| 摘要 |
PURPOSE:To obtain a lens of a high performance by making at least two successive focusing electrodes have openings larger than those of other focusing electrodes and alternately applying a first and a second electric potential to at least four focusing electrodes including the front and the back ones. CONSTITUTION:The third focusing electrode 16 has a large opening 36 and the fourth focusing electrode 17 has a large opening 37. The openings 36 and 37 are larger than the openings 32a, 32b and 32c of a cup-like electrode and have an elliptic shape. The second focusing electrode 15 consists of a cup-like electrode the bottom of which has openings 39a, 39b and 39c arranged in a line. The side openings 39a and 39c are located slightly apart from the central electron beam hole 39b. A positive high electric potential of about 6.5kV is applied to the first and the fourth focusing electrodes 14 and 17 while a positive high electric potential of about 25kV is applied to the second and the third focusing electrodes 15 and 16. As a result, the electronoptical magnification and the spherical aberration coefficient of this lens decreases, thereby remarkably improving the performance of the lens. |
