Depends on what type of reactor you have. If it is a single wafer rie system you
probably will not be able to obtain 50:1 selectivity but you can establish a two
step process that will yield a higher selectivity on the overetch step which
will remove much less silicon. I would try a CHF3+He 20 sccm:100sccm with low
power < 50 watts and determine how much time it takes to clear to silicon. If
per chance you have an Applied batch reactor you can obtain the selectivity you
are looking for using the same chemistry with little trouble. Bob Henderson