Piezoelectric Cantilever Microphone, Microspeaker, and Voltage-to-Frequency Converter

A micromachined piezoelectric cantilever device that can be used as a microphone, a microspeaker, and a voltage-to-frequency converter, has been fabricated and tested. The 2000x2000x4.7 μm3 cantilever has a 1.3-μm-thick zinc oxide (ZnO) piezoelectric thin film on a supporting layer of low-pressure chemical-vapor-deposited (LPCVD) low-stress silicon nitride. The cantilever has no low-temperature oxide (LTO) layer over the ZnO layer, unlike our previous devices. This structural modification frees the fabrication processes from the problems brought about by the use of ZnO in integrated circuit fabrication facilities. The measured cantilever microphone sensitivity is around 3 mV/μbar in the low-frequency range which extends up to 2 kHz. The cantilever microspeaker also can be used as a voltage-to-frequency converter by the addition of a silicon top plate and an aluminum (Al) sputtered layer on the backside of the cantilever. The resonant frequency of the microspeaker is changed by the potential applied between the top plate and the backside Al layer. As the potential is changed from 0 to 40 V, the resonant frequency shifts down from 14.5 kHz to 11.5 kHz. The frequency shift of the microspeaker can be detected remotely with a microphone.