High-pass energy-filtered photoemission electron microscopy imaging of dopants in silicon

Differently doped areas in silicon can show strong electron-optical contrast in dependence on the dopant concentration and surface conditions. Photoemission electron microscopy is a powerful surface-sensitive technique suitable for fast imaging of doping-induced contrast in semiconductors. We report on the observation of Si (100) samples with n- and p-type doped patterns (with the dopant concentration varied from 10¹⁶ to 10¹⁹ cm⁻³) on a p- and n-type substrate (doped to 10¹⁵ cm⁻³), respectively. A high-pass energy filter of the entire image enabled us to obtain spectroscopic information, i.e. quantified photo threshold and related photoyield differences depending on the doping level. Measurements have confirmed the possibility of resolving areas at a high contrast even with the lowest dopant concentration when employing the energy filter. The influence of electron absorption phenomena on contrast formation is discussed.     

Elemental imaging by electron energy loss microscopy

Electron energy loss images obtained by parallel filtration at selected energies are used to derive qualitative and quantitative distributions of elements in specimens as diverse as low alloy steel, cartilage, and ribosomal particles. Partial ionization cross-sections for electron scattering from phosphorus L-shells are calculated.     

High resolution imaging by 1 MV electron microscopy

High voltage electron microscopy is well suited to the high resolution imaging of local structural changes in a material. New information at atomic resolution can be obtained on a broad range of material problems. Investigations of the structure of dislocations and planar faults in metallic and covalent crystals is presented.     

Quantitative energy-filtered electron microscopy of biological molecules in ice.

The theoretical and experimental bases for quantitative electron microscopy of frozen-hydrated specimens are described, with special considerations of energy filtration to improve the images. The elastic and inelastic scattering from molecules in vacuum and in ice are calculated, and simple methods to approximate scattering are introduced. Multiple scattering calculations are used to describe the scattering from vitreous ice and to predict the characteristics of images of frozen-hydrated molecules as a function of ice thickness and accelerating voltage. Energy filtration is predicted to improve image contrast and signal-to-noise ratio. Experimental values for the inelastic scattering of ice, the energy spectrum of thick ice, and the contrast of biological specimens are determined. The principles of compensation for the contrast transfer function are presented. Tobacco mosaic virus is used to quantify the accuracy of interpreting image intensities to derive the absolute mass, mass per unit length, and internal mass densities of biological molecules. It is shown that compensation for the contrast transfer function is necessary and sufficient to convert the images into accurate representations of molecular density. At a resolution of 2 nm, the radial density reconstructions of tobacco mosaic virus are in quantitative agreement with the atomic model derived from X-ray results.     

Aberration-corrected multipole Wien filter for energy-filtered x-ray photoemission electron microscopy

The aberration of a multipole Wien filter for energy-filtered x-ray photoemission electron microscopy was analyzed and the optimized Fourier components of the electric and magnetic fields for the third-order aperture aberration corrections were obtained. It was found that the third-order aperture aberration correction requires 12 electrodes and magnetic poles.     

High resolution dark-field electron microscopy

In the last few years some promising images of biological specimens have been obtained using the high contrast and resolution of dark-field electron microscopy. However, important problems of image interpretation and difficulties in specimen preparation limit at the present time, the usefulness of this mode of image formation. The destruction of the sample by the electron beam, is of utmost importance. Some possibilities of partly overcoming it are discussed.     

High resolution and conventional electron microscopy studies of repeated wedge microtwins in monoclinic rare earth sesquioxides

High resolution and conventional studies have been made on the inclined twin boundaries occurring near the tip of wedge microtwins in the case of repeated wedge microtwins occurring at twin intersections. These inclined boundaries appear to be continuous and all the atomic planes are also continuous so that we suggest the use of the term ‘inclined coherent boundaries’ instead of ‘incoherent’ which one generally employs, in analogy to the case of precipitates in a matrix. This is different from what occurs in the case of isolated wedge microtwins for the same material and for the same kind of twins where the inclined twin boundary is really not coherent, being made of non-inclined coherent regions separated by steps containing a dislocation. The coherent inclined boundaries of repeated wedge microtwins appear to be rotated about two perpendicular axes of the habit plane (tilt and twist) giving rise to a distribution of microdislocations whose role has been studied. The values of the tilt and twist angles are related to a minimization of the total energy by a mechanism that is likely to appear only in the case of thin foils. A comparative study of the different roles related to the different structures of inclined twin boundaries for isolated and repeated wedge microtwins has been made, and also a comparative study of the mechanisms of stress relaxation in each case. The studies have been made on (313) twins of a monoclinic samarium sesquioxide.     

Mapping of valence energy losses via energy-filtered annular dark-field scanning transmission electron microscopy

The advent of electron monochromators has opened new perspectives on electron energy-loss spectroscopy at low energy losses, including phenomena such as surface plasmon resonances or electron transitions from the valence to the conduction band. In this paper, we report first results making use of the combination of an energy filter and a post-filter annular dark-field detector. This instrumental design allows us to obtain energy-filtered (i.e. inelastic) annular dark-field images in scanning transmission electron microscopy of the 2-dimensional semiconductor band-gap distribution of a GaN/Al₄₅Ga₅₅N structure and of surface plasmon resonances of silver nanoprisms. In comparison to other approaches, the technique is less prone to inelastic delocalization and relativistic artefacts. The mixed contribution of elastic and inelastic contrast is discussed.     

High resolution in-focus Lorentz electron microscopy

The Foucault in-focus method for viewing magnetic domains in a conventional electron microscope has been modified. The main feature of our modification is to introduce an aperture below the intermediate lens and to use it for stopping out one part of the split central spot instead of the objective aperture. This arrangement substantially reduces the axial astigmatism due to the objective aperture and makes it possible to reach the point resolution 5–6 nm in both light and dark domains. The image resolution is limited by the small image magnification achieved using the three-stage optical system of a Tesla BS 413 microscope.     

Achievement of atomic resolution electron microscopy

Atomic-level details are easily resolved in the latest generation of intermediate voltage electron microscopes, but structural information on the same scale can only be extracted under certain specific conditions. Some understanding of imaging theory, as well as an awareness of correct operating conditions, is required for reliable image interpretation. Several representative examples are chosen to illustrate the possibilities for atomic-resolution imaging of materials, and perspectives and outlook for the technique are briefly discussed.     

Combining accurate defocus with low-dose imaging in high resolution electron microscopy of biological material

High resolution (< 2 nm) electron microscopy of biological specimens requires three exacting conditions to be met simultaneously: (a) fine specimen detail must be protected from destruction by the electron beam (low dose), (b) the electron optics must be adjusted to be capable of imaging that detail interpretably (accurate defocus), and (c) a suitable field of interest must be identified. We describe a method encompassing all three with an 80% success rate using only minor modifications to a transmission electron microscope, and no expensive on-line computing.     

Emission microscopy and related techniques: resolution in photoelectron microscopy, low energy electron microscopy and mirror electron microscopy.

A unified treatment of the resolution of three closely related techniques is presented: emission electron microscopy (particularly photoelectron microscopy, PEM), low energy electron microscopy (LEEM), and mirror electron microscopy (MEM). The resolution calculation is based on the intensity distribution in the image plane for an object of finite size rather than for a point source. The calculations take into account the spherical and chromatic aberrations of the accelerating field and of the objective lens. Intensity distributions for a range of energies in the electron beam are obtained by adding the single-energy distributions weighted according to the energy distribution function. The diffraction error is taken into account separately. A working resolution is calculated that includes the practical requirement for a finite exposure time, and hence a finite non-zero current in the image. The expressions for the aberration coefficients are the same in PEM and LEEM. The calculated aberrations in MEM are somewhat smaller than for PEM and LEEM. The resolution of PEM is calculated to be about 50 A, assuming conventional UV excitation sources, which provide current densities at the specimen of 5 x 10(-5) A/cm2 and emission energies ranging up to 0.5 eV. A resolution of about 70 A has been demonstrated experimentally. The emission current density at the specimen is higher in LEEM and MEM because an electron gun is used in place of a UV source. For a current density of 5 x 10(-4) A/cm2 and the same electron optical parameters as for PEM, the resolution is calculated to be 27 A for LEEM and 21 A for MEM.     

Digital imaging in transmission electron microscopy

The digital revolution currently under way, as evidenced by the rapid development of the Internet and the world‐wide‐web technologies, is undoubtedly impacting the field of transmission electron microscopy (TEM). Digital imaging systems based on charge‐coupled device (CCD) technologies, with pixel array size up to 2 k × 2 k at the present and increasing, are available for TEM applications and offer many attractions. Is it time to phase out film cameras on TEMs and close the darkrooms for good? This paper reviews digital imaging technologies for TEM at different voltages, and contrasts the performance of digital imaging systems with that of TEM film. The performance characteristics of CCD‐based digital imaging systems, as well as methods for assessing them, are discussed. Other approaches to digital imaging are also briefly reviewed.     

Backscattered electron imaging and scanning transmission electron microscopy imaging of multi-layers

Experimental and theoretical results on image contrast of semiconductor multi-layers in scanning electron microscopy investigation are reported. Two imaging modes have been considered: backscattered electron imaging of bulk specimen and scanning transmission imaging of thinned specimens. The following main results have been reached. The image resolution of the multi-layers is, in both cases, defined by the probe size. The contrast, governed by density and atomic number differences, is affected by the size of the interaction volume in backscattered electron imaging and by the beam broadening in scanning transmission. Operating in the scanning transmission mode, the contrast of bright field images can be easily related to local variation in atomic number and density of the specimen while the dark field image contrast is strongly affected by electron beam energy, detector collection angles and specimen thickness. All these factors are able to produce contrast reversals that are difficult to explain without the support of a suitable simulation code.     

Elemental and magnetic sensitive imaging using x-ray excited luminescence microscopy

We demonstrate the potential of x-ray excited luminescence microscopy for full-field elemental and magnetic sensitive imaging using a commercially available optical microscope, mounted on preexisting synchrotron radiation (SR) beamline end stations. The principal components of the instrument will be described. Bench top measurements indicate that a resolution of 1 μm or better is possible; this value was degraded in practice due to vibrations and∕or drift in the end station and associated manipulator. X-ray energy dependent measurements performed on model solar cell materials and lithographically patterned magnetic thin film structures reveal clear elemental and magnetic signatures. The merits of the apparatus will be discussed in terms of conventional SR imaging techniques.     

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.     

Problems of interpretation in high resolution electron microscopy

Current assumptions in wave aberration theory and specimen scattering theory are reviewed. More quantitative image simulations would be valuable as well as use of a wider range of imaging techniques, particularly STEM. The severe difficulties of high resolution three-dimensional reconstruction are described and illustrated.     

High resolution electron microscopy of carbon structure

An investigation is made of the inherent performance of a high-resolution transmission electron microscope applied to the study of developing graphitic-sheet structures in heat-treated, coal-tar pitch carbons. Image detail is shown to be highly dependant on instrumental defocus. It is not obvious which form will be assumed by artefacts in these images; consequently, anomalous features are illustrated by reference to a specific electron-optical case and a corresponding light-optical analogy. Despite the difficulties associated with locating an optimum level of focus, optical diffractometry confirms that adequate conditions of microscopy are attainable on a routine basis. In a quantitative analysis of morphology in coal-tar pitch carbons, the technique reveals the cause of incipient, thermally-induced, molecular distortions which evidently result in a regression in the improvement of order otherwise developing with progressive heat-treatment.     

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.