I suggest you look at the process you first started with. SF6 with He makes a
great Nitride etch process and the selectivity to photoresist depending on power
should be greater than 5:1. Try 20 sccm SF6 at 200 mtorr with 150 watts rf
power. Depending on the clearing pattern you can adjust edge to center or center
to edge clearing with pressure. It should take around 1 minute to etch to clear.
If this doesn't work try including He of 40 sccm into your plasma to help with
photoresist selectivity and it should also help with etch uniformity as well.
Drop me a line and let me know how it went. Bob Henderson