| Thin film bulk acoustic resonator (FBAR) has many distinctive features and advantages over other coundtparts such as high frequency, low insertion loss, high power capacity for RF applications, small size, compatible with CMOS technology and so on, it can meet the requirements of modern RF communication. The FBAR technology has been researched and its applications have been explored intensivey, and FBAR-based oscillators, frequency filters and duplexers have been commercialized. Furthermore, owing to its high sensitivity and reliability, FBARs have been used as mass sensors, pressure sensors, humidity sensors and bio-chemical sensorareas. However, the research on FBAR-based ultra-violet (UV) light sensors is very limited, and the performance of those UV-sensors developed is unsatisfactory, unable to meet the ever-increase dmands for high performance UV-sensors. In order to obtain higher sensitivity devices, we designed and fabricated various novel FBAR UV sensor structures and assessed their performances.Firstly, through careful initial study on the back trench type FBAR (from the material selection for the piezoelectric stack layers, the design of device structure, the fabrication process etc) and its associated processing problems, we proposed appropriate solutions for the FBAR device and UV sensor developments, and then developed the fabrication processes step by step, obtained a stable and reliable fabrication process in our laboratory conditions. The resonant frequency of the FBAR devices fabricated is around1.56±0.02GHz using a～2μm ZnO piezoelectric layer, the Quality factor is750+50with the best one of950. The temperature coefficient of the FBARs is-59.48±5ppm/℃. On this basis, the research then focused on the development of the FBAR UV sensors. We designed and fabricated several structures of UV sensors, tested the FBAR devices under UV light illumination, and then built a device circuit model to explain the bahaviour of the sensors. To increase the absorption of UV light, we proposed to use semi-transparent thin Au layer, transparent Al-dope ZnO (AZO) layer and grid metal structure as the top electrodes for the FBAR UV-sensors, the results show that the shift frequency is40KHz,50KHz,40KHz-50KHz, respectively for the corresponding FBAR sensors, much larger, hence sensitive than those reported. This research also developed a flexible FBAR device technology which uses the PET as the substrate to fabricate FBARs. We used COMSOL software to investigate the relationship between the thickness of PET substrate and the resonance frequency, and the impact of the electrode material on the performance of the devices. The results showed that harder metal material as the bottom electrodes can make the resonators with better performance. We then used Au, instead Al, as the bottom electrodes to fabricate flexible FBARs on PET. The series and parallel resonant frequencies of the devices with a ZnO of2um are1.18±0.05GHz and1.21±0.05GHz, and the coupling coefficients k2eff and Q are0.148%,30.48respectively. In order to increase the transmission signal amplitude of the FBARs, we proposed a filter structure FBAR and proved its viability both by simulation and experiment. We fabricated the flexible FBAR devices and characterized the devices in a bending state and UV sensing performance. Although the UV sensing performance of the flexible sensor is not as good as the devices on a solid Si substrate, the results underbending show that the FBAR devices work well even after bending for many times. Finally we fabricated a FBAR filter which has a bandwidth of80MHz and the operating frequency of1.25GHz. The thesis also highlighted the future work and tasks to improve the performances of FBARs sensors. |
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