Anisotropic etching of silicon in a two-component alkaline solution

The application of anisotropic alkaline etching for the fabrication of micromechanical parts is mostly based on the strong angular dependence of the etch rates in single-crystal silicon. In some applications, the smooth, defect-free etched surface is of high importance and from this point of view the strong anisotropy is disadvantageous. With this consideration, the composition of the alkaline etching solution has been changed by the addition of strong oxidizing agent. In such an etching solution, a mirror-like and defect-free silicon surface has been obtained, even after long etching times. In parallel, the reduction of the anisotropy, i.e. the decrease of the ratio (R_⟨001⟩/R_⟨111⟩) of the etch rates has occurred. In (001) oriented wafers, when the pattern alignment follows the ⟨100⟩ directions, this type of anisotropic etching produces vertical walls. The absolute values of the etch rates in all crystallographic directions are relatively low and they change with the doping concentration. The strong oxidizing component is supposed to ensure homogeneous oxidation (i.e. passivation layer formation) on the whole surface even though there are differences in the activation energy of the surface states. In this process the dissolution of the passivation layer is the rate-limiting chemical reaction.