Orientation dependent etching of silicon by fluorine molecules: a quantum chemistry computational study

Anisotropic etching is a widely used process in semiconductor manufacturing, in particular for micro- and nano-scale texturing of silicon surfaces for black silicon production. The typical process of plasma-assisted etching uses energetic ions to remove material in the vertical direction, creating anisotropic etch profiles. Plasma-less anisotropic etching, considered here, is a less common process that does not use ions and plasma. The anisotropy is caused by the unequal etching rates of different crystal planes; the etching process thus proceeds in a preferred direction. In this paper, we have performed quantum chemistry modeling of gas-surface reactions involved in the etching of silicon surfaces by molecular fluorine. The results confirm that orientation-dependent etch rates are the reason for anisotropy. The modeling of F2 dissociative chemisorption on the F-terminated silicon surfaces show that Si-Si bond breaking is slow for Si(111) surface, while it is fast for the Si(100) and Si(110) surfaces. The Si(100) and Si(110) surfaces incorporate a larger number of fluorine atoms resulting in the Si-Si bonds having a larger amount of positive charge which lowers the reaction barrier of F2 dissociative chemisorption, yielding a higher etch rate for the Si(100) and Si(110) surfaces compared to the Si(111) surfaces. Molecular dynamics modeling of the same reactions has shown that the chosen reactive bond order (REBO) potential does not accurately reproduce the lower reaction barriers for F2 dissociative chemisorption on Si(100) and Si(100) surfaces. Thus, reparameterization is necessary to model the anisotropic etching process that occurs at lower temperatures.