Ar ion milling would work for sure, it's not selective but purely physical. For a chemical attack I'd try Cl2, HCl, Br2 or HBr : FeCl3 and FeBr2 are volatile.

I would go for Chlorine plasmas...

thanks for your prompt answer. I wonder if there is any literature about that?

I concur with Dieter, something like HCl should do the trick and it is probably easier to work with and more accesable than some of the pure halogens.

Depending on thichness: Ion Beam etching 100% physicl with Ar can be the best if:

1-CD not critical

2-thichness lower than 0.5µm

Otherwise, prefer plasma etching By Cl2/Ar.