PZT thin films for MEMS devices: from in-situ sputter deposition to energy harvesting device

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.


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