Self-excited piezoelectric cantilever oscillators

We describe a micromachined self-excited piezoelectric cantilever device whose oscillating frequency is in the audio range. The 2000 mu m X 2000 mu m X 2.8 mu m cantilever has a zinc oxide (ZnO) piezoelectric thin film between two aluminum (Al) layers on a supporting layer of low-pressure chemical-vapor-deposited (LPCVD) low-stress silicon nitride. The transducer and an amplifier that drives the cantilever via a driving electrode constitute a self-excited unit. If the gain of the amplifier is sufficient to overcome the loss of the transducer and the resonance frequency is in the amplifier passband, the device oscillates and produces significant acoustic output. The self-excited acoustic output transducer could serve as a sensor for environmental quantities such as relative humidity or chemical vapor concentration. Simulation results show that the gravimetric sensitivity exceeds 5000 cm(2) g(-1) when the whole cantilever is covered with the sorptive film.