Electromechanical properties of ferroelectric films for MEMS

The possibility of miniaturization and integration with Si technology makes ferroelectric films promising for microelectromechanical systems (MEMS). Electromechanical properties of thin-layer ferroelectrics, which determine their performance in micro devices, are still poorly understood as compared to the properties of bulk materials. Small grain size, preferred orientation of the films and their strong clamping to the rigid substrate are the main factors, which are responsible for the large difference in electromechanical responses of thin-film and bulk piezoelectrics. This paper reviews author's recent investigations on electromechanical properties of ferroelectric films. The emphasis is given to the contribution of domain wall motion to piezoelectric properties, which is shown to be strongly suppressed in thin films as compared to bulk materials. Nevertheless, this contribution is found to be essential at sufficiently high ac electric field in PZT films of the morphotropic phase boundary and rhombohedral compositions. Large anisotropy of the piezoelectric properties is found in PZT films having different textures. This anisotropy is similar to that recently observed in relaxor single crystals and is explained based on the anisotropy of electrostriction coefficients. Electrostrictive properties of bulk and thin film ferroelectrics are compared and analyzed.