Experimental verification of the diffusion theory for wet isotropic etching of si via circular mask openings

Isotropic etching of silicon in HF based solutions is expected to be controlled by the diffusion of fluorine to the silicon surface. In order to gain quantitative understanding of the process we studied etching of Si in HF/HNO3/H2O via circular mask openings and compared the results with the theoretical expectations. The cavity edges due to etching under the mask were analyzed with a high precision by processing the optical microscope images. Dependence on the etching time and opening size was investigated. The time dependence was verified with 1% precision. Dependence on the opening size predicted theoretically is not fully supported by the experiment. There is a small (4%) but clearly observable deviation from the theory. A small anisotropy was observed in perfect agreement with the crystal orientation symmetry. The anisotropy becomes larger with the decrease of the opening size for (100) and (110) wafers.