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Accurate alignment of an electron microscope is important when the highest achievable resolution is demanded. In a conventional alignment for high-resolution electron microscopy, the incident beam direction is aligned for the object point imaged at a viewing-screen center. It should be noted however that the object point corresponding to the viewing-screen center does not usually located on the magnetic-field axis of an objective lens due to lens misorientation and/or beam deflection caused by a leakage field around the objective lens. Then, the voltage-center and coma-free conditions cannot be satisfied simultaneously, even if a magnetic-field is rotationally symmetric. We implemented last year a proposed scheme to satisfy both coma-free and voltage-center conditions. This alignment scheme however requires a suitable deflector at the objective image plane.In this report, we summarize ordinary alignment procedures for high resolution electron microscopy and propose a new alignment procedure, which we call a lens-field axis alignment. Using this alignment we can satisfy the coma-free and voltage-center conditions simultaneously.

It has been known for many years that materials such as zeolites, polymers, and biological specimens have crystalline structures that are vulnerable to electron beam irradiation. This radiation damage severely restrains the use of high resolution electron microscopy (HREM). As a result, structural characterization of these materials using HREM techniques becomes difficult and challenging. The emergence of slow-scan CCD cameras in recent years has made it possible to record high resolution (∽2Å) structural images with low beam intensity before any apparent structural damage occurs. Among the many ideal properties of slow-scan CCD cameras, the low readout noise and digital recording allow for low-dose HREM to be carried out in an efficient and quantitative way. For example, the image quality (or resolution) can be readily evaluated on-line at the microscope and this information can then be used to optimize the operating conditions, thus ensuring that high quality images are recorded. Since slow-scan CCD cameras output (undistorted) digital data within the large dynamic range (103-104), they are ideal for quantitative electron diffraction and microscopy.

In a conventional alignment for high-resolution electron microscopy, the specimen point imaged at the viewing-screen center is made dispersion-free against a voltage fluctuation by adjusting the incident beam direction using the beam deflector. For high-resolution works the voltage-center alignment is important, since this alignment reduces the chromatic aberration. On the other hand, the coma-free alignment is also indispensable for high-resolution electron microscopy. This is because even a small misalignment of the incident beam direction induces wave aberrations and affects the appearance of high resolution electron micrographs. Some alignment procedures which cancel out the coma by changing the incident beam direction have been proposed. Most recently, the effect of a three-fold astigmatism on the coma-free alignment has been revealed, and new algorithms of coma-free alignment have been proposed.However, the voltage-center and the coma-free alignments as well as the current-center alignment in general do not coincide to each other because of beam deflection due to a leakage field within the objective lens, even if the main magnetic-field of the objective lens is rotationally symmetric. Since all the proposed procedures for the coma-free alignment also use the same beam deflector above the objective lens that is used for the voltage-center alignment, the coma-free alignment is only attained at the sacrifice of the voltage-center alignment.

TEM hollow cone illumination can produce high resolution images having atomic number (Z) contrast within a lattice image. Inorder to produce these images, the contribution of four sources of electrons should be considered. These are the main, inelastically scattered, elastically scattered, and diffracted beams. This abstract discusses these sources of electrons to the hollow cone (HC) image, and then goes further to propose a possible method of extending the resolution of the electron microscope by using diffracted HC beams to form holograms which should remove the limitation on resolution imposed by the objective lens and inelastically scattered electrons. A Philips EM 430T was used to take the electron micrographs.

The alloy of composition Ni4Mo develops, at temperatures below 860 °C, an ordered Dla-structure which is based on the fcc-lattice. This alloy has been widely investigated with respect to its physical properties and its ordering behaviour. High resolution studies are rare and concentrated mainly on its short-range order structure. The aim of the present work was to develop a detailed understanding of image contrast and to apply the results to antiphase-boundary studies in ordered Ni4Mo by means of a JEOL 4000 EX electron microscope.In high-resolution electron microscopy, depending on defocus and foil thickness, a large variety of different images is obtained. Only a few of these allow a direct interpretation concerning the location and the type of the atoms. By computing a through-focus/through-thickness map (TFTT map) before starting experimental work it is possible to determine the proper conditions at which images can be obtained which are closely related to the projected potential.

Many of the most interesting specimens are so severely damaged during the electron microscopical imaging that no useful image emerges from the electron noise. One method of reducing this radiation damage is to cool the specimen, and if this helps it seems reasonable to cool it down as far as possible. This goal is reached most straightforwardly by using the Siemens helium-cooled superconducting objective lens designed by I. Dietrich and co-workers.

High resolution electron microscopy images of crystals are obtained by making some beams, diffracted by the crystal, interfere with the transmitted beam. For some experimental conditions, to be carefully defined for each specimen, the image can be shown to represent atomic columns; thus information in the atomic range is obtained, though it must be stressed that interpretation of these structure images must be done carefully as artefacts can give effects not linked with the crystal structure. In the last 3 years, this technique has been applied to the study of surfaces where it has given some striking results.

Once it is understood in terms of wave optical transfer theory, tilted beam bright-field illumination can be applied to a variety of high resolution microscope problems which encompass lattice imaging and imaging nonperiodic objects. The tilted beam method offers the advantages of increasing the resolution and allowing both elastically and inelastically scattered electrons to contribute to high spatial frequency detail in a micrograph. In the simplest case, one can demonstrate these effects by the conventional lattice imaging mode using a single diffracted beam and the direct transmitted beam positioned symmetrically on either side of the microscope optic axis. However, the transfer theory of the microscope for tilted beam illumination shows that the cancellation of chromatic aberration occurs not only for the one diffraction direction across the optic axis from the unscattered beam, but also for an entire circle centered about the axis in the back focal plane (BFP) of the microscope.

A system of computer programs has been written to simulate both bright-field and dark-field micrographs of a variety of materials of interest in both metallurgical and biomolecular applications of high resolution electron microscopy. Selected area diffraction has also been incorporated into this system of programs as well as the ability to orient the object of interest relative to the microscope coordinate system by specifying the Eulers angles between the microscope and object coordinate systems. For bright-field imaging both elastic and inelastic scattering can be included for the tilted beam or axial imaging modes. Optical diffraction programs yield the power spectra of these micrographs and filtering programs have also been designed for high pass filtering and correction of the microscope transfer function. Some of the variables which are input into this system of computer programs are: objective lens defocus, spherical aberration, chromatic aberration, beam tilt, objective aperture size and electron wave length.

The National Facility for High Resolution Electron Microscopy, within the Center for Solid State Science at Arizona State University, was established in 1979 by the National Science Foundation. The Facility was established for the purposes of advancing knowledge in the field of high resolution electron microscopy, and promoting the application of new research methods to problems of current scientific and technological importance. The current Director of the Facility is Dr. John M. Cowley.The major equipment at the Facility is as follows.JEM 4000EX 400keV ultra-high resolution electron microscope (installed 1984). Structure resolution limit of 1.7 Å, and information limit of 1.6 Å. The 4000EX can be placed under computer control (using a system developed at ASU) for the purposes of alignment; that is, the computer automatically corrects the focus, stigmation and beam tilt. At 1.7 Å resolution, direct structure images of virtually all crystalline materials can be obtained, and individual atomic columns may be resolved in many samples. The 4000EX has been applied to characterization of defects, interfaces and surfaces of ceramics, metals and semiconductors.

The phase contrast transfer function (CTF) for imaging of weak phase objects in high resolution electron microscopy is well known. It is an oscillating function, generating transfer intervalls with positive and negative contrast. Due to the influence of chromatic aberration and partial coherence its amplitude decreases at higher spatial frequencies. Several attempts have been made to improve the CTF by a posteriori optical filtering, e.g. zonal filtering or the use of Tsujiuchi-type phase filters. The method of holographic deconvolution, as proposed by Stroke and Halioua, employs phase and amplitude filters in such a way that the CTF approaches nearly a constant value.

The contrast observed in images of beam-sensitive, crystalline specimens is found to be significantly less than one would predict based on observations of electron diffraction patterns of the specimens. Factors such as finite coherence, inelastic scattering, and the limited MTF of the photographic emulsion account for some decrease in contrast. It appears, however, that most of the loss in signal is caused by motion of the specimen during exposure to the electron beam. The introduction of point and other defects in the crystal, resulting from radiation damage, causes bending and lateral motion, which degrade the contrast in the image. We have therefore sought to determine whether the beam-induced specimen motion can be reduced by reducing the area of the specimen which is illuminated at any one time.

Image processing can be considered as an attempt to aid image interpretation and to improve image quality by using additional information not contained in the image, or to make optimal use of redundant information contained in the image. Since high resolution electron micrographs contain a very high amount of noise from substrate, electron statistics and photographic grain, the separation of the signal from the noise part of the image is one of the most important problems. An operation that produces an output image with a higher signal-to-noise ratio than the input image may appear as witchcraft but is in fact feasible through clever use of a-priori information such as the knowledge of object and noise statistics.

Two main problems in high resolution electron microscopy are first, the existence of gaps in the transfer function, and then the difficulty to find complex amplitude of the diffracted wawe from registered intensity. The solution of this second problem is in most cases only intended by the realization of several micrographs in different conditions (defocusing distance, illuminating angle, complementary objective apertures…) which can lead to severe problems of contamination or radiation damage for certain specimens.Fraunhofer holography can in principle solve both problems stated above (1,2). The microscope objective is strongly defocused (far-field region) so that the two diffracted beams do not interfere. The ideal transfer function after reconstruction is then unity and the twin image do not overlap on the reconstructed one.We show some applications of the method and results of preliminary tests.Possible application to the study of cavitiesSmall voids (or gas-filled bubbles) created by irradiation in crystalline materials can be observed near the Scherzer focus, but it is then difficult to extract other informations than the approximated size.

It is well known that phase contrast in electron microscopy is normally due to the phase shift γ introduced by defocusing and spherical aberration of the objective lens. It is given by Scherzer's formula (2) :(1)where γ is the electron wavelength, Csthe spherical aberration coefficient, θ the diffraction angle and Δz the defocus value.However, in this way, only sections out of the spatial frequency spectrum contained in the abject are transferred to the image. In order to transmit the whole frequency spectrum, it has been proposed that a phase shifting plate is inserted into the back focal plane of the objective lens.

Modern electron microscopes provide film magnifications of X300,000 and higher at resolutions levels below 5 A. However, on these same microscopes it is difficult to obtain a magnification calibration of better than ± 5%, the value given by most microscope manufacturers for the reproducibility of their lenses.The following internal calibration procedure was developed that provides a magnification calibration of ± 1.6% at the 95% confidence level. The procedure utilized a graphitized ISAF carbon black specimen described in detail by Heidenreich et al.1 (see Figure 1). The particles of this calibration sample contain ordered carbon layer planes having an interplanar spacing (d002) o f 3.44 A, as measured by X-ray diffraction. In addition, these layer planes conform to the carbon particle contours and contain a large number of distortions, including curved and bent layer planes and various types of dislocations.