Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 1032 – 1035 30th Eurosensors Conference, EUROSENSORS 2016 Finite element modeling and synthesis of c-axis tilted AlN TFBAR for liquid sensing applications C, Caliendoa, M Hamidullaha, F Mattiolia a Institute of Photonics and Nanotechnology, IFN-CNR, Via Cineto Romano 42, 00156 Rome, Italy Abstract Thin film bulk acoustic resonator (TFBAR) with c-axis tilted AlN finite element modeling was performed using COMSOL Multiphysics simulation software Depending on the AlN c-axis tilt angle, dual mode TFBAR can be obtained that operate in longitudinal and shear mode: the former mode is a thickness-extensional mode, while the latter is a thickness-in plane-shear mode that is suitable for liquid sensing applications The acoustic wave displacement, the resonator frequency and equivalent admittance of the TFBARs with different tilt angles and electrode thicknesses were simulated to obtain the optimum design (high electroacoustic coupling efficiency and large quality factor Q) for shear mode operation Furthermore, AlN films having the caxis tilted up to about 30° from the substrate normal were successfully grown on silicon (100) substrates by reactive rf magnetron sputtering technique at 200°C © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license © 2016 The Authors Published by Elsevier Ltd (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: TFBAR, c-axis tilted, shear mode, liquid sensing, FEM Introduction Electroacoustic devices have demonstrated to be a powerful tool in a variety of physical and chemical sensors operating in liquid environments, for both laboratory research analysis and industry applications Devices based on the thin piezoelectric film technology, such as AlN or ZnO, have a small physical size, are low- cost, simple in operation (they use a low volume of liquid sample), and are applicable for in situ viscosity monitoring [1] Acoustic wave sensors based on the propagation of shear horizontal (SH) waves are suitable for operation in liquid environments, since they not suffer significant attenuation from liquid damping Shear mode TFBAR based on AlN thin film can be achieved when the c-axis of AlN thin films is tilted to certain angle μ with respect to the surface normal [2] 1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi:10.1016/j.proeng.2016.11.333 C Caliendo et al / Procedia Engineering 168 (2016) 1032 – 1035 The velocity v and electromechanical coupling coefficient K2 of the bulk acoustic waves (BAWs) propagating along AlN substrate were theoretically calculated for different c-axis tilt angle and the results are shown in figs 1a and b Fig (a) coupling coefficient K (b) acoustic wave velocity v angular dispersion curves The piezoelectric film thickness d, as well as the BAWs v and K2 are important parameters for the TFBAR design: the formers affect the operating frequency f=v/2d, while the latter affects the quality Q factor and thus the sensor sensitivity It was found that the TFBAR behaves as a single-mode resonator for certain μ values, such as 0° and 65° for the longitudinal resonance modes For all the others μ values the TFBAR behaves like a dual mode resonator, but the μ = 30° tilt angle seems to be the optimized angle for improved operation of shear mode resonator In this paper, TFBAR with AlN, molybdenum electrode and silicon nitride suspended layer was simulated using COMSOL Multiphysics The effect of tilt angles and molybdenum thickness onto the resonant frequency, impedance and Q were compared and discussed FEM Simulation Fig 2(a) shows the simulated structure of TFBAR consisting of a c-axis tilted AlN thin film, a Mo top and bottom electrode, and bottom SiN layer; the reference coordinated system (x1, x2, x3) is also shown in fig.2(a) To account for the tilted c-axis and the wave propagation direction along x3, the x2 axis was set out of plane with rotation angle about x2 axis Perfectly matched layer (PML) was applied in left and right side of structure to prevent wave reflection The particle displacement of the longitudinal and shear BAW (LBAW and SHBAW) is along the x3 and x1 directions COMSOL frequency domain simulation was performed with 20° tilt angle and the dual mode frequency behavior of the particle displacement components was achieved as shown in fig 2(b) Fig 2a TFBAR structure and 2b longitudinal and shear mode displacement at 20 degree c-axis tilt angle 1033 1034 C Caliendo et al / Procedia Engineering 168 (2016) 1032 – 1035 First frequency response has only x1 particle displacement which indicated a shear mode where the second frequency response has only x3 particle displacement for longitudinal mode The longitudinal frequency is expected to be higher than that of the shear mode because LBAW has higher velocity than that of the SHBAW The TFBAR admittance was simulated for different μ values to observe the effect of the tilt angle on the TFBAR resonant frequency f, modulus of impedance and Q As shown in the fig 3(a), for μ = 0° there is only a longitudinal mode that propagates at f = 1543 MHz; the shear mode appears for μ = 15° at f = 873 MHz The highest impedance modulus for the shear mode is at 30° tilt angle with a resonant frequency equal to 874 MHz At 45° tilt angle, the resonant frequency reduces back to 873 MHz and its impedance modulus becomes lower than 30°degree tilt angle The simulation results agree with the theoretical calculations shown in fig.1 where, at 30° tilt angle, the shear mode has the highest coupling coefficient and velocity Fig Longitudinal and shear horizontal mode at (a) various tilt angles and (b) various Mo thicknesses Further simulation was performed with different Mo thickness from 50 nm to 200 nm for μ = 30° As the frequency is inversely proportional to the total thickness of TFBAR including AlN, Mo electrodes and SiN, an increased resonant frequency is expected for thinner metal electrodes As shown in fig.3(b), the Mo electrode thickness affects the performances of the SH mode resonator With reducing the Mo electrode thickness, the shear mode resonant frequency increases from 874 to 1120 MHz, and the impedance modulus at resonance also increases; these two effects results in an enhanced sensitivity of the sensor However, the thickness of metal electrode cannot be arbitrarily reduced as it is limited by physical and electrical properties of the metal electrode A promising alternative to conventional metal electrode materials can be represented by a graphene layer: reference [3] describes a resonator based on a graphene electrode working at the predicted operating frequency with improved electrical performances [3] Synthesis of c-axis tilted AlN thin layer c-axis titled AlN thin layer were growth by rf reactive magnetron sputtering technique at 200°C by placing the substrate off-center from the target Additional tilt angle in the grown film can be achieved by tilting the substrate AlN films having the c-axis tilted up to about 30° from the substrate normal were successfully grown on Si (100) substrates: the SEM photos of fig show the cross section of the silicon wafer covered by a c-axis tilted AlN film The AlN layer shows a columnar structure where the cones are tilted around 20°(figure 4a) and 30° (figure 4b) relative to the film normal 1035 C Caliendo et al / Procedia Engineering 168 (2016) 1032 – 1035 (1) Fig Cross section SEM images of the AlN film deposited on Si substrate for AlN c-axis tilted around 20°(figure 4a) and 30° (figure 4b) with respect to the film normal Acknowledgements This work was supported by the EU H2020 Project MSCA SAWtrain Grant agreement n 642688 References [1] D.S Ballantine, et al (1997): Acoustic Wave Sensors: Theory, Design & Physico-Chemical Applications; Academic Press Inc.: San Diego, CA, USA, 120–132 [2] Qin, L., Chen, Q., Cheng, H., Chen, Q., Li, J F., & Wang, Q M (2011) Viscosity sensor using ZnO and AlN thin film bulk acoustic resonators with tilted polar c-axis orientations Journal of Applied Physics, 110(9), 094511 [3] Qian, Z., Liu, F., Hui, Y., Kar, S., & Rinaldi, M (2015) Graphene as a Massless Electrode for Ultrahigh-Frequency Piezoelectric Nanoelectromechanical Systems Nano letters, 15(7), 4599-4604 [4] Chen, Y C., Chang, W T., Kao, K S., Yang, C H., & Cheng, C C (2013) The liquid sensor using thin film bulk acoustic resonator with caxis tilted AlN films Journal of Nanomaterials, 2013, ... predicted operating frequency with improved electrical performances [3] Synthesis of c- axis tilted AlN thin layer c- axis titled AlN thin layer were growth by rf reactive magnetron sputtering technique... compared and discussed FEM Simulation Fig 2(a) shows the simulated structure of TFBAR consisting of a c- axis tilted AlN thin film, a Mo top and bottom electrode, and bottom SiN layer; the reference coordinated... of TFBAR including AlN, Mo electrodes and SiN, an increased resonant frequency is expected for thinner metal electrodes As shown in fig.3(b), the Mo electrode thickness affects the performances