Atomic layer etching of SiO₂ for surface cleaning using ammonium fluorosilicate with CF₄/NH₃ plasma

In this study, an atomic layer etching (ALE) process was developed and investigated for the removal of SiO₂ with CF₄/NH₃. An ammonium fluorosilicate [AFS, (NH₄)₂SiF₆] layer was successfully formed on SiO₂ with CF₄/NH₃ plasma and removed by subsequent thermal treatment above 100 °C using a lamp. An oxide removal rate of 2.7 nm/cycle was achieved with CF₄/NH₃ chemistry, and the self-limiting characteristic of the ALE process was demonstrated by the removal rates. After the ALE process of SiO₂ with CF₄/NH₃ plasma, no carbon residue was observed on the SiO₂ surface. The reaction characteristics of the CF₄/NH₃ plasma were compared with those of the NF₃/NH₃ plasma. The removal rate with NF₃/NH₃ was 9.1 nm/cycle, which is three times higher than that with CF₄/NH₃ without saturation at 600 s. The lower removal rate with the CF₄/NH₃ plasma is attributed to the stronger C-F bonding in CF₄ compared to the N-F bonding in NF₃. Moreover, the stronger bonding generates fewer fluorine radicals required to form HF and NH₄F reactants. This work demonstrates that CF₄ is suitable for the ALE process for the removal of oxide layers at the nanometer-scale; in addition, it provides an effective process for the nanoscale removal of oxides in three-dimensional devices.