Molecular dynamics simulations of silicon-fluorine etching

Molecular dynamics simulations of the reactions between gaseous fluorine atoms and (SiF_x)_n adsorbates on the Si{100} — (2 × 1) surface are performed using the SW potential and compared to simulations with the WWC reparameterization of the SW potential. Theoretical and experimental work has demonstrated that the reactive fluorosilyl layer during siliconfluorine etching is composed of tower-like adspecies of SiF, SiF₂, and SiF₃ groups. The objective of the simulations is to determine how the chemical composition, mechanism of formation, and energy distribution of the etched gas-phase products depend on the identity of the reacting adsorbate, the incident kinetic energy, and the parameterization of the potential energy function. Three reactions are simulated: F(g) + SiF₃(a), F(g) + SiF₂SiF₃(a), and F(g) + SiF₂SiF₂SiF₃(a). SiF₄ is the major product and Si₂F₆ and Si₃F₈ are minor products. In Si₂F₆ and Si₃F₈, the silicon-fluorine bond that is formed is stronger than the silicon-silicon bond in the molecule and, therefore, the majority of these products have enough energy to dissociate and will fragment before reaching the detector. An S_N2-like mechanism is the primary mechanism responsible for the formation of SiF₄, Si₂F₆, and Si₃F₈. In addition, at higher energies, the simulations have discovered a previously unknown mechanism for the formation of SiF₄, which involves an insertion between a silicon-silicon bond. The results of the simulations with the two potentials differ quite substantially in their prediction of the reactivity of the adsorbates. The SW potential predicts a 2- to 3-eV lower energy threshold for reaction and a much higher reaction cross-section, especially for the SiF₄ product. These results are explained in terms of the differences in the potential energy functions used to describe the silicon-fluorine interactions. In addition, the results are compared to experimental data on silicon-fluorine etching.