Flexible Surface Acoustic Wave Based Temperature and Humidity Sensors
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Flexible Surface Acoustic Wave Based Temperature and Humidity Sensors X.L. He,1 J. Zhou,1 W.B. Wang,1 W.P. Xuan,1 D.J. Li,2 S.R.Dong,1 H. Jin1, Y. Xu1 and J.K.Luo1,2* 1. Department of Information Science & Electronic Engineering, Zhejiang University and Cyrus Tang Center for Sensor Materials and Applications, 38 Zheda Road, Hangzhou 310027, China. 2. Institute of Renewable Energy and Environment Technology, Bolton University, Deane Road, Bolton BL3 5AB, United Kingdom * Corresponding authors E-mail: [email protected]
ABSTRACT: Flexible surface acoustic wave (SAW) based temperature and humidity sensors were fabricated and characterized. ZnO piezoelectric films were deposited on polyimide substrates by DC magnetron sputtering. ZnO films possess (0002) crystal orientation with large grain sizes of 50~70 nm. SAW devices showed two wave modes, namely the Rayleigh and Lamb modes, with the frequencies at fR ~132MHz and fL~427MHz respectively for a wavelength of 12 μm device. The two resonant frequencies have a temperature coefficient of frequency (TCF) of −423ppm/K and −258ppm/K for the Rayleigh and Lamb waves, respectively. The SAW sensors exhibited a good repeatability in responding to cyclic change of humidity. The responses of the sensors increase with the increase in humidity, and the sensitivity increases with the decrease in wavelength. A high sensitivity of 34.7 kHz/10%RH has been obtained from a SAW device without any surface treatment, demonstrated that the flexible SAW humidity sensors are very promising for application in flexible sensors and microsystems.
Keywords: Flexible SAW devices, ZnO, Temperature sensor, Humidity sensor. INTRODUCTION Flexible electronics and microsystems have been intensively studied recently as it is a very promising technology for widespread applications owing to its excellent flexibility, light-weight, low cost [1], and unique functions. They are able to fit to surfaces with any shape and structures, so that greatly increase the usability of electronics and systems. Great efforts have been made to develop various flexible electronic devices and microsystems such as field effect transistors [2], photodetector [3], piezoelectric actuators [4], infrared bolometers [5], thin film bulk acoustic wave resonators [6] and microgenerators [7] etc. Surface acoustic wave (SAW) resonators are one of the building blocks and essential components for electronic devices, microsensors, and microsystems. Owing to their high operating frequency, high sensitivity, simple fabrication process and device structure, and low cost, SAW devices have been intensively explored for applications as microsensors [8] to measure various physical parameters [9,10,11] and to detect chemical and biological substances, and as microactuators for microfluidics [12,13] and lab-on-a-chip [14]. SAW devices are also used to sensing temperature with very high sensitivity [15] and particularly useful for detect low humidity levels with precision [16,17]. SAW humidity sensors can also be integrated with other sensors on the sam
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