A Monte Carlo study of etching in the presence of a mask junction

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Abstract
Anisotropic wet chemical etching of silicon in alkaline solutions is a key technology in the fabrication of sensors and actuators. In this technology etching through masks is used for fast and reproducible shaping of micromechanical structures. Experimentally it has been found that near the junction between a slowly etching Si(111) surface and a mask, etching can be influenced by etch pit nucleation at this junction. In this paper the influence of the presence of such a junction on the etch rate and the surface topology is investigated by means of Monte Carlo simulations of etching of the Kossel (100) surface. To describe such a system only two parameters are needed: one parameter that describes the interaction between two bulk atoms and one parameter that describes the interaction between the mask and an adjacent atom. If the latter interaction is significantly smaller than the first, the nucleation rate at the mask junction is higher than throughout the crystal surface, which induces the formation of a stepped facet at the junction, which grows in time. An analytical expression for the misorientation of this facet is derived that agrees with the Monte Carlo simulations. The misorientation depends only on the interaction between the interface and the mask. The Si(111) surface is more complicated than the Kossel (100) surface. Underetching experiments have shown that a stepped facet is only formed for an obtuse contact angle of the Si(111) surface with the mask. This can be explained by comparing the topology of the mask junction for an obtuse and an acute contact angle.

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