Piezoelectric materials have excellent energy conversion efficiency between mechanical and electrical domains and have been used in a variety of electromechanical devices. Recently, the development of piezoelectric thin films has paved the way for creation of miniaturized electromechanical devices called piezoelectric MEMS (piezo-MEMS). Evolving around the development of piezo-MEMS are interdisciplinary and intertwined technologies, including materials science, surface and thin-film processes, measurement and device technologies, etc. Piezo-MEMS has been used in inkjet printer heads, gyro-sensors, and BAW (bulk acoustic wave) filters with new applications being developed for optical, acoustic, and micro-power generation devices.1–3
The performance of piezo-MEMS devices strongly depends on the piezoelectric properties of the materials, i.e., the piezoelectric thin films. These thin film materials, such as Pb(Zr,Ti)O3 (PZT) and AlN, have been vigorously deposited and intensively characterized. In addition to these two “traditional” piezoelectric thin films, new material candidates, such as lead-free (K,Na)NbO3 (KNN) or Sc-doped AlN piezoelectric thin films, have also been rapidly developed.4–7 These newcomers are expected to facilitate design and manufacturing of the next generation piezoelectric microdevices.
This APL Special Topic “Piezoelectric thin films for MEMS” features recent advances in piezoelectric thin films and related technologies aimed for MEMS applications. These technologies include new piezoelectric thin-film materials, innovative thin-film processes, and new evaluation methods with the goal of advancing the piezo-MEMS applications.
In recent years, research on piezoelectric thin film materials has made remarkable progress. PZT thin films have conventionally been deposited with a polycrystalline structure on Si substrates. However, now that orientation control using buffer layers and epitaxial growth techniques becomes possible, and improvements and tuning of piezoelectricity have been achieved.8,9 Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) relaxor thin films, which have the potential to surpass PZT thin films in terms of piezoelectricity, have also been intensively investigated. Moreover, Song et al. reported fabrication and characterization of epitaxial piezoelectric thin films with an artificial superlattice structure to explore the possibility of arbitrary design of piezoelectric properties.10 Last, in addition to conventional thin film deposition methods like sputtering, chemical solution deposition, and pulsed laser deposition, fabrication of thick polycrystalline films by aerosol deposition or screen-printing methods has been reported,11,12 together with characterization results of their piezoelectric properties.
As an alternative to Pb-based ceramics, lead-free piezoelectrics have been widely investigated. Among a variety of lead-free piezoelectric materials, KNN is considered by many to be the most promising one. Therefore, KNN piezoelectric thin films are attracting much research interest. Shibata et al. reported wafer-size deposition of high-quality KNN thin films for lead-free piezo-MEMS applications.13 Furthermore, epitaxial growth processes of KNN thin films on Si substrates using buffer layers and nanosheet seed layers have also been reported, leading to a significant improvement of the piezoelectric properties of the KNN thin films.14,15 In addition to KNN, piezoelectric properties of BiFeO3 (BFO) epitaxial thin films and (Na1/2Bi1/2)TiO3-Ba(Zr,Ti)O3 (NBT-BZT) solid solution polycrystalline thin films were also reported.16,17 Finally, piezoelectric properties of MXene-based materials, a new candidate of lead-free piezoelectrics, have been analyzed in detail by using first-principles calculations.18
For high-frequency resonators, Sc-doped AlN thin films have been studied as a key material. The piezoelectric properties of AlN thin films doped with 30% Sc have been investigated in surface acoustic wave (SAW) and Lamb wave resonators. The effect of oxide films on top of the Sc-AlN film was clarified.19–21 In addition, the effect of Sc doping into AlN on its electrical and mechanical properties was analyzed by using the Landau-Devonshire thermodynamic theory.22
In addition to achieving a high piezoelectric coefficient, maintaining a long-term reliability is as important for a piezoelectric thin film aimed for practical piezo-MEMS applications. This APL Special Topic “Piezoelectric thin films for MEMS” reports interesting and important research results on the long-term reliability of piezoelectric thin films. The degradation of piezoelectric properties of PZT thin films has been evaluated in terms of leakage current, heat generation, crack initiation, and their intertwined relationships.23,24 In particular, defects like vacancies in PZT and KNN thin films have a significant impact on their reliabilities, and the involved mechanisms are investigated.25,26
With the rapid progress in piezoelectric thin-film technologies, a variety of piezo-MEMS devices have been proposed. This APL Special Topic “Piezoelectric thin films for MEMS” presents interesting results from recent investigations, e.g., film bulk acoustic resonators (FBARs), SAWs, gyrators, and vibration power generators. Sekimoto et al. fabricated the parity-inverted resonators using SiAlN/AlN multilayer thin films.27 Su et al. deposited 32° Y-X LiNbO3 thin films on Si substrates and successfully fabricated SAW filters with a bandwidth exceeding 1 GHz.28 In addition, research on a prototype device of solidly mounted resonators (SMRs) with an acoustic Bragg reflector and the application of FBAR to viscosity measurement of liquids using quasi-shear waves is presented.29–31
Other interesting applications of piezo-MEMS include a piezoelectric thin-film fingerprint sensor using GHz-band resonance properties of an epitaxial PbTiO3 thin film and a Lorentz-force gyrator using Sc-AlN thin films, which are expected to be commercialized in the near future.32,33 Energy harvesting is also an area of prominent interest in piezo-MEMS applications. Aphayvong et al. have shown significantly increased output power in a prototype 2-DOF piezo-MEMS vibration energy harvester.34 Haptic actuators using PZT thick films, micro-mirrors for space applications, and MR sensors using the resonance of ZnO MEMS cantilevers are some of the newly proposed piezo-MEMS devices, which are expected to be developed in the near future.35–37
This APL Special Topic “Piezoelectric thin films for MEMS” collects the latest and noteworthy research results on piezo-MEMS, and we, the editors, believe this issue will be useful in predicting future trends in piezo-MEMS technologies. Finally, we would like to thank all the authors who contributed to this Special Topic, as well as the journal editors and staff who helped us put together this wonderful collection.