Improved lens systems for the electron microscope

Abstract

The general aim of the present investigation was to design and build an experimental electron microscope column based on miniature single-polepiece magnetic lenses, suitable for operation at an accelerating voltage of 1000 KV or more, but comparable in size to that of a 100 KV transmission electron microscope (TEM). An AE] EM6 100 KV TEM was modified for the purpose. In particular the standard viewing chamber was considerably shortened and all the original imaging lenses were replaced. Special attention was paid to the projection system. A novel wide-angle projection system was devised with a projection semi-angle of 30° compared with the usual value of 8° in commercial electron microscopes.The improved performance has been achieved by the simultaneous correction of spiral and radial distortion in a two lens system. This consists of two single-polepiece lenses excited by currents of opposite sign. Use is made of the asymmetric nature of the axial field distribution of single-polepiece lenses. Thus the final projector lens produces an unusually low amount of distortion. The corrector lens, on the other hand, produces a considerable amount of distortion, which enables it to contribute appreciably (3X) to the total magnification whilst reducing the overall distortion of the final image to negligible proportions. Such a correction system is especially suitable for reducing the size and weight of high voltage electron microscopes,as well as for providing greatly improved viewing facilities.Key Words:electron optics,