Abstract
One of the difficulties which must be overcome in the design of objective lens stigmators is that they necessarily take up space where it is most precious, in the vicinity of the specimen. The space following the specimen is needed for the contrast aperture while the space between magnetic poles in the gap is required for stage mechanism. To go appreciably below the objective back focal plane to position stigmators subjects the image to possible image distortion when correction is large. A further problem is caused by the trend to shorter objective focal lengths, e.g., 1.5 mm, thus further reducing the space available for the stigmator.To adapt the electrostatic stigmator to these physical restraints, a design has been completed which is only 0.75 mm thick and has a free bore diameter of the same size. Its sensitivity is 0.1 micron/volt with 100 kV electrons. The overall diameter is uncritical and may be adapted to the physical requirements of the system. It should be possible to make stigmators of 3 mm thickness with a simple reduction of critical dimensions (thickness and slot width).
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