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. @en

The effect of the contact angle and radius of a microsize droplet on the surface acoustic wave (SAW) response for microfluidic applications is reported. It is studied through the dynamic change of the droplet shape during the evaporation process. An aluminium nitride SAW device, operating at 125.7 MHz, is utilized to investigate the deformation of the droplet shape (contact angle and contact radius) caused by shrinking. The large cavity placed on the propagation path distorts the in-band SAW response one time at the centre frequency. The fractional coefficient of the SAW insertion loss, before and after dropping the liquid on the propagation path, is continuously recorded. The change in the fractional coefficient shows that the radiated acoustic kinetic energy depends on the contact area between the sessile micro-size droplet and the SAW device more than the contact angle of the droplet. Three droplet volumes have been considered, namely 0.05, 0.1 and 0.13 μl, and the electrical results show a better agreement with the theoretical data than the optical image data. The average duration of the fractional coefficient change for these cases is 420, 573 and 760 s, respectively. The effect of the hydrophobicity versus hydrophilicity of the contact surface on the duration of the fractional coefficient change is studied by coating the SAW with a silicon oxide or hexamethyldisilazane (HMDS) thin layer. For the same 0.05 μl sessile droplet on the hydrophobic surface, this duration is on average 110 s longer than that on the hydrophilic surface.

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A surface-acoustic-mode aluminum nitride (AlN) transducer is utilized to determine the type of liquid dropped on the propagation path. It is based on tracking the shrinking droplet radius and observing stagnant liquid molecules during and after the liquid evaporation process. The device configuration is suitable to test small amounts of liquids, in the microliter range. According to both mass loading and physical property mechanisms, eight samples of liquids, isopropanol (IPA), ethanol (ETH), deionized-water (DW), tap water (TW), heptane (HEP), propylene glycol monomethyl ether acetate (PGMEA), hexamethyldisilazane (HMDS) and acetone (ACE), which have different equilibrium vapor pressures, molecular weights and boiling points, are accurately detected. The experimental results show that the rate of the change in the energy loss including a slow and fast attenuation region depends on the change of physical properties, such as density, sound speed in liquids and evaporation rate, during the evaporation process. As the evaporation rate of the DW is rather slow, the slow attenuation region occurs for a longer time than the fast one. Consequently, the whole oscillation duration of the attenuation occurs for a longer time, whereas that of the other liquids studied, like ACE, ETH, and IPA, having a faster evaporation rate is shorter. Sensitivities of the surface-acoustic-mode transducer to the evaporation process of liquids such as DW, TW, PGMEA, HMDS, HEP, IPA, ETH and ACE are −29.39, −29.53, −31.79, −34.12, −33.62, −32.87, −32.67, and −32.82 dB μm−2, respectively. The concentration of stagnant liquid molecules causes a change in the surface mass of the micro-electro-mechanical transducer, which causes a frequency shift and increases the signal noise at the receiver after the liquid evaporation process. The average frequency shifts of ACE, HEP, HMDS, ETH, IPA, PGMEA, TW and DW are 241, 206, 172, 117, 76, 27.3, 11.6 and 0 kHz, respectively, coherent with the type of formed liquid pattern on the device surface, thus allowing to detect liquid samples effectively.@en

This work presents the mixing wave generation of a novel surface acoustic wave (M-SAW) device for sensing in liquids. Two structures are investigated: One including two input and output interdigital transducer (IDT) layers and the other including two input and one output IDT layers. In both cases, a thin (1 μm) piezoelectric AlN layer is in between the two patterned IDT layers. These structures generate longitudinal and transverse acoustic waves with opposite phase which are separated by the film thickness. A 3-dimensional M-SAW device coupled to the finite element method is designed to study the mixing acoustic wave generation propagating through a delay line. The investigated configuration parameters include the number of finger pairs, the piezoelectric cut profile, the thickness of the piezoelectric substrate, and the operating frequency. The proposed structures are evaluated and compared with the conventional SAW structure with the single IDT layer patterned on the piezoelectric surface. The wave displacement along the propagation path is used to evaluate the amplitude field of the mixing longitudinal waves. The wave displacement along the AlN depth is used to investigate the effect of the bottom IDT layer on the transverse component generated by the top IDT layer. The corresponding frequency response, both in simulations and experiments, is an additive function, consisting of sinc(X) and uniform harmonics. The M-SAW devices are tested to assess their potential for liquid sensing, by dropping liquid medium in volumes between 0.05 and 0.13 μl on the propagation path. The interaction with the liquid medium provides information about the liquid, based on the phase attenuation change. The larger the droplet volume is, the longer the duration of the phase shift to reach stability is. The resolution that the output change of the sensor can measure is 0.03 μl.@en

This paper presents the effect of an arbitrary interruption of the propagation path in Surface Acoustic Wave (SAW) microdevices on the intensity of the scattered surface waves. Using finite element modeling, simulations have been carried out to validate a new equivalent circuit based on the conventional Mason and Smith model. In addition, experimental results obtained with 30, 50 and 100 μm diameter microholes are reported. The comparison of theory, simulation and experiment proves that it is possible to fabricate an interruption like deep microcavities or microholes in the propagation path which results in an acceptable signal magnitude attenuation, but without shift in the operating frequency.@en

This paper presents a novel mixing surface acoustic wave (M-SAW) device using multiple Aluminum (Al) interdigital transducer (IDT) layers for liquid sensing applications. The stronger mechanical wave beam generated by the multiple input-IDT layers on the thin film (1 μm) Aluminum Nitride (AlN), consists of different-phase acoustic waves that can travel through a liquid medium. In contrast to the whole signal leaking into liquid medium as in typical SAW devices, the signal obtained at the receiver of the M-SAW device only has a decline in the Rayleigh component. The experimental results show that it is possible to interact with the liquid medium placed on the surface and extract information about the liquid volume based on the phase change. Phase shifts of -16.35°, -11.23°, -4.84°, and -0.86° are obtained for different volumes of demi-water after 10 seconds, 2, 4 and 5 minutes of vaporization, respectively.@en