A lot depends on the RIE process you're going to use.

For a start, you should expect the selectivity of some 1:1 for a chemistry like SF6/Ar. A better selectivity might be obtained by replacing SF6 with CHF3 or CF4. There was a long ago discussion here https://www.researchgate.net/post/PMMA_mask_for_SiO2_etching_by_RIE2 , maybe those guys might update us with their results.

Thanks for your response Victor V Solovyev . We are using CHF3, CF4 gases.

I read that discussion. I will check that and share the updates if I got any response.