Surface reactions induced by fluorocarbon plasmas were studied on SiO2 and Si3N4 substrates using an inductively coupled plasma source. C4F8 and C5F8 were employed as source gases to investigate their differences in etching performance and selectivity on both SiO2 and Si3N4 substrates. Polymer deposition was noticed in both gases by Fourier transform infrared ellipsometric measurements when substrate bias was not applied. When the bias was applied, etching started on the SiO2 surface from a certain threshold bias voltage and the rate increased with the increase in voltage. However, on the Si3N4 surface this increasing tendency in C5F8 plasma was much less than that in C4F8 plasma. This difference provides a greater selectivity of SiO2 to Si3N4 in C5F8 plasma. The reason for this increased deposition tendency in C5F8 plasma is attributed to the presence of higher mass species in a larger abundance than in C4F8 plasma, as confirmed by electron attachment mass spectrometry.