NPGS : Bad Coating
a close-up image of a wheel pattern after an unusual sputter coating of gold
from a tabletop sputtering system. Normally, a 2 to 20 nm thick coating of
metal is used for good imaging, however, this coating was accidentally made
much too thick, which was obvious because the coating looked like shiny gold
to the eye.
The most interesting aspect of this image is that where the lines are close together, the original narrow linewidths of ~50 nm can be still be seen, but farther away the lines appear to become wider. This same effect where isolated lines appear to be wider than closely spaced lines was also apparent in other patterns on the same sample, but this image most dramatically demonstrates the effect. A subsequent sample with an identical exposure and a good coating had uniform lines of ~50 nm throughout the entire pattern, as expected.
image shows another case where the wheel pattern appears to be distorted
after sputtering. However, in this case, the deposited metal was close
to the desired thickness, and the individual grains of the gold can be seen
in this image. By eye, the sample color did not appear to be gold at
The entire sample showed the systematic effect where closely spaced lines appeared to be reasonably sharp, while more isolated lines appear wide with rounded edges.
Also see the general SEM Optimization Guide and the SEM Lithography Setup Guide.