Hybrid Film Bulk Acoustic Resonator Fabricated Using a Single Bragg Reflector

Film bulk acoustic resonators (FBARs) with air cavities for sensing and high radio-frequency filters often exhibit reliability and yield strength issues during bulk etching of the silicon substrate. The handling of Si wafers after the complete removal of bulk Si below the active area is critical and distorts the performance characteristics of the resonator. This paper presents the design and microelectromechanical systems (MEMS)-based fabrication of a novel hybrid FBAR to enhance the reliability and yield strength of the MEMS-based resonator. A single Bragg reflector introduced between the bottom electrode and SiO₂ layer in the hybrid FBAR acts as a reflector for an acoustic signal and leads to improved mechanical strength of the active area. The λ/2 mode arrangement was used in the design of the hybrid FBAR. The mirror’s lower layer is made of high-acoustic-impedance material, while the upper layer is made of low-acoustic-impedance material. The acoustic mirror layer thickness is λ/4. Zinc oxide (ZnO) is used as a piezoelectric (PZE) film. The resonator based on zinc oxide PZE film measures a resonant frequency at 1.524 GHz with a coefficient of coupling and a quality factor of 7.53% and 331.2, respectively. The fabrication results indicate that the figure of merit (FoM) is enhanced by 208% and 366% and \(k_{{{\text{eff}}}}^{2}\) $k_{\text{eff}}^2$ is improved by 300% and 215% when compared to cavity-based FBAR and Bragg reflector-based FBAR, respectively. According to the measurement results of fabrication, the hybrid FBAR holds immense potential for the development of sensors, oscillators, and filters.