Axel Lubk (Dresden / DE), Renato Huber (Dresden / DE), Felix Kern (Dresden / DE), Dimitry Karnaushenko (Chemnitz / DE), Arsha Thampi (Dresden / DE), Bernd Büchner (Dresden / DE), Daniil Karnaushenko (Chemnitz / DE), Oliver Schmidt (Chemnitz / DE)
Abstract text (incl. figure legends and references)
Introduction: Tunable electromagnets and corresponding devices, such as magnetic lenses or stigmators, are the backbone of numerous charged particle optical (CPO) instruments, such as electron microscopes. However, the built-in electromagnets are typically macroscopic conducting coils, which cannot generate swiftly changing magnetic fields due to their large resistivity and inductivity, require active cooling, and are structurally bulky. These restriction make them unsuitable for fast beam manipulation and miniaturized applications. To overcome these limitations, miniaturization of tuneable magnetic devices is required.
Objectives: We seek on-chip microsized magnetic charged particle optics, which can be fabricated by microprocessing technologies, such as lithography. These devices should generate alternating magnetic fields of several 100 mT at frequencies up to GHz, supplying sufficiently large optical power for CPO applications. That particular includes fast spatio-temporal electron beam modulation such as electron beam deflection, focusing, and wave front shaping.
Methods: Recently developed microcoil technology [1] allows overcoming the obstacles haunting miniaturization of magnetic CPO devices[PR3]. These microcoils (diameter: several 10 µms) are fabricated via lithography-patterned microelectronic structures, which assemble into 3D tubular architectures due to strain engineering within the multilayered polymer structure (Fig. 1a). The current-carrying copper layer of more than 100 nm thickness sustains more than 100 mA current in vacuum that generates a field of several mT depending on the number of windings. That field is further increases by introducing a soft-ferromagnetic wire (radius 5μm,
saturation magnetization 0.2T, relative permeability 1000) into the zero-pitch microcoils (Fig. 1a).
Results: Adapting the previously described microcoil technology, the whole platform was fabricated directly around an aperture plate in a multipolar arrangement (i.e., as monopole, dipole and quadrupole, Fig. 1b). The micron-sized devices were then mounted in dedicated high-frequency aperture holders, facilitating fast switching of the devices within a TEM. With that, alternating magnetic fields of about ±100 mT up to a hundred MHz, supplying sufficiently large optical power for numerous CPO applications is feasible (Fig. 2), including fast beam blanking and focusing for stroboscopic imaging. Notably, the quadrupole devices allow switchable line foci of a focal length of ca. 5 cm at 300 kV acceleration voltage, which is comparable to macroscopic bulk devices [2].
[1] D. Karnaushenko, T. Kang, V. K. Bandari, F. Zhu, and O. G. Schmidt. Adv. Mat. 32.15 (2020), 1902994.
[2] R. Huber, F.L. Kern, D. Karnaushenko, E. Eisner, P. Lepucki, A. Thampi, A. Mirhajivarzaneh, C. Becker, T. Kang, S. Baunack, B. Büchner, D. Karnaushenko, O. Schmidt, A. Lubk, Nat. Comm. 13 (2022), 3220
[3] We acknowledge funding from the European Research Council (ERC) under the Horizon 2020 research and innovation program of the European Union (grant agreement no. 715620).
Fig.1: Fabrication of microcoil charge particle optic devices: (a) strain-engineered multilayer architecture and rolling process. (b) Schematics of quadrupole devices and SEM image of central part of QP device.
Fig.2: Performance characteristics of quadrupole device. Switchable line foci at 70 mA coil current and high-frequency deflection measurement demonstrating ca. 100 MHz cut-off frequency.