Linas:  Add about 5% CF4 or other fluorine source to the plasma. It  will
help to reduce the filler material probably carbon while you etch thru the
film.
If you really want to do it right your tool should have a pump capable of
around 150 CFM an ICP and RIE capability. The ability to run higher gas flows of
 oxygen at a reasonable pressure like 900 m/torr will give you much better
results than an rie system by itself. I build such a system but the price is
probably too much for your budget. Bob Henderson