The mechanism for control of the Si3N4 etch rate in a self-aligned contact process when C4F8/Ar/O-2 gas is used was investigated. The Si3N4 surfaces after dry etching were analyzed by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy. After dry etching, a polymer-like residue film less than 15 nm thick and an oxidized layer between the polymer layer and the Si3N4 surface were observed. In the CF polymer layer, the etching product of CN or CNO was characterized. CN or CNO components are widely distributed in the CF polymer layer, while the SiO2 layer is more localized on the Si3N4 surfaces. We found that the thicknesses of the CF polymer and SiO2 layer strongly depend on the Si3N4 etch rate, which is determined by the flow rate of the additional O-2 gas. The thickness of the CF polymer, which was decreased from 10.1 to 4.1 nm, is considered to act as an inhibitor. The thickness of the SiO2 layer, which is easy to etch, was increased from 1.4 to 2.7 nm. The CF polymer thickness and the etching path Of Si3N4 oxidation are important factors in controlling the Si3N4 etch rate.