| 摘要 |
Electron-beam sources are disclosed that exhibit substantially reduced spherical aberration compared to conventional sources. In a beam produced by the cathode of such a source, axially propagating electrons are subjected to a lens action by voltage applied to a Wehnelt electrode and an extraction electrode. The cathode includes a peripheral portion that is "drawn back" (displaced along the axis of the source away from the beam-propagation direction) relative to a center portion of the cathode. With such a cathode, the percentage of dimensions of the crossover involved in spherical aberration of the crossover is reduced. This improves the uniformity of beam current at a lithographic substrate and minimizes location-dependency of the aperture angle. Since the Wehnelt voltage can be reduced, positional changes in the electrical field at the cathode surface are reduced, and the distribution of electrons in the beam propagating from the cathode surface is made more uniform than conventionally.
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