Tomographic imaging ultra-thick specimens with nanometer resolution

Transmission electron microscopy (TEM) is a well established and powerful technique to explore matter down to the atomic scale in two dimensions. Since real world objects have a three dimensional character the need to obtain volume information on nanometer scale is increasing especially on the mesoscopic dimensions of devices or complex mixtures of multiphase objects. TEM and scanning TEM (STEM) tomography methods have been developed in the past to fulfil this need, but these methods are limited to relatively thin specimens. In TEM this is caused mainly by the chromatic aberration and the dramatic increase of inelastic scattering in the sample, which leads ultimately to a strong resolution loss. Only high acceleration voltage TEM (above 1 million volts) allows for investigation of thicker specimens; this technique is, however, extremely expensive, far from being routine and highly destructive, in particular for organic matter. STEM tomography can minimize the effect of chromatic aberration, but is limited by the focal depth when large convergence angles are used. In this paper we establish a novel method based on dark field STEM tomography that pushes the resolution in all three dimensions down to a few nanometers for ultra-thick specimens of several micrometers at 300kV acceleration voltage.