Residual Stress Effects in Ferroelectric Thin Films

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Residual Stress Effects in Ferroelectric Thin Films T. A. Berfield†, N.R. Sottos†, R.J. Ong††, and D.A. Payne†† † Department of Theoretical and Applied Mechanics, †† Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ABSTRACT Recent developments in soft lithographic patterning enable the integration of functional ceramic thin films on a chip, rather than by adding discrete components to the system. As integrated device applications push the characteristic length scale of materials smaller and smaller, surface and interface effects play an important role, producing significant scientific challenges to the characterization of mechanical properties, performance and reliability. In this paper, we investigate the complex roles of microstructure, interface effects and residual stresses on ferroelectric thin film performance. Pb(Zr0.53Ti0.47)O3 films ranging in thickness from 190 nm to 500 nm were deposited by a sequential build up of sol-gel derived thin layers onto platinized Si substrates. Residual stresses in the films after thermal processing were observed and calculated from laser reflectance measurements of wafer curvature. Field-induced displacements were then measured by interferometric methods for films with well-characterized residual stress states. Results indicate significant increases in film performance with a decrease in measured residual tensile stress.

INTRODUCTION Potential uses of ferroelectric and piezoelectric thin films are far ranging, from microactuators and motors to sensor and memory applications. Lead zirconate titanate (PZT) is one of the most commonly used material systems for many of these applications. Compositions at and around Pb(Zr0.53Ti0.47)O3 have high dielectric constants, ferroelectric and piezoelectric properties1 and were used in the present work. Development of reliable material properties depends not only on a suitable choice of chemical composition, but on several other factors. Among these are phase development and texture, grain size and distribution, interface effects with the electrodes, and residual stresses present after processing.2,3 While a number of thin film deposition techniques exist, including: chemical vapor phase deposition,4 physical sputtering,5 and soft-solution processing,6 the latter non-vacuum methods, such as sol-gel processing, may offer the best combination of excellent compositional control with reduced- temperature processing, with high resistivity values. Although the composition, phase, grain size, orientation, and stress may influence electromechanical properties in bulk specimens, some of these effects are more dominant in thin films. Property differences between thin films and bulk specimens are often attributed to interface and stress effects. Thin-film deposition results in high residual stresses and a large interaction area between the film and the top and bottom electrodes. Several researchers have

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