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.     

Contrast in the electron spectroscopic imaging mode of a TEM

The ratio of inelastic-to-elastic total cross-sections has been measured in an energy-filtering electron microscope for different elements. Formulae for the transmission of elastically and inelastically scattered electrons in part I were used to calculate the optimum conditions for a Z-ratio contrast in the electron spectroscopic imaging mode. Structure-sensitive contrast can be observed for all non-carbon atoms in biological sections when filtering with an energy loss at ΔE ～ 250 eV below the carbon K edge. Model experiments with evaporated layers of different elements on a carbon film allow measurement of the contrast increase. Filtering with the carbon plasmon loss shows a lower phase contrast than with zero-loss filtering. This can be explained by calculating contrast transfer functions for inelastically scattered electrons.     

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.     

Low voltage scanning electron microscopy

The scanning electron microscope (SEM) is usually operated with a beam voltage, V₀, in the range of 10–30 kV, even though many early workers had suggested the use of lower voltages to increase topographic contrast and to reduce specimen charging and beam damage. The chief reason for this contradiction is poor instrumental performance when V₀=1–3 kV, The problems include low source brightness, greater defocusing due to chromatic aberration greater sensitivity to stray fields, and difficulty in collecting the secondary electron signal. Responding to the needs of the semiconductor industry, which uses low V₀ to reduce beam damage, considerable efforts have been made to overcome these problems. The resulting equipment has greatly improved performance at low kV and substantially removes the practical deterrents to operation in this mode. This paper reviews the advantages of low voltage operation, recent progress in instrumentation and describes a prototype instrument designed and built for optimum performance at 1 kV. Other limitations to high resolution topographic imaging such as surface contamination, the de-localized nature of the inelastic scattering event and radiation damage are also discussed.     

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.     

Quantitative analysis of backscattered-electron contrast in scanning electron microscopy

Backscattered-electron scanning electron microscopy (BSE-SEM) imaging is a valuable technique for materials characterisation because it provides information about the homogeneity of the material in the analysed specimen and is therefore an important technique in modern electron microscopy. However, the information contained in BSE-SEM images is up to now rarely quantitatively evaluated. The main challenge of quantitative BSE-SEM imaging is to relate the measured BSE intensity to the backscattering coefficient η and the (average) atomic number Z to derive chemical information from the BSE-SEM image. We propose a quantitative BSE-SEM method, which is based on the comparison of Monte–Carlo (MC) simulated and measured BSE intensities acquired from wedge-shaped electron-transparent specimens with known thickness profile. The new method also includes measures to improve and validate the agreement of the MC simulations with experimental data. Two different challenging samples (ZnS/Zn(O_xS_1–x)/ZnO/Si-multilayer and PTB7/PC₇₁BM-multilayer systems) are quantitatively analysed, which demonstrates the validity of the proposed method and emphasises the importance of realistic MC simulations for quantitative BSE-SEM analysis. Moreover, MC simulations can be used to optimise the imaging parameters (electron energy, detection-angle range) in advance to avoid tedious experimental trial and error optimisation. Under optimised imaging conditions pre-determined by MC simulations, the BSE-SEM technique is capable of distinguishing materials with small composition differences.     

On the benefit of the negative-spherical-aberration imaging technique for quantitative HRTEM

Employing an aberration corrector in a high-resolution transmission electron microscope, the spherical aberration C_S can be tuned to negative values, resulting in a novel imaging technique, which is called the negative C_S imaging (NCSI) technique. The image contrast obtained with the NCSI technique is compared quantitatively with the image contrast formed with the traditional positive C_S imaging (PCSI) technique. For the case of thin objects negative C_S images are superior to positive C_S images concerning the magnitude of the obtained contrast, which is due to constructive rather than destructive superposition of fundamental contrast contributions. As a consequence, the image signal obtained with a negative spherical aberration is significantly more robust against noise caused by amorphous surface layers, resulting in a measurement precision of atomic positions which is by a factor of 2–3 better at an identical noise level. The quantitative comparison of the two alternative C_S-corrected imaging modes shows that the NCSI mode yields significantly more precise results in quantitative high-resolution transmission electron microscopy of thin objects than the traditional PCSI mode.     

Charge compensation by in-situ heating for insulating ceramics in scanning electron microscope

With a steady temperature increase under high vacuum (HV) in an environmental scanning electronic microscope, we observed charge-free characterization and fine secondary electron (SE) images in focus for insulating ceramics (alumina (Al₂O₃), aluminum nitride (AlN), pure magnesium silicate (Mg₂SiO₄)). The sample current I_sc increased from −8.18×10⁻¹³ to 2.76×10⁻⁷ A for Al₂O₃ and −9.28×10⁻¹² to 2.77×10⁻⁶ A for AlN with the temperature increased from 298 to 633 K. The surface conductance σ increased from 5.6×10⁻¹³ to 5.0×10⁻¹¹/Ω for Al₂O₃ and 1.1×10⁻¹² to 1.0×10⁻⁷/Ω for AlN with the temperature increased from 363 to 593 K. The SE image contrast obtained via heating approach in high vacuum with an Everhart–Thornley SE-detector was better than that via conventional approach of electron–ion neutralization in low vacuum (LV) with a gaseous SE-detector. The differences of compensation temperatures for charge effects indicate dielectric and thermal properties, and band structures of insulators. The charge compensation mechanisms of heating approach mainly relate to accelerated release of trapped electrons on insulating surface and to increase of electron emission yield by heating.     

Transient analysis of gaseous electron-ion recombination in the environmental scanning electron microscope

Most of the work carried out in relation to contrast mechanisms and signal formation in an environmental scanning electron microscope has yet to consider the time dependent aspects of image generation at a quantitative level. This paper quantitatively describes gaseous electron‐ion recombination (also known as ‘signal scavenging’) in an environmental scanning electron microscope at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector images of alumina (Al₂O₃) immediately after a region of enhanced secondary electron emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron‐ion recombination that takes into consideration transients caused by the time constant of the gaseous secondary electron detector electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks are then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients and (iii) electron drift velocities, as well as absolute values of the total time constant of the gaseous secondary electron detection system and external circuitry, to be determined as a function of microscope operating parameters such as gaseous secondary electron detector bias, sample‐electrode separation, imaging gas pressure, and scan speed. The results revealed, for the first time, the exact dependence that the effects of secondary electron‐ion recombination on signal formation has on reduced electric field and time in an environmental scanning electron microscope. Furthermore, the model implicitly demonstrated that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought.     

Electron-specimen interactions in low-voltage scanning electron microscopy

Measurements of the electron range R, and the backscattering coefficient η and the secondary electron yield δ at normal and tilted incidence for different elements show characteristic differences for electron energies in the range of 0.5 to 5 keV, compared with energies larger than 5 keV. The backscattering coefficient does not increase monotonically with increasing atomic number; for example, the secondary electron yield shows a lesser increase with increasing tilt angle. This can be confirmed in back-scattered electron (BSE) and secondary electron (SE) micrographs of test specimens. The results are in rather good agreement with Monte Carlo simulations using elastic Mott cross-sections and a continuous-slowing-down model with a Rao Sahib-Wittry approach for the stopping power at low electron energies. Therefore, this method can be used to calculate quantities of BSE and SE emission, which need a larger experimental effort. Calculations of the angular distribution of BSEs show an increasing intensity with increasing atomic number at high takeoff angles than expected from a cosine law that describes the angular characteristics at high electron energies. When simulating the energy distribution of BSEs, the continuous-slowing-down model should be substituted by using an electron energy-loss spectrum (EELS) that considers plasmon losses and inner-shell ionizations individually (single-scattering-function model). The EELS can be approached via the theory for aluminium or from EELS spectra recorded in a transmission electron microscope for other elements. Measurements of electron range Rα Eⁿ of 1 to 10 keV electrons are obtained from transmission experiments with thin films of known mass thickness. In agreement with other authors the exponent n is lower than at higher electron energies.     

A new experimental procedure to quantify annular dark field images in scanning transmission electron microscopy

A new procedure to quantify the contrast in annular dark field images recorded without lattice resolution in a scanning transmission electron microscope is proposed. The method relies on the use of an in‐column energy filter prior to the annular dark field detector and the acquisition of a series of energy‐filtered images as a function of the inner detection angle. When the image contrast of an interface between two materials in such energy‐filtered annular dark field images is plotted vs. camera length and extrapolated to zero (i.e. infinite scattering angle), the contrast is shown to behave exactly as predicted by Rutherford's scattering formula (i.e. intensity scales ∝Z²). This can then be used to determine the local chemistry at and the effective chemical widths of interfaces or thin films without any additional spectroscopy method for calibration, provided the global chemical composition is known. As examples, the systems SiGe/Si and InGaAs/Ge are considered in detail.     

A systematic investigation of the charging effect in scanning electron microscopy for metal nanostructures on insulating substrates

Scanning electron microscopy is perhaps the most important method for investigating and characterizing nanostructures. A well‐known challenge in scanning electron microscopy is the investigation of insulating materials. As insulating materials do not provide a path to ground they accumulate charge, evident as image drift and image distortions. In previous work, we have seen that sample charging in arrays of metal nanoparticles on glass substrates leads to a shrinkage effect, resulting in a measurement error in the nanoparticle dimension of up to 15% at 10 kV and a probe current of 80 ± 10 pA. In order to investigate this effect in detail, we have fabricated metal nanostructures on insulating borosilicate glass using electron beam lithography. Electron beam lithography allows us to tailor the design of our metal nanostructures and the area coverage. The measurements are carried out using two commonly available secondary electron detectors in scanning electron microscopes, namely, an InLens‐ and an Everhart–Thornley detector. We identify and discriminate several contributions to the effect by varying microscope settings, including the size of the aperture, the beam current, the working distance and the acceleration voltage. We image metal nanostructures of various sizes and geometries, investigating the influence of scan‐direction of the electron beam and secondary electron detector used for imaging. The relative measurement error, which we measure as high as 20% for some settings, is found to depend on the acceleration voltage and the type of secondary electron detector used for imaging. In particular, the Everhart–Thornley detectors lower sensitivity to SE₁ electrons increase the magnitude of the shrinkage of up to 10% relative to the InLens measurements. Finally, a method for estimating charge balance in insulating samples is presented.     

Reduction of edge penetration effect in the scanning electron microscope

Electron penetration into the sample in the scanning electron microscope can give rise to bright fringes close to sharp edges in the secondary electron image. This can make it difficult to see details close to the edge. These fringes can be considerably reduced by putting a positive control electrode (CE) between the specimen and the collector. This can be mounted with an insulated clip directly onto the specimen stub. The effectiveness of this technique is demonstrated for the case of a cleaved silicon wafer containing microelectronic structures. The action of the CE is explained in terms of the effective solid angle subtended by the collector at the surface of the specimen.     

Scanning reflection electron microscopy of surface topography by diffusely scattered electrons in the scanning electron microscope

In this paper, the technique of scanning reflection electron microscopy (SREM) by diffusely scattered electrons in the scanning electron microscope is described in detail. A qualitative account of the formation of image contrast in SREM is also described. It is assumed that, for grazing geometry, forward-scattered electrons reflect from regions close to the surface, following a few scattering events within the first few atomic layers, and lose very little energy in the process. The penetration depth of the primary electrons is very limited, resulting in strongly peaked envelopes of forward-scattered electrons. It is also assumed that a surface containing topographic features presents a range of tilt angles, resulting in different reflection coefficients. Tilt contrast results because each facet has a different scattering yield, which is dependent upon local surface inclination. Full details of the instrumentation designed for SREM are described, and to illustrate the technique, results recorded from an epitaxial GaAs on GaAs crystal, Pb₂(Zr,Ta)O₆ thin film on silicon, and SiO₂ amorphous film on silicon are presented.     

Are electron tweezers possible?

Positively answering the question in the title, we demonstrate in this work single electron beam trapping and steering of 20–300 nm solid Al nanoparticles generated inside opaque submicron-sized molten Al–Si eutectic alloy spheres. Imaging of solid nanoparticles and liquid alloy in real time was performed using energy filtering in an analytical transmission electron microscope (TEM). Energy-filtering TEM combined with valence electron energy-loss spectroscopy enabled us to investigate in situ nanoscale transformations of the internal structure, temperature dependence of plasmon losses, and local electronic and optical properties under melting and crystallization of individual binary alloy particles. For particles below 20 nm in size, enhanced vibrations of the dynamic solid–liquid interface due to instabilities near the critical threshold were observed just before melting. The obtained results indicate that focused electron beams can act as a tool for manipulation of metal nanoparticles by transferring linear and angular mechanical momenta. Such thermally assisted electron tweezers can be utilized for touchless manipulation and processing of individual nano-objects and potentially for fabrication of assembled nanodevices with atomic level sensitivity and lateral resolution provided by modern electron optical systems. This is by three orders of magnitude better than for light microscopy utilized in conventional optical tweezers. New research directions and potential applications of trapping and tracking of nano-objects by focused electron beams are outlined.     

Dynamic secondary electron contrast effects in liquid systems studied by environmental scanning electron microscopy

We report an investigation into a dynamic contrast phenomenon in water-oil emulsions imaged in the environmental scanning electron microscope. Secondary electron contrast between oil and water phases is shown to change with scan rate, even inverting in extreme cases. This effect is attributed to the fact that charge carriers in liquids have intermediate mobilities compared with those in metallic conductors and solid insulators. Thus, increasing the electron energy flux density (via slower scan rates) results in the temporary accumulation of excess charge, which in turn gives rise to increased secondary electron emission. Excess charge dissipates between frames, however, such that classical charging of the specimen is not observed. The oils used here have conductivities lower than that of water, making them more susceptible to the effect. However, the material within the primary electron interaction volume is a conductive medium. We demonstrate that charging effects are not seen in regions of the oil where the interaction volume is in contact with the more conductive continuous water phase. Secondary electron emission from these regions therefore approximates to the intrinsic yield.     

Standards for quantification of elements in the otolithic membrane by electron probe X-ray microanalysis: Calibration curves and electron beam sensitivity

An absolute quantitative standardization technique has been developed to measure Ca and K weight fractions (WF) in the otolithic membrane of the saccule and utricle by scanning electron microscopy and electron probe X-ray analysis using the peak-to-background (P/B) ratio method. Microcrystalline salt standards were used to calibrate Ca and K Kα P/B or Y = (P/B) · Z²/A (Z = atomic number; A = atomic weight) against WF at 10, 15, 20 and 25 kV accelerating voltage. The effect of voltage on the calibration, plotting the coefficient of correlation (r) as a function of voltage, was not dependent on the voltage in the range 10–25 kV for Ca standards. K standards were also independent when P/B was corrected for Z²/A. Background counts in the otoconia (B_o) were obtained at 5, 25, 50, 100, 200 and 500 s and used to test the electron beam sensitivity of saccular and utricular otoconia. B_o was not dependent on the spectra acquisition time, with the exception of B_o under Kα K peak in the saccule at 10 kV. Ca and K WF were determined at 10, 15, 20 and 25 kV in the saccule and utricle, showing similar values regardless of the voltage used. This method of calibration offers several advantages, such as stability, homogeneity, known composition of the standards, high reproducibility at different voltages even without Z²/A correction and the similarity between the otoconia and crystal standards. We recommend the application of this method for other elements and biomineral systems.     

Transmission electron microscopy characterization of the erbium silicide formation process using a Pt/Er stack on a silicon-on-insulator substrate

Very thin erbium silicide layers have been used as source and drain contacts to n‐type Si in low Schottky barrier MOSFETs on silicon‐on‐insulator substrates. Erbium silicide is formed by a solid‐state reaction between the metal and silicon during annealing. The influence of annealing temperature (450 °C, 525 °C and 600 °C) on the formation of an erbium silicide layer in the Pt/Er/Si/SiO₂/Si structure was analysed by means of cross‐sectional transmission electron microscopy. The Si grains/interlayer formed at the interface and the presence of Si grains within the Er‐related layer constitute proof that Si reacts with Er in the presence of a Pt top layer in the temperature range 450–600 °C. The process of silicide formation in the Pt/Er/Si structure differs from that in the Er/Si structure. At 600 °C, the Pt top layer vanishes and a (Pt–Er)Si_x system is formed.     

EBIC microscopy of double-heterostructure laser materials and devices

The electron beam induced current (EBIC) mode of the scanning electron microscope (SEM) has been used to characterize double heterostructure laser materials and devices in GaAs/Ga_1–xAl_xAs. Scanning the electron probe across the cleaved face of the laser structure shows that displacement of the p-n junction with respect to the heterojunctions is not uncommon with displacements ～ 1 μm occurring. Concurrent measurement of the minority carrier diffusion length gives very short lengths of 0·3–0·4 μm, differing from those in much thicker layers. Scanning the electron probe in the contact plane indicates clearly that long-lived lasers exhibit marked heterogeneity during degradation. Considerable complexity and variation is recorded depending upon the fabrication details and degradation conditions adopted.     

Boundary Lubrication Properties of Surface Modified Material Produced by Laser and Electron Beam Surface Alloying

The surface-modified material can be produced by laser and electron beam surface alloying of a plasma-sprayed molybdenum layer on Cr-Mo steel. The boundary lubrication properties and the metal structure of the surface alloy have been investigated. The main points of this study are as follows. 1. The surface alloy is thick, about 5 mm, and of uniform composition.

2. The surface alloy shows good wear resistance and good seizure resistance, in the same level as electroplated chromium and plasma-sprayed molybdenum layers.

3. Good wear resistance is attributable to the metal structure which is composed of hard M₆C phase and soft α?Fe phase.