Zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg x Zn 1Àx O) are piezoelectric materials that can be used for high-quality-factor bulk acoustic wave (BAW) resonators operating at GHz frequencies. Thin-film bulk acoustic resonators (TFBARs) are attractive for applications in advanced communication and in various sensors as they offer the capability of monolithic integration of BAW resonators with radio-frequency integrated circuits (RF ICs). In this paper we report Mg x Zn 1Àx O-based TFBAR biosensors. The devices are built on Si substrates with an acoustic mirror consisting of alternating quarter-wavelength silicon dioxide (SiO 2 ) and tungsten (W) layers to isolate the TFBAR from the Si substrate. High-quality ZnO and Mg x Zn 1Àx O thin films are achieved through a radio-frequency (RF) sputtering technique. Tuning of the device operating frequency is realized by varying the Mg composition in the piezoelectric Mg x Zn 1Àx O layer. Simulation results based on a transmissionline model of the TFBAR show close agreement with the experimental results. ZnO nanostructures are grown on the TFBAR's top surface using metalorganic chemical vapor deposition (MOCVD) to form the nano-TFBAR sensor, which offers giant sensing area, faster response, and higher sensitivity over the planar sensor configuration. Mass sensitivity higher than 10 3 Hz cm 2 /ng is achieved. In order to study the feasibility of the nano-TFBAR for biosensing, the nanostructured ZnO surfaces were functionalized to selectively immobilize DNA, as verified by hybridization with its fluorescence-tagged DNA complement.

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