Deposition and modelling of lead zirconate titanate thin films on stainless steel for MEMS applications

Technological development is permanently advancing in the direction of miniaturisation and energy consumption reduction. One of the keys to this pathway lies in the integration of ferroelectric thin films into Micro-Electro-Mechanical Systems (MEMS) applications. For this purpose, due to its large piezoelectric response, lead zirconate titanate is the material of choice in industry. To broaden the range of applications, substrates less brittle than the currently favoured silicon must be implemented into MEMS. In this work, lead zirconate titanate thin films on stainless steel substrates were both experimentally investigated and modelled. Lead zirconate titanate thin films of composition PbZr0.52Ti0.48O3 were grown on AISI 304 stainless steel substrates by pulsed laser deposition with {001} texture to improve their piezoelectric response. As demonstrated using X-ray and electron backscatter diffraction, the texture engineering is achieved through a selection of two buffer layers, Al2O3 and LaNiO3. The former buffer prevents the oxidation of the stainless steel substrate during deposition of the subsequent layers. The latter serves as both a bottom electrode and a growth template to promote the {001} texture of PbZr0.52Ti0.48O3 thin films. The lead zirconate titanate thin films ferroelectric properties are improved through 2 mol.% Nb-doping, resulting in dielectric constants and losses at 1 kHz for a 200 nm thick film of 350 and below 5 %, respectively, before measurement of polarisation versus electrical field hysteresis. Following these measurements, the Nb-doped lead zirconate titanate thin films permittivity increases up to 430. With a thickness of 400 nm, the films exhibit a remanent polarisation of 16.5 μC·cm−2 and a coercive field of 92 kV·cm−1. In the modelling, based on a ferroelectric switching criterion and the Euler-Bernoulli beam theory, it was investigated how the vertical deflection of ferroelectric bending tongues behaves with a load at their free end. The model developed here bridges the gap existing in literature between the modelling of ferroelectric switching and the mechanical description of linear piezoelectric structures. Furthermore, the ferroelectric switching criterion model is improved by the inclusion of strain saturation at high fields, which is inherent to ferroelectrics. The model includes parameters describing the geometry of the bending tongue, its mechanical and material properties, the crystallographic state and built-in strain of the ferroelectric thin film and the applied electrical field to determine the vertical deflection for MEMS applications based on ferroelectric bending tongues. In summary, the technically relevant {001} texture of lead zirconate titanate thin films on stainless steel substrates has been successfully engineered. The thin films, especially with 2 mol.% Nb-doping, are excellent candidates for MEMS applications on non-brittle substrates, in particular in sensor technology. Furthermore, the model established in this work indicates that ferroelectric bending tongues made of lead zirconate titanate on stainless steel substrates can develop sufficient vertical deflection for MEMS applications