Study of silicon etching in CF4/O2 plasmas to establish surface re-emission as the dominant transport mechanism

This article describes an investigation of the etching of polysilicon in a CF4/O2 plasma. The ‘‘undercut’’ observed in etch profiles is related to the surface transport of reaction precursors. The possible mechanisms for this transport include surface re-emission and surface diffusion of the precursors. Simulations of profile evolution, conducted with both mechanisms, are compared with experimental results. The surface reemission simulations are found to predict experimental profile evolution accurately, whereas surface diffusion simulations require unphysical values for the surface diffusion length. Novel test structures have been fabricated and etched under the same conditions as used for trench etching. Surface re-emission simulations accurately predict the etch rate deep inside the shadowed cavity of different structures. On the other hand, simulations assuming surface diffusion to be dominant do not capture even the qualitative trends in test structure etching. This is strong evidence that surface re-emission is the dominant mechanism for transport of etch precursors in CF4/O2 plasmas.