The advantage of providing amplitude and phase information of an object exit-wave makes off-axis electron holography a powerful tool for analyzing field and potential distributions up to an atomic scale. The ability to selectively manipulate the phase of the incoming electron wave additionally opens doors to new possibilities and  microscopic methods, such as the direct interferometric measurement of the coherence length of an electron wave packet [1, 2] or energy-loss magnetic chiral dichroism (EMCD) based investigations [3].

In principle, a selective phase shifting device is realized by directing one of two coherent electron beams through an electrostatic potential surrounded by a grounded electrode to shield stray fields, while the other beam propagates undisturbed in the vacuum. We present two different concepts to create such a device.

The first one is a variation of an experiment G. Moellenstedt suggested in 1980 [1] and was first realized in 1985 by H. Schmid [2]. Instead of using two concentric tubes, we realized a setup with two separated and electrically isolated 1 mm long micro tubes on an especially developed modular carrier chip (M1) to electrify them (figure 1).

The second concept includes two perforated metallic plates (20 µm X 20 µm X 3 µm) in a distance of 3 µm to each other. The micro device shown in figure 2 was produced with focussed ion beam (FIB) allowing both a selective phase shift as well as the investigation of influences of stray fields.

We performed first in-situ biasing TEM experiments with the phase shifting micro tubes. A big and necessary achievement was the enlargement of the hologram width from 2 µm to more than 25 µm. This was realized by using solely the TL22 lens of the image Cs corrector as objective lens of the FEI Titan 80-300 Berlin Holography Special TEM. However, the micrographs in figure 3 show strong artefacts which we attribute to beam tilting due to charging effects of the tube surfaces. These artefacts manifest in a bending-like projection of a biprism beneath the micro tubes.

1.    G. Möllenstedt, G. Wohland, Seventh European Congress on Electron Microscopy, the Hague (1980).

2.    H. Schmid, Dissertation (1985).

3.    P. Schattschneider  et al., Nature 441, 486–488 (2006).

4.   The authors kindly acknowledge the support from the Backend and Packaging Group of the HHI during the development of the M1.

Figures:

Photographic image (left) and a schematic drawing (right) of the phase shifting micro tubes on M1.

SEM images of the metallic plates with four 2 µm holes in diameter.

TEM micrographs of the phase shifting micro tubes (black) with an overlap (white) produced by one electron biprism beneath them showing bending-like artefacts due to charging.

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EMC Abstracts - https://emc-proceedings.com/abstract/concepts-for-an-electrostatic-phase-shifting-device/