Stephan Uhlemann

Electron microscopy image enhanced

One of the biggest obstacles in improving the resolution of the electron microscope has always been the blurring of the image caused by lens aberrations. Here we report a solution to this problem for a medium-voltage electron microscope which gives a stunning enhancement of image quality.

Residual wave aberrations in the first spherical aberration corrected transmission electron microscope

Here we demonstrate the optical properties of a spherical aberration corrected transmission electron microscope by means of beam tilt series. The high-resolution capabilities are characterized by the measured residual wave aberrations up to the fifth order. Limits for the wave aberration coefficients are determined. We compare the phase-contrast transfer function of the corrected versus the uncorrected objective lens with the help of diffractograms. The information limit and the improvement of the point resolution is discussed.

Upper limits for the residual aberrations of a high-resolution aberration-corrected STEM

The development of correctors for electron optical systems has already brought the improvement of resolution for a low-voltage scanning electron microscope and a commercially available transmission electron microscope and is anticipated in the near future for a dedicated scanning transmission electron microscope (STEM). The resolution attainable especially of a probe-forming system at 200 kV cannot only be estimated from calculations ignoring all non-rotationally symmetric axial aberrations in an electron optical system. For a certain resolution, one would like to attain, the influence of the deviations from the ideal, aberration-free system has to be investigated. Therefore, in the following we have carried out the evaluation of the required accuracy for the compensation of the various residual aberrations in order to achieve a resolution in the sub-Angstrom regime with a probe-forming system.

Advancing the hexapole Cs-corrector for the scanning transmission electron microscope.

Aberration correctors using hexapole fields have proven useful to correct for the spherical aberration in electron microscopy. We investigate the limits of the present design for the hexapole corrector with respect to minimum probe size for the scanning transmission electron microscope and discuss several ways in which the design could be improved by rather small and incremental design changes for the next generation of advanced probe-forming systems equipped with a gun monochromator.

First application of Cc-corrected imaging for high-resolution and energy-filtered TEM

Contrast-transfer calculations indicate that C(c) correction should be highly beneficial for high-resolution and energy-filtered transmission electron microscopy. A prototype of an electron optical system capable of correcting spherical and chromatic aberration has been used to verify these calculations. A strong improvement in resolution at an acceleration voltage of 80 kV has been measured. Our first C(c)-corrected energy-filtered experiments examining a (LaAlO(3))(0.3)(Sr(2)AlTaO(6))(0.7)/LaCoO(3) interface demonstrated a significant gain for the spatial resolution in elemental maps of La.

Information Transfer in a TEM Corrected for Spherical and Chromatic Aberration

For the transmission electron aberration-corrected microscope (TEAM) initiative of five U.S. Department of Energy laboratories in the United States, a correction system for the simultaneous compensation of the primary axial aberrations, the spherical aberration Cs, and the chromatic aberration Cc has been developed and successfully installed. The performance of the resulting Cc /Cs-corrected TEAM instrument has been investigated thoroughly. A significant improvement of the linear contrast transfer can be demonstrated. The information about the instrument one obtains using Youngs fringe method is compared for uncorrected, Cs-corrected, and Cc /Cs-corrected instruments. The experimental results agree well with simulations. The conclusions might be useful to others in understanding the process of image formation in a Cc /Cs-corrected transmission electron microscope.

CHAPTER 2 – Present and Future Hexapole Aberration Correctors for High-Resolution Electron Microscopy

A deeper understanding of the non-ideal hexapole corrector with manufacturing tolerances and alignment elements attained by computer algebraic perturbation methods was helpful to trim the tree of possibilities. Of equal importance was the increasing availability of hardware for numerical image processing to characterize the state of alignment in an efficient and reliable manner and to generate the required feedback for the corrector control. Therefore, the required rotation of the inner two hexapole elements might not matter too much because it can be fixed by design. However, this version of an aplanat has two more transfer lenses and therefore is longer than the other solutions. It seems very likely that an aplanatic system will be the next step in the future development of the hexapole corrector for applications that demand a large isoplanatic field of view.

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.

Thermal magnetic field noise: electron optics and decoherence.

Thermal magnetic field noise from magnetic and non-magnetic conductive parts close to the electron beam recently has been identified as a reason for decoherence in high-resolution transmission electron microscopy (TEM). Here, we report about new experimental results from measurements for a layered structure of magnetic and non-magnetic materials. For a simplified version of this setup and other situations we derive semi-analytical models in order to predict the strength, bandwidth and spatial correlation of the noise fields. The results of the simulations are finally compared to previous and new experimental data in a quantitative manner.

Overview of Commercially Available CEOS Hexapole-type Aberration Correctors

Aberration correctors have become essential equipment for high-resolution imaging and spectroscopy in STEM and CTEM. This is impressively documented by the large and still rapidly growing number of hexapole-type imaging and probe correctors installed all over the world. The optical design of the hexapole Cs-correctors in essence is based on the theoretical studies of Rose [1, 2]. It has been put into practice by Haider et al. during 1992–1997 [3, 4] and consists of a hexapole doublet and two transferlens systems. This design provided the basis for the CEOS imaging correctors (CETCOR) which are available for a variety of commercial TEM instruments. Subsequently, a similar design could be used to correct for the spherical aberration of the probe-forming system in a STEM, as well. Nowadays, it is not uncommon to have a double-corrected instrument with a probe corrector above and an imaging corrector below the objective lens.