Difference between C 4F 8 and C 5F 8 plasmas in surface reaction processes for selective etching of SiO 2 over Si 3N 4

Abstract

Surface reactions induced by fluorocarbon plasmas were studied on SiO 2 and Si 3N 4 substrates using an inductively coupled plasma source. C 4F 8 and C 5F 8 were employed as source gases to investigate their differences in etching performance and selectivity on both SiO 2 and Si 3N 4 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 SiO 2 surface from a certain threshold bias voltage and the rate increased with the increase in voltage. However, on the Si 3N 4 surface this increasing tendency in C 5F 8 plasma was much less than that in C 4F 8 plasma. This difference provides a greater selectivity of SiO 2 to Si 3N 4 in C 5F 8 plasma. The reason for this increased deposition tendency in C 5F 8 plasma is attributed to the presence of higher mass species in a larger abundance than in C 4F 8 plasma, as confirmed by electron attachment mass spectrometry.