I don't think that reducing the pressure will decrease the formation of
polymer.  Reducing the pressure will, however, increase the mean free
path of the ions resulting in an increase in the average energy of the
ions bombarding the substrate.  This ion bombardment tends to remove the
polymer, allowing the etch to proceed.

Roger Shile

-----Original Message-----

Dear All,

I am using CHF3 to RIE a thin Si film. I have wanted to do this without the
normal inclusion of O2 that prevents polymerization as I am trying to avoid
etching photoresist. By dropping the base pressure to 20 mtorr I have been
able to etch Si using CHF3 without the need to introduce O2 into the plasma.
However, I am not 100% sure as to why the dropping of the pressure has
worked. I understand the basic idea of the reaction - getting dissociation
of CHF3 to give F- ions that produce SiF4 and that O2 helps hinder
polymerization by reacting with carbon to increase the F/C ratio and prevent
CxFy polymer build up. But I do not understand how reducing the pressure in
a purely CHF3 plasma can help prevent polymerization. Can anyone help
explain this to me please?