Cosimo,

This is a long etching time process and I highly recommend to do wet etch instead of dry etch. However, if you want to do dry etch keep in mind the F2-based etching process has very similar etch rate for SiO2 and Si, since it is mostly physical etching rather chemical. To increasing selectivity you might need to chenge your mask material to a thick hard metal mask (Ni, Cr). But, as I pointed early the dry etch is used for small depth etching up to 10 um, and for deeper etching process the wet etch technique must be used. 

Thanks M. Behzadirad for the reply. But I have to use SiO2 mask, and I am using DRIE because I want vertical sidewalls so anisotropic etching.