Max Haider

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.

Experimental Contrast of Atomically-resolved Cc/Cs-corrected 20-80kV SALVE Images of 2D-objects Matches Calculations

Spherical aberration correction has become inevitable for atomic resolution imaging in transmission electron microscopy of conventional objects at medium accelerating voltages (100-300kV). However, there are classes of materials such as low-Z-number and/or low-dimensional materials that require lower accelerating voltages as their knock-on damage threshold is below 80kV. Unfortunately, the resolution of spherical aberration-corrected TEM at an accelerating voltage less than 80 kV with a conventional electron source is strongly limited by the chromatic aberration of the objective lens. There are two approaches to achieve atomic resolution at lower voltages: either develop a monochromated electron source with an particularly small energy width but reduced beam current [1, 2], or develop an aberration corrector that corrects for both, the spherical and the chromatic aberration of the objective lens [3, 4] which then can be used together with a standard field-emission gun. Here we report on a fruitful approach that combines the Cc/Cs-corrected 2080kV SALVE technology with experiments on two-dimensional objects aimed to compare experimental and calculated image contrast.

Towards High Resolution in TEM and STEM: What are the Limitations and Achievements

The resolving power of a microscope is one of the most important parameters especially when talking about modern high resolution instruments like TEM or STEM. In contrary, the attainable resolution depends on the resolving power of the microscope and the object and it can only be measured when imaging a certain object detail. Therefore, resolving power in this context means the object independent capability of an instrument to image a certain minimal size of an ideal object detail only limited by instrumental parameters. The resolution is the minimal detail of a certain object which can be resolved by an electron microscope.

On Proper Phase Contrast Imaging in Aberration Corrected TEM

Since the realization of the first aberration corrected TEM [1] the number of aberration corrected TEMs is still rapidly increasing. Two key benefits have enabled this tremendous success of spherical aberration correction: improved point resolution limit and vanishing delocalization. Many thoughts have been spent on the proper ‘design’ of the phase contrast transfer function (PCTF) of an ‘ideal’ microscope, where only round aberration coefficients (defocus C1 and spherical aberration coefficients of different orders (CS=C3, C5, . . . ) are taken into account, see e.g. [2, 3]. In practice, however, many users only pay attention to a (global) π/4-limit for all or part of the aberration coefficients. Here we want to bridge the gap between the two approaches and point the way to achieve proper phase contrast imaging even if unavoidable residual aberrations are present.