On the design and effective strength of stigmators for electron beam lithography

Past expressions for stigmator strength are applied to experimental data using three different stigmators; two having magnetic focusing with low geometric aspect ratios (bobbin length to diameter), the other using electrostatic focusing with a larger aspect ratio. While the functional dependence of the variables is confirmed, the measured effective lengths are found to be much less than predicted from heuristic formulae, which assume either the quadrupole field is defined by the physical length of the stigmator, or worse, by a longer length due to a fringe‐field effect. This fringe‐field assumption becomes progressively worse as the aspect ratio decreases or when the stigmator is placed near ferrous material. The use of a two‐stigmator column design to reduce blanker‐induced beam motion is also discussed. The stigmator located in the final lens is used to form an anastigmatic column section between the conjugate blanker and substrate. The upper stigmator independently stigmates the beam at the target.