Study On Flexible Surface Acoustic Wave Devices

Surface acoustic wave (SAW) devices have a very wide range of applications including their use in wierless communication, biochemical sensing, drug development, healthcare, microfluidics as well as in lab-on-a-chip applications. The traditional SAW devices are normally fabricated on the rigid substrates such as sillicon and glass. As the demand for skin-like devices that are flexible and attachable to irregular or arbitrary shape surface is rapidly increasing, it would be a huge benefit if a SAW device can be constructed at a low cost on a cheap and flexible substrate such as on plastic films. Here we report a method of making flexible SAW devices on a polyimide (PI) substrate, and demonstrate their potential applications in flexible electronic devices, sensing, microfluidics and lab-on-a-chip.The content of this thesis consists of five parts:① Proposed the flexible ZnO SAW device and demonstrated their application in sensors and microfluidics,② Crystalline structure effect on the performance of flexible SAW devices,③ Analysis of two modes of flexible SAW devices ④ Deposition of high quality AN on PI substrate and fabrication of flexible SAW devices.⑤ Proposed the transparent and flexible ZnO based SAW devices.1. Proposed the flexible ZnO SAW device and demonstrated their application in sensors and microfluidics. The flexible SAW devices exhibit two wave modes, with a resonant frequency and coupling coefficient of 198.1 MHz and 1.05%, and of 447 MHz and 0.8% respectively. The signal amplitudes of the two modes are larger than 18 dB. The flexible devices have a high temperature coefficient of frequency, and are thus useful as sensitive temperature sensors. Moreover, strong acoustic streaming with a velocity of 3.4 cm/s and particle concentration using the SAW have been achieved, demonstrating the great potential for applications in electronics and MEMS. These results were published on Scientific Reports.2. Investigated the effects of the deposition conditions, crystal qualit, and film thickness of the ZnO films on the performance of the flexible SAW devices. The results show that the flexible SAW device have the best performance when the deposition pressure is 2 Pa, deposition power of 200 W, bias voltage of -75 V and the thickness of ZnO film of 4 μm. These results were published on Journal of Applied Phyisics.3. FEM simulation and theoretical analysis showed that the first mode of the flexible SAW device is Rayleigh mode while the second mode is Generalized So Lamb wave. Fabricated the AlN/Si SAW device for validating the Generalized So Lamb wave for the structure of the SAW device with high speed velocity piezoelectric layer covering on the low speed velocity substrate. The wavelength and thickness of ZnO films have been investigated on the performance of flexible SAW deivces. These results were published on Scientific Reports and Microfludics and Nanofludics.4. Investigated the effects of deposition parameters on the crystal structure of AN thin films on polymer substrates deposited by reactive direct-current magnetron sputtering. The optimal deposition parameters in our experiments are:deposition pressure 0.38 Pa, N2/Ar flow ratio 23, sputtering power 414 W, and thickness of Al underlayer 18 nm. Development of AN-based flexible SAW devices on the polymer substrates is initiated and the experimental and simulated results demonstrate the devices showing the phase velocity of 9000～10000 m/s, which indicate the AN lamb wave. These results were published Thin Solid films.5. Reported the design of transparent SAW resonators using aluminum-doped ZnO (AZO) as the transparent electrode on ZnO/glass substrates. SAW devices with a resonant frequency up to 204.4 MHz and signal amplitude up to 25 dB were obtained with transparency above 80%. Temperature sensing shows that they had a temperature coefficient of frequency of ～50 ppm/K. Having fabricated the flexible and transparent SAW devices, with different wavelength and different thickness of ZnO film and the signal of the flexible and transparent SAW devices is larger than 25 dB, demonstrating the good potential in the sensors and microfluidic application. These results were published IEEE Electron Device Letters.