Pure SF6 will tend to result in an isotropic etch profile. I have never
tried adding Ar, but this may increase anisotropy.  You should be able
to achieve nearly vertical sidewalls by adding O2 to the SF6.  The
SF6/O2 etch will tend to have "grass" that may be undesirable.  After
adjusting the SF6/O2 ratio to get the desired sidewall profile, you can
clean up the grass with the addition of CHF3. With a capacitively
coupled RIE you can expect etch rates of 1,000 to 5,000 Angstroms per
minute.  This technique, called "The Black Silicon Method", is discussed
in a series of papers by Henri Jenson from University of Twente.

Roger Shile