Bright-field scanning confocal electron microscopy using a double aberration-corrected transmission electron microscope

Scanning confocal electron microscopy (SCEM) offers a mechanism for three-dimensional imaging of materials, which makes use of the reduced depth of field in an aberration-corrected transmission electron microscope. The simplest configuration of SCEM is the bright-field mode. In this paper we present experimental data and simulations showing the form of bright-field SCEM images. We show that the depth dependence of the three-dimensional image can be explained in terms of two-dimensional images formed in the detector plane. For a crystalline sample, this so-called probe image is shown to be similar to a conventional diffraction pattern. Experimental results and simulations show how the diffracted probes in this image are elongated in thicker crystals and the use of this elongation to estimate sample thickness is explored.     

Biological structures imaged in a hybrid scanning transmission electron microscope and scanning tunneling microscope

A hybrid scanning transmission electron microscope (STEM) and scanning tunneling microscope (STM) is described which allows simultaneous imaging of biological structures adsorbed to electron-transparent specimen supports in both modes of scanning microscopy, as demonstrated on uncoated phage T4 polyheads. We further discuss the reproducibility and validity of height data obtained from STM topographs of biomacromolecules and present raw data from topographs of freeze-dried, metal-coated nuclear envelopes from Xenopus laevis oocytes.     

Acquisition and processing of scanning electron microscope images using a PC and a framestore

A versatile, low-cost data acquisition and processing system for scanning electron microscope (SEM) images has been successfully developed and demonstrated. This is based on an Amstrad PC and a framestore interfaced to the SEM, and is an upgrade of an earlier BBC microbased system. In addition to supporting colour x-ray mapping, as reported previously, the PC-based unit incorporates new features which enhance the system's performance, flexibility, and overall user friendliness. An example of the system's capabilities in a very large-scale integrated circuit application is presented. This relates to the processing and analysis of a series of images taken on the surface of a Motorola MC 68000 microprocessor, with and without a voltage stimulus.     

Secondary electron detection in the scanning transmission electron microscope

In a dedicated scanning transmission electron microscope (STEM) secondary electron images with high spatial resolution and good contrast can be obtained. Two types of detector are described. These take into account the secondary electrons which depend on the post-specimen field strength of the objective lens. Due to the thinness of the samples and the collection geometry the images differ from those obtained in a convectional scanning microscope. Examples are given where secondary electron images augment the information obtained by the more commonly used imaging modes.     

Digital image acquisition in scanning electron microscopy

Digital image acquisition is of great interest in SEM, as can be proven by the number of ways in which images can be improved by processing with this technique (Desai and Reimer 1985, Hawkes 1981). In the case of the system described here, this interest is enlarged by the facility of storage and retrieval of images. In addition, a digital image becomes the only quantitative result when the “MEBIS” (Microscope Electronique à Balayage In Situ)is used as an automatic control tool whithin an industrial production line (Franceschi and Le Floch 1985). This paper describes a system for acquisition, storage and processing of the images given by “MEBIS”, or other SEM if the scanning system includes a remote command. Software for preliminary processing for local representation is presented. The main advantages of this system are its ease of use and its compactness; it is developed around the familiar and low-cost Apple IIe microcomputer, presented in a portable version including a 5 video screen and a single disk drive in a ruggedized housing.     

Annular electron energy-loss spectroscopy in the scanning transmission electron microscope

We study atomic-resolution annular electron energy-loss spectroscopy (AEELS) in scanning transmission electron microscopy (STEM) imaging with experiments and numerical simulations. In this technique the central part of the bright field disk is blocked by a beam stop, forming an annular entry aperture to the spectrometer. The EELS signal thus arises only from electrons scattered inelastically to angles defined by the aperture. It will be shown that this method is more robust than conventional EELS imaging to variations in specimen thickness and can also provide higher spatial resolution. This raises the possibility of lattice resolution imaging of lighter elements or ionization edges previously considered unsuitable for EELS imaging.     

High-resolution scanning transmission electron microscope at Johns Hopkins

The scanning transmission electron microscope constructed at Johns Hopkins follows the general layout of the first instrument at the University of Chicago. It is currently operating at 50 kV with a resolution of about 3 A. Its detector scheme consists of scintillation crystals coupled to photomultipliers in such a way as to eliminate introduction of unnecessary statistical noise. A unique alignment scheme utilizes the spherical aberration of the objective lens.     

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.     

The conversion of a field-emission scanning electron microscope to a high-resolution,high-performance scanning transmission electron microscope,while maintaining original functions

Recently, the reliability of field-emission electron guns has increased. In addition, the cost of computer systems for on-line processing has dropped. Hence, we should now consider the use of scanning transmission electron microscopy (STEM) for routine work, especially, in the field of biology where one may expect to utilize digital image processing techniques. An STEM has been constructed, without disturbing the original functions, by converting a commercial scanning electron microscope equipped with a fieldemission gun. The STEM is generally operated at accelerating voltage 30 kV, focal length 7.5 mm, and beam current 1?2 × 10^?10 A. Several improvements have been incorporated for removing the effects of vibration, contamination, and stray magnetic fields. Also, an adjustable detector aperture was utilized. The modified instrument was connected to an on-line digital image processing system for utilizing the information obtained from STEM images. The advantages of the modified system were studied from various viewpoints.     

Contrast and resolution of scanning transmission electron microscope imaging modes

Image blurring due to delocalization of inelastic events was studied for scanning transmission electron microscopy (STEM) of unstained thin sections. The delocalization probability was obtained from the angular distribution of inelastic scattering, which was calculated from experimental electron loss spectra of organic samples. This probability was implemented in a Monte Carlo program to simulate the effects of multiple scattering and delocalization for STEM images collected by either the annular detector or the spectrometer, and images generated by a combination of these two signals. Depending on the illumination, the detector geometry and the energy-loss range selected for imaging the annular detector image is blurred by a non-negligible fraction of inelastically scattered electrons. Simultaneous acquisition of an inelastic image using a spectrometer allows the blurring to be reduced by calculation of either the ratio or the difference of the two darkfield signals. While inherent nonlinearities reduce the interpretability of ratio-contrast images, difference-contrast improves the visibility of details submerged in a diffuse background without introducing artifacts.     

Quantitative analysis of scanning electron micrographs

Scanning electron microscopy (SEM) provides a considerably greater depth of field than optical microscopy at the same magnification. It is therefore particularly suitable for the examination of specimens with irregular surfaces. This review of procedures for quantitative measurements on SEM images falls into two parts. The first deals with mensuration; that is, with the determination of the lengths, dihedral angles and other properties of specific features in the image. Particular emphasis is given to means of correcting for non-linearity and other distortions caused by instrumental defects. The second part considers the analysis for bulk properties such as volume fraction and boundary area per unit volume. In the case where observations are made on a planar section, these properties can be determined by use of the standard stereological relationships after making a computable correction (where necessary) for the tilt of the specimen and non-linearities in the scan. However, for irregular surfaces (for example, those produced by fracture) rigorous estimations are not possible without certain assumptions which, unfortunately, are in many cases unrealistic.     

A flexible instrument control and image acquisition system for a scanning electron microscope

We have developed an instrument control and image acquisition system for use with scanning electron microscopes. By making the system flexible over a wide range of operating voltages, scan generation and image acquisition modes can be easily accommodated to a wide range of instruments. We show the implementation of this system for use with a custom‐built low‐voltage scanning electron microscope. We then explore the simple modifications that are required for control of two instruments intended for use as free electron lasers.     

A high-voltage scanning transmission electron microscope at Nagoya University

A high-voltage scanning transmission electron microscope (STEM) H-1250ST of the maximum accelerating voltage of 1.25 MV was constructed at Nagoya University in 1983. The microscope, equipped with a field-emission gun, is designed with high-level STEM performance as well as conventional transmission microscopy mode operation. The aim of developing the microscope, basic design schemes, principal instrumentation, and techniques developed are described.     

The spatial resolution of imaging using core-loss spectroscopy in the scanning transmission electron microscope

The 'delocalization' of inelastic scattering is an important issue for the ultimate spatial resolution of core-loss spectroscopy in the electron microscope. This paper investigates the resolution of scanning transmission electron microscopy images for single, isolated atoms. Images are simulated from first principles using a nonlocal model for electron core-loss spectroscopy. The role of the width of the probe relative to the delocalization of the underlying ionization interaction is considered.     

Integral hologram using scanning electron micrographs: a new application of display holography

We produced at 43 times 23 cm, 120° integral hologram of a marine protozoan test to investigate the museum exhibit potential of interfacing scanning electron microscopy with commercially available integral holography. We describe here our procedure and some of the strengths and weaknesses of this holographic medium in exhibit situations.     

Simulation of scanning transmission electron microscope images on desktop computers

Two independent strategies are presented for reducing the computation time of multislice simulations of scanning transmission electron microscope (STEM) images: (1) optimal probe sampling, and (2) the use of desktop graphics processing units. The first strategy is applicable to STEM images generated by elastic and/or inelastic scattering, and requires minimal effort for its implementation. Used together, these two strategies can reduce typical computation times from days to hours, allowing practical simulation of STEM images of general atomic structures on a desktop computer.     

Atomic imaging using secondary electrons in a scanning transmission electron microscope: experimental observations and possible mechanisms

We report detailed investigation of high-resolution imaging using secondary electrons (SE) with a sub-nanometer probe in an aberration-corrected transmission electron microscope, Hitachi HD2700C. This instrument also allows us to acquire the corresponding annular dark-field (ADF) images both simultaneously and separately. We demonstrate that atomic SE imaging is achievable for a wide range of elements, from uranium to carbon. Using the ADF images as a reference, we studied the SE image intensity and contrast as functions of applied bias, atomic number, crystal tilt, and thickness to shed light on the origin of the unexpected ultrahigh resolution in SE imaging. We have also demonstrated that the SE signal is sensitive to the terminating species at a crystal surface. A possible mechanism for atomic-scale SE imaging is proposed. The ability to image both the surface and bulk of a sample at atomic-scale is unprecedented, and can have important applications in the field of electron microscopy and materials characterization.