Voltage-center and coma-free alignment for high-resolution electron microscopy

Recently, it has become possible to carry out routinely a coma-free alignment with the aid of a personal computer. However, a coma-free alignment is only attained with a sacrifice of a voltage-center alignment, since both alignment procedures use the same beam deflectors above an objective lens. In this paper we overcome this difficulty by realizing the method proposed by Yanaka et al. [Proc. 8th Int. Congr. on Electron Microscopy, Canberra, 1974, Vol. 1, p. 128], where a coma-free alignment is carried out by using beam deflectors above the objective lens followed by a voltage-center alignment by adjusting beam deflectors below the objective lens. It is demonstrated that a voltage-center and coma-free condition can be achieved even for a 6-μm off-axis imaging which causes an appreciable difference between voltage-center and coma-free directions.