Isotropic etching of 111 SCS for wafer-scale manufacturing of perfectly hemispherical silicon molds

This paper reports the results of a side-by-side comparison study of HF-HNO₃ isotropic etching of circular pits in <;111>; and <;100>; single crystal silicon (SCS). These etched holes will be used as sacrificial molds for micro-scale hemispherical resonator gyroscopes (HRGs) made using hemispherical shell resonators. Geometric uniformity of the mold is critical for HRG applications in order to achieve degenerate resonant modes and high optical and mechanical quality factors (Q). <;111>; wafers were found to provide excellent isotropic etching in the surface plane under all tested etching conditions with an average of only -1.4% radial variation, as compared to -3.2% variation for <;100>; wafers. The molds tested had an average radius of 41 μm, and depth of 35 μm, with a maximum of radius 92 μm with a depth of 95 μm.