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The high-tone bulk acoustic resonator (HBAR), consisted of a Mo/ZnO/Pt/Ti/Si structure, is fabricated. The thickness of Si substrate under HBAR is controlled by a two-step process of wet- and dry-etching. The resonance frequency spacing (Deltaf) of HBAR is dependent on the etching duration. The frequency response of the HBAR is measured using an HP8720 network analyzer and a CASCADE probe station. The estimation of Si residue based on the high-tone resonant phenomenon coincides with practical measurements. A frequency response with no harmonic resonance, which is an extreme case of HBAR without Si residue, is revealed. Furthermore, a sensor of high-frequency bulk acoustic wave resonator is obtained.