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Novel, highly sensitive piezoelectric acoustic sensors based on partially unclamped Pb(Zrx, Ti1−x)O3 (PZT) coated cantilever and bridge have been fabricated by silicon micromachining. High sensitivity at low frequencies (5–100 Hz) has been achieved by patterning very narrow slits (3 to 5 µm) around the structures. A typical response of 100 mV Pa−1 and a noise equivalent pressure of 1.6 mPa Hz1/2 at 20 Hz have been measured using a 10 pF charge amplifier. Stress compensation, dry etching and integration of high performance piezoelectric thin films were the key issues. PZT/Pt/SiO2 stacks have been patterned by reactive ion etching and stress compensation has been achieved by compensating the PZT film's tensile stress by adjusting the thickness of a thermal SiO2 layer. The integration of sol–gel PZT films with a transverse piezoelectric coefficient e31,f of −12.8 C m−2 has been realized without any degradation of the properties. The microphones were successfully integrated into a miniature photoacoustic detector and tested for CO2 detection. Concentrations down to 330 ppm could be measured with significant signals.

Unexpectedly high-longitudinal piezoelectric coefficients have recently been reported in c-axis oriented Pb(Zr,Ti)O3 thin films with morphotropic phase boundary composition [Fu et al., Appl. Phys. Lett. 80, 3572 (2002)]. Our analysis and experimental results show that, in comparison with other methods commonly used to investigate piezoelectric response of thin films, the charge integration technique used by Fu et al. may lead to an order of magnitude higher apparent piezoelectric coefficients if the substrate on which the film was deposited is bent. When this is the case, the large response is due to the transverse piezoelectric effect caused by the substrate bending.

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