It looks like a fill, but seems to be on top of any
passivation (if the chip has any). A cross section
might be more informative structure-wise, as would
an idea of what deprocessing may have been done
thus far.
You can narrow down the composition with a little
wet bench work, like see if KOH chews it right off
(aluminum) or you need HF (oxide, but nobody
patterns oxide that way that I've ever seen).
If you knew the die product info and foundry you
could probably find docs that describe the process
technology.