Epitaxial Piezoelectric MEMS on Silicon

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(Zr0.2Ti0.8)O3 layer grown on 2'' silicon wafers through two oxide layers: SrTiO3 used as buffer and metallic SrRuO3 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 piezoelectric coefficient d31 of 130-140 pm V-1, allowing the realization of MEMS devices with enhanced actuation/detection properties, such as large amplitude actuation with lower driving voltage, high sensitivity, and high efficiency in energy conversion. The superior properties of the epitaxial PZT thin film and the effectiveness of the optimized microfabrication techniques have been demonstrated by two examples of epitaxial PZT MEMS devices.