A Brief Introduction to Phase Plates: Benefits and Types

Abstract

The purpose of a phase plate in the TEM is to enhance contrast when imaging specimens that have little amplitude (scattering) contrast. Such specimens may be very thin, or they may lack highatomic-weight elements, such as is the case with biological specimens for cryo-EM. Electrons are elastically scattered by the specimen through small angles and have only a small phase shift. A phase plate adds phase shift, such that the total phase shift relative to the unscattered beam approaches the contrast maximum at 90° ( /2). While such additional phase shift can also be accomplished by underfocusing the objective lens (and the presence of spherical aberration), such a method limits the enhanced contrast to narrow bands of periodicities (spatial frequencies), according to an oscillating contrast-transfer function. Use of a phase plate allows imaging to be done close to focus (e.g. at Scherzer focus), while still obtaining excellent contrast transfer over a very broad range of periodicities (e.g. 0.3 to 50 nm). The wide range of nearly uniform contrast transfer also simplifies image interpretation. The overall contrast gain is about two times, while at some spatial frequencies the gain is as much as five times. An excellent paper on the theory and practical use of phase plates has been published by Danev et al. [1].All phase plates that have so far been realized have consisted of some kind of physical structure in the objective-lens back focal plane -the diffraction plane -of the TEM. Thus, an artifact of some kind is always apparent in the power spectrum of the image, but the effects of the artifact can nearly always be removed by image processing.There are two types of "conventional" phase plates: (1) The "Zernike" type [2,3] consists of a thin film with a central hole for the unscattered electrons. The electrons scattered by the specimen pass through the film, the electrostatic inner potential of which shifts their phase. There is also a "holefree" version [4] of the Zernike type that uses local charge-buildup from the unscattered electrons to shift the phase. (2) The "Boersch" or electrostatic type of phase plate [5,6,7,8] has a central element to which a voltage is applied, creating en electrostatic field that shifts the phase of the unscattered electrons relative to the that of the electrons scattered by the specimen. Both of these types have a "cut-on periodicity", limiting the largest periodicity (lowest spatial frequency) that that is phase-shifted. This is due to the presence of a central feature which either does not shift the phase of these frequencies (Zernike) or can block them entirely (Boersch). While the thin-film type is (in principle) easy to fabricate, scattering within the film reduces the overall contrast by about 10%, and the phase-plate phase shift is not adjustable. The electrostatic type has the advantage that the phase shift can be adjusted by varying the applied voltage (typically a few volts). This is useful to compensate for the additional phase shift of thicker specimens, and for tuning imag...