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Low voltage electron microscopes working in transmission mode, like LVEM5 (Delong Instruments, Czech Republic) working at accelerating voltage 5 kV or scanning electron microscope working in transmission mode with accelerating voltage below 1 kV, require ultrathin sections with the thickness below 20 nm. Decreasing of the primary electron energy leads to enhancement of image contrast, which is especially useful in the case of biological samples composed of elements with low atomic numbers. As a result treatments with heavy metals, like post-fixation with osmium tetroxide or ultrathin section staining, can by omitted. The disadvantage is reduced penetration ability of incident electrons influencing the usable thickness of the specimen resulting in the need of ultrathin sections of under 20 nm thickness. In this study we want to answer basic questions concerning the cutting of extremely ultrathin sections: Is it possible routinely and reproducibly to cut extremely thin sections of biological specimens embedded in commonly used resins with contemporary ultramicrotome techniques and under what conditions? Microsc. Res. Tech. 79:512-517, 2016. © 2016 Wiley Periodicals, Inc.

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.

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.

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.

Studies concerning assessment of the image quality in scanning electron microscopes and studies evaluating the detective efficiency of the secondary electrical (SE) detectors in these microscopes must be based on statistics of SE emission. The vast majority of previous studies have applied Poisson statistics, although their prerequisites have not been satisfied in must cases. This paper is concerned with the limits to the applicability of Poisson statistics to SE emission. Adequate definition of a non-Poisson factor in the variance of the number of SEs emitted is discussed. and a simple formula for this factor is derived for the low yield case in which both the primary and the backscattered electron are assumed not to release more than one SE. These conditions are met with conductive specimens composed of light elements at primary electron (PE) energies of tens of keV. For the lightest specimens, such as carbon, the non-Poisson factor can even be neglected for PEs >10 keV.

The real point resolution of an SEM image is treated in a two-dimensional model where the decisive quantity is the root-mean-square distance of the emitted electron from the pixel centre. This quantity is computed taking into account the direct illumination of the specimen surface by the primary spot the dimensions of which are given by the electron optical column and the indirect illumination from a virtual source of the backscattered electrons in the specimen depth the properties of which depend on the specimen. Because the second order movement of the full distribution is considered here instead of some measure of the central narrow peak of secondaries, the backscattered electron influence significantly deteriorates the resulting resolution. Reasonable approximations regarding both contributions, particularly the Monte-Carlo modelling of the backscattering process by algorithms providing acceptable results down to 1 keV and the approximate relations describing the secondary and backscattered electron emission again down to 1 keV, enabled us to bring the considerations up to numerical results for some typical instruments, namely a "cheap" SEM, a "top" SEM and a low-energy SEM (LESEM) adapted from the cheap SEM by using the cathode lens. The optimum landing energy providing the best point resolution, computed for the individual chemical elements, falls into the range 1 to 5 keV for the cheap SEM and it remains around 1 keV (with some uncertainty caused by the approximations mentioned) for both the top SEM and LESEM. Similarly, the real resolution for the elements ranges between 16 and 45 nm when the cheap SEM with a 3.4 nm nominal spotsize at 30 keV is used, between 3.5 and 9 nm for the 0.7 nm top SEM and between 5.5 and 7 nm for the cheap SEM adapted to LESEM.

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.

We demonstrate the possibility to examine the free-standing foils of thicknesses in units of nm in the scanning low energy electron microscope, using both reflected and transmitted electrons. Very high contrast has been obtained in dependence on the thickness and structure of the foil. A contribution of secondary electrons to the forward scattered electron signal is discussed and a way of suppressing it is presented. Examples of reflected, total transmitted and dark field transmitted electron signal for two graphene-like samples are shown. Dependence of the transmitted signal on the electron energy is observed.

Minimum necessary adaptations are described that are sufficient for obtaining very low energy electron micrographs (VLEEMs) from commercially available routine scanning electron micrographs (SEMs) with the electrons accelerated to an energy of the order of tens of keV. A cathode lens inserted into the specimen chamber enables one to decelerate electrons in front of the specimen surface to a desired low landing energy, which can be freely varied even down to zero. When a potential slightly more negative than the accelerating voltage is applied, a scanning mirror electron microscopy mode can be effected. The achievable point resolution at very low energies proves to be not too dependent on the objective lens parameters, so that the physical limit of aberrations of the homogeneous field of the cathode lens is nearly attainable. The detection efficiency for the standard Everhart-Thornley secondary electron detector is discussed, and results for the routine Tesla BS 340 SEM are presented.

We have developed a Scanning Low Energy Electron Microscope (SLEEM) based on the Cathode Lens (CL) principle [1]. A resolution of 4.5 nm at 20 eV, 0.8 nm at 200 eV and 0.5 nm at 15 keV primary beam energy can nowadays be obtained in a commercially available instrument [2]. One of the main advantages of operation at low energies is the decrease in the interaction volume from approximately 1 μιη at 10 keV to 10 nm at 100 eV. The material contrast can be optimised and the charging effect suppressed at a tailored electron energy. Wave-optical contrasts are also available beneath 50 eV. The specimen may be immersed in a strong magnetic field in addition to an electrostatic field in order to obtain a small spot size across the whole energy range. The same fields influence the signal trajectories, so we can choose which part of the angular and energy distributions of emitted electrons are to be collected. Certain arrangements provide strong crystallographic contrast. Imaging conditions have been tailored to various material types. Experiments have been performed in an experimental ultrahigh vacuum (UHV) SLEEM and in an XHR SEM Magellan 400L.

Some problems of scanning Auger electron microscopy (SAEM) concerning instrumentation and experimental techniques are described and briefly discussed in the light of the fundamental limitations of the method set by the nature of the phenomena themselves.

One of the well-proven and efficient methods of obtaining a very low-energy impact of primary electrons in the scanning electron microscope is to introduce a retarding field element below the pole piece of the objective lens (OL). It is advantageous to use the specimen alone as the negatively biased electrode (i.e., cathode of the cathode lens). The optical power of the cathode lens modifies some of the standard parameters of the image formation such as relation of working distance to OL excitation or magnification to the scanning coils current, the impact angle of primary electrons, and so forth. In computer-controlled electron microscopes these parameters, particularly with regard to focusing and magnification, can be corrected automatically. Derivation of algorithms for such corrections and their experimental verifications are presented in this paper. Furthermore, a more accurate analytical expression for the focal length of an aperture lens is derived.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Scanning electron imaging of plan views of boron-doped patterns in silicon is examined, together with the mechanism of formation of the electronic contrast in this kind of structures. Main to-date published results are critically reviewed and new data are presented concerning the secondary, backscattered and total-emission electron contrasts, including their qualitative and quantitative behaviour, particularly in the low energy range achieved with the help of the cathode lens (the scanning low energy electron microscopy mode, SLEEM). Surface analysis of the structure by means of Auger electron spectrometer has been performed, too, both before and after ion beam bombardment. The scanning electron microscope micrographs, acquired after the oxide mask removal in HF, are examined in a variety of detection modes, aiming at identification of the signal component primarily bearing the contrast. The energy dependence of the contrasts is presented as well as its change owing to alteration in the vacuum conditions. The most important findings include an extremely high contrast obtained in the SLEEM mode and even more enhanced under medium vacuum conditions at which the carbonaceous layer of surface contamination plays its role. The observed phenomena are partly explained in the frame of the “flat band” model of a passivated surface. The increased contrast in the SLEEM mode is understood as connected with the above-surface electric field of the cathode lens, generating space charge layers inside the semiconductor. In addition, charge carriers, injected via the primary electron beam, are considered as influencing the contrast vs. energy dependence.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

A novel principle of an in-lens detector of very slow electrons is described and the detector efficiency discussed. The detector was built into a coaxial column for a Cylindrical Mirror Analyser for Auger electron microanalysis. In order to obtain a very low energy scanned imaging, a cathode lens was formed between the final electrode of the column and a negatively biased specimen. The signal electrons accelerated within the cathode lens field enter the column and after being mirrored back impact a micro-channel-plate based detector fitted around the optical axis. The acceptance of the detector, expressed as a ratio of the number of electrons impacting the detector to the full emission of a cosine source, was calculated to be 0.86 for 1 eV and 0.985 for 10 eV electrons. Then, the efficiency of conversion into output pulses is 0.35 and 0.31, respectively; these parameters are superior to those of conventional SEM detectors for secondary electrons. Micrographs taken at low energies ranging down to units of eV are presented.

The paper describes the physical basis and the structure of the algorithms included in the software package for automatic spectrum processing developed recently as an option to the Tesla BP 350.1 Automatic Auger electron spectrometer.

This paper concerns the problem of testing the image resolution in scanned electron micrographs, namely from the point of view of the image contrast for the standard gold/carbon test specimen within the full energy scale, down to eV units. Microscopes equipped with a cathode lens are considered and experimental results are presented for three devices differing in vacuum conditions and fields surrounding the specimen. At very low energies, the contrast is shown to either invert or disappear and this ambiguity is discussed and confronted with data simulated on the basis of measured electron yields from gold and carbon. It is concluded that the observations can be explained in terms of the surface cleanliness and strength of the surrounding fields.