Electron Optics of an Electron-Beam Lithographic System

This paper describes the electron optics of a practical scanning electron-beam lithographic system (EL1) that provides high-volume direct wafer exposure. The throughput limitation inherent in serial exposure is greatly reduced by exposing entire pattern segments with a shaped beam. The shaped-beam concept represents a combination of scanning and projection methods. Twenty-five image points are exposed simultaneously to increase the spot current accordingly over that of a Gaussian round-beam system that exposes one image point at a time. The higher total current could lead to intensified Coulomb interaction between beam electrons, causing excessive Boersch effect and additional aberrations. However, the imaging and deflection methods described here are shown, by theoretical and experimental means, to reduce both the effects of Coulomb interaction and the total aberration of the system. This results in improvement of the beam current and field coverage compared with those found in conventional systems.