THE SCANNING ELECTRON MICROSCOPE
An important variation is the scanning electron microscope (Fig. 41-9a). The electron beam is focused to a very fine line and is swept across the specimen, just as the electron beam in a TV picture tube traces out the picture. As the beam scans the specimen, electrons are knocked off and are collected by a collecting anode that is kept at a potential a few hundred volts positive with respect to the specimen. The current in the collecting anode is amplified and used to modulate the electron beam in a cathode-ray tube, which i,s swept in synchronization with the microscope beam. Thus the cathode-ray tube traces out a greatly magnified image of the specimen. This scheme has several advantages. The specimen can be thick because the beam does not need to pass through it. Also, the knock-off electron production depends on the angle at which the beam strikes the sur
face. Thus scanning electron micro graphs have an appearance that is much more three dimensional than conventional visible-light micro graphs. The resolution is typically of the order of 10 nm, still much finer than the best optical microscopes. Figure 41-9b shows a photograph made with a scanning electron microscope.