Investigation Of Piezoelectric Ultrasonic Technology Based On MEMS

In this thesis,we present the investigation of piezoelectric ultrasonic technology that is used into the microfluidic and MEMS ultrasonic testing device.New ultrasonic transducer,microfluidic device and ZnO piezoelectric thin films based on MEMS technology will be studied as follows:1,Resonant ultrasound spectrum(RUS) has been developed since 1990s,which is a dynamic method to test the mechanical properties of the solid materials and is currently considered by the scientists in the condensed matter physics as the most accurate method to determine the elastic constants of materials.This ultrasonic testing method can be applied into many fields,such as the testing of minimized device in MEMS.In RUS,a solid sample is clamped between two piezoelectric transducers, which are generally PZT or other bulk piezoelectric ceramics or crystals.However,a sample,especially a very small one,is hard to be clamped due to the smooth surface of transducers,and if the sample is clamped at different locations of the piezoelectric transducers,the measured spectra will be fluctuant,lowering the accuracy of RUS measurement.To solve this problem,we propose a new type of PZT/Si composite transducer for RUS.The new type PZT/Si composite transducer is analyzed by comparing with traditional single PZT transducer.The results show that the error of displacement caused by voltage and static displacement is very small,about 2%.The error will decrease by reducing the size of silicon die.The resonant frequency of PZT/Si transducer is almost equal to that of single PZT plate.It is concluded that the proposed device can be used in ultrasonic testing.In the end,the PZT/Si composite traducer is fabricated by using MEMS technology primarily.2,With the rapid development of MEMS technology during the past 20 years, many microfluidic devices have been proposed and developed for biomedical applications,especially in biomedical engineering,such as bimolecular detection, human tissue environmental monitoring,drug delivery and cooling of "LAB ON CHIP" and IC chips so on.Among various types of technologies for manipulating liquids,acoustic/ultrasonic technique is proved to own higher efficiency than the other methods on driving and controlling fluid motion.Ultrasound wave propagating in a viscous fluid will induce acoustic streaming owing to the absorption of the fluid, which can cause unidirectional fluid motion in microchannel.Due to the size of the microdevices that is greatly reduced by using MEMS technology,fluid motion induced by acoustic streaming are quite different from that in the conventional scale device.Many experimental researches on acoustic streaming have been studied. However,very few are studied on acoustic streaming in the microscale.Therefore,it is a challengeable and important task to investigate acoustic streaming flow in microchannel.In this thesis,acoustic radiation force,acoustic streaming in microchannel actuated by ultrasonic vibration are studied in the proposed device which is composed of a PZT plate,microcavity and microchannel.Basing on the fundamental of fluid dynamics,an analytical study of fluid motion acoustically induced in microchannel is investigated.We have analytically studied the fluid motion driven by acoustic field and forced heat transfer by acoustic streaming in a microchannel in this paper.The maximum velocity of fluid motion in the microchannel can reach up to 1-9mm/s when the frequency of vibration is between 200 KHz and 1 MHz.The results show that acoustic streaming induced by ultrasonic vibration in the microscale is feasible and has potential for developing acoustic MEMS devices for driving fluid motion, drug delivery and micro cooling applications.3,ZnO piezoelectric thin film has many applications in the MEMS field such as MEMS microphone,micro accelerometer and bulk acoustic wave resonator.For these applications,high-quality ZnO piezoelectric thin film,which has a tendency to grow with strong preferential orientation,is need.In this thesis,we present the growth and characterization of ZnO piezoelectric thin films to obtain the high quality piezoelectric thin films used in piezoelectric ultrasonic application.Meanwhile, aluminum is also used as the bottom electrode to acquire the compatibility of fabrication of ZnO piezoelectric thin films device and IC technology.Magnetron RF sputtering technology is used to grow ZnO thin film for piezoelectric applications.We study the influence of annealing process and different substrates,including silicon,aluminum layer and silicon nitride thin film on silicon, on the piezoelectric performance of ZnO thin film by using XRD and SEM to analyze C-axis orientation and crystalline quality of ZnO thin film.The experimental results show that the crystal quality of ZnO thin films deposited on aluminum is worse than that on silicon substrate and silicon nitride layer.It is concluded that the crystal quality of ZnO grains will be influenced by the bottom electrode in the fabrication of ZnO piezoelectric device.To solve this problem,we primarily develop a fabrication process for which the aluminum is still used as the bottom electrode but isolated from the ZnO thin film by a layer of silicon nitride thin film,to meet the requirements of the high quality of the ZnO thin film and compatibility with CMOS technology.