This paper presents a flexible radiofrequency filter with a central frequency of 2.4 GHz based on film bulk acoustic wave resonators (FBARs). The flexible filter consists of five air-gap type FBARs, each comprised of an aluminum nitride piezoelectric thin film sandwiched between two thin-film electrodes. By transfer printing the inorganic film structure from a silicon wafer to an ultrathin polyimide substrate, high electrical performance and mechanical flexibility are achieved. The filter has a peak insertion loss of -1.14 dB, a 3 dB bandwidth of 107 MHz, and a temperature coefficient of frequency of -27 ppm °C . The passband and roll-off characteristics of the flexible filter are comparable with silicon-based commercial products. No electrical performance degradation and mechanical failure occur under bending tests with a bending radius of 2.5 mm or after 100 bending cycles. The flexible FBAR filters are believed to be promising candidates for future flexible wireless communication systems.

In this paper, a 2.6 GHz air-gap type thin film piezoelectric MEMS resonator was fabricated on a flexible polyethylene terephthalate film. A fabrication process combining transfer printing and hot-embossing was adopted to form a free-standing structure. The flexible radio frequency MEMS resonator possesses a quality factor of 946 and an effective coupling coefficient of 5.10%, and retains its high performance at a substrate bending radius of 1 cm. The achieved performance is comparable to that of conventional resonators on rigid silicon wafers. Our demonstration provides a viable approach to realizing universal MEMS devices on flexible polymer substrates, which is of great significance for building future fully integrated and multi-functional wireless flexible electronic systems.

flexible organic or inorganic transistors. However, in many of the emerging applications, such as the Internet of Things and implantable electronic systems, in addition to the aforementioned basic building blocks, functional elements that include wireless RF electronic devices are also essential elements for wireless interconnection and data transmission. RF resonators, such as film bulk acoustic resonators (FBARs), which are traditionally used as the basic building blocks for modern RF filters [12] and oscillators, [13] are a natural candidate for wireless flexible electronics. FBAR-based electronic systems have also been extensively used in the biochemical sensing and actuating domains, [14] e.g., chemical vapors [15] and antibody-antigen reactions for immunosensors. [16] Although FBARs are small in size and light in weight, they have traditionally not been amenable to mechanical bending or stretching because of their rigid and brittle silicon substrate. Therefore, incorporating flexible electronic technology into FBARs is highly demanding and offers much more functionality for next-generation flexible electronic systems. However, since the fabrication process of these FBARs is intrinsically incompatible with the majority of plastic substrates, it is difficult to fabricate flexible FBARs with prominent electrical performance and mechanical flexibility. Previously, a flexible air-gap-type FBAR on a silicon substrate was demonstrated by thinning the silicon wafer to 50 µm. [17] By means of directly fabricating the resonator on polyimide, FBARs integrated on arbitrary substrates (polyimide, glass, and silicon) have also been achieved but with compromised electrical performance. [18] In our previous work, free-standing FBARs on polyethylene terephthalate substrates were demonstrated. [19] However, none of these devices is fully qualified for the rigorous requirements of satisfactory mechanical flexibility and uncompromised electrical performance.In this work, a highly bendable FBAR was introduced to meet these rigorous requirements by placing the FBAR at the mechanical neutral plane using two polyimide thin films. Furthermore, to achieve a high quality factor, the acoustic energy should be efficiently trapped in the material stack by a significant acoustic impedance difference between the FBAR and its surroundings. Therefore, air cavities were created both above

In this work, we presented a thin-film piezoelectric acoustic gas sensor with enhanced sensitivity by a surface modification strategy of oxygen plasma treated graphene oxide (GO) functionalization. By exposing to ammonia vapor (NH) of various concentrations at controlled temperature and humidity, the characteristics of the GO-coated acoustic sensor were investigated, that is, sensitivity, linearity, response, and recovery time. Oxygen plasma treatment of the GO-coated sensor further enhanced the sensitivity compared with the freshly prepared GO-coated sensor. The mechanism of oxygen plasma treatment effect on the GO-coated sensor was discussed based on characterizations of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscope (SEM), and precise weighing of the acoustic sensor. It was found that the oxygen plasma treatment introduces numerous defects to GO flakes, which are uniformly distributed across the GO surface, providing more gas molecule binding sites.

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