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Precise control of ferroelectric and multiferroic domain states at the nanoscale is of considerable interest due to the potential to boost the development of next-generation low-energy-consumption ...

Here we demonstrate the use of nanofabricated grating holograms to diffract and shape electrons in a scanning electron microscope. The diffraction grating is placed in an aperture in the column. The entire diffraction pattern can be passed through the objective lens and projected onto the specimen, or an intermediate aperture can be used to select particular diffracted beams. We discuss several techniques for characterizing the diffraction pattern. The grating designs can incorporate features that can influence the phase and intensity of the diffracted SEM probe. We demonstrate this by producing electron vortex beams.

Two-dimensional materials, such as graphene, are usually prepared by chemical vapor deposition (CVD) on selected substrates, and their transfer is completed with a supporting layer, mostly polymethyl methacrylate (PMMA). Indeed, the PMMA has to be removed precisely to obtain the predicted superior properties of graphene after the transfer process. We demonstrate a new and effective technique to achieve a polymer-free CVD graphene — by utilizing low-energy electron irradiation in a scanning low-energy electron microscope (SLEEM). The influence of electron-landing energy on cleaning efficiency and graphene quality was observed by SLEEM, Raman spectroscopy (the presence of disorder D peak) and XPS (the deconvolution of the C 1s peak). After removing the absorbed molecules and polymer residues from the graphene surface with slow electrons, the individual graphene layers can also be distinguished outside ultra-high vacuum conditions in both the reflected and transmitted modes of a scanning low-energy (transmission) electron microscope.

The transmission of electrons with energies 15 keV and 30 keV through Si and Au films of 100 nm thickness each have been studied in a Scanning Transmission Electron Microscope. The electrons that were transmitted through the films were detected using a multi-annular photo-detector consisting of a central Bright Field (BF) and several Dark Field (DF) detectors. For the experiment the detector was gradually offset from the axis and the signal from the central BF detector was studied as a function of the offset distance and compared with MC simulations. The experiment showed better agreement between experiment and several different MC simulations as compared to previous results, but differences were still found particularly for low angle scattering from Si. Data from Au suggest that high energy secondary electrons contribute to the signal on the central BF detector for low primary beam energies, when the STEM detector is in its usual central position.

Scanning electron microscopy with very slow electrons offers a novel tool enabling one to image the graphene samples at nanometer or even sub-nanometer lateral resolution in transmitted as well as reflected electrons and to count reliably the atomic layers in both imaging modes. The study was performed on graphene prepared by chemical vapor deposition on thin copper foil. Observation by slow electrons has also confirmed the underlayer mechanism of nucleation and growth bellow already existing graphene layers on copper. Moreover, electrons with impacted energy below 100 eV can be used for “cleaning” of a material. It leads to elimination of the contamination process during the measurement, which enables to observe the predicted oscillations in reflection mode and to measure the transmissivity of various stacks of layers in transmission mode down to units of eV.

Methods available for the mapping of dopants in silicon-based semiconductor structures with p-type as well as n-type doped patterns using low and very-low-energy electrons are reviewed together with the results of demonstration experiments.

Single crystalline tungsten oxides in a form of W5O14 and W18O49 nanowires were synthesized by iodine transport method. The morphology, work functions and field emission properties of these nanowires were investigated. Work functions of the W5O14 (4.20–4.34 eV) and W18O49 (4.55–4.57 eV) nanowires (NWs) have been measured by Kelvin probe force microscopy (KPFM) in ultra-high vacuum. Field emission (FE) measurements of individual nanowires were performed in ultra-high vacuum at microscopic and macroscopic distances between the emitter and electron collector. The obtained FE curves at microscopic distances were analyzed in the framework of the Fowler–Nordheim (F–N) theory. Field enhancement factors of W5O14 at the emitter-collector distance of 2, 4 and 5 μm were calculated to be 110 ± 10, 180 ± 25 and 210 ± 30, respectively, and 125 ± 15 for W18O49 at 2 μm. At macroscopic distances, the F–N theory revealed unrealistic high field enhancement factors: for W5O14 at 1 mm it was 17,000 ± 500, and for W18O49, the field enhancement factors were 5050 ± 30 and 6450 ± 30 at 600 μm and 800 μm emitter-collector distance, respectively. Therefore, more realistic model was discussed. The lower work function and typically smaller diameter of the W5O14 nanowires in comparison with the W18O49 wires, range the W5O14 nanowires to the promising sources of electrons in field emission devices.

Scanning electron microscopes come equipped with different types of detectors for the collection of signal electrons emitted from samples. In-lens detection systems mostly consist of several auxiliary electrodes that help electrons to travel in a direction towards the detector. This paper aims to show that a through-the-lens detector in a commercial electron microscope Magellan 400 FEG can, under specific conditions, work as an energy band-pass filter of secondary electrons that are excited by the primary beam electrons. The band-pass filter properties verify extensive simulations of secondary and backscattered electrons in a precision 3D model of a microscope. A unique test sample demonstrates the effects of the band-pass filter on final image and contrast with chromium and silver stripes on a silicon substrate, manufactured by a combination of e-beam lithography, wet etching, and lift-off technique. The ray tracing of signal electrons in a detector model predicate that the through-the-lens detector works as a band-pass filter of the secondary electrons with an energy window of about 3 eV. By moving the energy window along the secondary electron energy spectrum curve of the analyzed material, we select the energy of the secondary electrons to be detected. Energy filtration brings a change in contrast in the image as well as displaying details that are not otherwise visible.

Progress in materials science is inseparably connected with development of new analytical methods which make possible to observe the materials microstructure with high sensitivity. The aim of the present study is shown that scanning low energy electron microscopy (SLEEM) has a significant impact in advance of a fundamental understanding of the evolution of microstructure upon semi-solid processing. This paper deals with the application of the ultra high vacuum scanning low energy electron microscopy (UHV SLEEM) to the study of microstructure of X210Cr12 steel after the formation in semi-solid state and the study of the annealing of deformed metastable austenite. Examples from these specimens show that the contrast between differently oriented grains in polycrystalline materials is very sensitive to the parameters such as energy of the primary beam, working distance and detection of high angle backscattered electrons.

Summary Commercially available graphene samples are examined by Raman spectroscopy and very low energy scanning transmission electron microscopy. Limited lateral resolution of Raman spectroscopy may produce a Raman spectrum corresponding to a single graphene layer even for flakes that can be identified by very low energy electron microscopy as an aggregate of smaller flakes of various thicknesses. In addition to diagnostics of graphene samples at larger dimensions, their electron transmittance can also be measured at very low energies.

Summary The advent of new imaging technologies in Scanning Electron Microscopy (SEM) using low energy (0–2 keV) electrons has brought about new ways to study materials at the nanoscale. It also brings new challenges in terms of understanding electron transport at these energies. In addition, reduction in energy has brought new contrast mechanisms producing images that are sometimes difficult to interpret. This is increasing the push for simulation tools, in particular for low impact energies of electrons. The use of Monte Carlo calculations to simulate the transport of electrons in materials has been undertaken by many authors for several decades. However, inaccuracies associated with the Monte Carlo technique start to grow as the energy is reduced. This is not simply associated with inaccuracies in the knowledge of the scattering cross-sections, but is fundamental to the Monte Carlo technique itself. This is because effects due to the wave nature of the electron and the energy band structure of the target above the vacuum energy level become important and these are properties which are difficult to handle using the Monte Carlo method. In this review we briefly describe the new techniques of scanning low energy electron microscopy and then outline the problems and challenges of trying to understand and quantify the signals that are obtained. The effects of charging and spin polarised measurement are also briefly explored. SCANNING 9999:1–17, 2016. © 2016 Wiley Periodicals, Inc.

The high negative bias of a sample in a scanning electron microscope constitutes the “cathode lens” with a strong electric field just above the sample surface. This mode offers a convenient tool for controlling the landing energy of electrons down to units or even fractions of electronvolts with only slight readjustments of the column. Moreover, the field accelerates and collimates the signal electrons to earthed detectors above and below the sample, thereby assuring high collection efficiency and high amplification of the image signal. One important feature is the ability to acquire the complete emission of the backscattered electrons, including those emitted at high angles with respect to the surface normal. The cathode lens aberrations are proportional to the landing energy of electrons so the spot size becomes nearly constant throughout the full energy scale. At low energies and with their complete angular distribution acquired, the backscattered electron images offer enhanced information about crystalline and electronic structures thanks to contrast mechanisms that are otherwise unavailable. Examples from various areas of materials science are presented.

An overview of recent developments in very low energy scanning electron microscopy is presented. Electron optical aspects are briefly summarized including the low energy beam formation in a cathode lens equipped column, comparison of the sequential and overlapped electric and magnetic fields in the objective lens, and detection issues including extension to the transmitted electron mode as well as to the multichannel detection of signal sorted according to the polar angle of emission. In addition to the acquisition of contrasts specific for the very low energy range, advantages of detection of electrons backscattered to large angles from the surface normal are demonstrated on selected application examples.

The cathode lens (CL) is a frequently used electron optical element in the photoemission electron microscopy (PEEM), low energy electron microscopy (LEEM), and recently in the scanning electron microscopy (SEM) [1]. Lenc and Mullerova [2,3] studied the basic optical parameters of the CL as such, as well as in combination with a magnetic focusing lens, and derived approximate analytical expressions for the axial aberration coefficients for both cases. For the combination, the sequential arrangement of electrostatic and magnetic fields was considered.

The surface morphology of pure Mg was studied by means of the cathode lens mode equipped scanning low energy electron microscope after bombarding with Ga+ ions at various energies (10, 20, 30, and 40 keV) and incident angles (0°, 30°, 45°, and 60°). In accordance with the Bradley-Harper theory at off-normal angles of incidence ripples were observed on the irradiated areas. The Monte Carlo program SRIM2008 was used to estimate the sputtering yield and damage depth.

The use of the scanning low energy electron microscopy (SLEEM) has been slowly making its way into the field of materials science, hampered not by limitations in the technique but rather by relative scarcity of these instruments in research institutes and laboratories. This paper reports the results obtained from an investigation of the microstructure of ultra fine-grained (UFG) copper fabricated using equal channel angular pressing (ECAP) method, namely in the as-pressed state and after annealing. SLEEM is very sensitive to the perfection of crystal lattice and using SLEEM, local strain can be effectively imaged.

Study of the grain structure in the equal channel angular pressing processed copper by means of the cathode lens equipped ultrahigh vacuum scanning low energy electron microscope is reported. The grain contrast was found achieving its maximum at electron energies below about 30 eV where it alternated its sign and exhibited dependence on electron energy specific for the grain orientation. The energy dependence of the electron reflectance seemed to be capable of serving as a fingerprint enabling determination of the crystalline orientation. In the cathode lens mode at hundreds of eV fine details of the microstructure are also observable including twins and low angle grain boundaries. This is explained by acquisition of high-angle backscattered slow electrons, normally not acquired in standard scanning electron microscopes. The very low energy electron reflectance is promising as an alternative to the EBSD method owing to its high resolution and fast data acquisition.

Instrument and methodology is presented for very low energy scanning transmission electron microscopy. The detector system provides simultaneous acquisitions of total reflected and transmitted electron fluxes. Introductory experiments incorporated examination of ultrathin foils of gold, carbon and graphene flakes. Extremely sensitive thickness contrast obtained at units of eV is demonstrated. The phenomenon of electron transmissivity apparently exceeding 100% owing to the contribution of secondary electrons released near to the exit surface is described and discussed.

Collection of the secondary electrons in the scanning electron microscope was simulated and the results have been experimentally verified for two types of the objective lens and three detection systems. The aberration coefficients of both objective lenses as well as maximum axial magnetic fields in the specimen region are presented. Compared are a standard side-attached secondary electron detector, in which only weak electrostatic and nearly no magnetic field influence the signal trajectories in the specimen vicinity, and the side-attached (lower) and upper detectors in an immersion system with weak electrostatic but strong magnetic field penetrating towards the specimen. The collection efficiency was calculated for all three detection systems and several working distances. The ability of detectors to attract secondary electron trajectories for various initial azimuthal and polar angles was calculated, too. According to expectations, the lower detector of an immersion system collects no secondary electrons I and II emitted from the specimen and only backscattered electrons and secondary electrons III form the final image. The upper detector of the immersion system exhibits nearly 100% collection efficiency decreasing, however, with the working distance, but the topographical contrast is regrettably suppressed in its image. The collection efficiency of the standard detector is low for short working distances but increases with the same, preserving strong topographical contrast.

Summary The time response of a scintillator-light guide-photomultiplier combination was measured with a time-constant of 3 ns. Single detected electrons were recognizable at the output of the photomultiplier. The distribution of the number of photoelectrons produced by one detected electron and the pulse-height distribution of the photomultiplier output pulses were analysed. Statistical noise computed from these distributions was compared with the noise produced by the dark current of the photomultiplier.

By introducing a cathode lens below or inside the objective lens of a scanning electron microscope, many experiments similar to those typical of the LEEM method can be performed. The conditions for the diffraction of slow electrons are modified by the convergence of the primary beam, and challenges include the necessity of managing the signal species propagating along the optical axis in a direction opposite to that of the primary beam. However, even a simple implementation, providing the integral dark-field signal only, has not only delivered plenty of results in the very low energy range below 50 eV, but the performance in the range of hundreds of eV and units of keV has also been substantially improved. The scanning LEEM method is illustrated using experimental results acquired by additionally employing multichannel detection and detection of transmitted electrons.

In the present talk, MgAl2O4 in the Al2O3/Al-1.0mass%Mg2Si alloy composite was also observed by a scanning electron microscope equipped with the low energy electron (SLEEM) adaptation aiming at examination of its morphology and orientation relationships to the Al2O3 particles. Owing to its much smaller interaction volume of signal exciting electrons in the target and hence more localized information, together with a favorable combination of secondary (SE) and backscattered (BSE) electron signals, the SLEEM method provided much better readable and detailed images of all particles, their shapes and mutual orientations, in comparison with conventional SE and BSE images at the electron energies usually used in the SEM. MgAl2O4 (spinels) were formed on facets of Al2O3 as small particles, and their shape well corresponded to an octahedron consisting of 8 equiaxial triangles.

The cathode lens (CL) mode of the SEM, employing sample as a cathode of the beam-decelerating electrostatic lens, enables one to preserve the image resolution down to lowest electron energies and in the same time secures an excellent collection efficiency of signal species. In the range of tens and units of eV, new image contrasts become available, based on the quantum mechanical character of scattering and the electron wavelength comparable with inter-atomic distances. However, already in the low keV and hundreds of eV ranges the CL mode has proven itself very efficient in many materials science applications, overcoming some weak points the conventional SEM modes suffer from. Selected material structures are presented as demonstration examples.

New type of detection system has been designed and tested in the Scanning Electron Microscope. Backscattered electrons (BSE) at low energies of electron impact are collected by an annular detector above the specimen, consisting of eight concentric ring-shaped collectors arranged around the optical axis so that polar angles of the initial velocity vectors of electrons can be distinguished. Backscattering of electrons, in particular around and below 1 keV of impact energy, provides rich information content that even enhances toward higher angles with respect to the surface normal. Slow, high-angle BSE are normally not detected with standard detectors. The cathode lens principle was used to secure high lateral resolution, high collection efficiency and large amplification of the detector at low energies. Prospective applications include the grain orientation contrast, contrast between amorphous and crystallinic phases, etc.

Mechanisms responsible for the contrast between differently doped areas in semiconductors, which is observed in electron micrographs, is discussed as regards the key factors determining the sign and magnitude of the contrast. Experimental data obtained by means of the scanning electron microscope (SEM), scanning low energy electron microscope and photoelectron emission microscope are reviewed together with hints following from them for compilation of a model of the contrast mechanism.

A method for scanning electron microscopy imaging of nonconductive specimens, based on measurement and utilisation of a critical energy, is described in detail together with examples of its application. The critical energy, at which the total electron yield curve crosses the unit level, is estimated on the basis of measurement of the image signal development from the beginning of irradiation. This approach, concentrated onto the detected signal as the only quantity crucial for the given purpose of acquiring a noncharged micrograph, evades consequences of any changes in an irradiated specimen that influence the total electron yield curve and possibly also the critical energy value. Implementation of the automated method, realised using a cathode lens-equipped scanning electron microsope (SEM), enables one to establish a mean rate of charging over the field of view and its dependence on the electron landing energy. This dependence enables one to determine the energy of a minimum damage of the image of the given field of view. Factors influencing reliability and applicability of the method are discussed and examples of noncharged micrographs of specimens from both life and material science fields are presented.

The cube-shaped phase (cube-phase) in an aged Al-Mg-Si alloy including Mg in excess has been investigated by an energy-filtering transmission electron microscope (EFTEM) equipped with an energy dispersive X-ray spectrometer (EDS) to determine its chemical composition. The quantitative chemical compositions of cube-phase and β-phase (Mg2Si) were determined from EELS elemental maps. These data were also in good agreement with their EDS data. The cube-phase contained higher Mg content than the equilibrium β-phase (Mg2Si), although they had the same crystal lattice of fcc, as reported by some previous researchers.

Although doped regions in semiconductors have been shown to give a different secondary electron yield in low-voltage scanning electron microscopy, the basic interpretation of this contrast has been difficult. It is accepted that this contrast stem from electronic phenomenon rather than atomic number differences between differently doped regions. However, the question is whether variations in the patch fields above the sample surface, balancing variations in the inner potentials, or surface coatings and/or surface states are the mechanisms responsible for the observed contrast. The present study reports on comparative experiments of these two models and demonstrates that the image contrast can be controlled by the presence of thin-surface metallic coatings. These results are the first evidence of the adlayer contacts, i.e., the subsurface electric fields instead of the patch fields above the surface, being responsible for the secondary electron contrast of doped semiconductors imaged in low voltage scanning electron microscopes under standard vacuum conditions, and they pave the way for the routine use of this method in semiconductor research and industry.

Significant differences in backscattered electron (BSE) yields exist between the surfaces cleaned by methods used in electron microscopy and spectroscopy. These differences have been observed for Au, Cu and Al specimens, and are interpreted on the basis of simulated BSE yields. Composition and thickness of the surface contamination layers, responsible for the differences, are estimated. The results (7 nm of carbon on Au or 3 nm of oxide on Al) remain within expectation and indicate that the BSE yield measurements and BSE images should be interpreted cautiously. Peculiar results are obtained for Cu, perhaps due to a different cleaning procedure. A new concept of an information depth for the BSE signal is introduced as a depth within which the total BSE yield can be modelled as composed of the yields of layers proportional to their thickness weighted by the escape depths. This concept proved satisfactory for thin surface layers and brought the information depth values 2 to 4 times smaller than first estimated, i.e. half the penetration depth.

At a landing energy of 10 eV it is possible to achieve spatial resolution of the same order as at the nominal energy, which is usually 15 keV in the classical scanning electron microscope, by taking advantages of the optical properties of the cathode lens. Two different types of the detection system were designed and tested to learn as much about the optical properties of this system as possible and to start to understand the contrast mechanisms at very low energies. Great changes in the contrast take place when the landing energy is changed from 10 eV to an energy of about 2 keV.

Low energy microscopy is treated as the low energy limit of electron microscopy as a whole in all its basic branches, i.e. the emission, transmission and scanning microscopies. The instrumental and methodological aspects are briefly discussed. These include the interaction of electrons with a solid, the contrast formation mechanisms, the instrumentation problems and actual progress achieved in all three microscopies, from the point of view of lowering the energy of electrons, impacting or leaving the specimen, down to the low energy range below 5 keV and the very low energy range below 50 eV.

High resolution can be obtained in the scanning electron microscope (SEM) for a very low landing energy of electrons, even for that below 50 eV, when a cathode lens is used with the specimen as a cathode held at a high negative potential but the detection of signal electrons is totally different compared with classical SEM. Primary electrons with an energy of the order of tens of keV are decelerated in the field of the cathode lens to a very low landing energy and signal electrons originating in the specimen are accelerated and collimated by the same field to a narrow beam with an electron energy nearly the same as that of the primary beam. To detect these signal electrons we must deflect them from the optical axis without deteriorating the properties of the primary beam. The design of a novel type of separator of the primary and signal electrons consisting of two stages, each of them formed by the electric and magnetic crossed fields, is presented, together with calculated trajectories for both primary and signal electrons.