High-Sensitivity Strain Sensing Over an Ultrawide Range Achieved on Temperature-Compensated Multimodal Flexible SAW Device

In this work, flexible surface acoustic wave (SAW) devices are prepared based on ZnO in thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu $ </tex-math></inline-formula> m deposited on a polyimide (PI) film. The ZnO piezoelectric film possesses superior piezoelectric properties (piezoelectric coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{{33}}$ </tex-math></inline-formula> of 22 pm/V at 1 MHz) and an ideal orientation (002). The devices with the wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20~\mu $ </tex-math></inline-formula> m exhibit two wave modes, including the Rayleigh and Lamb waves with resonant frequencies of 68.4 and 212.3 MHz and electromechanical coupling coefficient (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}^{{2}}$ </tex-math></inline-formula>) of 9.71% and 4.82%, respectively. These characteristics bring the potential for developing a high-performance strain sensor with our device, which significantly improves the strain sensitivity to 1330 Hz/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> in Lamb wave mode with an ultrawide testing range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3000~\mu \varepsilon $ </tex-math></inline-formula>. Moreover, based on the device’s dual-mode response, an on-chip temperature compensation scheme was therefore proposed. All these results indicate that this developed sensor is an excellent candidate for strain measurement and can be proposed for noninvasive conformal monitoring.