Design of a hybrid double-sideband/single-sideband (schlieren) objective aperture suitable for electron microscopy

Abstract

A novel design is described for an aperture that blocks a half-plane of the electron diffraction pattern out to a desired scattering angle, and then – except for a narrow support beam – transmits all of the scattered electrons beyond that angle. Our proposed tulip-shaped design is thus a hybrid between the single-sideband (ssb) aperture, which blocks a full half-plane of the diffraction pattern, and the conventional (i.e. fully open) double-sideband (dsb) aperture. The benefits of this hybrid design include the fact that such an aperture allows one to obtain high-contrast images of weak-phase objects with the objective lens set to Scherzer defocus. We further demonstrate that such apertures can be fabricated from thin-foil materials by milling with a focused ion beam (FIB), and that such apertures are fully compatible with the requirements of imaging out to a resolution of at least 0.34 nm. As is known from earlier work with single-sideband apertures, however, the edge of such an aperture can introduce unwanted, electrostatic phase shifts due to charging. The principal requirement for using such an aperture in a routine data-collection mode is thus to discover appropriate materials, protocols for fabrication and processing, and conditions of use such that the hybrid aperture remains free of charging over long periods of time.

Towards an optimum design for electrostatic phase plates

“…Although the gold surface of the BPP is highly conductive and held at ground potential, we observed severe charging, possibly as a result of uncompensated surface charges trapped in insulating contaminants on the gold surface. Such insulating contaminants could be diamond-like carbon that might build up under the intense electron radiation [1], gold oxide that might form due to ionization and reactive dissociation of adsorbed water when hit by electrons [10,20], or polymerized hydrocarbons that cover the gold surface [1]. In an attempt to reduce the effect of possible surface charges, we developed a PP goniometer equipped with a gas injection system consisting of metallic nozzles pointing towards the PP chip (Fig.…”

“…Ionized argon molecules directly interact with trapped charges on the BPP surface, clean the BPP surface, or even modify the surface structure. It has been observed that sputter cleaning with argon reduces the grain size and creates a more amorphous gold structure [10]. The origin of changes in the Fourier pattern observed after argon injection remains elusive and needs further investigation.…”

“…It would be interesting to examine the effect of higher argon pressure on BPP charging, but this would require differential pumping apertures in the TEM, similar to the ESEM design [18]. The implementation of such a gas injection system in a TEM would make it possible to study the influence of reactive gases on the charging behavior of PPs, such as the injection of H 2 to remove possible oxide layers [10] or O 2 to remove carbon contaminants.…”

“…In the past decade, a variety of PP types have been implemented into TEMs and generation of in-focus contrast has been demonstrated [3][4][5][6][7][8][9][10][11]. When placed in the back-focal plane of the TEM, PPs introduce an additional 90°phase shift between the scattered and the unscattered electrons, and thus generate phase contrast without the need for defocusing the objective lens.…”

A Brief Introduction to Phase Plates: Benefits and Types

“…(2) The "Zach" type [14] has an electrode that projects near the axis of the diffraction plane, producing an electrostatic field next to its tip. (3) The "tulip" type [15] has a small, central semi-circular element that blocks a range of the lowest spatial frequencies -only on one side of the diffraction plane -thus creating a single-sideband type of contrast transfer for these frequencies, while retaining conventional double-sideband imaging for the higher frequencies.…”