Importance of fluorine surface diffusion for plasma etching of silicon

Dry etching of silicon is an important process in the manufacturing of integrated circuits and micromachined devices. Traditionally, the etch rate limiting step for an isotropic silicon etching process is considered to be the arrival of fluorine atoms directly from the gas phase onto the silicon surface, and the mechanism to promote anisotropy is the prevention of lateral etching by the formation of an inhibiting layer on the vertical walls. Furthermore, isotropic dry etching is considered to etch features in the same way as isotropic wet etching. Conventional mechanisms cannot explain, however, the perfect anisotropic etching of silicon with pure SF6, when no polymer is formed. Neither can it be understood how a deep (>50 µm), isotropic, dry etching process applied to silicon can result in structures with a pinched neck and sharp ridges, in contrast with a wet etching process, where the corners are rounded and no pinching of the neck is observed. It is proposed that long-range diffusion of fluorine atoms can precede the eventual binding to a silicon atom. The rate of binding increases if the silicon is bombarded with high energy ions. Tests were performed to corroborate this model which is also consistent with the findings of others