Enhancement of low-spatial-frequency components by a new phase-contrast STEM using a probe formed with an amplitude Fresnel zone plate

Electron microscopy is a powerful tool for visualizing the shapes of sub-nanometer objects. However, contrast is not in proportional to density distribution, and therefore achieving a quantitative understanding of specimens is not straightforward, especially for low-contrast subjects such as biological specimens. To overcome this problem, we have developed a new phase-contrast scanning transmission electron microscope (STEM) in which a probe beam formed with an amplitude Fresnel zone plate (FZP) and the resulting interference patterns produced by the zeroth and first order diffracted waves generated by the FZP are detected. We name it FZP-PC-STEM hereinafter. The amplitude FZP was manufactured by using focused ion beam (FIB) equipment, and the diffraction data were collected by using diffraction imaging technique. The validity of our proposed optical model was confirmed by comparing experimental and simulated images. Observations of carbon nanotube (CNT) bundles by this method showed that the contrast of low-spatial-frequency components in the CNT image was enhanced, unlike the case in conventional bright-field STEM. This method does not, in principle, require the post-image processing used in the diffraction imaging method, and it can be easily introduced into a conventional STEM system without major modifications. The stability and robustness of the method toward intense electron irradiation during long-time operation were also confirmed. We expect that the FZP-PC-STEM will be widely applicable to quantitative observations of radiation-sensitive light-element specimens, with simple and easy operation.