Harald Rose

Conditions and reasons for incoherent imaging in STEM

Abstract The origin of incoherent imaging in STEM has been analysed by investigating the effects of the detector geometry and of the thermal vibrations of the atoms on the image formation. The conditions for incoherent imaging are discussed. In this case the Fourier transforms of the intensities at the exit plane of the object and at the image plane are linearly related with each other. The corresponding transfer function coincides with the modulation transfer function for incoherent imaging in TEM. By analysing the properties of the degree of coherence, the reasons for the suppression of the interference terms are shown and detector arrangements are found which yield largely incohenrent images. The validity of the semianalytical results for thin objects are also confirmed numerically for thick objects by means of a modified multislice algorithm. With increasing object thickness the phonon scattered electrons dominate the image intensity. Detector arrangements were found for which the elastic part of the image shows contrast reversal. The dependence of the Z -contrast on the geometry of the annular detector and on the atomic number Z is investigated in detail.

XPEEM WITH ENERGY-FILTERING: ADVANTAGES AND FIRST RESULTS FROM THE SMART PROJECT

The second development step of the SMART project, i.e. an energy-filtered but not yet corrected photoelectron emission microscope, operates at the undulator U49/1-PGM beamline at BESSY II. It already demonstrates the variety of methods of the final version: microscopy, spectroscopy and electron diffraction. Some recent experimental results are reported for these three operation modes. In addition, the theoretical improvement of lateral resolution and transmission of PEEMs in general by using an energy filter is discussed for systems without and with aberration correction.

SMART: An Aberration-Corrected XPEEM/LEEM with Energy Filter

A new UHV spectroscopic X-ray photoelectron emission and low energy electron microscope is presently under construction for the installation at the PM-6 soft X-ray undulator beamline at BESSY II. Using a combination of a sophisticated magnetic beam splitter and an electrostatic tetrode mirror, the spherical and chromatic aberrations of the objective lens are corrected and thus the lateral resolution and sensitivity of the instrument improved. In addition a corrected imaging energy filter (a so-called omega filter) allows high spectral resolution (ΔE=0.1 eV) in the photoemission modes and back-ground suppression in LEEM and small-spot LEED modes. The theoretical prediction for the lateral resolution is 5 A; a realistic goal is about 2 nm. Thus, a variety of electron spectroscopies (XAS, XPS, UPS, XAES) and electron diffraction (LEED, LEEM) or reflection techniques (MEM) will be available with spatial resolution unreached so far.

Correction of aberrations, a promising means for improving the spatial and energy resolution of energy-filtering electron microscopes

Abstract It is shown that the correction of aberrations is the most promising procedure for improving significantly the performance of electron microscopes. The basic parts of an analytical sub-angstrom transmission electron microscope (SATEM) are discussed. The proposed analytical SATEM should yield a resolution limit of about 0.6 A and allow isochromatic energy filtering with energy windows below 1 eV. For achieving these limits in practice, the incoherent parasitic aberrations must be sufficiently suppressed. In addition a monochromator for reducing the energy width of the incident electrons is mandatory. The correction of the aberrations is discussed for the objective lens, the energy filter and the projector system.

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(111) surface.

Historical aspects of aberration correction

A brief history of the development of direct aberration correction in electron microscopy is outlined starting from the famous Scherzer theorem established in 1936. Aberration correction is the long story of many seemingly fruitless efforts to improve the resolution of electron microscopes by compensating for the unavoidable resolution-limiting aberrations of round electron lenses over a period of 50 years. The successful breakthrough, in 1997, can be considered as a quantum step in electron microscopy because it provides genuine atomic resolution approaching the size of the radius of the hydrogen atom. The additional realization of monochromators, aberration-free imaging energy filters and spectrometers has been leading to a new generation of analytical electron microscopes providing elemental and electronic information about the object on an atomic scale.

Future trends in aberration-corrected electron microscopy

The attainable specimen resolution is determined by the instrumental resolution limit di and by radiation damage. Solid objects such as metals are primarily damaged by atom displacement resulting from knock-on collisions of the incident electrons with the atomic nuclei. The instrumental resolution improves appreciably by means of aberration correction. To achieve atomic resolution at voltages below approximately 100 kV and a large number of equally resolved image points, we propose an achromatic electron–optical aplanat, which is free of chromatic aberration, spherical aberration and total off-axial coma. Its anisotropic component is eliminated either by a dual objective lens consisting of two separate windings with opposite directions of their currents or by skew octopoles employed in the TEAM corrector. We obtain optimum imaging conditions by operating the aberration-corrected electron microscope at voltages below the knock-on threshold for atom displacement and by shifting the phase of the non-scattered wave by π/2 or that of the scattered wave by −π/2. In this negative contrast mode, the phase contrast and the scattering contrast add up with the same sign. The realization of a low-voltage aberration-corrected phase transmission electron microscope for the visualization of radiation-sensitive objects is the aim of the proposed SALVE (Sub-Å Low-Voltage Electron microscope) project. This microscope will employ a coma-free objective lens, an obstruction-free phase plate and a novel corrector compensating for the spherical and chromatic aberrations.

Elastic and inelastic scattering cross-sections of amorphous layers of carbon and vitrified ice

Abstract Elastic and inelastic scattering cross-sections of amorphous layers of carbon and vitrified ice have been determined by electron spectroscopic diffraction (ESD). Using an energy-filtering TEM (EFTEM), elastic and inelastic differential scattering distributions were recorded separately on Image Plates (IP) and quantified. The thickness of carbon films was measured photometrically, that of ice layers by direct imaging. The elastic cross-sections are in good agreement with theory and previous experimental data. The measured inelastic scattering cross-sections are higher than the values derived from theoretical models for free atoms because these models do not account for collective excitations and binding effects. The short mean free path length for inelastic scattering indicates the importance of zero-loss energy filtering for imaging of biological samples embedded in amorphous ice.

Acceptance of imaging energy filters

Abstract The acceptance of imaging filters used in energy-filtering transmission electron microscopy is discussed. It is shown that the acceptance of all presently available filters is nearly equal. The acceptance of in-column filters can be increased by two orders of magnitude if the dispersion is enlarged and the second-order aberrations are eliminated.

Mirror corrector for low-voltage electron microscopes

Publisher Summary The beam separator and the electron mirror have been constructed and assembled. This chapter discusses the mechanical setup of mirror corrector. The testing of the components was carried out in a conventional scanning electron microscope. The theoretical resolution limit of present, uncorrected, direct-imaging low-energy electron microscopes is limited to 5 nm. In the case of electron illumination (LEEM), the best edge resolution achieved is 10 nm, while with photon illumination (PEEM), resolution limits of only 20 nm have been observed. The difference is mainly caused by the decreased electron yield in the latter case that leads to long recording times and hence to problems with mechanical and electrical stability. With the aid of a mirror corrector for simultaneous correction of chromatic and spherical aberrations, the spatial resolution limit can, in principle, be lowered to 0.5 nm, or alternatively it is possible—at a resolution comparable to that of uncorrected microscopes—to increase the limiting aperture angle by a factor of 4–10. The electron gain then ranges between 16 and 100. This application of the corrector is, therefore, especially interesting for the PEEM mode.