One of the most challenging tasks for high-resolution electron microscopy (HREM) is the atomistic investigation of defects and interfaces in thin films of semiconductors and ceramics. In particular, the application of these materials in electronic devices requires the understanding of the atomistic structure responsible for electronic and optical properties. The imaging of these structures requires a point resolution down to 0.1 nm. For example the smallest atomic spacing of GaAs in the (110) projection is 0.14 nm. According to Scherzer the point resolution of the TEM is proportional to spherical-aberration coefficient Cs1/4 and to the wave length λ3/4. Commercial medium-voltage microscopes up to 400 kV offer only a point resolution of 0.16 nm due to the high spherical-aberration constant Cs of the electromagnetic objective lens. Decreasing the wave length λ by increasing the electron energy improves the point resolution, but has the drawback of severe radiation damage and high costs.
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.
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