Mechanical and Ferroelectric Behavior of PZT-Based Thin Films

The mechanical and ferroelectric behavior of freestanding lead zirconate titanate (PZT) -based thin film unimorph structures for microelectromechanical systems was investigated with uniaxial tension specimens of SiO₂-TiPt-PZT-Pt, SiO₂-TiPt-PZT, SiO₂-TiPt, and individual SiO₂ and Pt films. Full-field strains obtained with digital image correlation were used to compute the stress versus strain curves for each film combination and the properties of the PZT layer. The mechanical response of the 1-μm thick PZT films was nonlinear but nonhysteretic, and it deviated from linearity at 0.3-0.35% axial strain. The elastic modulus of the PZT was 84 ± 3 GPa and 60.5 ± 5 GPa in open and short circuit conditions, respectively. Failure of the PZT stacks initiated in the PZT layer at 510 ± 35 MPa and 356 ± 55 MPa, in open and short circuit conditions, respectively, while the presence of the top Pt electrode delayed failure initiation in the SiO₂-TiPt-PZT-Pt stack compared to the SiO₂-TiPt-PZT stack. The high field effective electroactive coefficient e_31,eff calculated from biased PZT stacks subjected to various prestress amplitudes was in the range of -50 to -30 NV^-1 m^-1. The e_31,eff was strongly dependent on the electric field for most applied stress amplitudes, but it remained constant with the applied field when the applied stress reached 400 M Pa.