Is there a Road Map of Aberration Correction towards Ultra-High Rsolution in TEM and STEM?

Abstract

Until the mid of the nineties it was assumed aberration correction is too complicated to be ever used for routine work. However, with the emergence of aberration correctors for high-resolution electron microscopes a new generation of microscopes has been made available. This stimulating development of aberration correctors was first successful with a dedicated Low Voltage Scanning Electron Microscope for which the chromatic and the spherical aberration have been cancelled [1]. About two years later a spherical aberration corrected 200 kV TEM could be finished [2] and a Cscorrected STEM was completed [3] again one year later. The first installations of spherical aberration corrected EMs have shown new results and have proven the possibility to improve the resolution of an EM by means of an aberration corrector [4]. These developments are, however, not finished and the current state and possible future directions will be discussed.In addition to these exciting developments several groups demonstrated the reduction of the energy spread of the electron beam by means of a monochromator. This has proven to further improve the information limit of a TEM equipped with a Schottky-emitter as has been shown by [5] and recently by ZEISS [6]. Additionally, aberration corrected energy filters with increased dispersion became available for spectroscopy and spectroscopic imaging during the last years.Today, after the installation of the first generation of Cs-corrected TEMs and STEMs the question arises: What are the next developments and how will the future microscope look like? The "Road Map" splits in two trends for the future: On one side the improvement of the ultimate resolving power is always the driving parameter for future developments and on the other side the extended applicability with, for example, larger pole-pieces gaps will become at least as equally important.The availability of Cs-correctors for TEM and STEM already had a strong impact on the imaging science. For TEM bright-field imaging, for example, the imaging of light elements and, furthermore, the measurement of the occupancy of oxygen in ceramics [7] with a precision of about 10% has been demonstrated. Delocalisation effects at interfaces are no longer present and TEM dark-field imaging with atomic resolution is now possible. The improved resolving power of a Cs-corrected STEM helps to image single atoms with improved contrast [8], since the beam current at a given probe size is increased by about one order of magnitude, owing to the increased illumination cone.After having demonstrated the possibilities and advantages of spherical aberration correction in electron microscopes it was immediately clear that the achievable resolution is now set by the instrument itself. Unfortunately, the information limit of the currently existing modern instruments set by the incoherent perturbations, for example internal and external time varying fields and chromatic image blur, has not improved as dramatically as their electron-optical properties have. It turns out, th...

A Practical Solution for Eliminating Artificial Image Contrast in Aberration-Corrected TEM

“…In the previous section, we proposed a novel method to overcome the problems based on HRTEM imaging theory and have shown the efficiency of the method by experiments and simulations. In this section, we simulate the results expected in the near future when the limitations from the chromatic aberration and instability of instruments should be improved~Mook & Kruit, 1999;Haider & Muller, 2005!. If an instrument is stabilized enough that mechanical vibration considered in Figure 6b can be ignored, then the results shown in Figure 6c are expected.…”

“…In this section, we simulate the results expected in the near future when the limitations from the chromatic aberration and instability of instruments should be improved~Mook & Kruit, 1999;Haider & Muller, 2005!. In this section, we simulate the results expected in the near future when the limitations from the chromatic aberration and instability of instruments should be improved~Mook & Kruit, 1999;Haider & Muller, 2005!.…”