One of the critical items in doing more than one layer lithography is how accurately the second (or further masks) can be aligned to the first layer. If the optical mask aligners are abused, this overlay accuracy becomes a problem.
As part of the qualification of optical lithography, we have a two mask set that can be used to test the layer to layer accuracy. Jolene is writing a Standard Operating Procedure for this test and she is the expert on this test. If you feel there is a problem with optical mask overlay, you can before the measurement yourself or talk to Jolene.
At present, there are no problems with the overlay on the ABM optical mask aligners.