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

Abstract

Anisotropic wet chemical etching of silicon in alkaline solutions isa key technology in the fabrication of sensors and actuators. In thistechnology etching through masks is used for fast and reproducible shaping ofmicromechanical structures. Experimentally it has been found that near thejunction between a slowly etching Si(111) surface and a mask, etching can beinfluenced by etch pit nucleation at this junction. In this paper theinfluence of the presence of such a junction on the etch rate and the surfacetopology is investigated by means of Monte Carlo simulations of etching of theKossel (100) surface. To describe such a system only two parameters areneeded: one parameter that describes the interaction between two bulk atomsand one parameter that describes the interaction between the mask and anadjacent atom. If the latter interaction is significantly smaller than thefirst, the nucleation rate at the mask junction is higher than throughout thecrystal surface, which induces the formation of a stepped facet at thejunction, which grows in time. An analytical expression for the misorientationof this facet is derived that agrees with the Monte Carlo simulations. Themisorientation depends only on the interaction between the interface and themask. The Si(111) surface is more complicated than the Kossel (100) surface.Underetching experiments have shown that a stepped facet is only formed for anobtuse contact angle of the Si(111) surface with the mask. This can beexplained by comparing the topology of the mask junction for an obtuse and anacute contact angle.