Abstract
Shear mode solidly mounted resonators (SMRs) are fabricated using an inclined c-axis ZnO grown on a rough Al electrode. The roughness of the Al surface is controlled by changing the substrate temperature during the deposition process to promote the growth of inclined ZnO microcrystals. The optimum substrate temperature to obtain homogeneously inclined c-axis grains in ZnO films is achieved by depositing Al at 100 °C with a surface roughness ~9.2 nm, which caused an inclination angle of ~25° of the ZnO c-axis with respect to the surface normal. Shear mode devices with quality-factors at resonance, Qr and effective electromechanical coupling factors, {{boldsymbol{k}}}_{{bf{eff}}}^{{bf{2}}}, as high as 180 and 3.4% are respectively measured. Mass sensitivities, Sm of (4.9 ± 0.1) kHz · cm2/ng and temperature coefficient of frequency (TCF) of ~−67 ppm/K are obtained using this shear mode. The performance of the devices as viscosity sensors and biosensors is demonstrated by determining the frequency shifts of water-ethanol mixtures and detection of Rabbit immunoglobin G (IgG) whole molecule (H&L) respectively.
Highlights
Electro-acoustic sensors resonating at high frequencies (~1–5 GHz) are promising for low-cost, label-free and high sensitivity detection of chemical and biological species in liquid media
Earlier works have shown that the morphology of sputtered metals such as Al can be controlled by the substrate temperature, TS during deposition[18, 19]
Al electrodes sputtered at different TS are compared and solidly mounted resonators (SMRs) operating in the quasi-shear mode at 1.1 GHz are fabricated to determine the suitability of this method
Summary
Electro-acoustic sensors resonating at high frequencies (~1–5 GHz) are promising for low-cost, label-free and high sensitivity detection of chemical and biological species in liquid media. The higher operating frequency of thin film bulk acoustic wave (BAW) resonators compared with the well-established quartz crystal microbalance (QCM) leads to higher mass sensitivities, Sm, in the order of several kHz · cm2/ng compared to QCM, which have Sm of several Hz · cm2/ng only[1, 2]. This is because Sm increases with the square of fr according to Sauerbrey’s equation[3]. An application of the SMRs as a sensitive gravimetric biosensor in liquid is investigated by functionalizing the surface with streptavidin to detect biotin conjugated Rabbit immunoglobin G (IgG)
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