Focus-variation image reconstruction in field-emission TEM

Abstract

The use of a field emission gun (FEG) in high resolution TEM (HRTEM) improves the information limit much below the point resolution. This is due to the FEG’s high brightness and low energy spread which yield a very good coherence. In the area between point and information resolution of the FEG-TEM, image interpretation is complicated by the lens aberrations and focus effects, which cause scrambling of the information from the specimen. This problem is solved by ‘holographic’ techniques, which retrieve aberration-corrected amplitude and phase information of the electron wave ϕ at the exit plane of the specimen. We follow the route of ‘in-column’ or ‘nonlinear’ holography by digital processing of a focal series of HRTEM images. Different reconstruction algorithms can be used for that purpose depending on the assumptions that are made in the HRTEM imaging model. We have devised two workable algorithms. A first method, called the “paraboloid method” (PAM), aims at filtering out recursively the nonlinear contributions in the images, so that high-speed linear reconstruction can be applied.