| 摘要 |
PURPOSE:To reduce chromatic aberration by constituting lens electrodes in the order of the first - fourth electrode having the center opening, applying a charged-particle acceleration voltage across the first and fourth electrodes, the same potential to the second and fourth electrodes, and a negative or positive potential across the third and fourth electrodes. CONSTITUTION:Electrodes 6-9 are arranged in rotational symmetry with respect to a light axis to constitute lens electrodes. A beam having the energy of a volage V1 in relation to the electrode 6 is generated from a charged beam generation source. A voltage V2 is applied across the electrodes 6, 9, a voltage V3 across the electrodes 8, 9, and the same potential to the electrodes 8, 9. The polarity of the voltage V3 is selected so that a beam is accelerated between the electrodes 7, 8 and decelerated between the electrodes 8, 9. When the distance S between a light source and the electrode 9 is limited to Sm, a distance Lm between the electrodes 6, 7 or a diameter d1m of the electrode 6 is calculated, while the focal distance is kept constant and S=Sm with the distance L between the electrodes 6, 7 or the diameter d1 of the electrode serving as a parameter, than L and d1 are set to 50-100% of Lm and d1m respectively. Accordingly, chromatic aberration can be reduced. |
