One of the limiting parameters in high-resolution transmission electron microscopy (HRTEM) are the high values for the spherical aberration Cs of the objective lens, which are the reason, that for TEM's with field-emitter the point resolution at Scherzer defocus is about two times lower than the information limit. Another effect of Cs-values of about one mm is the rather large disc of least confusion, which contributes to a high amount of contrast derealization. Furthermore the images are very sensitive towards beam tilt. These disadvantages contribute to a difficult interpretation of HRTEM-images especially at interfaces and defects. In-situ applications as well as diffraction contrast experiments require a gap of at least ten mm between the pole pieces. For the present TEM's this degrades the resolution at an acceleration voltage of 200 kV to approximately 0.27 nm. Cs-correction offers the ability to combine high resolution with a large space for the sample, which can be used for in-situ experiments.
Recently, we have constructed a lens system for Cs-correction based on hexapole lenses for a commercial 200 kV instrument equipped with a field emission gun. This system allows choosing the Cs-value between +2.0 mm and -0.05 mm. The objective lens of the microscope has a Cs value of 1.2 mm resulting in a point resolution of 0.24 nm.