In-liquid bulk acoustic wave resonators for biosensing applications

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 $\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 ^\circC, an optimum surface roughness of 9.2 nm yields homogeneously inclined 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, k²_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$\cdot$cm²/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 ^\circC 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 $\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.

Name dc:type.qualificationname

PhD

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2017

Author dc:creator

• Rughoobur, Girish

Advisor dc:contributor.advisor

• Flewitt, Andrew

• Bulk acoustic wave resonators
• Biosensors
• Gravimetric sensors

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d7ce1fc2-532a-4146-a35a-8a8dbdf6f555/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

en

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.69855

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/322398