Piezoelectric and dielectric reliability of lead zirconate titanate thin films
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This work was directed toward developing a database for the long-term reliability of the transverse piezoelectric coefficient d31 under both unipolar and bipolar drive. Under unipolar drive, the films showed excellent reliability, with 99% of the devices surviving to 109 cycles. However, both aging and low amplitude bipolar drive resulted in rapid degradation of d31 due to backswitching of the ferroelectric domains. Both thermal and ultraviolet (UV) imprint prevented backswitching and resulted in improved aging and bipolar degradation behavior. Additionally, the UV imprinted samples showed nonlinear aging due to the presence of an internal space charge field that developed from photo-induced charge carriers.
I. INTRODUCTION
There is significant interest in lead zirconate titanate (PZT) thin films for sensors, actuators, and microelectromechanical systems (MEMS). With piezoelectric coefficients an order of magnitude larger than aluminum nitride (AlN) and zinc oxide (ZnO),1,2 PZT films can provide low voltage, high amplitude actuation. One of the key questions for device development is the reliability of PZT-based microdevices. To date, however, only a few electromechanical reliability studies have been reported. A majority of the available literature to date has focused on determining the aging and large alternating current (ac) electric field reliability of the longitudinal piezoelectric coefficient d33.3,4 The results during unipolar drive are the most promising, with little degradation witnessed in the piezoelectric response out to 109 cycles. Unfortunately, aging and bipolar degradation result in substantial losses in the longitudinal piezoelectric coefficient over time. Separate studies on both d33 and d31 have concluded that “spontaneous depoling” is the cause of the rapid aging of the piezoelectric coefficients.3,5,6 While the work on the longitudinal piezoelectric coefficient is illustrative, a majority of MEMS device structures require the transverse piezoelectric coefficient d31 for sensing and actuation. Consequently, this work focused on studying the factors that affect the stability of the d31 coefficient. Using the wafer flexure technique7 to evaluate the transverse piezoelectric response, an analysis of aging, high field unipolar, and small amplitude bipolar drive reliability was performed to assess the lifetime of PZT thin-film devices. This work was designated to determine the piezoelectric reliability of PZT thin films under variJ. Mater. Res., Vol. 15, No. 11, Nov 2000
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ous electrical stresses and determine the specific mechanism(s) contributing to electromechanical degradation. To improve upon piezoelectric reliability, a mechanism to prevent depoling in PZT thin films is necessary. Creating a preferred polarization state within the ferroelectric is known to be effective in preventing depoling in bulk ceramics, and is widely utilized in hard PZT compositions for actuator applications. A preferred polarization state may be achieved by stabi
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