Piezoelectric Pb(Zrx, Ti1 x)O3thin film cantilever and bridge acoustic sensors for miniaturized photoacoustic gas detectors

Novel, highly sensitive piezoelectric acoustic sensors based on partially unclamped Pb(Zr x , Ti 1− x )O 3 (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 Hz 1/2 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/SiO 2 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 SiO 2 layer. The integration of sol–gel PZT films with a transverse piezoelectric coefficient e 31, 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 CO 2 detection. Concentrations down to 330 ppm could be measured with significant signals.