etch receipt question

[Chong Xie]

Hello AMT users,
I have a question about the etching receipts. I need to etch oxide with selectivity over Si and vertical sidewall. The receipt 3 (std Ox) has enough selectivity for me but I don't want the tapered sidewall profile, while receipt 4(via) has a good profile but very low selectivity over Si. 
The difference between the two is
                std     via
Gas 1-O2 	6 	30 
Gas 2-CHF3 	85 	50 

I assume the low O2 flow in std contribute to the high selectivity and sidewall protection. Higher flow of CHF3 also helps organic layer formation on the sidewall. 
I was wondering whether anyone has tried to tune the gas flows and get a vertical etch profile and a fairly good selectivity. Any input is appreciated. Thanks!

chong