Effect of a Physical Phase Plate on Contrast Transfer in an Aberration-Corrected Transmission Electron Microscope

TL;DR

This theoretical study demonstrates that a physical phase plate in an aberration-corrected transmission electron microscope enhances contrast transfer, especially at low and medium resolutions, and increases tolerance to defocus and aberrations.

Contribution

It provides a detailed analysis of contrast transfer improvements with a physical phase plate in Cs-corrected TEMs, highlighting optimal conditions and practical benefits.

Findings

Physical phase plate converts sine to cosine phase contrast transfer.

Enhances low- and medium-resolution contrast in high-resolution TEM.

Increases tolerance to defocus and lens aberrations.

Abstract

In this theoretical study we analyze contrast transfer of weak-phase objects in a transmission electron microscope, which is equipped with an aberration corrector (Cs-corrector) in the imaging lens system and a physical phase plate in the back focal plane of the objective lens. For a phase shift of pi/2 between scattered and unscattered electrons induced by a physical phase plate, the sine-type phase contrast transfer function is converted into a cosine-type function. Optimal imaging conditions could theoretically be achieved if the phase shifts caused by the objective lens defocus and lens aberrations would be equal zero. In reality this situation is difficult to realize because of residual aberrations and varying, non-zero local defocus values, which in general result from an uneven sample surface topography. We explore the conditions - i.e. range of Cs-values and defocus - for most favourable contrast transfer as a function of the information limit, which is only limited by the effect of partial coherence of the electron wave in Cs-corrected transmission electron microscopes. Under high-resolution operation conditions we find that a physical phase plate improves strongly low- and medium-resolution object contrast, while improving tolerance to defocus and Cs-variations, compared to a microscope without a phase plate.