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 $\textit{c}$-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

A novel split mode TFBAR device for quantitative measurements of prostate specific antigen in a small sample of whole blood.

Easy monitoring of prostate specific antigen (PSA) directly from blood samples would present a significant improvement as compared to conventional diagnostic methods. In this work, a split mode thin film bulk acoustic resonator (TFBAR) device was employed for the first time for label-free measurements of PSA concentrations in the whole blood and without sample pre-treatment. The surface of the sensor was covalently modified with anti-PSA antibodies and demonstrated a very high sensitivity of 101 kHz mL ng-1 and low limit of detection (LOD) of 0.34 ng mL-1 in model spiked solutions. It has previously been widely believed that significant pre-processing of blood samples would be required for TFBAR biosensors. Importantly, this work demonstrates that this is not the case, and TFBAR technology provides a cost-effective means for point-of-care (POC) diagnostics and monitoring of PSA in hospitals and in doctors' offices. Additionally, the accuracy of the developed biosensor, with respect to a commercial auto analyser (Beckman Coulter Access), was evaluated to analyse clinical samples, giving well-matched results between the two methods, thus showing a practical application in quantitative monitoring of PSA levels in the whole blood with very good signal recovery

Carbon nanotube isolation layer enhancing in-liquid quality-factors of thin film bulk acoustic wave resonators for gravimetric sensing

A thickness longitudinal mode (TLM) thin film bulk acoustic resonator biosensor is demonstrated to operate in water with a high quality-factor, Q. This is achieved using a layer of carbon nanotubes (CNTs) on top of the resonator which has a significantly different acoustic impedance to either the resonator or liquid whilst being susceptible to the binding of biological molecules. This allows the resonance to be decoupled from direct energy loss into the liquid, although still retaining mass sensitivity. AlN solidly mounted resonators (SMRs) having a thickness shear mode (TSM) at 1.1 GHz and TLM at 1.9 GHz are fabricated. CNTs with different forest densities are grown by chemical vapor deposition on the active area with Fe as catalyst and resulting devices compared. High forest density CNTs are shown to acoustically decouple the SMRs from the water and in-liquid TLM Q values higher than 150 are recorded even exceeding TSM SMRs without CNTs. The TLM Q in water is remarkably improved from 3 to 160 for the first time by dense CNT forests, rendering the large-scale fabrication of TLM SMRs for liquid-phase sensing applications possible. Despite this partial isolation, SMRs with CNT forests ~15 μm tall can still detect binding of bovine serum albumin.This work was supported by the European Community's Horizon 2020 Programme [grant number SPIRE-01-2014-636820 (RECOBA)]; and the Ministerio de Economía y Competitividad del Gobierno de España [grant number MAT2013-45957-R]. G.R. and S.Z. also wish to acknowledge funding from the Cambridge Commonwealth, European and International Trust

Nanofabrication of Conductive Metallic Structures on Elastomeric Materials.

Existing techniques for patterning metallic structures on elastomers are limited in terms of resolution, yield and scalability. The primary constraint is the incompatibility of their physical properties with conventional cleanroom techniques. We demonstrate a reliable fabrication strategy to transfer high resolution metallic structures of <500 nm in dimension on elastomers. The proposed method consists of producing a metallic pattern using conventional lithographic techniques on silicon coated with a thin sacrificial aluminium layer. Subsequent wet etching of the sacrificial layer releases the elastomer with the embedded metallic pattern. Using this method, a nano-resistor with minimum feature size of 400 nm is fabricated on polydimethylsiloxane (PDMS) and applied in gas sensing. Adsorption of solvents in the PDMS causes swelling and increases the device resistance, which therefore enables the detection of volatile organic compounds (VOCs). Sensitivity to chloroform and toluene vapor with a rapid response (~30 s) and recovery (~200 s) is demonstrated using this PDMS nano-resistor at room temperature

Gravimetric and biological sensors based on SAW and FBAR technologies

This presentation will describe the development of Gravimetric and Biological Sensors based on SAW and FBAR Technologies. The SAW devices were fabricated on polycrystalline ZnO thin films deposited using both standard R.F. sputtering techniques and a novel High Target Utilisation Sputtering System (HiTUS). This system ensures that we can produce the low stress films at the high deposition rates necessary for such structures to operate efficiently. However in order to further improve the sensitivity of our sensors we have also investigated the use of Thin Film Bulk Acoustic Resonators (FBARs) . We will describe standard gravimetric sensors based on such material and also gravimetric sensors for use in liquid environments through the use of inclined c-axis ZnO material. The talk will conclude with a discussion of dual mode thin film FBARs for parallel sensing of both mass loading and temperature

Split resonances for simultaneous detection and control measurements in a single bulk acoustic wave (BAW) sensor.

A self-referenced resonator consisting of two distinct areas of the top electrode made from Mo and a thin (5-30 nm) functional Au layer is shown. The fundamental frequencies for both the shear (∼1 GHz) and longitudinal (∼2 GHz) modes are split in two, such that mass attachment on the functional layer region causes frequency shifts in only one of the resonances, allowing a new approach of using the difference between the two frequencies to be used to measure mass attachment; this reduces the importance of device-to-device variability in absolute resonant frequency as a result of device fabrication.Cancer Research UK Cambridge Institute Early Detection Programme Pump-Priming 2016 award. Cambridge Commonwealth Trusts

Electron transmission through suspended graphene membranes measured with a low-voltage gated Si field emitter array

We experimentally demonstrate the transmission of electrons through different number (1, 2, and 5) of suspended graphene layers at electron energies between 20 and 250 eV. Electrons with initial energies lower than 40 eV are generated using silicon field emitter arrays with 1 μm pitch, and accelerated towards the graphene layers supported by a silicon nitride grid biased at voltages from −20 to 200 V. We measured significant increase in current collected at the anode with the presence of graphene, which is attributed to the possible generation of secondary electrons by primary electrons impinging on the graphene membrane. Highest output current was recorded with monolayer graphene at approximately 90 eV, with up to 1.7 times the incident current. The transparency of graphene to low-energy electrons and its impermeability to gas molecules could enable low-voltage field emission electron sources, which often require ultra-high vacuum, to operate in a relatively poor vacuum environment.AFOSR/MURI (Contract FA9550-18-1-0436)DARPA (Contract N66001-16-1-4038

Nanofabricated Low-Voltage Gated Si Field-Ionization Arrays

We demonstrate high-density (1-μm pitch) silicon field-ionization arrays (FIAs) with self-aligned gate apertures (350 nm in diameter) and integrated nanowire current regulators. Our FIAs achieved high field factors (>0.1 nm⁻¹) and significantly lower ionization voltages (<100 V) than the devices with lower tip densities previously reported. Ion currents were measured in argon, deuterium, and helium at pressures from 1 to 16 mTorr. The FIAs turned on between 70 and 85 V, and the ion currents of around 0.4 nA were measured at 100 V. Higher currents of 7 nA were obtained at 147 V and 16 mTorr, but with the risk of gate damage by the ions energized in the intense gate-ionizer field. Si FIAs coated with Pt resulted in higher field factors due to sharper tips, but lower ion currents. Surface states, coupled with molecular adsorption and transport to the ionizer, are the possible mechanisms for lower voltage ionization in the uncoated Si FIAs