Integration of new functional materials into silicon microsystems is a key factor to enable technology for a wide range of innovative MEMS devices. Piezoelectric materials are of primary interest for integrating sensing and actuation functions in MEMS due to their high forces and high energy densities. The use of PZT thin films in MEMS applications offers the possibility of increasing the sensitivity or actuation capabilities of the devices compared to alternatives such as AlN and ZnO. In general, PZT thin films exhibit smaller piezoelectric coefficients and polarizations than PZT bulk materials due to grain size, composition, crystallographic orientation, non-defined stoichiometry and mechanical boundary conditions. Moreover, PZT thin films are typically grown onto amorphous surfaces resulting in polycrystalline structures, which often lead to degraded performance due to fatigue and aging characteristics. Since epitaxial PZT films exhibit properties, including piezoelectric coefficients, polarizations, and dielectric constants, generally superior to polycrystalline films, it is of high interest to consider their application in MEMS devices, however, there are some real challenges to be solved. Once these issues are overcome, epitaxial PZT thin films could offer interesting potentials in the realization of high performance piezoelectric MEMS.In this thesis, several aspects related to the development of epitaxial piezoelectric MEMS on silicon are investigated, which cover the following topics: the deposition and integration of high quality epitaxial PZT thin films on silicon wafers; the establishment of microfabrication techniques with associated process flows; and the FEM supported design, and characterization of epitaxial piezoelectric MEMS. A short overview is first given on the current state-of-the-art of piezoelectric MEMS. The integration of epitaxial oxide films on silicon wafers and their properties is then briefly described. The epitaxial oxide thin film heterostructures are based on a piezoelectric Pb(Zr 0.2 Ti 0.8 )O 3 layer grown on 2″ silicon wafers through two oxide layers: SrTiO 3 used as buffer and metallic SrRuO 3 used as bottom electrode. The optimized microfabrication process for these oxide layers with specific attention in maintaining the piezoelectric properties of the epitaxial PZT films is presented. The polarization was measured to optimize their processing with at the end no degradation of the piezoelectric properties throughout the process. The epitaxial PZT thin films exhibit a large load. The second application is based on an epitaxial PZT membrane to produce a resonating device.The study of basic characteristics of such device has shown excellent results as it shows a strong harmonic oscillation response with a high quality factor at atmospheric pressure. The finite element model of the epitaxial PZT membrane has then been developed for localized-mass sensing application to determine the resonant frequency, and the effect of the position of the mass and of the resonant mode o...

This paper focuses on the fabrication and evaluation of vibration energy harvesting devices by utilizing an epitaxial Pb(Zr 0.2 Ti 0.8 )O 3 (PZT) thin film. The high quality of the c-axis oriented PZT layer results in a high piezoelectric coefficient and a low dielectric constant, which are key parameters for realizing high performance piezoelectric energy harvesters. Different cantilever structures, with and without a Si proof mass, are realized using micro-patterning techniques optimized for the epitaxial oxide layers, to maintain the piezoelectric properties throughout the process. The characteristics and the energy harvesting performances of the fabricated devices are experimentally investigated and compared against analytical calculations. The optimized device based on a 0.5 μm thick epitaxial PZT film, a cantilever beam of 1 mm × 2.5 mm × 0.015 mm, with a Si proof mass of 1 mm × 0.5 mm × 0.23 mm, generates an output power, current and voltage of, respectively, 13 μW g −2 , 48 μA g −1 and 0.27 V g −1 (g = 9.81 m s −2 ) at the resonant frequency of 2.3 kHz for an optimal resistive load of 5.6 k . The epitaxial PZT harvester exhibits higher power and current with usable voltage, while maintaining lower optimal resistive load as compared with other examples present in the literature. These results indicate the potential of epitaxial PZT thin films for the improvement of the performances of energy harvesting devices.

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In studies using piezoresponse force microscopy, we observe a nonzero lateral piezoresponse at 180°d omain walls in out-of-plane polarized, c-axis-oriented tetragonal ferroelectric Pb͑Zr 0.2 Ti 0.8 ͒O 3 epitaxial thin films. We attribute these observations to a shear strain effect linked to the sign change of the d 33 piezoelectric coefficient through the domain wall, in agreement with theoretical predictions. We show that in monoclinically distorted tetragonal BiFeO 3 films, this effect is superimposed on the lateral piezoresponse due to actual in-plane polarization and has to be taken into account in order to correctly interpret the ferroelectric domain configuration. © 2009 American Institute of Physics. ͓doi:10.1063/1.3226654͔ Ferroelectric materials, characterized by their reversible spontaneous electric polarization, show great potential for multifunctional applications ranging from nonvolatile memories 1,2 to nanoscale sensors and actuators. 3 Controlling the structure and stability of ferroelectric domains in these materials is a key requirement for device implementation. In particular, the dynamics of domain walls, the interfaces separating regions with differently oriented ferroelectric polarization in the films, can significantly affect performance. 4 Understanding domain wall behavior at the nanoscopic scales of current and future devices, however, requires techniques with the requisite nanoscale resolution.One such technique is piezoresponse force microscopy ͑PFM͒, 5 in which a metallic atomic force microscope ͑AFM͒ tip is used to apply an alternating voltage across the ferroelectric material, resulting in a local mechanical response at the film surface due to the inverse piezoelectric effect. This piezoelectric response can be detected from the induced displacement of the AFM cantilever, recorded by the position of a laser beam reflected onto a quadrant-split photodetector. The vertical deflection and angular torsion of the tip are referred to as vertical and lateral PFM, respectively. The response phase provides information on the polarization, while its amplitude is related to the polarization magnitude. 6 Depending on the piezoelectric coefficient tensor d ij , linked to the crystal symmetry, a combination of these two measurements allows access to both out-of-plane and in-plane components of polarization. Although quantitative measurements of piezoelectric coefficients via PFM are challenging, the technique has provided valuable information about the behavior of domain walls and switching dynamics both in thin films 7,8 and in device structures. [9][10][11] In this context, understanding the origins of the PFM signal observed at ferroelectric domain walls is an important issue.Considering only piezoelectric effects, in a c-axis-oriented tetragonal ferroelectric film with an electric field applied along the polarization axis, the piezoelectric response is determined by the d 33 coefficient, leading to a purely vertical PFM signal. However, in such films, a nonzero lateral PFM response has been ...

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