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

Abstract 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.