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

Abstract

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.