Thin-film piezoelectric MEMS
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troduction Piezoelectricity and piezoelectric materials Piezoelectric materials enable electromechanical transduction. They generate electrical signals in response to applied mechanical stress, with applications ranging from the flashing lights in children’s shoes to motion- or vibration-powered wireless sensors. Conversely, piezoelectrics also respond to applied electrical fields by changing shape—for example, piezoelectric transducers are used to generate high-frequency acoustic waves for ultrasound medical imaging (see Figure 1) or to enable nanoscale positioning in scanning probe microscopes. More details on the definition of piezoelectricity and the standard notation used to describe it are given in the sidebar. Bulk piezoelectric materials are widely employed for a diverse range of applications. Figure 1 shows ultrasonic images of a fetus taken using a single piezoelectric piston in the 1960s and by ceramic Pb(Zr,Ti)O3 (PZT) two-dimensional (2D) transducer arrays. The improved image quality in modern ultrasonic imaging systems and the capability of three-dimensional (3D) rendering were enabled by the development of more sensitive, broader bandwidth, higher frequency piezoelectric transducer arrays. The last 15 years have seen an explosion in the development of new piezoelectric single crystals1 and lead-free piezoelectric materials,2 resulting in large improvements in material property coefficients and new application areas.
Thin films and multilayered heterostructures of piezoelectric materials have great potential for micro- and nanoscale devices due to the added functionality provided by the electromechanical transduction coupled with the ability to micromachine using standard processing tools. Advances in bulk piezoelectric materials have rapidly been incorporated into thin films through the development of improved growth techniques. Moreover, the last decade has seen significant growth in the ability to design and micromachine functional devices. At the same time, foundries are beginning to provide services for piezoelectric films, increasing the breadth of the community able to access such functional materials. This issue of MRS Bulletin highlights the integration of next-generation piezoelectric thin films into micromechanical systems (MEMS) and nanoelectromechanical systems (NEMS).
Piezoelectric MEMS overview Numerous means of generating motion in micromachined structures have been reported, including electrostatic, magnetostrictive, thermal, and piezoelectric approaches. State-of-the-art MEMS devices often use silicon, SiO2, SixNy, SiC, or diamond-like carbon micro- and nanomechanical structures actuated by electrostatic forces, which exist between charged surfaces at a distance.3–7 This technology has opened many new application possibilities, for example, in the area of MEMS spatial light modulators being
Chang-Beom Eom, Department of Materials Science and Engineering, University of Wisconsin–Madison; [email protected] Susan Trolier-McKinstry, Pennsylvania State University; [email protected] DOI: 10.1557
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