First principles study of Si etching by CHF3 plasma source

A model of CF₃ etching Si (2 × 4) surface has been developed based on density functional theory. We find that the reconstruction Si surface tends to be fully F-terminated. Meanwhile, C-C chain forms spontaneously on the top of the surface to resist further F-Si interacting. Over-saturated Si bonds could still be stable as well due to the strong F-Si bonding, however, it needs to overcome an energy barrier of 1.85 eV to achieve this doubly saturated bonding. Two reaction paths are found to investigate chemical reaction of CF₃ with the full F-terminated Si surface. The first path displays that all fully saturated F-Si bonds could be over-saturated and thus produce F-Si-F bonds with an energy barrier of ∼1.85 eV. For this path, there is no product of SiF₄; the second path indicates a formation of SiF₄ with a much lower surface energy than the first path. The formation of SiF₄ shows the possible etching mechanism.