Joachim Zach

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.

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.

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.