Spatial resolution and energy filtering of backscattered electron images in scanning electron microscopy.

Numerical simulations of energy filtering effects on backscattered electron images of semiconductor multilayers are reported. The theoretical investigation has been performed for a wide range of energies, 1-40 keV, and for beam incidence angles between 90 degrees (normal incidence) and 20 degrees. Quite a general purpose of this research concerns the investigation of the optimum energy conditions and of their implications. It will be shown that the optimum energy defines an operating context suitable to ensure a compositional contrast enhancement; i.e. a minimum threshold current and a maximum resolution, without energy filtering, independent of the beam incidence angle. This optimum energy, depending on the specimen and its details, is, however, of the order of a few keV or less for specimen details having a size of the order of few nm. When the performance of the electron gun does not allow to work at low energy it is necessary to operate at an energy higher than the optimum one, the energy filtering can produce positive effects. Yet in those circumstances there is an optimum energy loss window suitable to minimise the threshold current. It spreads from 10-30%, depending on the primary energy and size of the compositional detail, for normal incidence, to a few per cent for high incidence angles and high energy. The simulation results for these last conditions are in agreement with the well-known experimental results obtained with the low-loss methods.     

High resolution biological scanning electron microscopy: A comparative study of low temperature metal coating techniques

Structural information on the surface of biological specimens can be resolved within molecular dimensions by “in-lens” field emission scanning electron microscopes when cryo-methods are used to adequately preserve the native state of the specimen. The visual definition of molecular surface structures depends largely on the metal coating. The thickness of the coating, as well as the temperature at which it is deposited, are among the most important parameters affecting visual definition. These were evaluated on T4 polyheads and T4D phages using chromium double-axis rotary shadowing (DARS). Micrographs of optimally DARS coated T4 polyheads and T4D phages were compared with chromium planar-magnetron sputtering (PMS) and unidirectional shadowing with platinum/carbon. Metal deposition was carried out at low temperatures during all three procedures. Optimal visual definition of structural details on the surface of DARS coated T4 polyheads and T4D phages (capsomeres of T4 polyheads and their subunits with diameters of 8 and 3 nm; T4D phage tail fibres with a thickness of 3 nm) is achieved at a thickness of the chromium film greater than the minimum required for metal film coalescence. Chromium DARS coating at room temperature resulted in poor structural definition, whereas DARS at specimen temperatures of ?85°C and ?150°C, with the chromium thickness optimized for each temperature, yielded good visual detail of polyhead substructures. The visual definition was slightly reduced when DARS coating was carried out at a specimen temperature of ?250°C. Adequate structural visibility of T4D phage and T4 polyhead surface structures was achieved with the three coating techniques tested. Smaller filamentous structures, however (e.g., phage tail fibres), were more clearly identified after chromium DARS coating or unidirectional platinum/carbon shadowing than after optimized PMS with chromium. Each method has its own merits.     

Selected area low energy electron diffraction and microscopy

The possibility of using the cathode lens for selected area low energy electron diffraction (LEED) and microscope in selected reflection is discussed. The column design for separated primary and diffracted beams is described.     

Prospects in high resolution X-ray photoelectron microscopy

After a brief review of the present state of the art in X-ray photoelectron microscopy, the prospects to improve the nominal value of the spatial resolution are described. Progress in this method is also related to the development of dense X-ray sources, but possible radiation damage effects have to be considered.     

Development of methodology for low exposure, high resolution electron microscopy of biological specimens.

Specimen damage resulting from inelastic scattering is one of the factors that limits high-resolution electron microscopy of biological specimens. We have, therefore, sought to develop a method to record images of periodic objects at a reduced electron exposure in order to preserve high-resolution structural detail. The resulting image will tend increasingly to be a statistically noisy one, as the electron exposure is reduced to lower and lower values. Construction of a statistically defined image from such data is possible by spatial averaging of the electron signals from a large number of identical unit cells. In this paper, we have first investigated the theory pertaining to the attainable resolution as a function of the electron exposure, the magnification, and several other relevant parameters. In addition, we report experimental results obtained with a commercial image intensifier and with nuclear track photographic emulsion, both of which are highly sensitive recording devices. Usable images can be recorded and processed at exposures in the image plane as low as 10(-3) electron/micron2 (1.6 x 10(-14) coulomb/cm2).     

Very low energy electron microscopy of graphene flakes

Commercially available graphene samples are examined by Raman spectroscopy and very low energy scanning transmission electron microscopy. Limited lateral resolution of Raman spectroscopy may produce a Raman spectrum corresponding to a single graphene layer even for flakes that can be identified by very low energy electron microscopy as an aggregate of smaller flakes of various thicknesses. In addition to diagnostics of graphene samples at larger dimensions, their electron transmittance can also be measured at very low energies.     

Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy. An introduction to the proceedings of the Second International Symposium and Workshop on Emission microscopy and Related Techniques

Emission microscopes and related instruments comprise a specialized class of electron microscopes that have in common an acceleration field in combination with the first stage of imaging (i.e., an immersion objective lens, also called a cathode lens or emission lens). These imaging techniques include photoelectron emission microscopy (PEEM or PEM), electron emission induced by heat, ions, or neutral particles, mirror electron microscopy (MEM), and low-energy electron microscopy (LEEM), among others. In these instruments the specimen is placed on a flat cathode or is the cathode itself. The low-energy electrons that are emitted, reflected, or backscattered from the specimen are first accelerated and then imaged by means of an electron lens system resembling that of a transmission electron microscope. The image is formed in a parallel mode in all of the above instruments, in contrast to the image in scanning electron microscopes, where the information is collected sequentially by scanning the specimen. A brief history and introduction to emission microscopy, MEM, and LEEM is presented as a background for the Proceedings of the Second International Symposium and Workshop on this subject, held in Seattle, Washington, August 16-17, 1990. Current trends in this field gleaned from the presentations at that meeting are discussed.     

Enhanced spatial resolution in vector potential photoelectron microscopy

The spatial resolution of the vector potential photoelectron microscope is determined by the maximum size of the cyclotron orbits of the imaged electrons at the surface of a sample. It is straightforward to calculate the spatial resolution for any imaged electron energy given the magnetic field strength. However, in low‐energy secondary photoelectron images from an aluminium–calcium metal matrix alloy, we find the apparent spatial resolution is significantly higher than expected. A possible explanation for the enhanced resolution is that the low‐energy cyclotron orbits are distorted when passing from one area of work function to another and the image is dependent on the surface field distribution.     

The resolution of the low energy electron reflection microscope

The resolution attainable by imaging a surface with reflected electrons with energies from zero to several hundred eV in an immersion lens is discussed. It is shown that resolutions in the nm range can be obtained.     

Kodirex for high resolution electron microscopy.

Measurement of the important performance parameters shows that Kodak Kodirex film is more suitable than conventional ones for dark-field transmission electron microscopy of molecules at very high mignification. Results are cited for 120 kV; but the relation ship is valid up to 3 MV.     

Medipix 2 detector applied to low energy electron microscopy

Low energy electron microscopy (LEEM) and photo-emission electron microscopy (PEEM) traditionally use microchannel plates (MCPs), a phosphor screen and a CCD-camera to record images and diffraction patterns. In recent years, however, MCPs have become a limiting factor for these types of microscopy. Here, we report on a successful test series using a solid state hybrid pixel detector, Medipix 2, in LEEM and PEEM. Medipix 2 is a background-free detector with an infinite dynamic range, making it very promising for both real-space imaging and spectroscopy. We demonstrate a significant enhancement of both image contrast and resolution, as compared to MCPs. Since aging of the Medipix 2 detector is negligible for the electron energies used in LEEM/PEEM, we expect Medipix to become the detector of choice for a new generation of systems.     

Very low voltage electron microscopy.

We conclude that a 150 V scanning microscope with a resolution of 10 A is quite feasible and could have considerable value. It might consist of a field emission source, an electron gun to decelerate the electrons, a condenser lens to produce a parallel beam, a multipole corrector and a short focal length objective lens. Electrons reflected from the specimen surface would pass through a spectrometer whose principal features would be a large collecting power and low (1/200) energy resolution. Finally, we should add that such a microscope presents a considerable challenge and new opportunities for the electron optician in both lens and spectrometer design. We cannot refrain from pointing out that the Scherzer theorem does not necessarily hold for such a lens since the constraints of the theorem do not apply to this case.     

Resolving power of lens-less low energy electron point source microscopy

Simulations show the resolving power of lens-less low energy electron point source microscopy to be approximately 15 Å  for electron energies of between 14 and 105 eV. This resolution can be improved to 10 Å  by employing a composite hologram method. However, these values fall short of predictions of at least 3 Å  resolution for electron energies in this range because the limited divergence angle of the electron beam had not previously been taken into account. Also shown is that electron coherence from field emitting tips that terminate in a single atom will not limit the resolving power as much as the beam divergence angle. The penetration depth of electrons with energies of between 50 and 100 eV, into biological molecules, was estimated from the inelastic mean free path length to be 25 Å . This places an upper limit on the size of biological molecules for which internal structural information can be obtained using low energy electrons.     

Visualization of bloch waves of high energy electrons in high resolution electron microscopy

By applying the Bloch wave theory of electron diffraction to the problem of lattice images in high resolution electron microscopy, it is shown that those characteristic type of images as found for Ge <110> can be interpreted as images of individual Bloch waves formed by the incident electron inside the lattice.     

On dark field techniques in transmission electron microscopy.

Different dark field techniques in transmission electron microscopy are investigated. In particular, a displaced aperture technique is developed, which produces high quality off-axis dark field images. The essential feature of the method is that the chromatic aberration of the objective and intermediate lenses is compensated for with the aid of a one-stage beam deflector in the object plane of the intermediate lens. In addition, the image distortion and the effect of the curvature of the image field are minimized by reducing the coefficient of sperical aberration of the intermediate lens. Diffracted beams, forming angles alpha up to 2.5 degrees with the optical axis, can be used for dark field imaging. Resolutions better than 30 A for alpha = 2 degrees, and 17 A for alpha = 1 degree, are obtained.     

Sialolith characterization by scanning electron microscopy and X-ray photoelectron spectroscopy

The objective of this study has been to characterize sialolith, a calcium phosphate deposit that develops in the human oral cavity, by high-resolution field emission scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The morphological and chemical data obtained helped in the determination of their formation mechanism in salivary glands. Sialoliths in the submandibular salivary glands may arise secondary to sialodenitis, but not via a luminal organic nidus. We believe this is the first study that characterizes a sialolith by XPS.     

Procedures for focusing,stigmating and alignment in high resolution electron microscopy

Several different approaches to the alignment, stigmating and focusing of a high resolution electron microscope are evaluated theoretically and experimentally. Ambiguities in the interpretation of diffractograms are pointed out which make it necessary to explore a range of incident beam directions before the correct alignment can be established. The variation of image contrast with the imaging conditions is examined in detail, and its global minimization is shown to be a reliable criteria for adjustment of all parameters. Recommendations are made as to the best procedures to adopt in various circumstances, and a computer-controlled procedure based on the image contrast is described which sets all parameters automatically in less than a minute.     

Elemental imaging and resolution in energy-filtered conventional electron microscopy

Energy-filtered transmission electron microscopy (EFEM) was used to image the distributions of uranium and carbon in uranyl acetate stained catalase crystals. The spatial resolution obtained from inelastic C K-edge and U O4,5-edge images, determined from the highest-order reflection in the computed diffraction pattern, was 3.4 nm for both carbon and uranium. The resolution limit imposed by the delocalization of inelastic scattering was estimated from cross-section measurements to be 0.6 nm for U and 0.2 nm for C. Considering both delocalization and the effects of microscope aberrations, for an objective lens chromatic aberration coefficient of 2.8 mm and 10 eV energy window, the calculated resolutions are 2.0 nm for C and 1.2 nm for U. The effects of plural inelastic and elastic-inelastic scattering were sufficiently large to show crystalline structure in unprocessed pre-edge inelastic images. Previously suggested methods for eliminating these artifacts were applied to obtain the compositional information in the catalase EFEM images.     

High resolution shadowing for electron microscopy by sputter deposition.

The sputtering process by an ion beam well collimated and highly accelerated provides a valuable means of high resolution shadowing, replication of a fine object by a combination of pre-shadowing and deposition as well as a preparation of supporting films. High resolution shadowing and films with grains smaller than 1 nm can be obtained by argon ion-sputtering targets of tungsten and tungsten/tantalum alloy. The resolution of carbon replicas pre-shadowed with tungsten/tantalum is determined from the radius of curvature of replicated magnesium oxide crystal corners.     

On-line image processing in high resolution electron microscopy

On-line processing and analysis of high resolution electron microscope (HREM) images has been implemented using a software controlled image processor based on a commercial digital video framestore. It directly digitizes the output from the low light TV camera on the HREM and applies any processing necessary. The conditions under which images from amorphous specimens are being obtained can be established on-line. The fast fourier transforms (FFTs) of 256 times 256 pixels produced in 15 s with the system off-line are comparable to those from diffraction analysis of the same data on a laser optical bench. The precision of analysis of defocus, aberrations and astigmatism on-line is discussed, together with possible approaches to routine analysis of crystalline specimens and for interactive control.