COMPUTER-SIMULATION FOR A LARGE-FIELD ELECTRON-BEAM-REDUCING IMAGE PROJECTOR

Computer simulation is used to optimize the design of a large-field electron image projector system. Aberrations from mask to target are smaller than 0.02 mum in distortion and 0.12 mum in beam resolution for a 27 X 27 mm2 field size, 3:1 reduction system, with a 4-m optical path distance. The lenses are given nonuniform bores. The bottom of mask-side lens and the top of wafer-side lens are reduced in diameter to 1/4 of the other bore diameters. The axial direction tolerance for lenses, mask, and target positions is found to be much larger than errors that are anticipated to occur during assembly. The reduction factor can be adjusted to the value designed by adjusting lens positions or mask and target positions, and can be adjusted for the wafer linear distortion through adjusting the lens current ratio of two lenses. We have identified, but not solved, two remaining problems in the lithography system. One is the large emittance electron gun and the other is a condenser lenses spherical aberration.