Fool proof technique for that is to pattern alignment mark as a first
step using a lift-off and Aluminum. Use you first mask and expose the
lit-off resist while masking everything but the alignment marks. Kapton
tape on the mask will block UV for this first step, on simply put a
silicon wafer cleave the  right size. Then with the second layer of SU8,
the marks are very well visible. It's an extra step I know, but the time
you same not looking for the alignment mark is worth it.

Matthieu
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