| 摘要 |
PURPOSE:To correct the focal error of a crossover image by providing a lens wherein the focus is changed according to the dimensions of a beam or the data of an area at the incident side of the beam of an aperture for determining an aperture angle. CONSTITUTION:An einzell type electrostatic lens 20 is provided at the charged beam incident side of a limiting aperture 11 which determines the aperture angle of the last stage lens 9. The focal length of the electrostatic lens 20 is automatically corrected by changing applied voltage according to data in a table which shows the corresponding relations of the dimensions of a beam or the area of the beam with a focal lengths stored previously in a memory. In this construction, the generation of dose irregularity can be prevented since the focal error of a crossover image 19 is corrected at a high speed.
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