| Anisotropic etching of silicon is a fundamental process in micro-systems technology (MST) and in the fabrication of micro-electromechanical systems (MEMS). This work addresses the fundamental atomic mechanisms of anisotropic etching of single-crystal silicon, by wagon-wheel-based under-etch experiments of {lcub}110{rcub} and {lcub}100{rcub} silicon in TMAH at 25wt%, 19wt%, 17wt%, 15wt%, 12wt% and 9wt% at 80°C. The under-etched surfaces often consist of two to three facets. The inclination angles of these facets are categorized in two modes, as being 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 (fp and fk), and based on steps on flat {lcub}111{rcub} planes. Variations of under-etched surfaces near enough to {lcub}111{rcub} planes are well-matched by the model. Planes near {lcub}110{rcub} and {lcub}100{rcub} cannot be matched by this simple formulation. Effective fk and fp are calculated for all the experimental cases. The etching of the same crystallographic features can vary substantially at different geometrical attitudes, in identical etchant conditions. |
