Improved HNA isotropic etching for large-scale highly symmetric toroidal silicon molds with <10-nm roughness

Abstract. Microsystem technology is well suited to batch fabricate microhemispherical resonator gyroscopes (HRG) to reduce cost and volume. In the processing of micro-HRG, a crucial step is to get a 3-D hemispherical mold with the large-scale, high-symmetry, and smooth surface. Compared with the hemispherical resonator, the toroidal resonator has the smaller frequency split and larger effective resonance mass under the same processing accuracy. A wafer-scale etching method for the toroidal resonator mold was presented, which is based on the deep reactive ion etching and improved HNA isotropic etching. The advantages of this method include low cost, time savings, and easy operation. With this method, toroidal molds with an average diameter over 1900  μm, asymmetry <0.2  %  , and roughness <10  nm were successfully fabricated. The uniformity and surface smoothness of the molds are mainly determined by the parameters of HNA etching. A series of controlled experiments were conducted to optimize isotropic etching parameters that include mask design, bath agitation, HNA composition, and temperature. The influence of these parameters on etching rate and uniformity was discussed. The result shows that the composition of 2.5:7:1 (HF  :  HNO3  :  CH3COOH), temperature of 30°C, and bath agitation of 20 revolutions per minute are optimal etching conditions to achieve high-performance molds.