New Type Acoustic Wave Resonators And Their Applications As Sensors

Motivated by the fundamental advancement of material science and the progress of the-state-of-the-art fabrication technology, flexible/stretchable electronics have progressed rapidly in recent years. Flexible/stretchable electronics are not just as a supplement to the well-established rigid substrate-based electronics but also a vital and innovative branch of electronics, which opens the door for variety of applications. Several types of flexible devices have been developed, but there has been limited research on flexible electromechanical systems (MEMS). Surface acoustic wave (SAW) devices and film bulk acoustic wave resonators (FBAR) are not only essential electronic devices, but also are the building blocks for sensors and MEMS.In this thesis, a technology to make flexible SAW devices using ZnO nanocrystals deposited on low-cost and bendable PI and PET substrates was proposed and developed. The ZnO films were deposited by magnetron sputtering and exhibited highly (0002) orientation with large grain size. High performance flexible SAW devices with two main resonant modes were fabricated on the ZnO/Polymer substrates. The two modes correspond to the symmetric (A0) and antisymmetric (So) modes of generalized Lamb wave. The influences of the ZnO film thickness, device wavelength and acoustic wave traveling distance to the performance of the flexible devices were investigated systematically. These SAW devices are bendable and can endure strains up to 2500?s for hundreds of times, showing the feasibility of these devices as flexible electronics. These flexible SAW devices were utilized to develop a variety of sensors, and the performance of the devices as temperature, humidity, ultra-violet (UV) light and strain sensors were investigated in detail. The types of SAW sensors showed excellent performance for the detection of the corresponding physical parameters, with some of them even better than those SAW sensors fabricated on rigid substrates, demonstrated the potential of these flexible devices for sensing applications.Besides the flexible SAW devices, another ZnO based acoustic resonator, FBAR, was also investigated. The FBARs were fabricated on silicon substrates with a back--cavity structure. The FBARs exhibited two main resonance modes corresponding to the piezoelectric ZnO layer and ZnO-on-SiO2 layer. The responses of the dual mode FBAR to changes of temperature and pressure were studied, the resonant frequency of both modes were found to shift linearly with temperature and pressure changes. By utilizing the FBAR's back trench as a sealed cavity, an on-chip single FBAR sensor suitable for measuring pressure and temperature simultaneously was proposed and studied. This dual mode on-chip pressure sensor is simple in structure and operation, can be fabricated at low cost, and yet requires no specific package for sealing, therefore has great potential for applications.