Image feature delocalization in defocused probe electron ptychography

Electron ptychography can in principle convert a conventional scanning electron microscope (SEM) into a good quality transmission electron microscope (TEM). An improvement in resolution of about a factor of 5 over the lens-defined resolution of an SEM was first demonstrated by Humphry et al. (2012). However, the results from that work showed some delocalization in the atomic fringes of the gold particles used as a test specimen for the technique. Here we explore factors that result in the delocalization effect when a defocused probe is used for the ptychographic data collection: source incoherence, the effects of detector faults, data truncation and a poorly calibrated illumination step size (or camera length). Various mitigation strategies are tested, including modal decomposition of the incoherence in the beam. We reprocess the data from the original SEM experiment to show that these refinements significantly improve the reconstruction.