Towards an optimum design for electrostatic phase plates.

Charging of physical phase plates is a problem that has prevented their routine use in transmission electron microscopy of weak-phase objects. In theory, electrostatic phase plates are superior to thin-film phase plates since they do not attenuate the scattered electron beam and allow freely adjustable phase shifts. Electrostatic phase plates consist of multiple layers of conductive and insulating materials, and are thus more prone to charging than thin-film phase plates, which typically consist of only one single layer of amorphous material. We have addressed the origins of charging of Boersch phase plates and show how it can be reduced. In particular, we have performed simulations and experiments to analyze the influence of the insulating Si3N4 layers and surface charges on electrostatic charging. To optimize the performance of electrostatic phase plates, it would be desirable to fabricate electrostatic phase plates, which (i) impart a homogeneous phase shift to the unscattered electrons, (ii) have a low cut-on frequency, (iii) expose as little material to the intense unscattered beam as possible, and (iv) can be additionally polished by a focused ion-beam instrument to eliminate carbon contamination accumulated during exposure to the unscattered electron beam (Walter et al., 2012, Ultramicroscopy, 116, 62-72). We propose a new type of electrostatic phase plate that meets the above requirements and would be superior to a Boersch phase plate. It consists of three free-standing coaxial rods converging in the center of an aperture (3-fold coaxial phase plate). Simulations and preliminary experiments with modified Boersch phase plates indicate that the fabrication of a 3-fold coaxial phase plate is feasible.