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. First, different epitaxial PZT cantilevers with and without a Si proof mass have been developed for vibration energy harvesting applications. A high power density of up to 14 µW g-2 was obtained with high current generation and usable voltage, while maintaining lower optimal resistive 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 on the mass sensitivity. The mass sensitivity is of the order 10-12 g Hz-1, which is in excellent agreement with the simulated value. The minimum detectable mass of ∼5 ng can be achieved. These results indicate that the integration of epitaxial thin films on silicon is a promising technology, which improves the performances of piezoelectric MEMS devices. Finally, the concept of charge integration technique for static measurement in piezoelectric sensors is proposed. This technique improves the detection sensitivity of piezoelectric sensors in low frequency measurements, which makes them suitable for chemical and biological detection in a liquid environment.

Internal electric fields and electrode effects in ferroelectric thin films for piezoelectric energy harvesting

Robin Nigon

The continuing decrease of power requirements of electronic circuits offers the potential to deploy wireless systems as embedded sensors for cars or industrial tools, and implanted medical devices. Harvesting ambient vibrations by an appropriate energy harvesting (EH) device allows to avoid an undesirable battery replacement. At the scale of micro-electromechanical systems, where severe size constraints must be met, microfabricated EH devices with piezoelectric thin films offer the best energy density. Ferroelectric lead zirconate titanate (PZT) thin films with interdigitated electrodes (IDE) appear as the most promising device design for this purpose. Accurate characterization of the thin film response is necessary to determine the PZT composition and doping, the electrode geometry, and the stack design for maximum EH efficiency. Unfortunately, there is no rigorous description of the physical behavior of the IDE system to date. One goal of this thesis was thus to provide a better understanding of the experimental observations made by previous researchers. In addition, only a limited PZT composition and doping range has been investigated in this configuration. It was the second goal of this thesis to widen this range in order to determine the combination that yields the best EH efficiency. Finally, the risk of partial or total depoling over the device lifetime is always present in ferroelectric materials. The phenomena of aging and self-poling are of great interest to ensure proper retention of the poled state and, thus, the reliability of the harvesting device. Neither of the two are well understood. It was the third goal of this thesis to investigate the aging behavior and methods to promote self-poling. In this thesis work, we have have proposed a description of the physical behavior of the IDE system, and we have developed an analytical model for extracting the effective material properties from standard characterization measurements, which is well supported by both finite element (FE) simulations and experimental data. We found that if the substrate is conductive enough, a parasitic capacitance is present in parallel to the material response. We have provided a method to subtract the contribution of the parasitic capacitance, which has an accuracy of better than 4% in a wide range of IDE geometries as determined by FE simulations. We have investigated the performances of doped PZT thin films with IDE for several combinations of dopant and composition. We have improved an existing fabrication route to obtain textured PZT films on an insulating MgO layer. We found that dopants systematically reduced the piezoelectric response and retention capability, and increased the dielectric constant. All three are detrimental for EH. Undoped compositions should be chosen. We have studied methods to improve the stability of the poled state through aging and self-poling. Introducing the latter into the IDE configuration did not provide sufficiently strong effects to be of practical interest. On the contrary, the aging process may allow to tune the extrinsic contributions to the dielectric and piezoelectric response. It is likely caused by polarization discontinuities at grain boundaries. Further work is needed for fully optimizing this phenomenon. Finally, from the previous investigations, we could deduce and propose golden rules for the design of IDE structures, and discuss typical applications where they are advantageous.

EPFL2017

Piezoelectric acoustic sensors and ultrasonic transducers based on textured PZT thin films

Nicolas Ledermann

This work dealt with the establishment of a reproducible and industrially exploitable fabrication technology for piezoelectric MEMS based on PZT thin films. {100}-textured piezoelectric 1-4 μm thick films were found to be the most suited materials for MEMS applications where a strong transverse e31,f coefficient is required. Values of 11.5 to 12.5 C/m2 throughout the investigated thickness range have been obtained. PZT MEMS requires some special microfabrication processes. The patterning of platinum electrodes in ICP reactor with an Ar/Cl2 gas mixture gave satisfying results. For 1 μm PZT films, excellent results were achieved with dry etching in an ECR/RF low-pressure reactor. Above 1 μm thickness, wet etching technique was used. The chosen process flow was such that all layers up the top electrode were first deposited, and then sequentially patterned. In this way, the functional properties were completely maintained. Finally, the process flow related to PZT was successfully combined with deep silicon etching techniques to produce free standing structures. The applicability of the processes has been demonstrated by means of two piezoelectric micro-electro-mechanical devices. First, a cantilever acoustic sensor for low frequency applications (< 50 Hz) has been developed for integration in photoacoustic gas detectors. A high device sensitivity of up to 150 mV/Pa has been obtained with noise level corresponding to a few mPa, mainly due to the preamplifier. The second device was based on a silicon/PZT diaphragm to produce a vibrating structure in view of fabricating ultrasonic transducers for position sensors and non-destructive testing probes. The study of the basic properties of such structures has shown excellent results as coupling factors k2 up to 5% for PZT 2 μm / Si 4 μm SOI membranes held with four small bridges. Ultrasonic wave transmission between two devices has been achieved up to a distance of 200 mm. The microfabrication of piezoelectric MEMS was found to be a complex task where all aspects from materials, microfabrication, and device design to assessment of performance are closely interconnected. Piezoelectric MEMS can offer exciting opportunities to fabricate innovative and performing devices.

EPFL2003

Resonating large angle and low consumption micromachined optical scanner

Philippe Renaud, Sandra Schweizer

A monolithic silicon integrated optical micro-scanner is presented. The device consists of a mirror located an the tip of a thermal bimorph actuator beam. The fabrication process is very simple and compatible with IC fabrication techniques. The device is excited electrothermomechanically at its resonance frequency, enabling large angular deflections at low power consumption. The technological process consists of basic frontside silicon micromachining steps requiring only three mask levels. The moving part is defined by selective silicon bulk etching. The bimorph beam is made of silicon dioxide and a thin film conductor. The residual stress in the two layers is used to achieve a 45 degrees out-of-plane rest position of the mirror. This allows optical components (e.g, laser diode, collimating lens) to be placed directly on the silicon substrate. Mirrors of 500300 and 1000500 mu m(2) with resonant frequencies at 300 and 100 Hz respectively were realized. Mechanical scan angles of above 90 degrees were acheived. The devices are very robust and have run through fatigue tests of billions of cycles at 300 Hz and 90 degrees deflection. The power consumption of the device is typically 1mW for 30 degrees mechanical scan angles.