Xiaodong,

You will probably have the most success with hydrogen bromide and chlorine,
if you have it available. Hydrogen bromide is slow (~0.2 microns per
minute), but is very selective to SiO2 (~100:1) and selective enough to
photoresist for a 1-2 to micron etch. If you don't want to mess with
corrosive chemistries, you might consider using CF4 and O2 with perhaps some
argon. If the selectivity to resist is not good enough, you can deposit a
metal mask.

Hope that helps,

Michael Marrs
Trion Technology