Bimorph material/structure designs for high sensitivity flexible surface acoustic wave temperature sensors

R Tao; S A Hasan; H Z Wang; J Zhou; J T Luo; G McHale; D Gibson; P Canyelles-Pericas; M D Cooke; D Wood; Y Liu; Q Wu; W P Ng; T Franke; Y Q Fu

doi:10.1038/s41598-018-27324-1

Bimorph material/structure designs for high sensitivity flexible surface acoustic wave temperature sensors

Sci Rep. 2018 Jun 13;8(1):9052. doi: 10.1038/s41598-018-27324-1.

Authors

R Tao¹, S A Hasan¹, H Z Wang^{1

2}, J Zhou³, J T Luo^{4

5}, G McHale¹, D Gibson⁶, P Canyelles-Pericas¹, M D Cooke⁷, D Wood¹, Y Liu², Q Wu¹, W P Ng¹, T Franke⁸, Y Q Fu⁹

Affiliations

¹ Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK.
² State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, P.R. China.
³ College of Intelligent Science and Engineering, National University of Defense Technology, Changsha, HuNan, 410073, P.R. China.
⁴ Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. luojt@szu.edu.cn.
⁵ Shenzhen Key Laboratory of Advanced Thin Films and Applications, College of Physics and Energy, Shenzhen University, Shenzhen, 518060, P.R. China. luojt@szu.edu.cn.
⁶ Institute of Thin Films, Sensors & Imaging, University of the West of Scotland, Scottish Universities Physics Alliance, Paisley, PA1 2BE, UK.
⁷ Department of Engineering, Durham University, South Road, Durham, DH1 3LE, UK.
⁸ Biomedical Engineering, University of Glasgow, Rankine Building, G12 8LT, Glasgow, UK.
⁹ Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. richard.fu@northumbria.ac.uk.

Abstract

A fundamental challenge for surface acoustic wave (SAW) temperature sensors is the detection of small temperature changes on non-planar, often curved, surfaces. In this work, we present a new design methodology for SAW devices based on flexible substrate and bimorph material/structures, which can maximize the temperature coefficient of frequency (TCF). We performed finite element analysis simulations and obtained theoretical TCF values for SAW sensors made of ZnO thin films (~5 μm thick) coated aluminum (Al) foil and Al plate substrates with thicknesses varied from 1 to 1600 μm. Based on the simulation results, SAW devices with selected Al foil or plate thicknesses were fabricated. The experimentally measured TCF values were in excellent agreements with the simulation results. A normalized wavelength parameter (e.g., the ratio between wavelength and sample thickness, λ/h) was applied to successfully describe changes in the TCF values, and the TCF readings of the ZnO/Al SAW devices showed dramatic increases when the normalized wavelength λ/h was larger than 1. Using this design approach, we obtained the highest reported TCF value of -760 ppm/K for a SAW device made of ZnO thin film coated on Al foils (50 μm thick), thereby enabling low cost temperature sensor applications to be realized on flexible substrates.