Summary

In virtually every kind of electron beam device, some provision is made for deflection, that is, for lateral shift of the electrons with as little disturbance of the beam structure as possible. In fixed-beam instruments, essentially in conventional transmission electron microscopes, deflection plays a minor role and is provided only to permit nonmechanical alignment of the column. In scanning devices, however, the role of the optics of the deflection system is to move a focused spot (often called the electron probe) in a raster pattern over a prescribed area of a specimen or a viewing screen, or to sweep a two-dimensional image over a small detector, and its design is at least as important as that of the lenses. The obvious examples here are cathoderay oscillographs and television tubes, together with the various types of scanning electron and ion microscopes. In the more recent generations of electron microscopes, provision is made for both fixed-beam (conventional) transmission imaging and scanning transmission microscopy, so that the deflection system becomes just one member of the complex sequence of optical components that make up these hybrid instruments. In the past decade, deflection systems, mainly of the magnetic type, have acquired new importance, for it is their properties that determine the optical performance of electron lithography devices. Here, not only is the raster technique employed but—far more advantageously—the shaped-beam technique: instead of a narrow beam, focused into a small probe, a comparatively broad beam, shaped by masks, is deflected. The accuracy required in such devices is very high. More details will be found in Chapter 40.