nitride dry etching

I have a 400 nm thick TiN/GaN multilayer film on a Al2O3 substrate and
i want to do a cross-plane electrical measurement of this film for
which i have to make a back contact to the bottom TiN layer (since the
substrate is insulating). Any ideas?
I was planning to dry etch the multilayer from top using either laser
etching, reactive ion etching or FIB, but all of them have
complications. All ideas/opinions are welcome?

~vijay

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