Correlating Microscopic Ferroelectric Properties and Macroscopic Thin Film Device Performance

Citation

Ruglovsky, Jennifer Lynn (2007) Correlating Microscopic Ferroelectric Properties and Macroscopic Thin Film Device Performance. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/K3BG-N315. https://resolver.caltech.edu/CaltechETD:etd-02252007-153131

Abstract

The relationship between thin film device performance and crystallographic microstructure is one of fundamental importance in materials science. Ferroelectric materials that show an electromechanical response via domain switching, such as the perovskites BaTiO₃ and PbTiO₃, are discussed. In this work, we focus on thin film MEMS actuators fabricated from four different ferroelectric thin film microstructures: poorly oriented, fiber textured, biaxially textured, and single crystal. The microscale properties of these thin film materials are characterized and correlated to macroscale mechanical device behavior.

We have modeled each of these four microstructures to determine the effect of grain-scale crystallographic texture on device-scale electromechanical constants. The method enables the effective electromechanical properties to be obtained for a polycrystalline film via a self-consistent approach. Using this model, we show that most electromechanical constants depend primarily on the out-of-plane texture of the ferroelectric thin film.

We have used surface micromachining to create free-standing bridge geometries in ferroelectric thin films of polycrystalline and biaxially textured PbTiO₃. The material properties of these thin films are characterized with various techniques to confirm the texture at the grain scale. We have utilized a custom experimental apparatus that can apply a loading force to a single microdevice via magnetostatic interaction while measuring the resulting displacement. The force-displacement curves that we measure provide insight into the initial stress and modulus of our composite beam devices and the role of the underlying crystalline microstructure.

In order to study cantilever actuators, BaTiO₃ active layers are grown monolithically on SrRuO₃ electrodes and devices are patterned via focused ion beam (FIB) milling exclusively or with a subsequent XeF₂ etch. Using this fabrication method, we study cantilevers consisting of fiber, biaxial, and single crystalline microtextures. The cantilevers are actuated by applying a voltage across the active layer and the resulting displacement is measured via inspection with optical microscopy. We are able to relate the macroscopic device performance to the microscopic piezoelectric constants via multimorph calculations.

Our experiments show that ferroelectric thin film device performance may be enhanced by improving the underlying grain scale crystalline microstructure - from fiber to biaxial to single crystal texture.

Thesis Files