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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snipped-for-privacy@gmail.com wrote:

Ion gun etch in ultrahigh vacuum. Not sure how you'd know where you are in the cross-section without removing from vacuum and doing ellipsometry or something like that. You should consider doing the electrical measurements in ultrahigh vacuum as well, as I think the TiN will at least partially revert back to the oxide in the presence of oxygen.
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