Computer-Assisted Microscope Alignment

Abstract

Full alignment of a high resolution electron microscope, including alignment of the beam tilt, is a very difficult process. Whereas astigmatism correction is relatively straightforward, correcting the beam tilt is by no means so simple and there is a strong interaction between astigmatism and tilt so that it is possible to apparently correct the two at one defocus. To overcome this problem, the method of applying a ± tilt oscillation to the beam has been introduced previously, the principle being to adjust the texture of an amorphous carbon film image so that it is symmetric with respect to the tilt oscillations. This procedure works, but in practice is not easy to use and there are additional experimental problems in terms of loss of intensity due to insufficiently corrected beam shift/tilt purity.As an extension of this process, and as a general extension of astigmatism correction as well, we have developed a procedure for providing a computer generated optical diffraction pattern to compliment the standard TV image. One critical problem was providing optical diffraction patterns at a sufficiently rapid speed to make the process experimentally viable; one optical diffraction pattern every 5 seconds for example is too slow. The numerical procedure can be broken down into a number of different steps: