There are many choices depending on the results you are looking to obtain.
CF4+4to8% O2 is pretty standard for oxide etching. You will probably need to
combine that with Helium to produce uniform results. We have also used SF6 +
Helium and it has some advantages with respect to etch rate and selectivity
to resist. If you would like to discuss further with things like etch
thickness desired, masking material and other details just drop me an email.
Bob Henderson