Piezoelectric cantilever acoustic transducer

Abstract

We present a piezoelectric acoustic transducer fabricated on a bulk-micromachined cantilever diaphragm. Use of the cantilever as a supporting diaphragm produces a highly sensitive microphone. In addition, when the device is driven electrically as an output transducer, a microspeaker, the relatively large deflections produce significant acoustic output. A voltage-to-frequency converter has also been demonstrated with piezoelectric cantilever transducers. The 2 x 2 x 0.0047 mm(3) micromachined transducer has a zinc oxide (ZnO) piezoelectric thin film on a 1.5 mu m thick cantilever diaphragm, made of LPCVD low-stress silicon nitride. The measured cantilever microphone sensitivity is fairly constant around 3 mV mu bar(-1) in the low-frequency range below the first resonant frequency, which occurs at 1.8 kHz. The microspeaker output is approximately 100 dB SPL at 4.8 kHz and 12 VP-P (peak-peak) input drive. The voltage-to-frequency conversion is accomplished by the addition of a conducting plate and an aluminum (Al) sputtered layer on the underside of the cantilever. The resonant frequency of the microspeaker is changed by the potential applied between the top conducting plate and the lower Al layer. As the potential is changed from 0 to 40 VP-P, the resonant frequency shifts down from 14.5 kHz to 11.5 kHz while the amplitude of the output pressure is increased by 12.5 dB SPL. In the potential range of 15 to 25 VP-P, the frequency shift is fairly linear with the potential change and the sensitivity (frequency shift per unit applied potential change) is 200 Hz V-1 around 13 kHz.