Novel Surface Acoustic Wave Devices Based Sensors And Microfluidic Actuators

With the approach of aging society, medical electronic devices and systems have attracted more and more attentions in recent years, especially those lab-on-a-chips, point-of-cares and wearable electronic systems as they can be used to diagnose diseases at early stage and monitor health conditions continuously etc. Microsensors and actuators are the key components for these systems, therefore the development of high sensitivity and high performance sensors and actuators becomes more and more important. Surface acoustic wave (SAW) device is one of the building blocks for modern electronics. Recently, SAW has been explored for many applications as sensors, actuators and microfluidic devices. Owing to the charming advantage of high sensitivity and performance, low cost, easy operation, good stability and CMOS compatible process and materials used, SAW-based sensors and actuators are terrificly fit for the applications in medical electronics like lab-on-a-chip systems. In this research, piezoelectric thin films were used to develop low cost and high performance SAW devices on various low cost substrates including flexible polymer and transparent glass, and their applications in sensors and microfluidic actuators were explored and investigated. Furthermore a novel SAW-based cell lysis device and transparent ultra violet (UV) sensor were proposed and developed, their performances were investigated.The main achievements of this PhD research are highlighted as follows:(1) High quality ZnO thin films were successfully deposited on Si, glass and polyimide (PI) substrates, and SAW devices were fabricated on the deposited thin films. From the transmission properties of these devices, five resonant peaks were observed, corresponding to the Rayleigh mode and Sezewa mode of ZnO/Si devices, the Rayleigh mode of ZnO/glass devices and the Rayleigh and Lamb mode of ZnO/PI devices. The Sezewa mode of ZnO/Si devices demonstrated the best performance in transmission and microfluidic application, and a～12.8 cm/s streaming velocity and a ～5.8 sec time for micropaticles concentration have been achieved. ZnO/glass devices have a comparable microfluidic performance with those of ZnO/Si ones, while ZnO/PI devices have much weaker transmission and inferior microfluidic performance compared to those of the other two types of devices.(2) AIN and AlScN with 27% Sc thin films were deposited by sputtering on Si and used to fabricate SAW deivces. Due to elastic softening introduced by Sc doping, d33 of the AlScN thin film and transmission of the SAW devices were improved significantly, leading to a more than 300% enhancement in electromechanical coefficient (K2) for Sc doping devices. The microfluidic performance of AlScN SAW devices was also significantely improved, and a double streaming velocity and a trible droplet moving velocity were achieved compared to those by AIN devices. Meanwhile the threshold RF powers for acoustic streaming and droplet movement decreased to ～40% and ～70% of those AIN devices. These results demonstrated that Sc doping can indeed solve the problems of low piezoelectric property and low electromechanical coupling factor which make AIN thin film based SAW devices with poor performance.(3) By utilizing the high impact force generated by high speed acoustic streaming, a SAW-based droplet type cell lysis actuator was proposed and fabricated. The devices showed up to 95% lysis efficiency for a droplet volume of 30μl, demonstrated its great potential for application in multi-functional lab-on-a-chip systems.(4) A SAW based transparent UV sensor has been developed and the post annealing effect for the sensing performance was investigated. The devices, with 400 ℃ post-deposition annealing performed best. The frequency response to UV light increased by more than 20 times compared to those without annealing, and a response time was less than 2.4 sec, sufficiently good for application. Considering the merits of low cost and highly transparency, the proposed UV sensors show great potential in future transparent electronics.