In this thesis, we present a micromachined piezoelectric acoustic transducer fabricated on a cantilever. 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 (microspeaker), the relatively large deflections produce significant acoustic output. A voltage-to-frequency converter and a self-excited oscillator have also been demonstrated with piezoelectric cantilever transducers.
The 2 x 2 x 0.0045 mm^3 micromachined transducers have a zinc oxide (ZnO) piezoelectric thin film on a 1.5 um-thick bulk-micromachined cantilever diaphragm, made of low-pressure chemical-vapor-deposited (LPCVD) low-stress silicon nitride.
The measured microphone sensitivity is fairly constant at 2 mV/ubar 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 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 plate and the lower A1 layer. As the potential is changed from 0 to 40 Vpp, 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.
The multi-electrode cantilever transducer constitutes a self-excited oscillator with an amplifier that amplifies the feedback signal to drive the cantilever via drive electrode. If the gain of the amplifier is sufficient to overcome the loss of the transducer and the resonant frequency is in the amplifier passband, oscillation occurs and significant acoustic output is produced. The output pressure is higher than 90 dB SPL at 14.54 kHz at 0.5 cm from the transducer.
December 31, 1995
Lee, S. S. (1995). Micromachined Piezoelectric Cantilever Transducers. United States: University of California, Berkeley.