Walter Andreas, Steltenkamp Siegfried, Schmitz Sam, Holik Peter, Pakanavicius Edvinas, Sachser Roland, Huth Michael, Rhinow Daniel, Kühlbrandt Werner
Max Planck Institute of Biophysics, Department of Structural Biology, Max-von-Laue-Str. 3, D-60438 Frankfurt, Germany.
Caesar Research Center, Ludwig-Erhard-Allee 2, D-53175 Bonn, Germany.
Ultramicroscopy. 2015 Jun;153:22-31. doi: 10.1016/j.ultramic.2015.01.005. Epub 2015 Feb 3.
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.
物理相板的充电问题一直阻碍着它们在弱相位物体透射电子显微镜中的常规应用。理论上，静电相板优于薄膜相板，因为它们不会衰减散射电子束，并允许自由调节相移。静电相板由多层导电和绝缘材料组成，因此比通常仅由单层非晶材料组成的薄膜相板更容易充电。我们已经研究了博尔施相板充电的起源，并展示了如何减少充电。特别是，我们进行了模拟和实验，以分析绝缘Si3N4层和表面电荷对静电充电的影响。为了优化静电相板的性能，制造出具有以下特点的静电相板将是理想的：（i）给未散射电子赋予均匀的相移；（ii）具有低截止频率；（iii）使尽可能少的材料暴露在强烈的未散射电子束下；（iv）可以通过聚焦离子束仪器进行额外抛光，以消除在暴露于未散射电子束期间积累的碳污染（Walter等人，2012年，《超微结构》，116，62 - 72）。我们提出了一种满足上述要求且优于博尔施相板的新型静电相板。它由三根在孔径中心汇聚的独立同轴杆组成（三折同轴相板）。对改进的博尔施相板进行的模拟和初步实验表明，制造三折同轴相板是可行的。
