{"id":{"repo_id":"cambridge","oai_identifier":"oai:www.repository.cam.ac.uk:1810/322398"},"canonical_url":"https://search.dev.ndltd.org/etd/cambridge/oai:www.repository.cam.ac.uk:1810/322398","repository":{"repo_id":"cambridge","name":"Cambridge University","base_url":"https://api.repository.cam.ac.uk/server/oai/request"},"display":{"title":"In-liquid bulk acoustic wave resonators for biosensing applications","abstract":"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.","abstract_html":"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 <span class=\"etd-inline-math\"><em>c</em></span>-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 <span class=\"etd-inline-math\"><em>c</em></span>-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 <span class=\"etd-inline-math\"><em>c</em></span>-axis ZnO (inclination of up to $\\sim$45<span class=\"etd-inline-math\"><sup>\\circ</sup></span>) 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<span class=\"etd-inline-math\"><sub>s</sub></span> = 100 <span class=\"etd-inline-math\"><sup>\\circ</sup></span>C, an optimum surface roughness of 9.2 nm yields homogeneously inclined <span class=\"etd-inline-math\"><em>c</em></span>-axis ZnO films with angles $\\sim$25<span class=\"etd-inline-math\"><sup>\\circ</sup></span>. Solidly mounted resonators (SMRs) operating in a shear mode at $\\sim$1.1 GHz with the Al electrodes have resonant quality factors (Q<span class=\"etd-inline-math\"><sub>r</sub></span>) higher than 150 and effective electromechanical coupling coefficients, k<span class=\"etd-inline-math\"><sup>2</sup></span><span class=\"etd-inline-math\"><sub>eff</sub></span>, 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<span class=\"etd-inline-math\"><sub>m</sub></span> of (4.9 $\\pm$ 0:1) kHz$\\cdot$cm<span class=\"etd-inline-math\"><sup>2</sup></span>/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 <span class=\"etd-inline-math\"><sup>\\circ</sup></span>C using Fe/Al layers on the active area of inclined <span class=\"etd-inline-math\"><em>c</em></span>-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<span class=\"etd-inline-math\"><sub>r</sub></span> 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.","abstract_has_math":true,"creators":["Rughoobur, Girish"],"institution":"University of Cambridge","degree_name":"PhD","degree_level":"doctoral","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Flewitt, Andrew"],"committee_chairs":[],"committee_members":[],"year":2017,"date_issued":"2017-01-16","date_published":"2017-01-16","updated_at":"2026-07-22T22:24:17Z","subjects":["Bulk acoustic wave resonators","Biosensors","Gravimetric sensors"],"languages":["en"],"rights":[],"rights_urls":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d7ce1fc2-532a-4146-a35a-8a8dbdf6f555/download","https://www.rioxx.net/licenses/all-rights-reserved/"],"identifier_entries":[]},"links":{"outbound_url":"https://doi.org/10.17863/CAM.69855","outbound_label":"DOI","outbound_source":"dc:identifier.doi"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Flewitt, Andrew"]},{"key":"dc:creator","label":"Author","values":["Rughoobur, Girish"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.issued","label":"Date","values":["2017-01-16"]},{"key":"dc:publisher.institution","label":"Dc Publisher Institution","values":["University of Cambridge"]},{"key":"dc:relation.isreferencedby.uri","label":"Dc Relation Isreferencedby URI","values":["https://www.repository.cam.ac.uk/handle/1810/322398"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["doctoral"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["PhD"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Bulk acoustic wave resonators","Biosensors","Gravimetric sensors"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d7ce1fc2-532a-4146-a35a-8a8dbdf6f555/download","https://www.rioxx.net/licenses/all-rights-reserved/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.doi","label":"DOI","values":["10.17863/CAM.69855"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/14477a15-2c83-4944-af2d-10b9f0a5b3fe/download"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["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."]},{"key":"dc:format.checksum.md5","label":"Dc Format Checksum Md5","values":["54cdef7a528d94e330d2bb156825b164","87eda9de84448d1f82354d60eee3eb5f"]},{"key":"dc:title","label":"Title","values":["In-liquid bulk acoustic wave resonators for biosensing applications"]}]}],"canonical_facts":{"dc:contributor.advisor":["Flewitt, Andrew"],"dc:creator":["Rughoobur, Girish"],"dc:date.issued":["2017-01-16"],"dc:description.abstract":["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."],"dc:format.checksum.md5":["54cdef7a528d94e330d2bb156825b164","87eda9de84448d1f82354d60eee3eb5f"],"dc:identifier.doi":["10.17863/CAM.69855"],"dc:identifier.uri":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/14477a15-2c83-4944-af2d-10b9f0a5b3fe/download"],"dc:language":["en"],"dc:publisher.institution":["University of Cambridge"],"dc:relation.isreferencedby.uri":["https://www.repository.cam.ac.uk/handle/1810/322398"],"dc:rights":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/d7ce1fc2-532a-4146-a35a-8a8dbdf6f555/download","https://www.rioxx.net/licenses/all-rights-reserved/"],"dc:subject":["Bulk acoustic wave resonators","Biosensors","Gravimetric sensors"],"dc:title":["In-liquid bulk acoustic wave resonators for biosensing applications"],"dc:type":["Thesis"],"dc:type.qualificationlevel":["doctoral"],"dc:type.qualificationname":["PhD"]},"updated_at":"2026-07-22T22:24:17Z"}