Hi Julie,

I am also working on DRIE in SOI. I would like to remind you of the fact
that DRIE is a pretty expensive process and that SiO2 (Mask) / Si
selectivity is roughly about 1:100. So I would suggest that you first plasma
clean your trench on either side for prolonged time (I do it for about 30
min @ 980 W) to completely remove the passivating layer and then do a HF
vapour etch or even a buffered HF wet etch (after you have etched top and
bottom trenches). This should actually work. Do let me know how you solved
this issue.

Cheers,
Vijay