Surface Acoustic Mode Aluminum Nitride Transducer for micro-size liquid sensing applications

The thesis focuses on the investigation of thin-film surface acoustic wave (SAW) devices for liquid sensing applications. The piezoelectric material is a thin film of Aluminum Nitride (AlN), a CMOS compatible material, deposited by pulse DC reactive sputtering technique. A CMOS compatible process is developed and employed to fabricate the AlN/Si surface acoustic wave (SAW) devices which operate in a liquid medium. The applicability of the SAW device in sensing liquid is proved by numerical analysis, simulations and experimental results.
In the first chapter, the development of liquid sensors based on MEMS fabrication is introduced together with the wide range of applications for these devices. Also, the motivation to investigate the SAW device based on thin film AlN for liquid sensing is presented. In chapter 2, sensing mechanisms in general and applicable mechanisms of SAW sensors for liquid are presented. To determine the most suitable design of the SAW devices, three-dimension (3D) modeling based on the finite element method (FEM) is performed and analyzed.
Chapter 3 reports on the effect of a micro-size droplet shape, specifically the liquid contact angle, radius (area) and wettability of the contact surface on the SAW response. The numerical analysis and experimental results explain the interaction mechanism between the attenuated SAW beam and micro-droplets. The beam, which is emitted into the droplet, is expressed by the fraction coefficient. The change in contact radius influences the fraction coefficient more than the change in contact angle, especially on hydrophilic and super-hydrophilic surfaces.
In chapter 4, the first applicability of the SAW sensor is demonstrated by identifying the kind of liquid present on the propagation path. The sensing mechanism is based on physical properties (liquid density, sound speed in liquid and evaporation rate) and mass loading (concentration of stagnant liquid molecules). This also suggests a potential method to identify liquid samples of microliter volumes in microfluidic biosensors based on this SAW device.
In chapter 5, a SAW device equipped with an embedded microhole is proposed for the control and monitoring of the contact area between the piezoelectric material and the liquid medium. The device is miniaturized to be integrated on a printed circuit board (PCB). The device response to changes in density and pressure as well as to the evaporation of the liquid inside the microhole is studied. These initial indirect experimental results show the applicability of the SAW device for the state of liquid flow inside the microhole.
In chapter 6, some optimized structures of the SAW device are proposed. The simulation and experimental results showed that SAW devices with circular shape FIDTs have better performance, and provide a good method to detect micro-size droplets due to the better concentration of the energy traveling through the propagation path. Also in this chapter, a mixing IDT structure for SAW devices, which includes two layers of input IDTs, is proposed to reduce the longitudinal component in SAWs and generate novel mixing acoustic waves by mixing surface waves and plate waves on the piezoelectric material.
Finally, in chapter 7 concluding remarks and recommendations for future work are given.