When
writing fine patterns, the most difficult step in the microscope optimization
is to properly minimize the astigmatism in the beam. Consequently, NPGS includes
a test pattern consisting of "wheels" as shown here which will make any
astigmatism during the pattern writing very easy to identify in the resulting
exposure.
This image shows a very good exposure at 30 kV of the wheel pattern where the linewidths are ~20 nm. When lines of this size are viewed, astigmatism of the beam during the imaging can appear identical to lines that had astigmatism during the writing. In this case, it is apparent that the beam during writing and the beam during imaging had very little astigmatism.
Also, notice that where the spokes meet at the center and where they hit the outer rim, there is little, if any, broadening of the lines. This is a sign that the beam was well focused during the pattern writing, while broadening at such junctions indicates that the beam was not well focused.
Note that this pattern was exposed on a thermal field emission SEM with no blanker. The only artifacts from not having a blanker are the slight positioning errors where the spokes at 1, 9, and 11 o'clock meet the rim.
This
image shows a higher magnification view of the wheel that is displayed above.
The sample was evaporated with ~30 nm of gold before viewing, and the grain size of the gold can be seen to be ~10 nm. To see the results of lift-off on this sample (on a wheel of a slightly higher dose), click Lift-Off.
To see examples of this same pattern when the focus and astigmatism were not well optimized during the pattern writing, select the following: "Wheel #2", "Wheel #3", "Wheel #4", or "Wheel #5".
To see the results of the same wheel written at 10 kV (but with the dose set for the lower kV), click "10 kV Wheel".
Also see the general SEM Optimization Guide and the SEM Lithography Setup Guide.