One of the most challenging tasks for high-resolution electron microscopy is the investigation of the atomic structure of defects, interfaces, and grain boundaries. In particular, the application of thin films in electronic devices requires detailed knowledge of the microstructure, which can influence the electronic and optical properties. These tasks require a point resolution down to about 0.1 nm because this enables structure imaging for many semiconductors and metals. The resolution of transmission electron microscopes is limited, besides other parameters, by the spherical aberration coefficient Cs of the objective lens and the wavelength λ of the electrons. 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. Controlling the value of Cs by a corrector lens system can offer the required resolution even at an acceleration voltage of 200 kV. Recently, a lens system for Cs-correction, based on hexapole lenses, has been developed at the European Molecular Biology Laboratory in Heidelberg and incorporated in a commercial 200 kV instrument equipped with a field emission gun (Haider M, Rose H, Uhlemann H, Schwan E, Kabius B and Urban K Nature 392: 768 (1998) ). This system allows the Cs-value to be set between +2.0 mm and -0.05 mm. For the present investigation, we have adjusted a Cs-value of 50 μm in order to achieve optimum phase contrast, which improved the point resolution to 0.14 nm. Structure imaging can be performed down to atomic distances of 0.14 nm. A further unique advantage of Cs correction is the reduction of contrast delocalization, which is a major obstacle in the interpretation of images of defects and interfaces. The influence of Cs on contrast delocalization is demonstrated for a Si/CoSi2 interface. The unique combination of a low acceleration voltage and aberration-free imaging down to the information limit offers new prospects for the interpretation of non-periodic structures as defects and interfaces in crystalline materials.
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