| 摘要 |
PURPOSE:To lessen spherical aberration and improve resolution by forming an aperture of a main electron lens for each of two outer electron guns among three in-line guns, larger than that of a central electron gun. CONSTITUTION:A selection is made so that an aperture Ds of electrodes 4, 5 for main electron lenses of outer electron guns 21, 23 among in-line three guns is larger than an aperture Dc of the main electron lens of a central electron gun 22 and also the ratios of a length Ls of electrodes for outer electron guns 21, 23 to the aperture Ds and of a length Lc of an electrode for the central electron gun 22 to the aperture Dc are the same. Further, the outer electron guns 21, 23 are made from a permalloy and the central electron gun is made from a stainless steel, besides a focusing coil 6 is arranged on the outside of a container 10. Accordingly, resolution may be remarkably improved by putting the outer electron guns in only an electrostatic focusing action and the central electron gun in a cooperative action of electrostatic focusing and electromagnetic focusing. |
