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A 2-D silicon phononic crystal (PnC) slab of a square array of cylindrical air holes in a 10-μm-thick freestanding silicon plate with line defects is characterized as a cavity-mode PnC resonator. A piezoelectric aluminum nitride (AlN) film is employed as the interdigital transducers to transmit and detect acoustic waves, thus making the whole microfabrication process CMOS compatible. Both the band structure of the PnC and the transmission spectrum of the proposed PnC resonator are analyzed and optimized using finite-element method. The measured quality factor (Q factor) of the microfabricated PnC resonator is over 1000 at its resonant frequency of 152.46 MHz. The proposed PnC resonator shows promising acoustic resonance characteristics for radio-frequency communications and sensing applications.