Practical Electron Microscopy and Database

In off-axis electron holography, the best resolution, and thus the highest spatial frequency q_max can be reconstructed from the sideband and is given by the spacing s_hol = 1/q_c of the interference fringes, [1] given by,

          --------------------------- [2556a]
where,
           k -- The electron wave vector,
           a -- The distance of the back focal plane of the objective lens to the biprism,
           U_F -- The biprism voltage,
           f -- The focal length,
           γ₀U_F -- The deflection angle at the biprism filament. [2]

The biprism voltage U_F is the only variant in Equation 2556a. From the resolution point of view, the biprism voltage U_F should be set to as low as possible.

On the other hand, the width W of the hologram can be given by, [1]

           ----------------------- [2557b]

However, in practice, the width W of the hologram is limited by the coherent current of the electron beam and by the strength of excitation of the subsequent intermediate lens. [1]

The biprism voltage is provided by a highly stabilized DC power supply (e.g. rel. 10^-5) with output voltages of 70 - 300 V for 200 keV incident electrons and 1000 V for 300 keV incident electrons, respectively. [1,3] Zheng et al. [3] set the biprism voltage in their 200 keV microscope in the range of 70 – 80 V, resulting in an overlapping width (W) of interference fringes from 2.5 µm to 3 µm.

[1] Michael Lehmann, and Hannes Lichte, Tutorial on Off-Axis Electron Holography, Microsc. Microanal. 8, 447–466, 2002.
[2] Lichte, H. (1991). Electron image plane off-axis holography of atomic structures. Adv Optical Electron Microsc 12, 25–91.
[3] C. L. Zheng, K. Scheerschmidt, H. Kirmse, I. Häusler, W. Neumann, Imaging of three-dimensional (Si,Ge) nanostructures by off-axis electron holography, Ultramicroscopy 124 (2013) 108–116.