Paper PS+EM-TuM4
Mechanism of Highly Selective SiCN Etchings Using NF₃/Ar-based Gases

Tuesday, October 22, 2019, 9:00 am, Room B131

Highly selective etchings over various other materials are increasingly required to achieve self-aligned processes, which provide higher density devices without shrinkage of the pattern dimensions in three-dimensional devices, such as fin-based field-effect transistors. In a self-aligned process, SiCN etching is required to achieve high selectivity to both SiO₂ and Si₃N₄. However, it had been difficult to achieve high selectivities to both SiO₂ and Si₃N₄ using fluorocarbon gas chemistries. For example, selectivity to SiO₂ increased using a hydrofluorocarbon plasma, while selectivity to Si₃N₄ decreased. So, it is important to investigate gas chemistries to simultaneously control selectivities to various materials.

In this study, mechanisms for highly selective SiCN etchings with microwave ECR plasma using NF₃/Ar-based gases were investigated over various materials. The rate of SiCN etching using NF₃/Ar plasma was higher than that of other materials, which were TiN, poly-Si, SiO₂, and Si₃N₄. The SiCN was etched with NF₃/Ar plasma, which formed SiF_x and FCN. On the other hand, other materials were etched with low rates. To achieve higher selectivities, the effects of adding gases to NF₃/Ar plasma on various materials to inhibit etching were analyzed by X-ray photoelectron spectroscopy (XPS).

Firstly, a highly selective SiCN etching over poly-Si was achieved by adding O₂ to NF₃/Ar plasma. This was because poly-Si etching was inhibited by the formation of a 1.0 nm-thick oxidized layer, which protected the poly-Si surface from the etching reaction with F radicals. The SiCN etch rate also decreased when the poly-Si etching was stopped. However, C atoms contained in the SiCN layer reacted with O radicals and controlled oxidization of the SiCN surface.

Next, highly selective SiCN etchings over SiO₂ and Si₃N₄ were achieved by using a NF₃/Ar-based plasma by which deposited layers were formed on the surfaces. The deposited layers formed on the SiO₂ and Si₃N₄ protected the surfaces from being etched by reacting with F radicals. On the other hand, the deposited layer was thought to be more difficult to be formed on the SiCN.

Lastly, highly selective etching over TiN was achieved by using H₂-added plasma. XPS result showed that a thin protective layer containing TiF_x and ammonium fluoride, which is decomposed over 673K, had been formed on the TiN surface. The protective layer formed on the TiN surface was very effective at protecting the TiN from being etched by F radicals.

In conclusion, we achieved extremely highly selective SiCN etchings over various materials by forming protective layers, which were formed on non-etched materials by adding gases to NF₃/Ar plasma.