NPGS : Wheel Pattern #4
This image shows an AFM image of the diagnostic "wheel" pattern written in PMMA after evaporation and liftoff. This exposure was done with a line dose of about 8 nC/cm. This high dose, the wide lines, and the fact that the pattern is broader where the spokes meet the rim and in the center where they are close together indicates that the beam was significantly out of focus during the pattern writing. The wheel diameter is ~10 microns.
This image also shows that astigmatism was present in the beam during the pattern writing. This is apparent because the spokes in the ~4 o'clock to ~10 o'clock direction have a noticeably different width then the spokes at 90 degrees. The variations in the circular rim also match the variations in the spokes, which is expected from astigmatism during pattern writing.
Astigmatism in the beam during pattern writing will manifest itself as a 90 degree asymmetry to the pattern. In this "wheel" pattern, such an effect can easily be seen by comparing the spokes. All spokes are given identical exposures, however, they may appear quite different in two directions 90 degrees apart if the beam does not have a circular cross section. Such a 90 degree variation in this "wheel" pattern is the classic sign of astigmatism in the beam during the pattern writing.
Also see the general SEM Optimization Guide and the SEM Lithography Setup Guide.