  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.
