Research Of New Electrically Tunable Film Bulk Acoustic Resonator(FBAR)

With the rapid development of microelectronics and progress of wearable devices/systems, the demands for advanced, multi-functional and high performance electronic components increase at ever-fast speed. As a high frequency resonator, thin film bulk acoustic resonator (FBAR) possesses the advantages of very small dimensions, easy process, and high frequency and quality factor, which are particularly suitable for the applications in modern RF communication and sensing. Today, FBARs have been widely used in wireless transceiver circuits. Owing to its very high sensitivity, various sensors based on FBARs have also been developed and their applications have been explored. It is believed that FBARs will play an even more important role in future electronic products.The purpose of the research is to develop high performance FBAR devices to meet the ever-increase market demands for communication and sensing. In this study, we proposed a new electrically tunable FBAR structure to realize frequency tuning by applying DC bias which could be utilized for the application of RF communication with varying frequency at different locations of countries. Some applications based on the new FBAR are demonstrated in this research. The research details and results obtained can be summarized as follows:1、We have developed a process which allows us to fabricate FBAR devices with resonant frequency, fr, in the range of1-3GHz, and the quality factor, Q, greater than500. The largest Q value obtained was2068, much larger than those reported by others and the commercial ones.2、For FBARs with fr～1.5GHz, we found that the resonant frequency can be electrically tuned by applying a DC bias at a rate between15kHz/V to30kHz/V which is around0.03%of the resonant frequency of the devices for a bias change from+10V to-10V. Theoretical analysis showed the frequency tune is mainly associated with the change of the elastic coefficient of ZnO piezoelectric film, which has a linear positive correlation with the electric field applied. The bias tuning rate for frequency is off application3、A new type of FBARs based on an Au/ZnO/n-ZnO/Au structure has been proposed and fabricated in this study. By introducing an n-ZnO semiconductor film into FBAR’s "sandwich structure", a schottky diode is formed and improves the tuning range significantly. A frequency shift up to200kHz/V was observed which is about ten time larger than that of traditional FBARs. We have also developed an equivalent circuit for the new type of FBARs which would be a useful tool for further researches.4、Finally, some applications based on the new electrically tunable FBAR device such as UV-light sensing, frequency self-calibration etc. have been explored. The UV sensor with the new FBAR structure showed a frequency downshift up to140kHz/V at a UV light intensity of1mW/cm2, while that was only40kHz/V at the same intensity for the traditional FBARs, demonstrating that the new structure FBARs can improve UV sensing sensitivity significantly. By utilizing on the electrical tuning ability, we have also designed temperature compensation and frequency self-calibration circuits which can also be integrated with CMOS circuits. Furthermore, a RF tunable filter array which can realize fine tuning and work at multi-band has been proposed and investigated theoretically using the new electrically FBAR. All these applications have brilliant market prospect.