Flexible and Transparent Aluminum-Nitride-Based Surface-Acoustic-Wave Device on Polymeric Polyethylene Naphthalate

The development of wearable technology increasingly requires bendable sensing devices operating across multiple domains for opto-electro-mechanical and biochemical transduction. Piezoelectric materials integrated into flexible and transparent device architectures can enable multiple-sensing platforms. It is shown that flexible and compliant surface-acoustic-wave (SAW) piezoelectric devices include all these features and can be applied to the human body. A flexible and transparent aluminum-nitride-(AlN)-based SAW device on a thermoplastic polyethylene naphthalate (PEN) substrate, fabricated by low-temperature sputtering deposition of a multilayered AlN-based stack, is reported for the first time. Two resonant modes, corresponding to Rayleigh and Lamb wave propagation, are shown and compared with a control SAW device on a silicon substrate. A large transmission-signal amplitude, up to 20 dB, is achieved for the Lamb resonance mode around 500 MHz at an acoustic velocity of 10 500 m s−1. The technology is applied to the fabrication of a wearable temperature sensor. Compared to the same piezoelectric stack and SAW technology onto silicon substrates, the AlN/PEN SAW shows better performance and a temperature coefficient frequency as high as ≈810 ppm °C−1. The potential of this flexible SAW device as a wearable temperature sensor based on Rayleigh modes is demonstrated.