The damage generated in reactive ion etched Si using CHF3 added with O2 is studied using a metal-contamination-free reactor. The measurements of the photoconductive decay curve on the etched Si substrates indicate that carrier recombination lifetime decreases as the flow rate of oxygen increases. Physical analysis x-ray photoelectron spectroscopy and secondary ion mass spectrometry show that plasma polymerized film consisting of carbon and fluorine can be seen on the etched Si substrate in the low oxygen flow rate region. The thickness of the film depends inversely on the oxygen flow rate. It becomes clear that thick plasma polymerization film protects the substrate from ion bombardment that results in damage. © 1997 American Vacuum Society.