Plasma atomic layer etching of SiO2 and Si3N4 using low global warming hexafluoropropene

In this work, plasma atomic layer etching (ALE) process was developed for silicon dioxide (SiO2) and silicon nitride (Si3N4) in an inductively coupled plasma (ICP) reactor with octafluorocyclobutane (c-C4F8), trifluoromethane (CHF3) or low global warming hexafluoropropene (C3F6) plasma. The SiO2 and Si3N4 surface was fluorinated with the c-C4F8, CHF3 and C3F6 gases in the surface modification step and the fluorinated surface was etched by ion bombardment using argon (Ar) plasma in the removal step. c-C4F8 plasma resulted in the formation of a fluorine-rich fluorocarbon layer after the modification step, followed by C3F6 and CHF3 plasma, indicated by the highest F 1s/C 1s ratio of the fluorocarbon layer. Self-limiting characteristics were observed at 90 s of Ar plasma time for both SiO2 and Si3N4. The etch per cycle (EPC) was investigated depending on ion energy in the removal step. The ALE window region was identified in the energy range of 35 similar to 45 V and the EPC was found to be 17.2 angstrom/cycle for SiO2 and 6.9 angstrom/cycle for Si3N4 with C3F6 plasma. Infinite SiO2/Si3N4 etch selectivity was achieved at early stages of etching due to the higher fluorocarbon consumption rate of SiO2 during the removal step. The surface roughness was reduced from 0.35 nm to 0.28 nm for SiO2 and from 0.26 nm to 0.21 nm for Si3N4 before and after the etch process with C3F6 plasma, confirming the advantage of the ALE process.