Silicon etch anisotropy in tetra-methyl ammonium hydroxide: Experimental and modeling observations

This work considers wagon-wheel-based under-etch experiments of {110} and {100} silicon in tetra-methyl ammonium hydroxide (TMAH) at 25 wt% and 12 wt% at 80 deg C. The under-etched surfaces often consist of two to three facets. The inclination angles of these facets are categorized in two modes, defined either by periodic bond chains or by rows of atoms, each having two dangling bonds. Using the facet information, a simple atomic model is applied to the under-etch rates, based on removal frequencies of the chains or rows (f sub p and f sub k ), and based on steps on flat {111} planes. Variations of under-etched surfaces sufficiently near to {111} planes are well-matched by the model. Planes near {110} and {100} cannot be matched by this simple formulation. Effective f sub k increases more than f sub p at low TMAH concentration. The etching of the same crystal planes can vary substantially at different geometrical attitudes under identical etchant conditions. For example, {110} surfaces are etched at several different rates in 12 wt% TMAH at 80 deg C.