Paper PS+EM-WeM6
Etching Mechanisms of Si Containing Materials in Remote Plasma Source using NF3 based Gas Mixture

Wednesday, October 24, 2018, 9:40 am, Room 104A

The introduction of new 3D designs (fin FETs, nanowire..) for sub-10 technological nodes bring new challenges for etch applications. Contrary to planar devices, 3D devices require more isotropic etch capabilities with high selectivity between different materials . Remote plasma source (RPS) which is based only on chemical mechanisms offers great capability for etch applications requiring high etch selectivity such as removal of SiN spacer in 3D device or fabrication of Si or SiGe horizontal nanowire for gate all around device. NF3 based gas mixtures are frequently used to etch Si containing materials in a RPS process. In this paper, we propose to investigate the etching and selectivity mechanisms of Si containing materials (SiN, SiO₂, SiGe and Si) in RPS process using NF3/NH3 or NF3/H2 gas mixture. In this study, the hydrogen content of SiN and SiO₂ materials is modulated by using different deposition techniques (LPCVD, PECVD..). The etching experiments are performed in an industrial RPS reactor. The substrate temperature can be varied between 40 and 200°C. The etching kinetics are in real time thanks to in situ kinetic ellipsometry. The results show that the etching of both SiN and SiO₂ materials in NH₃/NF₃ remote plasma proceeds through the formation of (NH₄)₂SiF₆ salts on the material surface that consume the pristine material. Similarly to oxidation processes, the consumed thickness of material is proportional to the thickness of the salt layer. The real time monitoring of the SiN and SiO₂ etching reveals that the etching proceeds in three phases. First, the reactive species absorb on the material surface but without consuming it. The delay before the materials starts to be etched increases with substrate temperature and decreases if the surface is rich in O and/or H, conveying that these elements act as a catalysis of the salt formation. During the second phase, the material is etched rapidly through the salt layer. During this phase, the material consumption depends on the substrate temperature and nature, as well as the plasma conditions. Finally during the third phase, the materials are consumed less rapidly because the reactive species have to go through the salt layer before reaching the salt/material interface. The etching kinetics in the third phase are almost independent on the substrate temperature, material, and plasma conditions. The key to get infinite etch selectivity of SiN over SiO₂ and SiO₂ over SiN is to tailor the substrate temperature and the surface functionalization. High etch selectivity of SiGe over Si can be easily achieved in NH3/NF3 remote plasma. Adding H2 in the mixture allows to reverse the trend.