Beam characteristics for various sizes of annular aperture on scanning electron microscope

Using an analogy between light optics and electron optics, we have calculated beam characteristics such as the beam profile and the optical transfer function for several sizes of annular and circular apertures on a scanning electron microscope (SEM). It has been found that an annular aperture improves the image quality with regard to several kinds of image resolution and the depth of focus at the price of good low-frequency (ν) contrast. In contrast with conventional circular-aperture SEM images, a combination of a low-ν-pass filtered, circular-aperture SEM image with a high-ν-pass filtered, annular-aperture SEM image has the potential to enhance the image quality in terms of both the image resolution and the depth of focus.