Practical aspects of hollow-cone imaging

Hollow-cone imaging (HCI) was proposed in the late 1940s in order to improve the resolution of TEM. This method reduces the influence of the spherical aberration of the objective lens on the image quality, leading to a theoretical improvement of resolution up to 100%. However, it is shown that the HCI contrast decreases with increasing cone radius and in effect, with increasing resolution. Another difficulty in applying this technique lies in the required accurate alignment of the microscope. The limitations in hollow-cone imaging an outlined and optimum imaging parameters are determined. Considering all the limitations and assuming a coherent electron beam, HCI may lead to an improvement of the resolution by 20-30% compared to the point-to-point resolution at axial illumination. The effects of the limited coherence of the electron beam upon HCI are discussed, and experimentally obtained images with a Philips CM30 ST electron microscope (where the information limit lies close to the point-to-point resolution) are shown. Image simulations show that substantial improvement in resolution can be obtained only using such a microscope fitted with a field emission electron source.