We developed a Monte Carlo simulation of electron trajectories in a cell-projection electron beam lithography optical system. In the simulation the emission angle, the velocity and the emission timing of electrons from the electron gun are taking into account. The Coulomb interaction among electrons in the beam is calculated. There are five lenses in the system. Assuming that the stencil mask pattern is a series of line-and-space, the exposure pattern is obtained when the current densities are 5, 10, and 13 A/cm
2 at the specimen surface. The probability of electrons to enter the designed pattern as a function of the defocusing distance are calculated. The best refocus length is obtained to give the maximum probability in the designed pattern at the specimen surface.
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