Characteristics and effectiveness of a foil lens for correction of spherical aberration in scanning transmission electron microscopy

A side-entry type foil lens was developed for correction of spherical aberration of a probe-forming lens in a 200 kV scanning transmission electron microscope (STEM). Measurements of the spherical aberration coefficient and recording of STEM images were performed in parallel for various foil lens voltages. The measured third order spherical aberration coefficient of the probe-forming lens combined with the foil lens decreased with increasing foil lens voltage and reached negative values at foil lens voltages higher than similar to 400 V. The quality of images observed at a large beam convergence semi-angle of 20 mad was improved correspondingly to the reduction of the spherical aberration coefficient. Under the condition of a constant probe current, the best image observed at the foil lens voltage of 350 V showed smaller blurring compared with the image observed without the foil lens at the optimum beam convergence semi-angle of 10 mrad.