Flexible Surface Acoustic Wave Based Temperature and Humidity Sensors

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 f R ~132MHz and f L~4 27MHz 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.

Flexible dual-mode surface acoustic wave strain sensor based on crystalline LiNbO3 thin film

“…The AlN/SOI based SAW pressure sensor developed by Mu et al features very close TCFs between the Rayleigh and Lamb waves [32]. The ZnO/PI based SAW humidity and temperature sensors developed by He et al have very different TCFs between the Rayleigh and Lamb waves [15]. However, in these literature, few explanations were given to why the multiple modes have similar or different temperature characteristics.…”

“…Flexible SAW devices have been mostly realized by using piezoelectric thin films deposited on flexible substrates, distincting from those conventional SAW devices fabricated on rigid substrates. Our group has developed flexible SAW devices on various flexible substrates, including polyethylene terephthalate (PET), polyimide (PI), and ultra-thin glass, and explored their applications as strain [11,12], humidity [13], UV-light [14], and temperature sensors etc [15]. These flexible SAW sensors achieved excellent performance, with some of them even outperformed those on bulk materials such as quartz, LiNbO 3 and LiTaO 3 .…”

Current Trends on Surface Acoustic Wave Biosensors

“…[214][215][216] So far, while much of this manuscript had revolved around the use of solid-state materials to produce SAW devices, it is important to note that another interesting trend in the development of SAW devices in recent years had been the utilization of flexible material such as polymers for the development of more versatile, reusable, and biocompatible SAW biosensors. Despite the fact that most work on flexible SAW sensing devices have so far focused on the detection of factors such as pressure, temperature, and humidity, [217][218][219][220][221][222][223][224] some studies have also been reported on SAW biosensors using piezoelectric thin film such as AlN and ZnO deposition on different polymer substr ates. [13,60,134,219,225,226] The use of piezoelectric thin film material permits for the deposition and the localization of the film solely on the SAW propagation region through micromachining [227] and also makes it possible to incorporate various functions such as wearability and tactility.…”