Image formation in the scanning electron microscope

Abstract

Image formation in the scanning electron microscope (SEM) was discussed in theory by von Ardenne. It was reduced to practice by von Ardenne, Knoll, McMullan, Smith and Oatley and Smith. The basic ideas are as follows. An electron beam (EB) of diameter dEBis scanned over a solid specimen. This penetrates for a distance dPENbelow the surface. A signal is collected and modulates the brightness of a cathode ray tube scanned in sympathy with the EB. While the resolution is always limited by dEB, it is not always limited by dPEN.Secondary electron (SE) imaging. von Ardenne distinguished between the secondary electrons (SE) with an energy lower than 50 eV and the backscattered electrons (BSE) having energies between 50 eV and the primary energy E0. He proposed that SE that escape at the point of entry of the EB (now called SE-I) should provide a high resolution component to the SE image, while SE excited at the exit point (now called SE-II) escape from a larger area and so give rise to an image with a poorer resolution. (Also see Peters.6)