NPGS : Wheel Pattern #3
This image shows an SEM image of the diagnostic "wheel" pattern written in ~320 nm thick, 950k PMMA after sputtering, but without liftoff. This exposure was done with a line dose of about 3 nC/cm. This high dose and the fact that the lines are almost one micron wide, but still underexposed, indicates that the beam was not well focused during the pattern writing. Also, note that the "dots" around the perimeter and the "star" in the middle are the only places where the PMMA has been fully exposed, which again indicates that the beam was not well focused. The marker at the bottom is 1 micron.
In PMMA, fine lines (50 nm or less) are typically obtained at a line dose of about 1.3 nC/cm. If the pattern only shows features where lines cross or come close together and the dose is much higher than the expected dose, it is the classic sign of not having the beam focused well on the surface of the resist.
If you look closely, this image also shows that astigmatism was present in the beam during the pattern writing. This is apparent because the spokes in the ~5 o'clock to ~11 o'clock direction have a significantly different width/depth than the spokes at 90 degrees. Also, the "dots" and "star" show asymmetry that matches the 90 degree variation in the linewidths.
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.
(This image was acquired with the NPGS Digital Imaging Feature.)
Also see the general SEM Optimization Guide and the SEM Lithography Setup Guide.