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?

Regards,

Martyn Gadsdon