This
image shows an SEM image of the diagnostic "wheel" pattern written in ~150
nm thick, 950k PMMA after sputtering, but without liftoff. This exposure
was done with a line dose of about 2.2 nC/cm. This somewhat
high dose and the fact that the lines are somewhat over 0.1 microns wide
indicates that the beam was not ideally focused during the pattern writing.
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 significantly different exposure than the spokes at 90 degrees. The exposure of the circular rim also matches the asymmetry of the spokes, which is expected from astigmatism during the 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, if lift-off were attempted on this pattern, it is likely that only the four darkest spokes and the two dark parts of the rim would remain attached to the sample.
(This image was acquired with the NPGS Digital Imaging Feature.)
To see an example of a similar wheel after liftoff, click here.
Also see the general SEM Optimization Guide and the SEM Lithography Setup Guide.
