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Influence of Electron Density Distribution at the Electron Source in a Projection Exposure System

The influences of the size and the density distribution at the electron source on the electron exposure characteristics in a projection lithography system are studied. The individual electron trajectory is traced in a realistic optical system considering the Coulomb interaction effect among electrons along with the lens aberration to discuss these effects quantitatively. The uniform, the Gaussian, and the ring density distributions are calculated as the spatial distribution of the electron density at the source. Based on the results obtained above, the most preferable conditions for the electron source are clarified for the projection exposure system. A simple equation is obtained for estimating the most preferable diameter and the broadness of the ring. If the ring source is used in the projection electron optical system, it is shown that the minimum displacement distance at the wafer surface can be largely improved, and it may be approximately 50 nm at the acceleration voltage of 50 kV with the current density of 10 A/cm2.