Fabrication of piezoelectric diaphragm using lead zirconate titanate (PZT) films

Piezoelectric diaphragms were fabricated using bulk micromachining. The diaphragms had a unimorph structure, where Pb(Zr_0.52Ti_0.48)O₃ (PZT) and thermally grown silicon oxide (SiO₂) films were used as the active and passive layers, respectively. To actuate the diaphragms, two modes were designed: d₃₁ and d₃₃-mode. For d₃₁-mode diaphragms, a Si wafer with Pt/Ti/SiO₂ (0.5 μm) was coated with ∼1.2 μm PZT. A Cr/Au top electrode was then evaporated. Each layer including the bottom electrode was patterned into a circular shape. To fabricate d₃₃-mode diaphragms, a Si wafer with thermal SiO₂ (0.5 μm) was coated with ∼0.3 μm ZrO₂ and ∼1.6 μm of PZT. On top of these layers, a Cr/Au top electrode was deposited and patterned into a ring-shaped interdigitated transducer. Finally, both d₃₁ and d₃₃-mode diaphragms were released using deep reactive ion etching. Diameters of the fabricated diaphragms were in the range of 600 μm and 1000 μm. For d₃₁-mode diaphragms, the dielectric constant and loss of the released piezoelectric layer at 1 kHz were > 800 and < 2%, respectively. The remanent polarization was ∼20 μC/cm² and the coercive field was ∼61 kV/cm. Ferroelectric measurements showed well-developed hysteresis loops for the d₃₃-mode diaphragms. Both d₃₁ and d₃₃-mode diaphragms behave as membranes rather than plates. Their measured resonance frequencies were consistent with calculations from an analytic model for circular membranes and ANSYS finite element analysis.