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In-liquid bulk acoustic wave resonators for biosensing applications



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Rughoobur, Girish 


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 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 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 c-axis ZnO (inclination of up to $\sim45^{\circ})filmswithoutsignificantequipmentmodifications.SputteredAlelectrodeswithcontrolledroughnessarethensubstitutedfortheparasiticAlNseedlayerstoimprovetheelectromechanicalperformance.Atasubstratetemperature,T{s}$ = 100 C, an optimum surface roughness of 9.2 nm yields homogeneously inclined c-axis ZnO films with angles $\sim25^{\circ}$. Solidly mounted resonators (SMRs) operating in a shear mode at $\sim1.1GHzwiththeAlelectrodeshaveresonantqualityfactors(Q{r})higherthan150andeffectiveelectromechanicalcouplingcoefficients,k^{2}$${eff},of2.9−3.4{m}$ of (4.9 ± 0:1) kHzcm2/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 C using Fe/Al layers on the active area of inclined c-axis AlN SMRs designed for improved thermal and chemical stability. The dense CNT forest (with 0.5/8 nm Fe/Al) of $\sim15μmheightprovidesanacousticisolationtoDIwaterwithonly50−70_{r}$ 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 $\sim30μmandnosignificantfrequencyshiftsduetomassattachmentareobserved.WiththeCNTsthelongitudinalmodeisshownforthefirsttimetobemoresensitivetomass(\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.




Flewitt, Andrew


Bulk acoustic wave resonators, Biosensors, Gravimetric sensors



Awarding Institution

University of Cambridge