A model of CF3 etching Si (2 x 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 similar to 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. Published by Elsevier B. V.