Analysis of off-axis shaped beam systems for high throughput electron beam lithography

For high-throughput electron beam lithography, projection systems using symmetric magnetic doublet lenses can produce images with zero distortion. However, the projected pattern area is limited by beam blur at large off-axis distances. If an off-axis shaped beam pattern is imaged in a projection system, the aberrations can be greatly reduced by introducing deflectors, which steer the beam through the projection lenses in a modified path. In this paper, the principle of this type of projection with in-lens deflectors is first outlined. The method for computing the optical properties of such systems, based on an extension of our previously published unified aberration theory, is then described. To provide accurate simulation of systems with such large field sizes, our new software computes both the third and fifth-order aberrations. The computation of dynamic corrections, which can not only correct deflection field curvature and astigmatism but also reduce stitching errors, is also described. A design example of an off-axis shaped beam projection system with deflectors is presented, which has been optimized by the damped least squares method. The results show that such systems can have extremely small beam blur, distortion and stitching errors. The presented design images a 0.25 mm square shot over a 3 mm square region of the wafer, with 2 mad beam half-angle, with a beam blur less than 26 nm, and distortions and stitching errors less than 19 nm.