Miniature components for electron optical instruments with two parallel optical axes

Abstract text (incl. figure legends and references)

This poster introduces miniature electron-optical components for e.g. a double mirror corrector or quantum electron microscope [1,2]. Krielaart"s QEM design has two parallel optical axes; a microscope axis and, shifted by typically a millimetre, a mirror axis. A beam separator pair is needed to tilt the entering beam towards the mirror while the reflected exiting beam passes straight, or vice versa. Electrostatic mirrors are needed to cancel aberrations and diffract the beam in a QEM cavity [2].

The components are designed for electron beam energies up to 5 keV. Experimental data on the performance of a beam separator (EBE unit) and a miniature electron mirror will be presented.

The EBE unit is a compact bi-axial beam separator with low higher order deflection aberrations [4]. The EBE unit consists of three dipole layers that are placed spatially along the parallel optical axes. This (di)pole arrangement reduces the inherent hexapole fields. In each layer, a mu-metal pole pair provides for deflection in the two lateral directions by either an electric and/or magnetic field. Experiments at E = 2 keV beam energy show a deflection sensitivity of 0.275 mrad/mA and 1.98 mrad/V for magnetic respectively electric excitation.

Our miniature mirror unit design combines classic parts machining (e.g., for the holder frame and the liner tube that guides the beam through the mirror electrode plane) with micro-lithographic technologies as developed by the MEMS industry. The high precision of MEMS manufacturing techniques yields well-defined bur-free free-form flat electrodes. These can be used to build low-cost high-quality electrostatic deflectors, stigmators lenses and mirrors. Here, electrostatic mirrors and lenses are made using metalized silicon elements as electrodes, which are sandwiched in between electrically insulating flat quartz spacers. Typical layer thicknesses are a few hundred micrometres. To avoid electrical fields above the breakdown limit of 10 kV/mm, the beam energy is typically limited to 5 keV. Before fixing their position in the full stack, each electron optical element is accurately aligned to both axes using a dedicated 6-DOF Hexapod stacking tool [5].

The beam is injected into the QEM cavity through a grounded metal liner tube, thus offering electrons a field-free passage through the voltage barrier of the mirror and lens electrodes. The beam is shifted from the microscope axis to the mirror axis by a MEMS electrostatic deflector and vice versa by electro-magnetic deflectors in the EBE unit. To set the focal length and aberrations of the mirror, 3-5 accurately stacked electrodes are energized up to ±5 kV [1-3].

In conclusion, when properly designed, aligned and energized, these new compact electron optical components can together conceive a round-trip cavity as needed to coherently build up the signal in a quantum electron microscope.

References

[1] Dohi, H. and Kruit, P., Ultramicroscopy 189 (2018) 1-23

[2] Krielaart, M.A.R., and Kruit, P., Ultramicroscopy 233 (2022) 113424

[3] M.A.R. Krielaart and P. Kruit, Phys. Rev. A 98 (2018) 063806

[4] Krielaart, M.A.R., Maas, D.J., Loginov, S.V., and Kruit, P., J. Appl. Phys., 127 (2020) 234904

[5] Chapter 5, PhD thesis Zonnevylle, A.C., https://doi.org/10.4233/uuid:23221904-c9c1-4537-af7f-abdbb9df06ed

MK, SL and PK acknowledge funding from the Gordon and Betty Moore Foundation. DM gratefully acknowledges support by Hitachi High Technologies.