The correction of spiral distortion in the transmission electron microscope

Abstract

The present investigation is concerned with the improvement of the image viewing and recording system of the transmission electron microscope, by means of a wideangle projector lens of low distortion.: A critical investigation of a previously proposed system for the correction of distortion by means of conventional magnetic lenses has been carried out; this showed that such lenses are not capable of providing a distortion free image with a projection semi-angle greater than about 10°. Theoretical and experimental investigation has shown that with projection semi-angles in the region of 30°, third order aberration theory is inadequate and can even be misleading. It was therefore desirable to calculate the actual axially symmetric trajectories of the electrons forming the projected image. Special attention was therefore paid to the development of a computer program for calculating the electron trajectory in a system of magnetic lenses with the aid of the general ray equation. Such a calculation allows one to determine the appearance of the final image directly without recourse to aberration theory. This direct image simulation has been achieved by using the output of the general ray program to print out the image of a square mesh grid on an x-y plotter. This enables a direct comparison to be made between the calculated and observed images. An experimental wide-angle lens has been constructed by computer-aided design, using the above principle. Essentially distortion-free images have been obtained over a semi-projection angle of 30°. It is believed that this represents an ultimate limit for a twin-lens design of this type. The projection lens system can find immediate application in TEMs with transmission fluorescent screens and also in high voltage electron mircoscopes.