CONFIGURED DETECTORS FOR STEM IMAGING OF THIN SPECIMENS

A new detector system built for a scanning transmission electron microscopy (STEM) instrument allows images to be obtained with variable detector geometry. A simple theoretical formulation in one dimension, readily extended to two dimensions, allows the image contrast and resolution to be evaluated for various detector configurations. In particular it is shown that with a thin-ring annular detector, giving a configuration equivalent to hollow-cone illumination in TEM, an improvement in bright-field resolution for thin specimens by a factor of about 1.6 can be achieved relative to that for a conventional axial detector. This mode is achieved in the STEM by magnifying the transmitted beam, limited by an objective aperture, until it just overlaps the inner edge of the detector used for high-angle annular dark-field imaging. If the transmitted beam diameter is just less than the inner diameter of this detector, bright-line contrast proportional to the magnitude of the differential of the projected phase distribution is obtained. Preliminary results obtained with the new detector system illustrate the use of these imaging modes.