Low selectivity is when two materials etch at relatively similar rates. When there is low selectivity a mask will not provide as much protection for etching the material of interest. This means a thicker mask may have to be used to etch to a required depth.

What are the factors affecting the etch rate?

Etching rates are determined by the flux and energy distribution of the bombarding ions and energetic neutrals [charge transfer products), while anisotropy is determined by their directionality.

How can I increase my etch rate?

Consider the addition of oxygen (O2) to the plasma: C can be removed by forming CO and CO2 gases which are easily pumped away (higher vapor pressure than solid C or CFx gas). This decreases the amount of C available to form CFx radicals, increasing the relative F concentration in the plasma, increasing the etch rate.