Piezoelectric micro-resonators for low-power and low-cost internet of things communication nodes

Abstract

Internet of Things (IoT) is a set of technologies that collect distributed data on anything, from daily life objects to more sophisticated devices such physical/chemical sensors, vehicles, cell phones, to interconnect with a vastly broad network to accomplish much advanced functionalities to improve certain existing processes or to enable a new scenario not previously possible. It is being widely applied in different areas and applications such as smart cities, smart agriculture, smart healthcare, smart manufacturing, and so on. IoT is currently growing tremendously, and it has been predicted that global IoT connections will reach 30 billion, and the market will be worth more than 1.7 trillion dollars by 2020. The heart of IoT is the radio frequency (RF) wireless modules, which are responsible for data exchange through wireless networks and share the same wireless technologies as mobile phones, i.e. 4G/5G. They must feature low power consumption and do so at a low cost, preferably realized in a small form factor, to overcome the challenges posed by massive deployment IoT nodes. IoT is also moving forward into new frontiers other than terrestrial areas relying on air as the propagating medium, such as underwater (Internet of Underwater Things, IoUT) and intra-body networks (Body Area Network, BAN). For such applications, ultrasonic signals are the most promising candidate as RF signals suffer from severe propagating attenuation in these scenarios. In this perspective, this dissertation presents the design and experimental verification of aluminum nitride (AlN)-based micro-resonator technologies for the implementation of next generation low-power and low-cost IoT wireless communications. AlN Cross-Sectional Lamé Mode Resonators (CLMRs), exploiting a 2-dimensional (2D) vibration mode, are developed and the potential to deliver high performance RF passive components with lower fabrication complexity with COMS integration compatibility for sub-6 GHz is demonstrated. We also developed a new acoustic resonator technology, AlN Combined Overtone Resonators (CORs), capable of addressing the filter requirements for the 5G mmWave spectrum in the 6-40 GHz range. The COR exploits the multimodal excitation of two higher-order Lamb waves in a suspended thin-film AlN plate to transduce a 2D overtone mode with high electromechanical coupling coefficient kt2, groundbreaking quality factor Q, and lithographic frequency tunability at twice the frequency of a conventional thickness-extensional mode excited in the same structure. The next part of the dissertation provides a substantially innovative approach, near-zero standby power RF wake-up system, realized by AlN Resonant Switches (ResoSWs) with CLMRs, to eliminate the standby power to significantly prolong the battery-life for event-driven IoT applications. Lastly, AlN Piezoelectric Micromachined Ultrasonic Transducers (pMUTs) are being explored as promising candidates for wireless communications in aqueous media to overcome the limitations of macroscale acoustic transducer on size, power efficiency, and bandwidth. The four resonator technologies presented here set a stepping stone towards the development of high performance, miniaturized wireless modules for the massive deployment of next generation low-cost and low-power IoT for a wide frequency range from ultrasound to mmWave waves for a variety of applications.