PIEZOELECTRICITY IN FERROELECTRIC THIN-FILMS - DOMAIN AND STRESS ISSUES

As ferroelectric thin films are investigated as alternative sensors an d actuators for microelectromechanical systems, it is becoming importa nt to understand which mechanisms control the magnitude of the observe d piezoelectric properties. It is well known that in bulk soft lead zi rconate titanate actuators, over half the room temperature response is in fact associated with domain wall contributions to the properties. However, recent studies on bulk ceramics have demonstrated that the co mplexity of the domain structure, and the mobility of the twin walls d epend on the grain size. This leads to appreciable degradation in the dielectric and piezoelectric properties for grain sizes below a micron . This has significant consequences in thin film actuators since a lat eral grain size of one micron is often the upper limit for the observe d grain size. In addition, since the pertinent domain walls are ferroe lastic as well as ferroelectric, the degree of stress imposed on the f ilm by the substrate can also clamp the observed piezoelectric respons e. To investigate the importance of these factors, controlled stress l evels were imposed on several types of ferroelectric thin films while the dielectric and electromechanical properties were measured. It was found that for undoped sol-gel lead zirconate titanate thin films, the extrinsic contributions to the dielectric and electromechanical prope rties make very modest contributions to the film response. No signific ant enhancement in the properties was observed even when the him was b rought through the zero global stress condition. Comparable results we re obtained from laser ablated films grown from hard and soft PZT targ ets. Finally, a similar lack of twin wall mobility was observed in ato mic force microscopy experiments. The consequences of this, as well as several alternative methods to increase the available piezoelectric c oefficients and achievable strains in ferroic films will be presented.