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This paper reports a novel flexible film bulk acoustic resonator (FBAR) based on β-phase polyvinylidene fluoride (PVDF) piezoelectric polymer. The proposed device was simulated and evaluated; then, a low-temperature photolithography process with a double exposure method was developed to pattern the electrodes for the device, which enabled the device to retain the piezoelectric properties of the β-phase PVDF film. Results showed that the β-phase PVDF FBARs had a resonant frequency round 9.212 MHz with a high electromechanical coupling coefficient (k 2 ) of 12.76% ± 0.56%. The device performed well over a wide bending-strain range up to 2400 µε owing to its excellent flexibility. It showed good stability as a strain sensor with a sensitivity of 80 Hz/µε, and no visible deterioration was observed after cyclic bending tests. The PVDF FBAR also exhibited an exceptionally large temperature coefficient of frequency (TCF) of −4630 ppm/K, two orders of magnitude larger than those of other FBARs based on common inorganic piezoelectric materials, extraordinarily high sensitivity for temperature sensing. All results showed that β-phase PVDF FBARs have the potential to expand the application scope for future flexible electronics.with high-cost equipment for the deposit of these highly crystalline films can limit their widespread application and commercialization.Polyvinylidine difluoride (PVDF), a well-known high-performance piezoelectric polymer, could be a strong candidate as a new flexible FBAR material due to its remarkable inherent flexibility, lightness, and robustness, as well as its high piezoelectric response and low acoustic impedance, similar to those of biological tissue. Since the discovery of the piezoelectric activity of the PVDF material by Kawai in 1969 [9], PVDF-based piezoelectric polymer has been used in a wide range of actuators and sensors, such as underwater acoustic transducers [10], ultrasonic inspection sensors [11], acceleration sensors [12], surface acoustic-wave devices [13], pressure sensors [14], and energy harvesters [15]. In addition, it has been utilized in medical and biological applications such as artificial muscles and organs [16], medical imaging [17], and blood-flow monitors [18]. However, a PVDF polymer cannot be processed by a standard lithography process, limiting its application in MEMS devices and electronics.In this paper, we report on the feasibility of using a β-phase PVDF polymer for the development of FBARs. In this research, an FBAR device was designed with a 100 µm thick β-phase PVDF film sandwiched between two metal electrodes with a simulated resonant frequency of 9.418 MHz. A facile and reliable fabrication method was developed to successfully fabricate β-phase PVDF FBAR, which is compatible with the standard lithography process. The stability of the crystalline phase and the piezoelectricity of the β-phase PVDF film after the fabrication process were verified by X-ray diffraction (XRD) and piezoelectric coefficient d 33 measurement, respectively. The performance of the β...

This Letter reports a high‐performance film bulk acoustic resonator (FBAR) that can be applied in 5G wireless communication. The FBAR has a back‐etched free‐standing structure using an ultra‐thin single‐crystalline lithium niobate (LiNbO3 ) as the piezoelectric film. The thin LiNbO3 was obtained by the smart cut method and FBARs were fabricated by a standard MEMS process. Owing to the superior bulk‐like properties of the single‐crystal thin film, the fabricated FBARs have a resonant frequency of 5.0 GHz, a quality factor above 1800 and an electromechanical coupling coefficient of 2.66%, demonstrating a promising potential of FBAR filters in 5G and future 6G applications.

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