Highly sensitive strain sensors using surface acoustic wave on aluminum nitride thin film for wireless sensor networks

We newly developed surface acoustic wave (SAW)-based strain sensors using aluminum nitride (AlN) films to integrate with oscillator circuits. An interdigital transducer (IDT) was fabricated on a layered structure consisting of AlN, Pt/Ti, and SiO2 deposited on a Si substrate. A SAW was excited in the piezoelectric AlN film and resonated at 107.8 MHz. The Q-factor and effective electromechanical coupling coefficient k_eff² were 700 and 0.46% at the resonance, respectively. When we applied stress to the sensor, the resonant frequency was linearly changed with a sensitivity of 107.2 MHz/strain below the applied strain of the order of 10^-5. The sensor can be fully fabricated at low temperature (<;400 °C) and can be embedded in a one-chip oscillator by post-IC-process.