Mechanical and piezoelectric behavior of thin film PZT composites for MEMS applications

The elastic and failure mechanical properties, the d₃₁ piezoelectric coefficient and the effect of applied stress on the hysteresis curves of freestanding PZT composite films, comprised of SiO₂, Pt, PZT and Pt, were measured from microscale tension specimens. The d₃₁ coefficient was measured from the out-of-plane deflection of biased PZT specimens with dimensions similar to those of MEMS components. An analytical solution for the bending of a multilayered piezoelectric beam was used to compute a first estimate of d₃₁ as 176±27 pm/V. The field induced in-plane stress hysteresis loops were asymmetric at small in-plane stresses becoming of similar magnitude as the applied stress was increased beyond 300 MPa. Similarly, the intersection of the hysteresis loops shifted from negative to positive electric field at stresses larger than 150 MPa. The applied stress resulted in reduction of the hysteresis magnitude due to mechanical constraints imposed on 90° domain switching. The effect of high in-plane stress on domain switching was also the reason for the hysteretic non-linear stress-strain curves that were recorded for unbiased PZT films.