Evolution of etch profile of <111> silicon in HNA solution

In purpose of building highly symmetric hemispherical molds for 3D shell resonator fabrication, the evolution of etch profile of (111) silicon in HNA (HF/HNO3/CH3COOH) solution has been reported. According to experimental data, the etch profile first evolves into an annular groove and returns to smooth surface as the etching period increases. The duration of the annular groove phase could last from seconds to minutes, depending on the etching configurations and mask window dimensions. It is suggested that the hemispherical molds can only be formed after the annular groove phase, where the etch is considered as isotropic. Based on the experimental conditions used in this study, hemispherical molds have been fabricated with etch window radii lower than 10 mu m. The fluorophore-assisted confocal laser measurement shows that the molds possess an average radius of 38.93 mu m with a variance of 0.79 mu m, which builds the basis to fabricate 3D shell resonators with high structure symmetry.