Flexoelectricity in PVDF and TiOx thin films : material characterization for MEMS applications

Abstract

In this work, two cantilevered micro-electro-mechanical systems (MEMS) device architectures were evaluated, whereas the actuation was done with the flexoelectric effect. The first approach is based on a non-polar, α-phase polyvinylidene fluoride (PVDF) structure sandwiched between two gold electrodes. In the second approach, iridium oxide (IrOx) serves as electrode material, and titanium oxide (TiOx) as a functional material. For the Au/∝-phase PVDF/Au transducer, ∝-phase PVDF was produced with a spin-on method on a heated chuck, which allows the realization of ultra-thin (< 200 nm) PVDF films with an average roughness below 20 nm. It was found that this soft material exhibits a significant electrostrictive response. Consequently, it was demonstrated that electrostrictive PVDF cantilevers could compete with their piezoelectric counterparts while drastically reducing fabrication complexity. Temperature-dependent cantilever tip deflection measurements were performed, which showed that the impact on the tip deflection from room temperature up to 160°C is linked to the changes in the effective Young’s modulus of the cantilever. Even though ∝-phase PVDF was reported in literature to exhibit a giant flexoelectric effect, this finding could not be verified. The latter findings from PVDF films differ, however, entirely from the sputter-deposited rutile TiOx devices fabricated and analysed in addition in this study. On device level, a linear dependence of curvature as a function of voltage was measured when exciting cantilevered MEMS transducers sinusoidally. The actuation was identified as flexoelectric, with a flexoelectric coefficient of ~2 nC/m. Segmented top electrodes were designed to stimulate additional electric field gradients to improve the performance of these devices even further. In addition, the pure electrical performance of the IrOx/TiOx/IrOx stack is determined in detail on mechanically non-released test structures, focusing on capacitance and leakage current measurements. On average, the devices in this thesis showed a permittivity range between 60 and 95. The leakage current measurements revealed capacitive-memristive leakage current hysteresis effects, which were linked to the electrical charging and discharging of oxygen vacancies. Additionally, a dynamic generation of these vacancies with an applied electric field was shown, which led to a repeatable formation and dissociation of low-resistivity vacancy filaments in the TiOx layer. A drastic change in the temperature dependence of the leakage current characteristics was found, depending on the atmosphere to which the IrOx bottom electrode was exposed before sputter-depositing the TiOx thin film. When getting in contact with air the formation of OH-bridges on the surface of the IrOx is a leading factor in the temperature-dependent leakage current behaviour. The temperature dependence of the leakage current is most probably due to the deprotonation of OH-bridges. The herein-released oxygen can passivate electrically active oxygen vacancies, thus decreasing the leakage current at increasing temperatures. With this knowledge, the first temperature-dependent cantilever measurements were performed, demonstrating this passivating behaviour.