I prefer using SF6 over CF4 as it has a higher selectivity to photoresist if
used properly. The faster etch rate makes for a cleaner etch and reproduces
the crititcal dimension of the pr better. A typical process would be:

Flow 20 sccm SF6
Pressure  250 mtorr
Power 150 watts of 13.56 Mhz rf power
50 sccm He if available. Helps with uniformity

If you have endpoint capability try to change to a lower power say 50 watts
to finish clearing once you start to notice clearing of the nitride. For higher
selectivities to the silicon you can finish the etch with CF4 and O2 which
will protect the silicon better than SF6 alone.
bob henderson