Acoustic Wave Resonators And Their Applications As Sensors

With the rapid development of microelectronics and MEMS(Microelectromechanical systems)technologies,and broad applications of the IoT technology(internet of things),sensors are playing more and more important roles in modern society in various areas such as in manufacture industries,space exploration and our daily life.Sensors can be found everywhere nowadays,and have become the third sense like eye or ear of human beings.There exist many types of sensors based on various operation mechanisms,such as resistive type sensors,electrical and optical sensors etc.Acoustic wave devices are a type of high-performance sensors,and are one of the building blocks of modern electronics with very broad applications.This thesis focuses on development of the acoustic wave devices for sensing applications,including the innovative flexible and transparent surface acoustic wave devices(SAW),and film bulk acoustic wave resonator devices(FBAR).In this thesis,the delay line SAW devices were analyzed,designed and fabricated using high quality ZnO piezoelectric films that were deposited on glass substrates or polyimide(PI)substrates using reactive magnetic sputtering method.Using the SAW devices on glass substrate,we developed high performance transparent SAW humidity sensor with graphene oxide nanomaterial as the sensitive film,and their sensing mechanism and properties under various conditions were investigated in details.For the flexible SAW devices based on PI substrate,numerical analysis and experimental results were carried out first.Combining the theoretical analysis and experiments,it was confirmed that the two different resonant peaks are the symmetric(AO)and antisymmetric(SO)modes of the Lamb wave.The two resonant peaks were then utilized for humidity sensing,and their performances using graphene oxide as the sensing material were investigated in detail.The humidity sensing performance of the flexible SAW devices under bending state(with the strain about 2000?)and normal state were investigated,both showed similar behavior with good humidity responses.It was also shown that the flexible SAW sensors remain unchanged with excellence performance after bending at a strain of about 2000 ? for up to 8000 times,demonstrating a promising potential for flexible electronic applications.Similarly,to understand the working mechanism of FBAR devices and to design better FBAR-based sensors,the Mason model was firstly developed for the FBAR devices using the ADS software,showing that the FBAR device with a SiO2 support layer could have three up to resonant peaks with different vibration modes up to the frequency range of 4 GHz.The back trenched FBAR devices were then designed,fabricated and characterized.The fabricated FBAR devices have a base resonant frequency of 1.95 GHz with a very high quality factor of 2500,and their characteristics are consistent with those of the Mason model simulated.By sealing the back trench of the FBAR device on a glass substrate using a UV curable glue,a new type of pressure sensor structure was developed which has a very small device size,compact and integratable with electronics on chips.The pressure and temperature responses of the FBAR pressure sensor with three different resonant peaks were investigated in details.The results showed that the first resonant peak with low operation frequency has the best pressure response among the three resonant peaks,and is particularly suitable for pressure sensing application.Furthermore,a new design of the top electrode with many holes was proposed for FBAR UV-light sensor.UV sensing experiments show that the FBAR UV-sensor with multi-hole top electrode has much better UV-light response than the normal one with flat metal top electrode,indicating the multi-hole top electrode structure has enhanced the absorption of UV light and improved the UV sensing performance.FBAR devices have been widely used as gas sensors,mostly based on the mass loading effect,for better understanding and design of FBAR based gas sensors,FBAR sensor was theoretically and numerically analyzed using Mason model and FEM model,showing consistent results from each other.Based on these,a Pierce structure based FBAR oscillator circuit and a FPGA-based frequency counter circuit were then designed and fabricated for humidity sensor,without using network analyzer for measurement.Its humidity response was investigated using graphene oxide as the sensing material,and demonstrated its function in chip structures with similar performance to those of FBAR discrete device using network analyzer.Moreover,a complete automatic system with an array of FBAR sensors for multiple gas sensing was developed,which was composed of a gas generation system,sensor testing chamber and a data acquisition system,showing its good function for multiple sensing.Finally,we designed a CMOS and FBAR integrated sensor array system based on the MEMS PDK provided by Invensense and 0.18 ?m CMOS PDK provided by Global Foundry.The integrated sensor array include 23 FBAR oscillators in an area of 4mm x 2.5mm.Each oscillator was composed of a key oscillator circuit,a current source,and a digital mixer based on the digital XOR gate.Both the pre-simulation and post-simulation showed excellent and consistent results.