The relationship between reactive species flux and their modified surfaces was studied in a SiO2 highly selective etching over Si and Si3N4. Sample specimens with large patterns and phi 0.35 mum contact holes were etched using C4F8/Ar/O-2 plasma in a dual-frequency (27/0.8 MHz) parallel-plate etching system. The amount of CFx reactive species was controlled by adjusting the C4F8 flow rate ratio while keeping the ion flux (3 X 10(16) cm(-2) s(-1)) and the V-pp of bias radio frequency (1450 V) constant. The highly selective etch process is attained in a certain condition of the radical flux. Quantitative analysis using x-ray photoelectron spectroscopy revealed that the etch rate strongly depended on the fluorocarbon (CF) film thickness formed during the etch reaction on SiO2 Si3N4, and Si. In the large-area-etching of Si and Si3N4, the CF film (< 2 nm) formed under conditions with low selectivity for SiO2 was thinner than the film (5-6 nm) formed in high-selectivity etch conditions. The CF film thickness on SiO2 were less than 1 mn under the high-selectivity etch conditions. The thinner CF film thickness on SiO2 is due to the additional oxygen which is an etch product from SiO2, and which can remove CF species from the SiO2 surface. Highly selective etching can be achieved under the proper CFx radical flux condition, in which the CF film is thin on SiO2, and the films on Si and Si3N4 are thicker than the ion projection range. Furthermore, we observed the same trend for etching at the bottoms of the phi 0.35 mum contact holes as with large area etching with result to the CF film thickness change. However, with a small increase in CF radical flux, the CF film thickness on SiO2 increased abruptly and caused a narrow process window for highly selective etching.