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The effect of thickness anisotropy on the degenerate elliptical resonance modes of micro-hemispherical shell resonators (µHSRs) created using the thermal oxidation process is investigated. This anisotropy arises from the variation in wet thermal oxide growth according to the exposed crystal planes of the single-crystal-silicon hemispherical mold used to generate the µHSRs. It is shown that, despite the presence of thickness anisotropy, the degenerate resonance modes of oxide µHSRs can exhibit zero intrinsic frequency split depending on the particular resonance mode and symmetry of the thickness anisotropy imparted from the underlying silicon wafer. Measured results verified by simultaneous electrical excitation on the 0° and 45° axes demonstrate less than 94 Hz intrinsic m=3 frequency split for a 1240 µm oxide µHSR (limited by measurement conditions), which is to the authors' knowledge the smallest as-fabricated frequency split reported to date for any µHSR.

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