AC and DC electrical stress reliability of Piezoelectric Lead Zirconate Titanate (PZT) thin films

This work was directed towards developing a database for the long-term reliability of piezoelectric PZT thin films under both ac and dc electric drive. Under unipolar ac drive, the transverse piezoelectric coefficient, d₃₁, exhibited excellent reliability with an increasing d₃₁ coefficient during cycling due to progressive poling of the capacitors. Cycling with field amplitudes ranging from 120 - 200 kV/cm, 99% of the approx. 1.0 μm thick capacitors examined survived to 10⁹ cycles. In contrast, bipolar ac excitation results in rapid degradation of the transverse piezoelectric response caused by a field-induced depoling mechanism. Additionally, a series of highly accelerated lifetime tests (HALT) was performed to determine the dc reliability of PZT thin films exposed to temperatures ranging from 120°C to 180°C and electric fields ranging from 250 kV/cm to 400 kV/cm. A graphical analysis of the results, assuming a lognormal distribution, revealed that breakdown was due to two distinct failure modes, early freak failures due to extrinsic defects and later failures that exhibit a rapid type of breakdown. The activation energy for failure was approx. 0.78 eV and the voltage acceleration factor was approx. 7.8. Lastly, samples that failed during the HALT experiments were analyzed by scanning electron microscopy. Analysis revealed that microcracking, film/electrode delamination, and arcing all contribute to failure during operation.