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

Abstract

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 ~ 45 V and the EPC was found to be 17.2 Å/cycle for SiO₂ and 6.9 Å/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.