Gravimetric sensors operating at 1.1 GHz based on inclined c-axis ZnO grown on textured Al electrodes

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, Q r and effective electromechanical coupling factors, , as high as 180 and 3.4% are respectively measured. Mass sensitivities, S m 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.

Film bulk acoustic resonators (FBARs) as biosensors: A review

“…As shown in Figure 3C, the longitudinal mode waves of the FBAR almost disappears in a liquid environment because of severe damping of resonant waves in liquid (accompanied with severe decrease of Q), which reduces the mass resolution and sensitivity substantially, whereas the shear mode waves travel in plane with little damping in liquid, hence the FBAR maintains high Q and sensitivity. Therefore, the former mode can only work under dry condition, while the latter is able to work in both dry and liquid environments (Rughoobur et al 2017;Rughoobur et al 2016).…”

“…The quality factor (Q) of a resonator is a dimensionless parameter that describes how underdamped a resonator is, and characterizes a resonator's bandwidth relative to its centre frequency. The standard definition is shown by the following equation (Rughoobur et al 2017):…”

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

“…Crucially, the scalability of fabricating inclined c-axis films is also limited compared to highly oriented c-axis films due to significant and expensive modifications necessary for the depositions as reported in previous works [23,24]. Alternative methods using either seed layers such as AlN [19,12] with specific crystallographic orientations or Al electrodes with controlled roughness [25] improve the uniformity of inclined c-axis piezoelectric films over a short range but still cannot achieve homogeneity over larger substrates. It would therefore be preferable to find a means of using the TLM for in-liquid sensing.…”

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

“…temperature, pressure, humidity) of these devices and cannot be differentiated from "real" mass attachment. [15][16][17] Such effects pose a major challenge for the reliable detection of low quantities of biological compounds. Several methods have been proposed to compensate or measure the surrounding conditions of gravimetric BAW resonators.…”

“…The fabrication process of the shear and longitudinal mode ZnO (with inclined c-axis ∼30°) based SMRs is described in our previous works (see ESI †). 17,21 After the SMR fabrication, the pentagonal region for the functional layer is defined by standard ultra-violet photolithography using AZ2020 nLoF photoresist. By means of thermal evaporation (E306A, Edwards, West Sussex, UK), 5 nm of Cr is deposited as an adhesion layer for the subsequent Au layer, the thickness of which is varied from 5 to 30 nm in 5 nm steps.…”