A method to reduce deflection aberrations in electron-beam lithography systems

Abstract

An imaging and deflection method is described which is intended for high-speed, submicron lithography. The basic electron-optical approach is similar to one described by Pfeiffer, comprising a large-bore lens and an in-lens deflection element. A saddle coil rather than toroidal deflector is utilized in order to facilitate the simultaneous reducton of deflection aberrations and beam deflection offset due to eddy currents and skin effect. Deflection aberrations usually are minimized by varying the relative positions of lenses and deflection elements against each other. In this paper, an optimization approach is described in which the shape of the deflection coil is varied. By changing the length of the coil wires, their radial and lateral positions, and by adjusting the axial coil position in the lens, low levels of field curvature, astigmatism, and distortion are reached. To minimize eddy currents, saddle-type deflection coils can be surrounded by secondary coils which generate the same magnetic dipole moment as the primary coils, but provide opposing polarity. This approach has been described earlier by Wardly; its extension to the correction of beam deflection offset due to skin effect is presented in this paper. Experimental tests of these concepts have been conducted. A specific result obtained is that all geometrical aberrations and all eddy current related effects are reduced to about 0.1 μm in the corners of a 2 mm scan field.