NPGS : Wheel Pattern #6
shows sub-20 nm lines that are not fully exposed.
It can be seen that where the 12 spokes meet at the center, the resist is developed through to the substrate, but the individual lines are underexposed.
The significance of this image is that the spokes do not show any sign of astigmatism in the beam during the writing. This is apparent, because the 12 spokes all appear to be uniform in depth and width.
When any pattern is underexposed, the response of the resist is extremely sensitive to any changes of the dose, which makes this underexposed wheel an excellent indicator that the beam was very well optimized when this pattern was written.
Note that a higher dose wheel in the same pattern produced uniform ~20 nm wide lines that were fully exposed, however, this underexposed pattern is a more sensitive test regarding the optimization of the beam. Often, higher dose patterns will appear to be fine, but the lower dose patterns will show any deficiencies in the optimization of the beam.
|This is a close-up image of the same wheel as shown above. The dark center "hole" is where the 12 spokes meet and it is exposed through to the substrate.|
Also see the general SEM Optimization Guide and the SEM Lithography Setup Guide.