Low Energy Scanning Electron Microscopy Research Articles

Scanning Low Energy Electron Microscopy and Time-of-Flight Spectroscopy Capabilities for Study of Advanced 2D Materials and Thin Foils

Scanning Low Energy Electron Microscopy and Time-of-Flight Spectroscopy Capabilities for Study of Advanced 2D Materials and Thin Foils

A soft touch with electron beams: Digging out structural information of nanomaterials with advanced scanning low energy electron microscopy coupled with deep learning

Nanostructured materials continue to find applications in various electronic and sensing devices, chromatography, separations, drug delivery, renewable energy, and catalysis. While major advancements on the synthesis and characterization of these materials have already been made, getting information about their structures at sub-nanometer resolution remains challenging. It is also unfortunate to find that many emerging or already available powerful analytical methods take time to be fully adopted for characterization of various nanomaterials. The scanning low energy electron microscopy (SLEEM) is a good example to this. In this report, we show how clearer structural and surface information at nanoscale can be obtained by SLEEM, coupled with deep learning. The method is demonstrated using Au nanoparticles-loaded mesoporous silica as a model system. Moreover, unlike conventional scanning electron microscopy (SEM), SLEEM does not require the samples to be coated with conductive films for analysis; thus, not only it is convenient to use but it also does not give artifacts. The results further reveal that SLEEM and deep learning can serve as great tools to analyze materials at nanoscale well. The biggest advantage of the presented method is its availability, as most modern SEMs are able to operate at low energies and deep learning methods are already being widely used in many fields.

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy.

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.

Contrast mechanism at landing energy near 0 eV insuper low-energy scanning electron microscopy.

In recent years, the technique of scanning electron microscopy (SEM) observation with low landing energy of a few keV or less has become common. We have especially focused on the drastic contrast change at near 0 eV. Using a patterned sample consisting of Si, Ni and Pt, threshold energies where the total reflection of incident electrons occurs were investigated by SEM at near 0 eV. In both the cases of in-situ and ex-situ sample cleaning, drastic changes in the brightness of each material were observed at near 0 eV, with threshold energies in the order Si < Ni < Pt. This order agreed with the order of the literature values of the work functions and the surface potentials measured by Kelvin force probe microscopy. This result suggests that the difference of the threshold energy is caused by the difference in surface potential due to the work function difference of each material. Although the order of the threshold energies also agreed with those of work functions reported in the literature, the work functions of air-exposed surfaces should be rather considered as 'modified work functions', since they could be significantly altered by the adsorbates, etc. Nevertheless, the difference of the threshold energy for each material was observed with commercial SEM at landing energy near 0 eV, which opens a new possibility to distinguish materials, although the difference should be rather recognized as 'fingerprints', since surface potentials are sensitive to conditions of surface treatments and atmospheric exposure.

Low-Energy Electron Inelastic Mean Free Path of Graphene Measured by a Time-of-Flight Spectrometer.

The detailed examination of electron scattering in solids is of crucial importance for the theory of solid-state physics, as well as for the development and diagnostics of novel materials, particularly those for micro- and nanoelectronics. Among others, an important parameter of electron scattering is the inelastic mean free path (IMFP) of electrons both in bulk materials and in thin films, including 2D crystals. The amount of IMFP data available is still not sufficient, especially for very slow electrons and for 2D crystals. This situation motivated the present study, which summarizes pilot experiments for graphene on a new device intended to acquire electron energy-loss spectra (EELS) for low landing energies. Thanks to its unique properties, such as electrical conductivity and transparency, graphene is an ideal candidate for study at very low energies in the transmission mode of an electron microscope. The EELS are acquired by means of the very low-energy electron microspectroscopy of 2D crystals, using a dedicated ultra-high vacuum scanning low-energy electron microscope equipped with a time-of-flight (ToF) velocity analyzer. In order to verify our pilot results, we also simulate the EELS by means of density functional theory (DFT) and the many-body perturbation theory. Additional DFT calculations, providing both the total density of states and the band structure, illustrate the graphene loss features. We utilize the experimental EELS data to derive IMFP values using the so-called log-ratio method.

Effect of native oxide on the crystal orientation contrast in SEM micrographs obtained at hundreds, tens and units of eV

This paper aims to elucidate the effect of native air-formed oxide on the crystallographic contrast between differently oriented copper grains in scanning electron microscope images obtained at energies from 0 eV up to 1 keV. The contrast between the Cu grains is strongly affected by the presence of native oxide. The crystallographic orientation contrast between the grains without covering the native oxide layer is relatively weak at hundreds of eV, negligible at tens of eV, and dramatically increases at energies below 10 eV. At extremely low landing energies, say below ~ 1 eV, the surface potential differences caused by work function variations between the differently oriented Cu grains affect the primary electrons, which enables us to obtain the micrographs with high crystallographic contrast. This contrast becomes surprisingly visible even if the grains are covered by a several nm thick native oxide layer. The presence of the native air-formed oxide layer on the Cu surface is inconsiderable for the contrast formation at energies close to the mirror conditions (< 1 eV). The surface potential differences originating in the substrate can affect the incident electrons through the native oxide film situated on the Cu surface. Scanning low-energy electron microscopy is a powerful tool for mapping local work function differences with a spatial resolution slightly better than 30 nm due to high sensitivity to local electrical potentials.

Вариация состояния поверхности в ходе сканирования в низковольтном РЭМ и ее влияние на размеры рельефной структуры

The emission variation of SSE from surface fragments of protrusions in a silicon plate is evaluated during prolonged scanning in low energy SEM. Emission dynamic variation that is especially complex near the edges of protrusion upper bases and slopes is discovered that affects the sizes of all protrusion fragments. Emission variation is explained with changes of unevenly localized charge state at different surface fragments of protrusions in a natural silica and a contamination film. Contamination is occurred in vacuum and depends on a surface charge state so it is not uniform along protrusion surface. The total absence of contamination on some protrusions is fixed so its initial linewidth size is occurred a constant during a scanning. The limiting uprising of a linewidth size to 4.5 at a one case and the limiting decrease to -0.8 at another case are fixed. Yu. V. Larionov, Senior Researcher, A.M. Prokhorov General Physics Institute, RAS, Moscow, 119991, Russian Federation, E-mail: luv@kapella.gpi.ru, Yu. V. Ozerin, Leading Engineer, yozerin@mikron.ru Mikron Co., Zelenograd, Russia

Observation of Dislocation Conversion in 4H-SiC Epitaxial Wafer by Mirror Projection Electron Microscopy

Dislocations and stacking faults in 4H-SiC (0001) si epitaxial wafer was inspected by mirror projection electron microscopy (MPJ) with the aid of low-energy SEM and FIB-STEM. MPJ observation found dislocation conversion near the wafer surface, and the conversion was confirmed by micro etch pit and low energy SEM method. Another conversion occurred in the epitaxial layer on array of TED half loops, which were detected by MPJ, was also observed by cross-sectional STEM.

Low-energy electron microscopy of graphene outside UHV: electron-induced removal of PMMA residues used for graphene transfer

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.

Visualization of three different phases in a multiphase steel by scanning electron microscopy at 1 eV landing energy

In this study, we investigated an observation technique by super low energy scanning electron microscopy (SLESEM) at below 5 eV and its contrast mechanism for analyzing complex microstructures of a multiphase steel consisting of ferrite, martensite and austenite. With SLESEM at 1 eV, the three phases were observed as different brightness levels, ferrite as the darkest contrast, martensite as the second brightest and austenite as the brightest. These contrasts disappeared at 2 eV or higher. Similar contrasts and phenomena were also observed in the results of low energy electron microscopy (LEEM). According to the energy dependences of the LEEM intensities of the three phases, the threshold energies of the transition from electron reflection to surface impact were determined to be 0.00 eV, 0.15 eV and 0.39 eV for ferrite, martensite and austenite, respectively. These differences in thresholds indicate that the potentials on the surfaces of each phase are different, which is considered to result in the different brightness of each phase. This potential differences are probably due to the contact potentials generated when phases with different work functions contact each other. Although the sample is covered by a thin native oxide film (several nm thickness), the potentials can affect the incident electrons through the oxide film.

Role of the Angular Distribution of Backscattered Electrons in Low Energy Scanning Electron Microscope

Role of the Angular Distribution of Backscattered Electrons in Low Energy Scanning Electron Microscope

Observation of a Latent Scratch on Chemo-Mechanical Polished 4H-SiC Wafer by Mirror Projection Electron Microscopy

A latent scratch which is an extremely shallow scratch induced on a SiC wafer during chemo-mechanical polishing (CMP) has been investigate by mirror projection electron microscopy (MPJ), low-energy scanning electron microscopy (LESEM), atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). The latent scratch, which is difficult to detect by using optical microscopes, was easily visualized by MPJ as a high contrast dark line. The morphology of detected latent scratch is less than 1nm in depth and about 30nm in full width at half depth by AFM evaluation. The STEM observation revealed the latent scratch was accompanied two dislocation arrays. One contains loop-like dislocations and the other contains spiky dislocations, both lying in the wafer at a few ten nm in depth.

Effect of Distribution of Fluorine Resin on Hydrophobic Property of Inorganic Coating on Steel Sheet

Covering metal with fluorine resin can create a hydrophobic surface on metals for many applications such as self-cleaning, anti-icing, and corrosion resistance. When fluorine resin is dispersed in a coating, the material's hydrophobicity can be controlled by altering the fluorine resin distribution in the coating. This study investigated fluorine resin dispersion effects on the hydrophobicity of an inorganic coating. The fluorine resin coverage was analyzed using X-ray photoelectron spectroscopy （XPS） and scanning low-energy electron microscopy （SLEEM）. Hydrophobicity was evaluated by the water contact angle. The contact angle on inorganic coatings in which fluorine resin was dispersed increased concomitantly with increasing amounts of fluorine resin. Furthermore, the surface coverage of the fluorine resin increased as the fluorine resin amount increased, but the coverage was not directly proportional to the resin amount. Results suggest that the fluorine resin is concentrated in the outermost layer of the coatings. The coating hydrophobicity is determined by the fluorine resin coverage on the outermost layer according to Cassie's law. The water contact angle also depends on the drying temperature of the coatings because the fluorine resin spreads in the outermost layer when the coating is dried at a temperature higher than the resin softening point.

Surface morphology and dislocation characteristics near the surface of 4H-SiC wafer using multi-directional scanning transmission electron microscopy.

To improve the reliability of silicon carbide (SiC) electronic power devices, the characteristics of various kinds of crystal defects should be precisely understood. Of particular importance is understanding the correlation between the surface morphology and the near surface dislocations. In order to analyze the dislocations near the surface of 4H-SiC wafers, a dislocation analysis protocol has been developed. This protocol consists of the following process: (1) inspection of surface defects using low energy scanning electron microscopy (LESEM), (2) identification of small and shallow etch pits using KOH low temperature etching, (3) classification of etch pits using LESEM, (4) specimen preparation of several hundred nanometer thick sample using the in-situ focused ion beam micro-sampling® technique, (5) crystallographic analysis using the selected diffraction mode of the scanning transmission electron microscope (STEM), and (6) determination of the Burgers vector using multi-directional STEM (MD-STEM). The results show a correlation between the triangular terrace shaped surface defects and an hexagonal etch pit arising from threading dislocations, linear shaped surface defects and elliptical shaped etch pits arising from basal plane dislocations. Through the observation of the sample from two orthogonal directions via the MD-STEM technique, a basal plane dislocation is found to dissociate into an extended dislocation bound by two partial dislocations. A protocol developed and presented in this paper enables one to correlate near surface defects of a 4H-SiC wafer with the root cause dislocations giving rise to those surface defects.

Observation of Basal Plane Dislocation in 4H-SiC Wafer by Mirror Projection Electron Microscopy and Low-Energy SEM

Basal plane dislocation (BPD) with dislocation lines in shallow areas near the surface in 4H-SiC epitaxial wafer was observed by mirror projection electron microscopy (MPJ) and low-energy scanning electron microscopy (LE-SEM). A contrast of dislocation line of BPD appeared in a MPJ observation as gradually weakened dark line toward the upper stream of offcut of wafer, and the contrast almost agreed with the LE-SEM image taken at the same BPD by not a morphology-sensitive imaging method but a potential-sensitive imaging method. Thus an origin of the contrast corresponding to BPD in MPJ is considered to surface potential change due to charging on dislocation line. MPJ observation can gives a BPD image with same quality as a potential-sensitive image by LE-SEM, in extremely short time and damage-and contamination-free condition at no electron irradiation on wafer.

Programmable set-up for electrochemical preparation of STM tips and ultra-sharp field emission cathodes

This paper describes a newly designed set-up which is intended for automated preparation both for STM tips and for ultra-sharp field emission cathodes made of polycrystalline and single crystal tungsten in laboratory conditions. The newly designed set-up incorporates electrochemical etching of a wire in the surface layer of an electrolyte and also the so called drop-off method which consists of two steps and requires more precise wire and current setting. Additionally, the method was extended for polycrystalline wires of tungsten using a special cut-off algorithm, which deals with variable etching speed for various crystallographic orientation of the current grain. Produced tips are examined using scanning low energy electron microscopy as for the surface and by the occurrence of electron field-emission as for the geometry based on the Fowler-Nordheim analysis. STM performance of the produced tip is discussed and examined as well.

Communication—Direct Imaging of Irradiation Damage in Semiconductors by Low-Energy SEM

The performance of the present-day Scanning Electron Microscopy (SEM) extends far beyond delivering electronic images of the surface topography. One of the most noticeable achievements of the technique was recognition of p- and n-type conductive channels on the semiconductor surface. Here, we report on the possibility of visualization of highly resistive layers produced by ion irradiation. The presented high-resolution imaging of damage-induced insulating layers was made possible through application of low energy of the primary beam (1 keV), optimization of the instrument parameters and effective separation of the so-called SE1 vs. SE2 secondary electrons.

Practical Use of Scanning Low Energy Electron Microscope (SLEEM)

Journal Article Practical Use of Scanning Low Energy Electron Microscope (SLEEM) Get access Ilona Müllerová, Ilona Müllerová Department of Electron Microscopy, Institute of Scientific Instruments ASCR, v.v.i., Brno, Czech Republic Search for other works by this author on: Oxford Academic Google Scholar Eliška Mikmeková, Eliška Mikmeková Department of Electron Microscopy, Institute of Scientific Instruments ASCR, v.v.i., Brno, Czech Republic Search for other works by this author on: Oxford Academic Google Scholar Šárka Mikmeková, Šárka Mikmeková Department of Electron Microscopy, Institute of Scientific Instruments ASCR, v.v.i., Brno, Czech Republic Search for other works by this author on: Oxford Academic Google Scholar Ivo Konvalina, Ivo Konvalina Department of Electron Microscopy, Institute of Scientific Instruments ASCR, v.v.i., Brno, Czech Republic Search for other works by this author on: Oxford Academic Google Scholar Luděk Frank Luděk Frank Department of Electron Microscopy, Institute of Scientific Instruments ASCR, v.v.i., Brno, Czech Republic Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 22, Issue S3, 1 July 2016, Pages 1650–1651, https://doi.org/10.1017/S1431927616009090 Published: 25 July 2016

Observation of Pair Structures of Threading Dislocation and Surface Defect in 4H-SiC Wafer by Mirror Projection Electron Microscopy

Two types of shallow surface defects associated with treading dislocation were found out by using mirror projection electron microscope. One was single groove with a dimension of about 4 nm in depth, 2 μm in width and 15 μm in length, named “as nanogroove”. The other was a shallow groove at 1.3nm in depth being between pair of hillocks at 2-3 nm in height and 1.5 μm in distance, named as “nanohillock pair”. Dislocations combined with the defects were found out by micro-KOH etching method with low-energy scanning electron microscopy. The dislocations were identified by g-b analysis using scanning transmission electron microscopy as threading edge dislocations converted from basal plane dislocation at bulk-epi layer interface or within epi layer.

Low-energy scanning electron microscope using a monochromator with double-offset cylindrical lenses

The nanoelectronics industry demands continuous improvement in the performance of scanning electron microscopes (SEMs). Extremely low energies of less than 1 keV are required for SEM observations to allow the subsurface and nanoscale information of target specimens to be measured with minimized charge-up and beam damage depths because of the reduced interaction volumes. In this article, the authors propose a new monochromator (MC) structure and investigate its applicability to SEMs operating at such extra-low energies. The proposed MC, which uses double-offset cylindrical lenses, can perform energy filtering in its midsection and form a stigmatic and nonenergy dispersive image at the exit. The energy resolution is expected to be better than 10 meV for a pass energy of 4 keV. The MC has the additional advantage of a simple but robust structure, which is essential for industrial applications. Assuming the use of ideal and high-performance SEM optics, for which the spherical aberration coefficient and the chromatic aberration coefficient are both 1 mm, beam diameters measured in terms of the full width that contains 50% of the beam current (FW50) are calculated at the specimen position. Use of the MC improves the beam diameter dramatically to 4.4 nm, as compared to the diameter of 19.7 nm for the SEM without the MC, at a landing energy of 100 eV. The chromatic aberration contribution also becomes negligible because of the MC. The beneficial effects of the MC with regard to the beam diameter become increasingly prominent at lower landing energies ranging down to 10 eV. A SEM using this MC can generate highly monochromatic (10 meV) electron probe beams with small size (5 nm) and low energy (100 eV), which indicates the additional possibility of a new surface electron microscope that uses phonon signals. Based on this theoretical investigation, the authors conclude that this MC can effectively improve the SEM's performance capabilities in the extra-low-energy region.