Electromechanical response of textured ferroelectric PZT thin film stacks

Thin film piezoelectric materials with high piezoelectric coefficients such as PbZr0.52Ti0.48O3 (PZT) offer several advantages to microelectromechanical systems (MEMS) due to their low power requirements, large displacements, high work and power densities, as well as high sensitivity in a wide dynamic range. The performance of PZT-based MEMS can be further improved by increasing the piezoelectric response of PZT polycrystals via texture control. However, freestanding PZT films, in particular for MEMS, are comprised of several other films forming a stack. These additional layers serve as seeding (TiO2), buffer (SiO2), and conducting (Pt) layers with substantial thickness and stiffness compared to the main PZT layer. As a result, quantitative understanding of the mechanical behavior of each layer is required in order to extract the electromechanical response of the PZT layer itself in a stack. This dissertation research investigated (a) the mechanical behavior of highly {111} textured Pt films grown on {100}-TiO2 which is required to achieve ~100% (001)-textured PZT films, and (b) the electromechanical behavior of freestanding textured PZT film stacks, with PZT texture varying from 100% (001) to 100% (111). PZT stacks in the form of d31-type actuators were comprised of an elastic SiO2 layer, an adhesion layer of {100}-textured rutile TiO2, a metallization layer of highly {111}-textured Pt, a seed layer of PbTiO3, the PZT layer, a second Pt metallization layer, and, finally, a thin ALD layer of Al2O3 and HfO2 deposited by atomic layer deposition. Microscale uniaxial tension tests were carried out on patterned SiO2 films and combinations of layers, such as TiO2-Pt, SiO2-TiO2-Pt, SiO2-TiO2-Pt-PZT and SiO2-TiO2-Pt-PZT-Pt-ALD to determine the properties of each layer. Experiments on TiO2-Pt stacks with different Pt thickness showed that a reduction in film thickness increases the flow stress of Pt. The evolution of flow stress with plastic strain as a function of film thickness and grain size was successfully modeled, providing insight into the deformation behavior of polycrystalline metal films grown epitaxially on polycrystalline underlayers. Mechanical experiments on (SiO2-TiO2-Pt-PZT) and full PZT stacks (SiO2-TiO2-Pt-PZT-Pt-ALD) showed that the mechanical, piezoelectric and ferroelastic properties of PZT thin films depend strongly on grain orientation. The open circuit PZT modulus varied linearly with %(001) and %(111) texture factors between the two texture bounds: a lower bound for 100% (001) and an upper bound for 100% (111). Pure (001) texture exhibited maximum non-linearity and ferroelastic domain switching, contrary to pure (111) texture with more linear behavior and the least amount of switching. A micromechanics model based on the Eshelby inclusion problem was employed to calculate the strain due to domain switching. The model reproduced the experimentally observed non-linearities in the stress vs. strain curves of (001) and (111) textured PZT films. Finally, the linear piezoelectric and ferroelectric properties of textured PZT films at low and high electric fields, respectively, were calculated using laser Doppler vibrometer measurements on PZT unimorphs. All samples, except one comprised of 73% (001) and 27% (111) texture, demonstrated saturation in transverse piezoelectric coefficients beyond ~150 kV/cm. Notably, the sample with the combination of 73% (001) and 27% (111) textures showed stable transverse piezoelectric coefficients at all electric field values with technologically significant implications to ultra-low-power MEMS. The ferroelectric and linear piezoelectric coefficients (with the exception of the aforementioned sample with stable linear properties) depended strongly on film texture, and the effective transverse strain and stress coefficients varied linearly with %(001) and %(111) texture factors. PZT films with 100% (001) orientation displayed 150%, 140%, and 80% larger linear piezoelectric strain coefficient, saturated strain coefficient and saturated stress coefficient, respectively, compared to films with 100% (111) orientation for the same electric bias and the same film thickness. Finally, PZT films with pure (001) texture showed 20% higher dielectric constant and 50% higher figure of merit in sensing than films with pure (111) texture. This dissertation research provided insight into material microstructure-electromechanical property relationships for freestanding PZT film stacks. The results will assist the development of reliable low power PZT-based MEMS devices with higher actuation and better sensing characteristics