It was recalled that the rate-limiting step for isotropic Si etching was traditionally considered to be the arrival of F atoms at the Si surface, directly from the gas phase. The mechanism for promoting anisotropy was the prevention of lateral etching by the formation of an inhibiting layer on the vertical walls. Isotropic dry etching was considered to etch features in the same way as isotropic wet etching. However, conventional mechanisms could not explain the perfect anisotropic etching of Si with pure SF6; when no polymer was formed. Neither was it understood how a deep (>50µm) isotropic dry etching process could result in structures with a pinched neck and sharp ridges (in contrast with a wet etching process, where the corners were rounded and no pinching of the neck was observed). It was proposed that long-range diffusion of F atoms could precede eventual binding to a Si atom. The rate of binding increased if the Si was bombarded with high-energy ions. Tests were performed in order to support this model; which was also consistent with other findings.

Importance of Fluorine Surface Diffusion for Plasma Etching of Silicon. P.Verdonck, A.Goodyear, R.D.Mansano, P.R.J.Barroy, N.S.J.Braithwaite: Journal of Vacuum Science and Technology B, 2002, 20[3], 791-6