Advanced sol-gel processing of PZT thin films for piezoelectric MEMS structures

Micro-electro-mechanical systems (MEMS) technology is revolutionizing industrial and consumer products by replacing bulky devices by miniaturized versions that are suitable for portable applications as for instance in mobile phones. Besides small size, the keys for the success are reproducibility, batch processing, and significantly cheaper prices due to mass production. Whereas standardMEMS processing relied fromits beginning on standard CMOS materials, increasing need in extreme requirements or exploitation of specific phenomena demands for the integration of new materials. The interest is particularly strong in piezoelectric MEMS devices due to their higher efficiency in converting electrical to mechanical energy and vice versa, and thus their ability to provide electro-mechanical transducers. Since most of the actively used piezoelectric materials are ceramic in nature, they must be synthesized at rather high temperatures, which pose challenges to integration routes. The recent years can be considered as the starting phase of product development of piezo-MEMS. As a result, a very high interest is put on further improving the quality of piezoelectric thin films, meaning higher sensitivity in MEMS sensors and larger stroke inmicro-actuators is remaining a key priority. Themost widely used piezoelectric thin filmmaterials forMEMS devices are zinc oxide (ZnO), aluminumnitride (AlN) and lead zirconate titanate (PZT). AlN is used extensively forMEMS resonator structures due to its high quality factor and low losses. It is also investigated for sensors where linearity and stability of the piezoelectric response is mandatory. PZT films are used in actuators and acoustic transducer applications because of their high piezoelectric coefficients and electromechanical coupling coefficients. In this work, energy harvesting is specifically considered as motivation for property evaluation and optimization. In this case, the piezoelectric thin filmis used like a sensor for vibrations andmotions, though with the optimization requirement that the product of current and voltage must be maximal. [...]