I believe that using a Cl2 plasma on an RIE system will do this if you
can avoid contamination.  ZnO tends to be fairly resistant to Cl2
plasma whilst Al2O3 and similar can be etched with BCl3/Cl2 mixture at
a 4:1 ratio.  Be warned that the selectivity is about 8:1 - it can be
made better, if your RIE system is older it will probably be worse.
It etches most PR at a 1.2-1.5:1 ratio, however, so be careful that
you don't lose your mask.  Wet etchants are a nightmare; ZnO is very
vulnerable to attack by acid.  I was not able to find a good solution
to that problem and had to go to dry etching because of it.

-Eric