A trick for stabilizing alignment marks on insulating substrates
Insulating substrates are usually covered with resist and then a layer of gold or aluminum (~15 nm). Unfortunately the metal can break up and peel when the mark is scanned repeatedly, or if the operator looks at it at high magnification.
Here's a trick for stabilizing the coating. It works for positive resists like PMMA and CSAR.
First coat the (positive) resist with gold, as usual, and expose large squares over the alignment marks. Large, meaning ~ 100 um. Then strip the gold and develop the resist. The alignment might be quite bad, but you don't care -- because in this step we are simply removing the resist around the marks.
Now sputter another thin layer of gold. The gold will be on top of the resist, and it will also land directly on the alignment marks. The marks are thick, and the resist coating is thin, so the contrast will still be very good.
This trick works because simple resists like PMMA, P(MMA-MAA), and CSAR can be exposed and developed twice.
Sadly, this trick does not work for negative resist. Also, this will not work when using conducting polymer, since the polymer itself breaks apart during alignment.