Epitaxial piezoelectric MEMS on silicon

Abstract

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...

Enhanced critical temperature in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 thin films on silicon

“…These results confirm the potential of a fully epitaxial approach for oxide growth on silicon for the fabrication of novel piezoelectric/ferroelectric devices. 28,29 In particular, the possibility to obtain high quality epitaxial PZT films exhibiting a transition temperature much higher than the bulk can be an advantage in terms of thermal budget for device microfabrication processing and operation.…”

Effect of fabrication conditions on phase formation and properties of epitaxial (PbMg1/3Nb2/3O3)0.67-(PbTiO3)0.33 thin films on (001) SrTiO3

“…Lead zirconium titanate (PbZr 1−x Ti x O 3 or PZT) is one of the frequently employed piezoelectric materials for the fabrication of such devices. 1,2 An alternate to PZT are relaxor ferroelectrics like Pb(Mg 1/3 Nb 2/3 )O 3 ) x -(PbTiO 3 ) 1−x (notably PMN-PT(x=0.67) also denoted as PMN-PT(67/33)), Pb(Zn 1/3 Nb 2/3 )O 3 ) x -(PbTiO 3 ) y (PZN-PT(91/09)). Recent developments demonstrating superior piezo response in the relaxor ferroelectrics compared to the well-known piezoelectric material PZT have propelled them to the forefront of sensor and actuator research and development.…”

Design Considerations for a Micromachined Proportional Control Valve

“…This is of interest to large-scale integration applications, but with layer thicknesses of the order of micrometers such actuators will not have enough volume to offer large work. Furthermore, the integration of piezoelectric films in the microfabrication process can significantly reduce the thermal budget available for further processing steps [49].…”