Secondary electron emission in the scanning electron microscope

The secondary electron emission induced by electrons in the energy range 2.5–25 keV was measured in a SEM. Values of the emission coefficient for C, Al, Cu, Mo, Ag and Au are presented showing that it varies systematically with atomic number. The coefficient is still appreciable at 25 keV beam energy. The signal from the secondary electron collector in the SEM includes large contributions from sources other than secondary electron emission from the specimen. These contributions are discussed and their relative importance measured. Physics Abstracts classification numbers: 0.690, 8.900     

The scanning low-energy electron microscope: First attainment of diffraction contrast in the scanning electron microscope

Using small Pb crystals deposited in situ on a partially contaminated Si (100) crystal, we demonstrate that a commercial scanning electron microscope (SEM) can easily be converted into a scanning low-energy electron microscope (SLEEM). Although the contrast mechanism is much more complicated than that in nonscanning LEEM because not only one diffracted monochromatic beam and its close environment are used for imaging, but several diffracted beams and a wide energy spectrum of electrons of different origin (secondary electrons, inelastically andelastically scattered electrons) are used, SLEEM is a valuable addition to the standard SEM because it provides an additional structure- and orientation-sensitive contrast mechanism in crystalline materials, a low sampling depth, and high intensity at low energies.     

Mechanical scanning of the specimen in the scanning electron microscope

A scanning electron microscope (SEM) system equipped with a motor drive specimen stage fully controlled with a personal computer (PC) has been utilized for obtaining ultralow magnification SEM images. This modem motor drive stage works as a mechanical scanning device. To produce ultra-low magnification SEM images, we use a successful combination of the mechanical scanning, electronic scanning, and digital image processing techniques. This new method is extremely labor and time saving for ultra-low magnification and wide-area observation. The option of ultra-low magnification observation (while maintaining the original SEM functions and performance) is important during a scanning electron microscopy session.     

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.     

Reduction of charging effects using vector scanning in the scanning electron microscope.

We describe a vector scanning system to reduce charging effects during scanning electron microscope (SEM) imaging. The vector scan technique exploits the intrinsic charge decay mechanism of the specimen to improve imaging conditions. We compare SEM images obtained by conventional raster scanning versus vector scanning to demonstrate that vector scanning successfully reduces specimen-charging artifacts.     

Electron spectroscopy in the scanning electron microscope

The combination of ultra high vacuum scanning electron microscopy with spectroscopy of the emitted electrons gives new possibilities for surface analysis. The paper surveys recent results in secondary, Auger, ionization loss, and elastic peak electron spectroscopy.     

Wear experiments in the scanning electron microscope

Wear experiments using a ball-on-flat configuration were performed in a scanning electron microscope. The sequence of events occurring during the initial or wear-in phase was followed for three different materials: these were mild steel, brass and aluminum. Differences in behavior among the materials were striking. Adhesion and prow formation were observed in the aluminum experiment. A change from sharp microstriations to increasing macrodeformation was characteristic of the behavior of the mild steel. In addition, readily identifiable microstriations persisted for many cycles of sliding contact on the steel surface, while on the brass surface these features changed with each cycle. Microscopic extrusions and lip formation were observed to progress from asperities to incipient wear debris. The tendency for debris to transfer to the harder surface and to influence striation formation in the opposing surface was observed.     

Nanotip electron gun for the scanning electron microscope

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.     

A robust micromanipulator for the scanning electron microscope.

A simple device for holding and moving mechanical tools in the region of a sample being viewed in the scanning electron microscope is described. The unit has a 20:1 mechanical reduction and when fitted with a tungsten carbide dental chisel, it is sufficiently rigid to cut biological hard tissues. Alternately, when fitted with an electro-etched tungsten needle, it can be used, in conjunction with specimen stage controls, to remove individual cells from the surface of soft tissues. Examples of these applications are illustrated.     

A fluorescence scanning electron microscope

Fluorescence techniques are widely used in biological research to examine molecular localization, while electron microscopy can provide unique ultrastructural information. To date, correlative images from both fluorescence and electron microscopy have been obtained separately using two different instruments, i.e. a fluorescence microscope (FM) and an electron microscope (EM). In the current study, a scanning electron microscope (SEM) (JEOL JXA8600 M) was combined with a fluorescence digital camera microscope unit and this hybrid instrument was named a fluorescence SEM (FL-SEM). In the labeling of FL-SEM samples, both Fluolid, which is an organic EL dye, and Alexa Fluor, were employed. We successfully demonstrated that the FL-SEM is a simple and practical tool for correlative fluorescence and electron microscopy.     

Use of the environmental scanning electron microscope for the observation of the swelling behaviour of cellulosic fibres

We have developed a method for observing transverse swelling of cellulosic fibres in the environmental scanning electron microscope (ESEM). The presence of liquid water in the ESEM specimen chamber allows the observation of in situ hydration without the need for coating, freezing, or drying of the sample. For reproducibility of the hydration and dehydration process, specialised mounting techniques are required and control of the conditions for condensation and evaporation of liquid water is necessary. The sensitivity of these cellulosic materials to the electron beam was investigated, showing that some damage mechanisms are enhanced by the continual presence of water vapour in the chamber. A discussion is presented of the effect of various experimental parameters on the extent and time of onset of the damage, and we outline steps to maximise the amount of useful experimental time for these fibres.     

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.     

Secondary electron imaging in the variable pressure scanning electron microscope

Secondary electron imaging is not possible in the variable pressure scanning electron microscope because the mean free path of the secondaries in the gas is too short to permit them to reach the detector. This paper therefore investigates an alternative strategy for producing an image containing significant amounts of secondary electron contrast. This involves modifying the microscope by the addition of a biased electrode above the sample and then collecting a specimen current signal. This system, originally described by Farley and Shah (1988), is found to produce true secondary electron detail over a wide range of conditions.     

Scanning tunneling microscope combined with scanning electron microscope

A scanning tunneling microscope (STM) has been installed in a usual scanning electron microscope (SEM) with a vacuum of 10⁻⁶ Torr. The STM image is displayed on the cathode ray tube of the SEM, 512 × 512 pixels, with a scanning rate of 80 s/picture. The spatial resolution of the STM is about 1 Å, while that of the SEM is several tens of ångströms. The combined scanning microscope covers a wide magnification range from 10 to 10⁷, where STM covers the high magnification region from 10⁵ to 10⁷.     

Resolution limits in the surface scanning electron microscope

An approximate theory is developed for estimating the limiting topographical resolution of the scanning electron microscope operating under certain idealized conditions. Limitations imposed by electron beam shot noise and electron diffusion effects within the specimen are considered for the case of an instrument incorporating a field emission source and in which there is ideal collection of the secondary electron signal. The specimen is assumed to be an homogeneous, isotropic solid with the beam incident normally to its surface. It is estimated that, under these ideal conditions, the limiting resolution for a metallic specimen lies in the region of 1 nm. The possibilities of realizing a resolution of this order in a practical instrument are discussed.     

Signal components in the environmental scanning electron microscope

We demonstrate that the gas-amplified secondary electron signal obtained in the environmental scanning electron microscope has both desired and spurious components. In order to isolate the contributions of backscattered and secondary electrons, two sets of samples were examined. One sample consisted of a pair of materials having similar secondary emission coefficients but different backscatter coefficients, while the other sample had a pair with similar backscatter but different secondary emission coefficients. Our results show how the contribution of the two electron signals varies according to the pressure of the amplifying gas. Backscatter contributions, as well as background due to gas ionization from the primary beam, become significant at higher pressure. Furthermore, we demonstrate that the relative amplification efficiencies of various electron signals are dependent upon the chemistry of the gas.     

Gigahertz stroboscopy with the scanning electron microscope

The application of the stroboscopic scanning electron microscope to gigahertz Gunn effect devices is discussed. Two modes of operation, the deflection mode and the bunching mode, are considered. In the bunching mode, using 1.5 ps beam pulses, two-dimensional voltage contrast in a Gunn effect device triggered at 1 GHz has been observed. A powerful technique for a device in pulsed operation is also presented. With this technique, the nonuniform domain propagation in the three-dimensional-structure Gunn device in pulsed operation has been clearly observed. The duty cycle of the pulsed operation has been 4x10(-3).     

Nanometric cutting in a scanning electron microscope

A nanometric cutting device under high vacuum conditions in a scanning electron microscope (SEM) was developed. The performance, tool-sample positioning, and processing capacity of the nanometric cutting platform were studied. The proposed device can be used to realize a displacement of 7 μm, with a closed-loop resolution of 0.6 nm in both the cutting direction and the depth direction. Using a diamond cutting tool with an edge radius of 43 nm formed by focused ion beam (FIB) processing, nanometric cutting experiments on monocrystalline silicon were performed on the developed cutting device under SEM online observation. Chips and machining results of different depths of cut were studied during the cutting process, and cutting depths of less than 10 nm could be obtained with high repeatability. Moreover, the cutting speed was found to exhibit a strong relationship with the brittle–ductile transition depth on brittle material. The experimental results of taper cutting and sinusoidal cutting indicated that the developed device has the ability to perform multiple degrees of freedom (DOFs) cutting and to study nanoscale material removal behaviour.     

Contrast in high-resolution scanning electron microscope images

Current scanning electron microscopes, equipped with field emission guns and high-performance immersion lenses, can achieve spatial resolutions of the order of 1 nm in both secondary and backscattered imaging modes over a wide range of operating energies. The generation and interpretation of images with nanometre-scale resolution relies on a detailed knowledge, and application, of electron-solid interactions. This paper develops the practical steps required to produce a high-resolution image, and discusses the principles which govern image interpretation. Attention is focused primarily on materials which are low in atomic number and density, such as biological tissue, but the results apply after appropriate scaling of the physical parameters to most other materials.