oa Editorial [Hot Topic: Sensing the World with Film Bulk Acoustic Resonators (Guest Editor: Xiaotun Qiu)]

Film bulk acoustic resonator (FBAR) usually consists of a sputtered piezoelectric thin film (ZnO) sandwiched by two metal layers (top and bottom electrodes). A resonance condition occurs if the thickness of the piezoelectric thin film is equal to an integer multiple of a half of the acoustic wavelength. The fundamental resonant frequency is then inversely proportional to the thickness of the piezoelectric material used. The first generation of FBAR was reported in the early 1980s. Since then, it has gained mainstream attention both as filters and as high sensitivity sensors. Due to the small size, low fabrication cost and high sensitivity, FBAR has great potential to be employed in a variety of sensing applications. On one hand, it can be used as a mass sensor, because the resonant frequency of FBAR decreases linearly with the mass accumulated on top of the resonator. Different coatings can be applied on FBAR for the absorption of various sensing targets, especially in biological applications. On the other hand, environmental factors, such as ultraviolet radiation, relative humidity and some harmful gases can alter the density of the ZnO film in FBAR. In this way, it is feasible to utilize FBAR as environment monitors. The aim of this special issue is to give a brief overview of the latest research progress on FBAR sensors. It will contain a collection of works from the most active research groups in this field. The first paper discussed the design of a FBAR oscillator to improve the sensitivity of the device, especially aimed at biological sensing applications. The second paper explored the response of FBAR to infrared radiation. In the third paper, the ZnO based FBAR sensor was applied to monitor ultraviolet radiation, relative humidity and different gases, such as acetone (reducing gas) and ozone (oxidizing gas). The last paper investigated different structures of FBAR (thickness field excitation FBAR and later field excitation FBAR) and compared their distinct response behaviors to DC voltage. These works can serve as a useful reference for researchers to have a comprehensive understanding of the current research status of FBAR sensors and the trends for future investigation.