To clarify the origins of high selectivity in SiO2-to-SiN etching in fluorocarbon gas plasma, mass-analyzed CFx+ (x=1,2,3) ions with a definite kinetic energy of 250–2000 eV were irradiated on SiN and SiO2 surfaces. Selectivity in SiO2-to-SiN etching varies greatly for different CFx+ ions. For CF3+ ions, the etch yield of SiN is almost the same as that of SiO2, causing poor selectivity. For CF+ ions, on the other hand, the etch yield of SiN is much smaller than that of SiO2. An amorphous fluorinated carbon (a-C:F) film grows without any neutral radicals on the SiN surface at energies below 1250 eV and on the SiO2 surface at energies below 500 eV due to CF+ ion irradiation. The difference in threshold energy of a-C:F film deposition causes high selectivity in SiO2-to-SiN etching. Slight etching of substrate films first takes place at the initial stage of deposition, then etching stops, and a homogeneous a-C:F film is grown. Accumulate- d carbons during the initial etching reaction modify the surface reaction layer, which causes drastic changes in reactions such as etching to “etch stop” and a-C:F film growth.