Platforms and Materials for QCM and SAW-Based Humidity Sensors

Acoustic wave (AW)-based humidity sensors, such as surface acoustic wave (SAW), quartz-crystal microbalance (QCM), or bulk acoustic wave (BAW), and cantilever-based devices create a specific class of humidity sensors. In all these devices, piezoelectric materials play one of the main roles. The main piezoelectric materials used in humidity sensors are α-quartz (SiO₂), nonstoichiometric lithium niobate (LiNbO₃), ZnO, and lead-zirconate titanate. Parameters of these materials are presented in this chapter. Piezoelectric materials such as AlN, GaPO₄, and La₃Ga₅SiO₁₄ (LGS), promising for high-temperature AW devices, are considered as well. Discussion of the approaches designed for fabrication of micromachining QCM-based sensors such as thin-film resonators (TFRs), the thin-film bulk acoustic resonator (FBAR), and solidly mounted resonator (SMR) structures is also included in the chapter.