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In-liquid bulk acoustic wave resonators for biosensing applications 10.17863/CAM.69855 Rughoobur, Girish Flewitt, Andrew Gravimetric sensors based on thin-film bulk acoustic wave (BAW) resonators operating between 1-5 GHz have tremendous potential as biosensors because they are inexpensive, label-free, fast and highly sensitive. The two main challenges in this objective are: the conventional longitudinal mode resonance in $\textit{c}$-axis oriented piezoelectric films suffers from more than 90% damping in liquid; the alternative is the shear mode resonance, with lower damping in liquid but which requires an inclined $\textit{c}$-axis piezoelectric film, a process that is still not fully scalable. In this thesis, seed layers such as AlN with mainly (103) orientations are used to promote the growth of homogeneously inclined $\textit{c}$-axis ZnO (inclination of up to $\sim$45$^{\circ}$) films without significant equipment modifications. Sputtered Al electrodes with controlled roughness are then substituted for the parasitic AlN seed layers to improve the electromechanical performance. At a substrate temperature, T$_{s}$ = 100 $^{\circ}$C, an optimum surface roughness of 9.2 nm yields homogeneously inclined $\textit{c}$-axis ZnO films with angles $\sim$25$^{\circ}$. Solidly mounted resonators (SMRs) operating in a shear mode at $\sim$1.1 GHz with the Al electrodes have resonant quality factors (Q$_{r}$) higher than 150 and effective electromechanical coupling coefficients, k$^{2}$$_{eff}$, of 2.9-3.4%, which are improved from only 2.2% with the AlN seed layers. This shear mode of the ZnO SMRs has mass sensitivities, S$_{m}$ of (4.9 $\pm$ 0:1) kHz$\cdot$cm$^{2}$/ng and temperature coefficients of frequency (TCF) of -(66$\pm$2) ppm/K. Viscosity sensing is carried out with different ethanol-water compositions; the SMRs are functionalised and successfully used in the detection of Rabbit Immunoglobin G. To mitigate the longitudinal mode damping in water, multi-wall carbon nanotube (CNT) forests are grown by chemical vapour deposition (CVD) at 600 $^{\circ}$C using Fe/Al layers on the active area of inclined $\textit{c}$-axis AlN SMRs designed for improved thermal and chemical stability. The dense CNT forest (with 0.5/8 nm Fe/Al) of $\sim$15 μm height provides an acoustic isolation to DI water with only 50-70% drop in the longitudinal mode Q$_{r}$ compared to 99% in SMRs without the CNTs. Mass loading is still detected and demonstrated by detecting bovine serum albumin (BSA) in water whereas with forest heights of $\sim$30 μm and no significant frequency shifts due to mass attachment are observed. With the CNTs the longitudinal mode is shown for the first time to be more sensitive to mass ($\sim$7x) than the shear mode in liquid, highlighting the potential of CNTs for the large scale use of the longitudinal mode for in-liquid sensing. University of Cambridge 2017-01-16 Thesis doctoral PhD en https://www.repository.cam.ac.uk/handle/1810/322398 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/14477a15-2c83-4944-af2d-10b9f0a5b3fe/download 54cdef7a528d94e330d2bb156825b164 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d7ce1fc2-532a-4146-a35a-8a8dbdf6f555/download 87eda9de84448d1f82354d60eee3eb5f https://www.rioxx.net/licenses/all-rights-reserved/ Bulk acoustic wave resonators Biosensors Gravimetric sensors