The Effects of Biaxial Stress on the Ferroelectric Characteristics of PZT Thin Films
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1Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park,
Pennsylvania 16802 2 Advanced MicroDevices Branch, Army Research Laboratory, Fort Monmouth, New Jersey, 07703 ABSTRACT The design and implementation of microelectromechanical (MEMS) systems requires a sound understanding of the influence of film stress on both the ferroelectric and piezoelectric characteristics of thin films to be used for sensing and/or actuation. Experiments were conducted in which thin film samples of sol-gel derived PZT were subjected to applied biaxial stresses from -139 to 142 MPa. Films were characterized at known stress states (derived from known values of residual stress and large deflection plate theory) in terms of their ferroelectric polarization (saturated and remnant), dielectric constants, coercive field strengths, and tan a.Results obtained indicate that domain wall motion in thin films contributes much less to the observed response than is typical for bulk PZT materials. Alternative mechanisms are proposed in an attempt to explain the discrepancies.
INTRODUCTION The recent advances in electroceramic thin film deposition technology has resulted in a great deal of research and development in the area of microelectromechanical systems (MEMS) since the sensing and/or actuation function of MEMS devices can be accomplished though the use of thin
film piezoelectric materials. Most piezoelectric-based devices are currently constructed with zinc oxide (ZnO) as their active material. The small piezoelectric coefficients of the ZnO when compared to those of the perovskite oxides (e.g. barium titanate and lead zirconate titanate) have led to inevitable attempts at implementation of thin film perovskites, most often lead zirconate titanate (PZT), in MEMS devices. It has been well documented in the past that thin film materials, be it metals, ceramics or polymers, are in a state of residual stress upon completion of the deposition procedure [1]. The state of stress is dependent upon a number of factors, a major component of which is the mismatch of thermal expansion coefficients between the substrate and film. Electroceramic thin films are no different in that respect and are subjected to residual stresses, either tensile or compressive in nature (depending upon the substrate and process conditions), on the order of hundreds of MPa [2]. Previous experiments with bare PZT films (i.e. no top electrode) have shown residual stresses to be tensile with magnitudes on the order of 100 MPa [3]. That fact becomes important when one designs micromechanical systems, for if the active material is subjected to an in-plane stress, the effective piezoelectric coefficients of the film may change relative to the unstressed values. In particular, the mechanical stress applied to the material may act to restrict domain wall motion, thus significantly reducing the extrinsic component of the piezoelectric response and making the material less desirable from an application point of view [4].
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