| 摘要 |
PURPOSE:To reduce S-shaped aberration of lens by using a triple magnetic pole electron lens for forming symmetrical magnetic field against a central magnetic pole under real image forcusing condition and exciting parameter of lens within predetermined range. CONSTITUTION:Magnetic poles 2, 3 are jointed at upper and lower ends of yoke 1 while a magnetic pole 4 is arranged between them. Exciting coils 5, 6 are contained in two hollow sections formed by the yoke 1 and the magnetic pole 4. The magnetic pole is vertically symmetrical against the center of the second magnetic pole 4 and has same diameter. The central portion of the second magnetic pole 4 is thined. When setting the exciting parameter SBU-1/2 (where S is the distance between magnetic poles, B is the strength of magnetic filed, and U is the acceleration voltage after correction) of said lens to be 0.35-0.5, minute focus distance fp1 and S- shaped aberration factor (DELTAS/r)2 are obtained, thereby the aberration is reduced and the distance between the lens and film can be shortened. |
