IMAGING MODES WITH AN ANNULAR DETECTOR IN STEM

An alternative to the use of electron holography as a means for enhancing bright-field image resolution in STEM is given by the use of a thin annular detector which intercepts the outer edge of the central beam spot and gives an image resolution 1.7 times better than that of normal bright-field STEM imaging with an axial detector. This has been confirmed experimentally for the case of a small limiting objective aperture. High-resolution images have also been obtained in the ''marginal imaging'' mode with the central beam spot just inside the annular aperture so that the image contrast approximates to the square of the differential of the projected potential distribution.