Benefits of a New Annular Darkfield Method Using Köhler Illumination in a Energy Filtering TEM

Darkfield illumination (DF) is a preferred method to visualize faint structures which are hardly seen in the brightfield (BF) mode. In DF the sample is illuminated by a tilted beam and the BF is blocked by an objective aperture. Thus, the image is formed by the scattered beam from the sample only. To provide a symmetrical (annular) DF two ways are possible: either a ring-shaped illumination aperture which provides an annular tilted beam on the specimen (Fig.3) is used, or the beam is conically rotated on the specimen (dynamical method). This paper describes a new DF which can be performed by a Köhler illumination system (KIS) and has numerous advantages over the standard methods provided by conventional illumination systems. In the KIS [1] the cross-over (emission pattern) is always imaged by the condenser lens system into the front focal plane of the final condenser lens (objective pre-field lens) which means the size (illumination angle), position (beam incidence), and the shape (conical DF) of this image defines the illumination conditions. Instead of a persistent conically rotated cross-over image also the emission pattern of an under-heated LaB6 filament can be used to generate a ring shaped illumination in the front focal plane of the objective pre-field lens (see Fig.1a). As visualised in Fig 1, a small central spot is surrounded by an outer emission ring (a) for very low filament currents. Increasing the current the emission pattern moves from a large spot (b) to a small spot (c) known as “tip mode”. If the current is furthermore increased four symmetrical off-axis emission areas appea