculations of the elastic cross-sections. Knowing the value of x_k allows the exponential decrease ∝ exp(—x/x_k) in transmission with increasing mass-thickness (x = ρt) of the specimen and the increasing gain of contrast for stained biological sections with decreasing electron energy to be calculated for brightfield and darkfield modes.     

Analysis of electron range versus energy relationship of insulators in low-voltage scanning electron microscopy

A theoretical analysis is made concerning the question of whether or not an insulator's surface is observable without the unstable disturbance due to negative charge-up in the secondary electron mode of a low-voltage scanning electron microscope. Introducing a simple modification into the elementary theory of secondary electron emission from solid materials, the threshold condition as to observability is formulated as a function of the energy E_p and the incident angle ?_p of the primary beam. It is shown that for insulators the material's constant n, which appears in the standard formula of the electron range (R) versus energy (E_p) relationship R Φ E, can be determined through experimental investigations into observability of the surface. Careful consideration is also given to the effectiveness of the present theoretical analysis.     

Recent developments and new strategies in scanning electron microscopy*

In addition to improvements in lateral resolution in scanning electron microscopy, recent developments of interest here concern extension of the incident beam energy, E₀, over two decades, from ≈ 20 keV to ≈ 0.1–0.5 keV and the possibility of changing the take-off emission, α, of detected secondary electrons. These two degrees of freedom for image acquisition permit a series of images of the same field of view of a specimen to be obtained, each image of the series differing from the others in some aspect. The origins of these differences are explored in detail and they are tentatively interpreted in terms of the change in the secondary electron emission yield δ vs. E₀, δ = f(E₀), and also of the change in δ vs. α, ∂δ/∂α. Various origins for the chemical contrast and topographic contrast have been identified. Illustrated by correlating a secondary electron image and a backscattered electron image, use of the scatter diagram technique facilitates image comparison. The difference between the lateral resolution and the size of the minimum detectable detail is outlined to avoid possible errors in nanometrology. Some aspects related to charging are also considered and possible causes of contrast reversal are suggested. Finally, the suggested strategy consists of the acquisition of various images of a given specimen by changing one parameter: primary beam energy and take-off angle for conductive specimens; working distance or beam intensity for high-resolution experiments; scanning frequency for insulating specimens.     

Material removal mechanism of Ni(200) when eroded by a slurry at 30° incidence

Ni(200) samples were eroded under steady state conditions by a slurry consisting of 30 wt.% 88–150 μm glass beads in 1 M NaClO₄ at 30° incidence angle and 30 m s^?1 velocity. Detailed optical and scanning electron microscopy examination was performed on the eroded surfaces and metallographic sections. A wavy surface topography formed by plastic deformation was observed. At the downstream sides of the wavecrests thin tongues are formed when the plastically deformed surface layer is gradually pushed over the crest of the waves. Material is removed when the tongues or fractions of them are torn off. Hence the material removal mechanism is a process of plastic flow and tearing of tongues.     

A spectroscopic study of some high-temperature superconductors using a low-temperature STM

We used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high-transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu₂O_x show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa₂Cu₃O_6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa₂Cu₃O_6·5+x and Y_0·5Al_0·05Ba₂Cu₃O_6·5+x, may indicate the presence of a lower temperature phase in these samples.     

Conditions for voltage contrast observation over passivated devices

Inspection of passivated microdevices using the scanning electron microscope (SEM) at a low primary electron energy has been increasingly studied. Previously we reported an empirical formula for conditions under which SEM observations over the passivation layer can be made without inducing serious charging disturbances. An investigation of charge balance between primary and secondary electrons was presented. The present work investigates the charging disturbance-free conditions of PSG, SiN, and Si₃N₄ passivated devices and how they are affected by beam irradiation. The storage time of the voltage contrast is measured for various values of primary electron energy and incident angle.     

A new method of scanning tunneling spectroscopy for study of the energy structure of semiconductors and free electron gas in metals

In this study, anew method for the scanning tunneling spectroscopy (STS) of direct and indirect gap semiconductors and free electron gas in metals is proposed. Band structures of Si, porous Si, and Ge were studied. The tunneling current-voltage characteristics of Si and porous Si surfaces were measured over different voltage intervals from tens of mV to 20 V under incident light from an Xe lamp and those of a Ge surface in the dark. The correlation between the shapes of the I-V curves and band structure of the materials was calculated. It was found that the curves are linear if measured in the voltage range V₀ E_g/(2e) and nonlinear when V₀ α E_g/(2e) (in the measurements the applied voltage was changed from -V₀ to V₀). The method was used for the observation of a new effect of tunneling of free electron gas having thermal energies from a metal tip to a band gap state of the semiconductor. The energy spectrum of free electron gas was measured.     

Spectrometers for recording electromagnetic showers emerging from oriented crystals

The design and main characteristics of combined shower spectrometers (CSSs) are presented: combined Cherenkov and lead-scintillation spectrometers that are intended for investigating shower processes in oriented crystals and that consist of ten independent lightproof counters with thicknesses of 1X₀ and 1.18X₀, respectively. At an electron energy E _e = 26 GeV, the energy resolutions of both CSSs together with a Cherenkov spectrometer with a thickness of 15X₀, which was placed behind them, were 2.4 and 4.3% for the combined Cherenkov and lead-scintillation shower spectrometers, respectively.     

Effect of microstructure on the erosion of steel by solid particles

The effect of microstructure on the erosion of AISI-SAE 1078 and 10105 steels by 240 grit A1₂O₃ particles was investigated at particle velocities V of 40–100 m s^?1 and angles of impingement a of 10°–90° relative to the target surface. The microstructures investigated included spheroidite, pearlite, martensite and tempered martensite.Spheroidite and pearlite microstructures eroded by the ductile mode at all velocities, exhibiting a maximum erosion rate at an impingement angle of 40°. The effect of the angle of impingement on the erosion rate of martensite and tempered martensite varied with particle velocity, the erosion mode tending towards a brittle mode with increasing velocity. At all angles of impingement the erosion rate tended to increase with the volume fraction of Fe₃C. Examination of the eroded surfaces by scanning electron microscopy showed the occurrence of localized plastic flow of appreciable magnitude. No subsurface cracking or void formation was evident. The erosion rate E_r could be considered to vary with particle velocity according to the power law E_r = kVⁿ where n has a value of about 2 independent of the microstructure and the angle of impact.     

Spectrometry analysis of fumes of (U<Subscript>0.80</Subscript>Pu<Subscript>0.20</Subscript>)O<Subscript>2</Subscript> hybrid nuclear fuel samples heated to a temperature of 2000°C

The results of the spectrometry analysis of a gaseous cloud that forms over (U_0.80Pu_0.20)O₂ hybrid uranium-plutonium fuel samples for reactors, which were heated to a temperature of >2000°C, are described. Samples were heated using a laser facility designed for conducting out-of-pile experiments with the nuclear fuel. The obtained results will make it possible to empirically forecast the isotopic composition of volley emissions from the reactor core during accidents on the 1000-MW power reactor with the (U_0.80Pu_0.20)O₂ hybrid fuel.     

A novel technique for visualizing electron beam induced charging

Charging is one of the most important problems encountered in scanning electron microscopy and as a result this phenomenon it has received a lot of both theoretical and experimental attention. Despite this, many questions remain about the nature and behavior of charging because of the limitations of the experimental techniques available to study it. For example, although it is now straightforward to determine in situ the surface potential of a sample that is charging during irradiation, it is difficult to measure the lateral extent of the charging, or its persistence once the incident beam is switched off. We describe here a simple technique which provides a rapid way of visualizing the temporal and spatial behavior of charging phenomena.     

An Electromagnetic Calorimeter with a Transversal Orientation of the Scintillating Fibers

The space and energy resolutions of the SPACAL electromagnetic calorimeter with a transverse orientation of the scintillating fibers have been measured. The main parameters of the calorimeter are presented. The results of experiments with an electron test beam of the DESY synchrotron at energies E = 1–6 GeV are discussed. The measured energy resolution of the calorimeter is found to be (/E) = 12.7%/E 2.0% (E is expressed in terms of GeV), and the space resolution is _{x, y} (E) = 1 mm at E = 4 GeV.     

Effect of elastic modulus mismatch on the contact crack initiation in hard ceramic coating layer

Effect of elastic modulus mismatch on the contact crack initiation is investigated to find major parameters in designing desirable surface-coated system. Silicon nitride coated soft materials with various elastic modulus mismatch,E _c /E _s = 1.06 — 356 are prepared for the analysis. Hertzian contact test is conducted for producing contact cracks and the acoustic emission detecting technique for measuring the critical load of crack initiation. The implication is that coating thickness and material strength are controllable parameters to prevent the initiation of contact cracks resulted from the elastic modulus mismatch in the hard ceramic coating layer on the soft materials.     

Characteristics of a fast neutral atom source with electrons injected into the source through its emissive grid from the vacuum chamber

In order to increase the equivalent current of a fast neutral atom beam the cold hollow cathode of the beam source is bombarded with electrons extracted from the plasma produced in the vacuum chamber and accelerated in the sheath between the plasma emitter of the source and its emissive grid. The cold cathode bombardment by accelerated electrons raises its electron emission current by an order of magnitude and as a result voltage U _c between the anode and the cathode of the source diminishes more than two times. This allows of increasing several times the beam equivalent current or decreasing the working gas pressure. A slight decrease in the U _c with increasing the accelerating voltage U at an overall cutoff of the electrons from the chamber reveals the influence of secondary electrons emitted by the grid. Measurement of the beam current is discussed.     

High-conductivity amorphous TiSi substrates for low-temperature electron microscopy

To avoid problems with electrostatic charging, electrically conductive specimen support films or coatings must be used when observing non-conductive organic specimens by electron microscopy. At room temperature, microscopists typically use either carbon films or carbon-coated plastic films as specimen substrates. Such films have also been adopted for low-temperature microscopy. Recent measurements have shown that conventional carbon films have high electrical resistivity at low temperature. Thin titanium films have been used to provide improved electrical conductivity, but the structure of titanium films is excessively granular for high-resolution electron microscopy. With the hope of solving both problems, thin films of Ti₈₈Si₁₂ have been produced using a standard evaporation system. The deposited films are amorphous, as shown by electron microscopy, electron diffraction and differential scanning calorimetry. The electrical conductivity of the binary alloy films is nearly as high as that of pure titanium films. Therefore, these films appear to be appropriate substrates for low-temperature electron microscopy.     

Alkyloxy‐s‐triazine derivatives with and without fluorine‐containing substituents as lubricants for steel–steel contact

Three alkyloxy‐s‐triazine derivatives were synthesized and their tribological properties as lubricants for steel–steel contact were evaluated using an Optimol SRV tester at 20°C and 100°C. Their thermal stabilities were also investigated by thermogravimetric analysis. The results show that the three alkyloxy‐s‐triazine lubricants have good thermal stability. Moreover, 2,4,6‐tris(1,1,5‐tri‐H‐octafluoropentyloxy)‐1,3,5‐s‐triazine (FPOT) possesses the best anti‐wear property and good load‐carrying capacity both at 20°C and 100°C. At 20°C the anti‐wear effectiveness of 2,4,6‐tris(n‐pentyloxy)‐1,3,5‐s‐triazine (POT) is the worst, while at 100°C that of the 1,1,5‐tri‐H‐octafluoropentyloxy and/or 1,1,7‐tri‐H‐dodecafluoroheptyloxy tri‐substituted s‐triazine mixture (FMOT) is the worst. In addition, the friction‐reducing properties of the two fluoroalkyloxy‐s‐triazines, FPOT and FMOT, are not as good as those of the non‐fluorine‐containing alkyloxy‐s‐triazine POT. Scanning electron spectroscopy with an energy dispersive analyzer of X‐ray and X‐ray photoelectron spectroscopy analyses of the worn surface indicate that during the rubbing process, tribochemical reactions occur between the lubricants and the metal surface to generate a complex boundary lubrication film comprised of FeF₂, Fe(OH)₂, organofluorine and organonitrogen compounds. Copyright © 2006 John Wiley & Sons, Ltd.