Comparison of Lead Zirconate Titanate Thin Films for Microelectromechanical Energy Harvester With Interdigitated and Parallel Plate Electrodes

Lead zirconate titanate (PZT) thin films on insulator-buffered silicon substrates with interdigitated electrodes (IDEs) have the potential to harvest more energy than parallel plate electrode (PPE) structures because the former exploit the longitudinal piezoelectric effect, which is about twice as high as the transverse piezoelectric effect used by PPE structures. In this work, both options are compared with respect to dielectric, ferroelectric, and piezoelectric properties, leakage currents, and figure of merit (FOM) for energy harvesting. The test samples were silicon beams with {100} PZT thin films in the case of the PPE geometry, and random PZT thin films for the IDE geometry. Both films were obtained by an identical sol-gel route. Almost the same dielectric constants were derived when the conformal mapping method was applied for the IDE capacitor to correct for the IDE geometry. The dielectric loss was smaller in the IDE case. The ferroelectric loops showed a higher saturation polarization, a higher coercive field, and less back-switching for the IDE case. The leakage current density of the IDE structure was measured to be about 4 orders of magnitude lower than that of the PPE structure. The best FOM of the IDE structures was 20% superior to that of the PPE structures while also having a voltage response that was ten times higher (12.9 mV/mu strain).

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

PZT thin films for MEMS devices

Andrea Mazzalai

The recent progress in synthesis and integration of highly piezoelectric lead-zirconate-titanate (PZT) thin films onto silicon substrates are paving the way for new, more efficient and faster devices in micro electro-mechanical systems (MEMS). In comparison to competing principles, piezo-MEMS devices need less power and are faster than thermally actuated devices, and work with lower voltage than electrostatic actuators. Within the frame of the development of mass production routines for the microfabrication of piezo-MEMS devices, there are several bottlenecks to be solved. Among them, the most relevant one is represented by the need of a high-quality, high-throughput PZT thin film deposition route. The popular sol-gel method results often in satisfactory performance in terms of piezoelectric activity, but the lack of automation makes this deposition method unsuitable for a large-scale production. Magnetron sputtering is instead a high-throughput and reliable technology, but high-performing PZT thin films are not straightforward to achieve. In this work we defined a high-quality, high throughput, sputtering process, directly on an industrial machine suitable for large silicon substrates. We show how the film orientation can be tuned between highly-(100) textured and (111), simply by playing with the thickness of a sputtered seed layer. The flexibility offered by magnetron sputtering in terms of microstructural tuning and its impact on the piezoelectric properties is discussed as well. With an optimized process we measured a transverse effective piezoelectric coefficient e31,f = -23 C/m2 on samples showing very low dielectric losses and leakage currents. Energy harvesting (EH) from ambient vibrations can represent a relevant technological breakthrough as it allows self-powering of portable electronic devices for which wiring or battery replacement is either too costly or unpractical. Examples are wireless sensor nodes for structural health monitoring, drug delivery devices, and many others. The most promising route to achieve adequate power levels and acceptable unit costs is to implement piezoelectric energy conversion at the MEMS scale. Whereas devices excited through a frequency coupling with the resonance of the harvester have made significant progress, only few studies have been performed for the exploitation of motion sources characterized by low frequencies and high amplitudes, such as human motion. In this work we report about design, modeling, microfabrication and characterization of an energy harvester based on PZT thin films equipped with interdigitated electrodes (IDE). The advantages of this configuration with respect to the standard parallel-plate (PPE) capacitor one have been studied in detail, and experimentally compared with the PPE configuration realized with AlN-ScN alloys. The PZT-IDE version realizes a similar high output voltage range, however, with a much larger electromechanical coupling coefficient. This means that our new version can be used in combination with lower quality factor resonant structures. As a consequence, one can realize efficient harvesting also in presence of air damping and thus avoid vacuum packaging. The remarkable resulting energy conversion efficiency makes this geometry particularly suitable for vibration energy conversion from human motion.

EPFL2014

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

Robin Nigon

EPFL2017

PZT thin films for MEMS devices

Andrea Mazzalai

EPFL2014