High-speed operation of scanning electron microscope lenses

The speed at which the focal properties of an iron-shrouded magnetic electron lens can be changed is fundamentally limited by Eddy currents in the magnetic material. The effects of such Eddy currents on the lens properties can be partially compensated by modifying the lens excitation to speed up the lens response. This work considers the application of excitation pre-emphasis techniques to scanning electron microscope lenses for small and large dynamic changes in excitation. Where transient material saturation effects come into play, attempts to further improve the lens response beyond that at the onset of saturation are futile. Under such circumstances, the use of a closed-loop feedback arrangement is proposed that can provide a degree of response improvement.     

S-4700型扫描电子显微镜的结构与操作注意事项

主要介绍了日立S-4700型扫描电子显微镜基本结构特点,以及该仪器在操作过程或日常维护中需要注意的几个方面。扫描电子显微镜的结构主要包括真空系统、电子光学系统和信息接收显示系统3个主要部分。本文针对S-4700型扫描电子显微镜分别对如何保证良好的真空条件;电子光学系统中的电子枪、电子透镜、扫描系统和消象散装置的结构与工作原理;以及信息接收显示系统中的二次电子检测器的结构与工作原理进行了简单的介绍。对应于以上的结构介绍,对仪器在日常维护中可能出现的问题进行了简单的分析。     

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.     

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     

Peculiarities of imaging one- and two-dimensional structures using an electron microscope in the mirror operation mode

Measurements performed in an electron microscope with the mirror operation mode are most sensitive to local electric fields and geometrical roughness of any kind of the object being studied. The object with a geometrical relief is equivalent to a smooth surface with an effective distribution of microfields. Electrons forming the image interact with the local microfields for an extended time: during approach to the object, deceleration and acceleration away from the object. As a result, the electron trajectories can be strongly distorted, and the contrast changes essentially, leading to image deformation of details of the object under investigation and to lowering of the resolution. These effects are theoretically described and are illustrated by experiments. An analysis of these effects enables the real size and the shape of the object involved to be reconstructed.     

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.     

Image synthesis in the electron microscope

A new method is described for transferring phase contrast in electron microscopy without artefacts due to oscillations of the phase contrast transfer function (PCTF). This is carried out by in situ image synthesis of two or three exposures transferred with complementary PCTF. The essence of the technique is to use optimized transfer attenuation functions to cut off the negative parts of PCTF.     

Optical shadowing in the electron microscope

Optical shadowing offers a valuable technique for the study of many transmission electron microscope specimens. Simply blocking half of the illumination with the objective aperture produces an image with a striking shadowed effect which gives a distinctly three-dimensional appearance to the specimen's surface topography. Theoretical analysis shows that this is due primarily to a discrimination between electrons refracted in opposite directions, and that the characteristic features of the effect are successfully explained by a refraction model. The capability of visualizing surface topography is applicable up to the full resolving power of the instrument and accordingly opens many new avenues of investigation.     

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.     

The future of the electron microscope*

The future of electron microscopy lies as much with the conventional as with the newer instruments. The point resolving power of the latter is likely to be pushed to 1 Å, but only after a considerable effort in solving problems of mechanical and electrical stability. Progress in correcting the lens aberrations is even slower. Techniques of specimen preparation and microscope operation are continuously being improved, but will need even greater refinement if proper use is to be made of a 1 Å resolving power, e.g. for identifying bases in a nucleic acid molecule. Extension of the working voltage to 1 MeV and above is increasing the usefulness of the conventional electron microscope, particularly in metallurgy, and plans are now being made for even higher voltages. Of the newer, unconventional instruments, the scanning electron microscope is already establishing a place for itself, especially in industrial applications where surface conditions on the microscale are important. It is likely to find increasing use in micro-circuitry, but also in some branches of biological research, even if its resolving power cannot be brought below 100 Å. The combination of X-ray spectrometry with electron microscopy holds promise of wide application, in the form of a hybrid electron microscope-microanalyser. Secondary-emission microscopes are slowly finding a place for themselves, mainly in applied science and technology. Mirror microscopes, for surface investigations, and ion microscopes, for microanalysis, are still in an early stage but have interesting possibilities. The field ion microscope, simplest in principle but sophisticated in technique, is unique in showing the position of individual atoms in a metal tip. It is already finding applications, especially in studies of radiation damage.     

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.     

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.     

Topographical contrast in the transmission electron microscope

An adaptation of the Foucault method for topographical imaging in the transmission electron microscope is described in detail. The image contrast is produced by selection of electrons which have suffered differential phase retardations in the specimen inner potential. Surface or interface displacements produce bright or dark image contrast, and the ultimate resolution approaches that of the atomic scale. The imaging method is applied in studies of both amorphous and crystalline objects. The possibility of performing quantitative measurements is demonstrated by the estimation of the inner potential of crystalline MgO.     

An electron microscope for the aberration-corrected era

Improved resolution made possible by aberration correction has greatly increased the demands on the performance of all parts of high-end electron microscopes. In order to meet these demands, we have designed and built an entirely new scanning transmission electron microscope (STEM). The microscope includes a flexible illumination system that allows the properties of its probe to be changed on-the-fly, a third-generation aberration corrector which corrects all geometric aberrations up to fifth order, an ultra-responsive yet stable five-axis sample stage, and a flexible configuration of optimized detectors. The microscope features many innovations, such as a modular column assembled from building blocks that can be stacked in almost any order, in situ storage and cleaning facilities for up to five samples, computer-controlled loading of samples into the column, and self-diagnosing electronics. The microscope construction is described, and examples of its capabilities are shown.     

Measurement of potential distribution function on object surface by using an electron microscope in the mirror operation mode

The quantitative theory of image contrast in an electron microscope in the mirror operation mode is given in this paper. This theory permits us to calculate the potential distribution on the object surface from the current density distribution on the microscope screen. The potential distribution results in image formation on the screen. Local electric fields existing on the object surface lead to a perturbation of electron trajectories above the object and to a redistribution of the current density on the screen, causing image contrast. Using the quantitative correlation between these fields and the function of current density distribution on the screen, it is possible to calculate the magnitude of these microfields as well. As illustration, a measured potential distribution on an object surface with spiral structures of adsorbates was analysed. These structures are formed during reaction of CO oxidation on Pt(110). The value of the measured contact potential difference comprised a few hundredths of volt.     

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.     

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).