STUDY OF ALIGNMENT SIGNALS FOR ELECTRON LITHOGRAPHY.

High quality alignment signals are required in electron lithography. The Monte Carlo method is used to study the alignment signals formed by electrons that are backscattered from tapered etched silicon marks. Effects of mark geometry and incident beam voltage are explored. Universal curves of the contrast and the average signal slope are obtained. The results indicate that although a step of depth greater than 40% of the Bethe range of the primary beam yields a better contrast, too large a step will result in the decrease of the average slope. The quality of the alignment signal can be improved if the detector is positioned to collect the backscattered electrons with small takeoff angles. The sum and difference signals from detectors in different azimuthal angles are examined. The results show that in detecting a topographical discontinuity, the difference signal is preferred rather than the sum signal. Experimental confirmation of the theoretical predictions shows good agreement with the universal curve of contrast versus the normalized step depth. The alignment marks were fabricated by using anisotropic etching techniques in silicon.