Temperature cross-sensitivity of AlN-based flexural plate wave sensors

In this paper, we investigate the temperature cross-sensitivity of aluminum nitride (AlN)-based flexural plate wave devices for sensing applications in contact with liquids. In our improved device topology, the interdigital transducers are designed as a buried electrode, and thus, are electrically shielded, enabling full immersion of the sensor into the liquid. The fabricated devices showed a mass sensitivity of 240 cm2/g for loading with deionized water which is in good agreement with the theoretical predictions. The temperature coefficient of frequency (TCF) was evaluated for devices with initially compressive and tensile stressed AlN layers. Devices with compressive film stress exhibited a TCF of −62 ppm/K to −28 ppm/K, while the devices with tensile film stress showed an increased TCF of −391 ppm/K to −72 ppm/K. It is shown that variations in in-plane tension are mainly accountable for the increased TCF, while the lower TCF is primarily caused by material softening.