Preparation of high fidelity holographic vortex masks using advanced <scp>FIB</scp> milling strategies

Holographic masks (HMs) with dislocation gratings placed in the condenser system of a TEM have been proven to be a reliable and robust method to impart quantized orbital angular momentum (OAM), as well as quantized magnetic moment onto the imaging electrons [1]. These so‐called electron vortex beams (EVBs) gathered a lot of attention due to some unusual properties like topological protection [2], peculiar rotation dynamics in magnetic fields [3] and intrinsic chirality. It has been suggested to use a holographic vortex mask as a vorticity filter after the specimen, in the selected area aperture holder, in order to detect spin polarized or other chiral transitions. This would bring up the unique chance to study magnetic properties of amorphous or nanocrystalline materials because the specimen's role as a crystal beam splitter – necessary in the standard energy‐loss magnetic chiral dichroism (EMCD) geometry ‐ is obsolete in this setup. High fidelity HMs are needed for such experiments. Also, in order to achieve high vortex order separation, the grating periodicity should be very fine. To improve the signal‐to‐noise ratio of the EMCD measurements, the dimensions of the HMs need to be large. FIB milling proved to be a robust and reliable technique to produce HMs, but with ever‐increasing demands on structure size and fidelity, the ordinary milling strategy using raster‐ or serpentine scanning showed limited success. Therefore, we developed a new threefold milling strategy. The first step is to employ a so called “vector scan” technique, where “stream”‐files provide the possibility to fully control the position and dwell time of the ion beam to generate spiral milling paths for every hole in the HM structure (see Fig. 1). The next step is to reverse the milling order and ‐direction after each pass [4]. Inspired by [5], the last part consists of a position‐dependent dwell time reduction in the proximity of the hole edges to enhance the HM bar edge fidelity. Fig. 2 shows an exemplary 30 µm vortex mask with a grating periodicity of 500 nm and a thickness of roughly 700 nm. One challenge encountered with this new strategy is limited digital‐to‐analog‐converter resolution as well as memory issues for large “stream”‐files. Using a state of the art FIB, it was possible to cut 50 µm HMs and to compare the ordinary raster scanning technique to the one proposed here, see Figs. 3 and 4. These results indicate that our new threefold scanning ansatz enhances the edge quality. Howerver, issues like sample‐ and beam drift as well as the crystallinity of the mask material have to be addressed in order to further improve the fidelity of the HMs' edges.

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