Hight F/C -  ration leads to more etching

Lower F/C –ration leads to more polymerisation

Adding H2 consumes F – leads to polymersisation

Addind O2 consumes C- leads to etching

You should use the recipe with CF4 and H2 to have a good selectivity to
silicium a the end you clean the chamber with O2

F/C=3  it is better for you to use CF4  and  H2.

Thanks

Naitbouda Abdelyamine
Dry etch ,Centrale Technologique / CDTA
Cité 20 Aout 1956 Baba Hassan Alger