Piezoelectric microelectromechanical systems
A piezoelectric microelectromechanical system (piezoMEMS) is a miniature or microscopic device that uses piezoelectricity to generate motion and carry out its tasks. It is a microelectromechanical system that takes advantage of an electrical potential that appears under mechanical stress. PiezoMEMS can be found in a variety of applications, such as switches, inkjet printer heads, sensors, micropumps, and energy harvesters.<ref>"High-Speed Switching Enhances Piezoelectric Response in piezoMEMS Devices". AZoSensors. August 30, 2017. Retrieved August 27, 2018.</ref>
Development
Interest in piezoMEMS technology began around the early 1990s as scientists explored alternatives to electrostatic actuation in radio frequency (RF) microelectromechanical systems (MEMS).<ref>"PZT". SINTEF. April 22, 2010. Retrieved August 27, 2018.</ref> For RF MEMS, electrostatic actuation specialized high voltage charge pump circuits due to small electrode gap spacing and large driving voltages. In contrast, piezoelectric actuation allowed for high sensitivity as well as low voltage and power consumption as low as a few millivolts.<ref name=":0">Polcawich, Ronald; Judy, Daniel; Pulskamp, Jeffrey; Trolier-McKinstry, Susan; Dubey, Madan (June 2007). "Advances in Piezoelectrically Actuated RF MEMS Switches and Phase Shifters". 2007 IEEE/MTT-S International Microwave Symposium. pp. 2083–2086. doi:10.1109/MWSYM.2007.380297. ISBN 978-1-4244-0687-6. ISSN 0149-645X. S2CID 19897747.</ref><ref name=":1">Metzger, Brian. "Piezoelectric MEMS (PiezoMEMS) Patent Portfolio: Thin Film PZT Deposition, Development, Device Designs, and Testing". TechLink. Retrieved August 27, 2018.</ref> It also had the ability to close large vertical gaps while still allowing for low microsecond operating speeds.<ref>Polcawich, Ronald; Pulskamp, Jeffrey; Judy, Daniel; Ranade, Prashant; Trolier-McKinstry, Susan; Dubey, Madan (December 6, 2007). "Surface Micromachined Microelectromechancial Ohmic Series Switch Using Thin-Film Piezoelectric Actuators". IEEE Transactions on Microwave Theory and Techniques. 55 (12): 2642–2654. Bibcode:2007ITMTT..55.2642P. doi:10.1109/TMTT.2007.910072. S2CID 15283027.</ref> Lead zirconate titanate (PZT), in particular, offered the most promise as a piezoelectric material because of its high piezoelectric coefficient, tunable dielectric constant, and electromechanical coupling coefficient.<ref name=":1" /> PiezoMEMS have been applied to various different technologies from switches to sensors, and further research have led to the creation of piezoelectric thin films, which aided in the realization of highly integrated piezoMEMS devices.<ref name=":2">Eom, Chang-Beom; Trolier-McKinstry, Susan (November 2012). "Thin-film piezoelectric MEMS". MRS Bulletin. 37 (11): 1007–1017. doi:10.1557/mrs.2012.273. S2CID 110494049.</ref>
The first reported piezoelectrically actuated RF MEMS switch was developed by scientists at the LG Electronics Institute of Technology in Seoul, South Korea in 2005.<ref name=":0" /> The researchers designed and actualized a RF MEMS switch with a piezoelectric cantilever actuator that had an operation voltage of 2.5 volts.<ref>Lee, Hee-Chul; Park, Jae-Yeong; Bu, Jong-Uk (April 11, 2005). "Piezoelectrically actuated RF MEMS DC contact switches with low voltage operation". IEEE Microwave and Wireless Components Letters. 15 (4): 202–204. doi:10.1109/LMWC.2005.845689.</ref>
In 2017, researchers from the U.S. Army Research Laboratory (ARL) evaluated the radiation effects in the piezoelectric response of PZT thin films for the first time. They determined that PZT exhibited a degree of radiation hardness that could be further extended by using conductive oxide electrodes instead of traditional platinum electrodes. Gamma radiation tests have also shown that actuated devices such as switches, resonators, and inertial devices could benefit from the radiation tolerance of PZT, suggesting the possibility that actuators and sensors can be integrated into platforms evaluating nuclear material and reduce human exposure to radiation.<ref name=":3">"Army Research Laboratory (ARL) Scientist Honored as the Department of Defense (DoD) Laboratory Scientist of the Quarter". Department of Defense Research & Engineering Enterprise. Retrieved August 27, 2018.</ref><ref>Brewer, Steven; Cress, Cory; Williams, Samuel; Zhou, Hanhan; Rivas, Manuel; Polcawich, Ronald; Glaser, Evan; Jones, Jacob; Bassiri-Gharb, Nazanin (July 13, 2017). "Phenomenological Model for Defect Interactions in Irradiated Functional Materials". Scientific Reports. 7 (1): 5308. Bibcode:2017NatSR...7.5308B. doi:10.1038/s41598-017-05071-z. PMC 5509665. PMID 28706227.</ref>
This experiment was part of a decades-long research investment effort at ARL to improve the use of PZT thin film technology for piezoMEMS.<ref name=":1" /> Other piezoMEMS-related work included developing a piezoelectric microphone based on PZT thin films,<ref>Polcawich, Ronald (November 2004). "A Piezoelectric MEMS Microphone Based on Lead Zirconate Titanate (PZT) Thin Films". U.S. Army Research Laboratory. Archived from the original on June 3, 2018 – via Defense Technical Information Center.</ref> creating new integrated surface micromachining processes for RF MEMS to incorporate thin film PZT actuators,<ref>Polcawich, Ronald; Pulskamp, Jeffrey; Judy, Daniel; Ranade, Prashant; Trolier-McKinstry, Susan; Dubey, Madan (December 6, 2007). "Surface Micromachined Microelectromechancial Ohmic Series Switch Using Thin-Film Piezoelectric Actuators". IEEE Transactions on Microwave Theory and Techniques. 55 (12): 2642–2654. Bibcode:2007ITMTT..55.2642P. doi:10.1109/TMTT.2007.910072. S2CID 15283027.</ref> providing the first experimental demonstration of monolithically integrated piezoMEMS RF switches with contour mode filters,<ref>Pulskamp, Jeffrey; Judy, Daniel; Polcawish, Ronald; Kaul, Roger; Chandrahalim, Hengky; Bhave, Sunil (January 2009). "Monolithically Integrated Piezomems SP2T Switch and Contour-Mode Filters". 2009 IEEE 22nd International Conference on Micro Electro Mechanical Systems. pp. 900–903. CiteSeerX 10.1.1.162.2655. doi:10.1109/MEMSYS.2009.4805529. ISBN 978-1-4244-2977-6. S2CID 18225031.</ref> and demonstrating the feasibility of vibrational energy harvesting using thin film PZT MEMS.<ref>Pulskamp, Jeffrey (February 2005). "Modeling, Fabrication, and Testing of a Piezoelectric MEMS Vibrational Energy Reclamation Device". U.S. Army Research Laboratory. Archived from the original on June 1, 2018 – via Defense Technical Information Center.</ref> In their work, researchers from ARL have also increased the overall electromechanical response of PZT thin films by 15-30% by incorporating iridium oxide electrode materials.<ref name=":3" />
Design
There exists three primary approaches to realizing PiezoMEMS devices:<ref name=":4">Tadigadapa, Srinivas (September 2010). "Piezoelectric microelectromechanical systems — challenges and opportunities". Procedia Engineering. 5: 468–471. doi:10.1016/j.proeng.2010.09.148.</ref>
- The additive approach: The piezoelectric thin films are deposited on silicon substrates with layers of insulating and conducting material followed by surface or silicon bulk micromachining.
- The subtractive approach: Single crystal or polycrystalline piezoelectrics and piezoceramics are subjected to direct bulk micromachining and then electrodes.
- The integrative approach: Micromachined structures are integrated in silicon or piezoelectrics by using bonding techniques on bulk piezoelectric or silicon substrates.
PiezoMEMS use two principal crystal structures, the wurtzite and perovskite structures.<ref name=":2" />
Challenges
PiezoMEMS still face many difficulties that impede its ability to be successfully commercialized. For instance, the success of depositing uniform films of piezoelectrics still depend heavily on the use of appropriate layers of proper nucleation and film growth. As a result, extensive device-specific development efforts are needed to create a proper sensor structure. In addition, researchers continue to search for ways to reduce and control the material and sensor drift and aging characteristics of thin film piezoelectric materials. Deposition techniques to create thin films with properties approaching those of bulk materials remain in development and in need of improvement. Furthermore, the chemistry and etching characteristics of most piezoelectric materials remain very slow.<ref name=":4" />
