Double aberration correction in a low-energy electron microscope

The lateral resolution of a surface sensitive low-energy electron microscope (LEEM) has been improved below 4 nm for the first time. This breakthrough has only been possible by simultaneously correcting the unavoidable spherical and chromatic aberrations of the lens system. We present an experimental criterion to quantify the aberration correction and to optimize the electron optical system. The obtained lateral resolution of 2.6 nm in LEEM enables the first surface sensitive, electron microscopic observation of the herringbone reconstruction on the Au(1 1 1) surface.     

Atomic imaging using secondary electrons in a scanning transmission electron microscope: experimental observations and possible mechanisms

We report detailed investigation of high-resolution imaging using secondary electrons (SE) with a sub-nanometer probe in an aberration-corrected transmission electron microscope, Hitachi HD2700C. This instrument also allows us to acquire the corresponding annular dark-field (ADF) images both simultaneously and separately. We demonstrate that atomic SE imaging is achievable for a wide range of elements, from uranium to carbon. Using the ADF images as a reference, we studied the SE image intensity and contrast as functions of applied bias, atomic number, crystal tilt, and thickness to shed light on the origin of the unexpected ultrahigh resolution in SE imaging. We have also demonstrated that the SE signal is sensitive to the terminating species at a crystal surface. A possible mechanism for atomic-scale SE imaging is proposed. The ability to image both the surface and bulk of a sample at atomic-scale is unprecedented, and can have important applications in the field of electron microscopy and materials characterization.     

Combining real and reciprocal space information for aberration free coherent electron diffractive imaging

Information from imaging and diffraction planes, or real and reciprocal spaces, of transmission electron microscopes (TEM) can be combined using iterative transformation algorithms to reconstruct the complex wave function, to improve image resolution and to remove residual aberrations in the case of aberration corrected TEM. Here, we describe the experimental and computation techniques needed for combining real and reciprocal space information. We demonstrate these techniques by reconstructing the complex wave function of quantum dots and carbon nanotubes beyond the microscope's resolution limit.     

Strategies for the compensation of specimen-induced spherical aberration in confocal microscopy of skin

We consider various strategies for confocal imaging of human skin which seek to reduce the effects of the specimen-induced aberrations. We calculate the spherical aberration introduced by the stratified structure of skin and show how the confocal signal is affected when attempting to image at various depths within the dermis. Using simple methods it is shown how images might be improved by compensating for the induced aberration. The methods include the use of an iris to reduce the pupil area, changing the refractive index of the immersion medium and using a lens with variable coverglass correction.     

Adaptive aberration correction in a two-photon microscope

We demonstrate aberration correction in two-photon microscopy. Specimen-induced aberrations were measured with a modal wavefront sensor, implemented using a ferro-electric liquid crystal spatial light modulator (FLCSLM). Wavefront correction was performed using the same FLCSLM. Axial scanned ( x z ) images of fluorescently labelled polystyrene beads using an oil immersion lens show restored sectioning ability at a depth of 28 µm in an aqueous specimen.     

A robust focusing and astigmatism correction method for the scanning electron microscope

This paper discusses a new approach to focusing and astigmatism correction based on the fast fourier transforms (FFTs) of scanning electron microscopy (SEM) images. From the FFTs, it is possible to obtain information on the severity of the defocus and astigmatism. This information is then processed by an algorithm to perform real-time focusing and astigmatism correction on the SEM. The algorithm has been tested on defocused and astigmatic images of different samples, including those with highly directional features. Experiments show that the images obtained after running the algorithm can be as good as those that an experienced SEM operator can achieve.     

Design of an aberration corrected low-voltage SEM

The low-voltage foil corrector is a novel type of foil aberration corrector that can correct for both the spherical and chromatic aberration simultaneously. In order to give a realistic example of the capabilities of this corrector, a design for a low-voltage scanning electron microscope with the low-voltage foil corrector is presented. A fully electrostatic column has been designed and characterised by using aberration integrals and ray tracing calculations. The amount of aberration correction can be adjusted relatively easy. The third order spherical and the first order chromatic aberration can be completely cancelled. In the zero current limit, a FW50 probe size of 1.0 nm at 1 kV can be obtained. This probe size is mainly limited by diffraction and by the fifth order spherical aberration.     

Retro‐fitting an older (S)TEM with two Cs aberration correctors for 80 kV and 60 kV operation

For the characterization of light materials using transmission electron microscopy, a low electron acceleration voltage of 80 kV or even 60 kV is attractive due to reduced beam damage to the specimen. The concomitant reduction in resolving power of the microscope can be restored when using spherical aberration (C_s) correctors, which for the most part are only available in the latest and most expensive microscopes. Here, we show that upgrading of existing TEMs is an attractive and cost‐effective alternative. We report on the low‐voltage performance on graphitic material of a JEOL JEM‐2010F built in the early 1990s and retro‐fitted with a conventional imaging C_s corrector and a probe C_s corrector. The performance data show C_s retro‐fitted instruments can compete very favourably against more modern state‐of‐the‐art instruments in both conventional imaging (TEM) and scanning (STEM) modes.     

Digital image processing for nonperiodic objects and an on-line image processing system for high resolution electron microscopy

A digital image processing method for noise removal and image enhancement in nonperiodic structural images is described. The method for noise removal uses a reversible transform between an image and image autocorrelation function. The Laplacian filter is then employed for image enhancement. Furthermore, an on-line image processing system for highresolution TEM is presented.     

Holographic measurement of the wave aberration of an electron microscope by means of the phases in the Fourier spectrum

The resolution limit achievable by holographic correction of the aberrations of an electron microscope depends critically on the information available about the microscope parameters when the hologram was taken. The measuring technique based on symmetry relations of the phases in the Fourier spectrum of the reconstructed electron wave is outlined and experimentally tested.     

Three‐dimensional optical transfer functions in the aberration‐corrected scanning transmission electron microscope

In the scanning transmission electron microscope, hardware aberration correctors can now correct for the positive spherical aberration of round electron lenses. These correctors make use of nonround optics such as hexapoles or octupoles, leading to the limiting aberrations often being of a nonround type. Here we explore the effect of a number of potential limiting aberrations on the imaging performance of the scanning transmission electron microscope through their resulting optical transfer functions. In particular, the response of the optical transfer function to changes in defocus are examined, given that this is the final aberration to be tuned just before image acquisition. The resulting three‐dimensional optical transfer functions also allow an assessment of the performance of a system for focal‐series experiments or optical sectioning applications.     

Linear versus non-linear structural information limit in high-resolution transmission electron microscopy

A widely used performance criterion in high-resolution transmission electron microscopy (HRTEM) is the information limit. It corresponds to the inverse of the maximum spatial object frequency that is linearly transmitted with sufficient intensity from the exit plane of the object to the image plane and is limited due to partial temporal coherence. In practice, the information limit is often measured from a diffractogram or from Young's fringes assuming a weak phase object scattering beyond the inverse of the information limit. However, for an aberration corrected electron microscope, with an information limit in the sub-angstrom range, weak phase objects are no longer applicable since they do not scatter sufficiently in this range. Therefore, one relies on more strongly scattering objects such as crystals of heavy atoms observed along a low index zone axis. In that case, dynamical scattering becomes important such that the non-linear and linear interaction may be equally important. The non-linear interaction may then set the experimental cut-off frequency observed in a diffractogram. The goal of this paper is to quantify both the linear and the non-linear information transfer in terms of closed form analytical expressions. Whereas the cut-off frequency set by the linear transfer can be directly related with the attainable resolution, information from the non-linear transfer can only be extracted using quantitative, model-based methods. In contrast to the historic definition of the information limit depending on microscope parameters only, the expressions derived in this paper explicitly incorporate their dependence on the structure parameters as well. In order to emphasize this dependence and to distinguish from the usual information limit, the expressions derived for the inverse cut-off frequencies will be referred to as the linear and non-linear structural information limit. The present findings confirm the well-known result that partial temporal coherence has different effects on the transfer of the linear and non-linear terms, such that the non-linear imaging contributions are damped less than the linear imaging contributions at high spatial frequencies. This will be important when coherent aberrations such as spherical aberration and defocus are reduced.     

5.4 nm spatial resolution in biological photoemission electron microscopy

We report a spatial resolution of 5.4 nm in images of sarcoplasmic reticulum from rabbit muscle. The images were obtained in an aberration-corrected photoemission electron microscope with a hyperbolic mirror as the correcting element for spherical and chromatic aberration. In-situ measurements and numerical simulations confirm the low residual aberration in the instrument and indicate the ultimate resolution in this type of microscopy to be below 2 nm.     

Quantitative phase-sensitive imaging in a transmission electron microscope

This paper presents a new technique for forming quantitative phase and amplitude electron images applicable to a conventional transmission electron microscope. With magnetised cobalt microstructures used as a test object, we use electron holography to obtain an independent measurement of the phase shift. After a suitable calibration of the microscope, we obtain quantitative agreement of the phase shift imposed on the 200 keV electrons passing through the sample.     

On the importance of fifth-order spherical aberration for a fully corrected electron microscope

Next generation aberration correctors will not only eliminate the third-order spherical aberration, but also improve the information limit by correction of chromatic aberration. As a result of these improvements, higher order aberrations, which have largely been neglected in image analysis, will become important. In this paper, we concern ourselves with situations where sub-A resolution can be achieved, and where the third-order spherical aberration is corrected and the fifth-order spherical aberration is measurable. We derive formulae to explore the maximum value of the fifth-order spherical aberration for directly interpretable imaging and discuss the optimum imaging conditions and their applicable range.     

A simple liquid-He-cooled specimen stage for an ultra-high resolution transmission electron microscope

A simple top-entry specimen holder for an ultra-high resolution transmission electron microscope which has demonstrated 2.5 Å point-to-point resolution at 30 K is described. The stage has proved useful in studying low-temperature solid phases and phase transformations and is expected to be effective in reducing radiation damage for some organic specimens.     

Low-cost image-capture system for a scanning electron microscope

We describe a PC-based active-capture system for recording digital images from a scanning electron microscope. The system is based on a National Instruments data-acquisition board and a Pentium computer, controlled by software that we have written in Visual Basic.     

High temperature controlled atmosphere experiments for catalysts in a transmission electron microscope

A new transmission electron microscopy (TEM) specimen preparation procedure for high temperature experiments using a controlled atmosphere specimen holder (HTCASH) has been developed. It is designed for studying the microstructure of catalyst specimens before and after treatments in various gases. The procedure involved (1) finding a new formula for the embedding material, (2) devising a new method of making specimen supports, and (3) developing a method for removing the embedding material after the specimen has been microtomed. These techniques were then brought together to produce the ideal specimens for the HTCASH experiments. As an extra benefit, this procedure is also suitable for preparing specimens for ultrahigh resolution imaging experiments. The application of the new procedure in HTCASH experiments is illustrated through a high temperature reduction of a Co/SiO₂-923 catalyst.     

Nanoanalysis by a high-resolution energy filtering transmission electron microscope

An energy-filtering transmission electron microscope with 300 kV acceleration voltage was developed and the spatial resolution of elemental distribution images was improved. Observing oxygen monolayers in Al(11)O(3)N(9), it was shown that the actual resolution attained is up to 0.5 nm. Surface plasmon loss images of silver particles were taken with a resolution of better than 0.4 nm. Furthermore, the sensitivity is sufficiently high to distinguish indium content differences of 2.5 atomic percent in In(x)Al(1-x)As. This performance is good enough to analyze elemental distribution with atomic-level resolution. Furthermore, since analysis with the energy-filtering microscope is easy and practical, nanoanalysis may come into wide use not only in academic fields but also in industry.