Piezoelectric aluminum nitride thin films for microelectromechanical systems
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Introduction Historically, the term microelectromechanical systems (MEMS) has been used to identify sensors and actuators that are microfabricated using manufacturing processes such as those employed for the making of integrated circuits (ICs). In most cases, MEMS devices are fabricated on or from silicon wafers and made out of the same type of materials that are conventionally used for the synthesis of complementary metal oxide semiconductor (CMOS) technologies, such as polysilicon, silicon dioxide, silicon nitride, and aluminum. In the early stages of MEMS development (starting around 1960), piezoelectric materials were rarely used and mostly restricted to ZnO, as it was hard to access deposition techniques that would guarantee repeatable results. Furthermore, the most common piezoelectric used for the making of macroscale transducers was lead zirconate titanate (PZT), a material that was more difficult to process and integrate as a thin film on silicon (although, as shown in the Pulskamp et al. article in this issue, many of these issues are now resolved), exhibits quality factors that are too low for some RF applications, and especially was of concern to IC fabrication facilities because of contamination risks in CMOS
lines. Thin-film zinc oxide (ZnO) also suffered from similar issues, and its reactivity with other IC materials made it hard to integrate with standard CMOS processes. With the introduction of aluminum nitride (AlN) and especially the commercial success of the thin-film bulk acoustic resonator (TFBAR or FBAR),1 interest in piezoelectrics for MEMS blossomed. It is fair to state that industry drives for more compact and cheaper radio frequency (RF) duplexers (for details, see next section), and the consequent investments in the development of repeatable physical vapor deposition (PVD) techniques for the growth of AlN films on silicon have spurred a great deal of activities in the field of piezoelectric and especially AlN MEMS. Practically, most conventional MEMS devices that used to be made out of silicon have recently been reproduced (in most cases with enhanced performance) by using AlN thin-film piezoelectric technology. For example, resonators,2–5 filters,6–8 switches,9–11 energy harvesters,12–14 ultrasonic transducers,15,16 microphones,17,18 strain sensors,19 chemical sensors,20 and accelerometers21 have been demonstrated using AlN thin films. This article reviews MEMS development using AlN, with a particular focus on RF applications and the development of
Gianluca Piazza, Carnegie Mellon University, Pittsburgh, PA; [email protected] Valeriy Felmetsger, OEM Group, Gilbert, AZ; [email protected] Paul Muralt, Swiss Federal Institute of Technology, Switzerland; paul.muralt@epfl.ch Roy H. Olsson III, Sandia National Laboratories, Albuquerque, NM; [email protected] Richard Ruby, Avago Technologies, San Jose, CA; [email protected] DOI: 10.1557/mrs.2012.268
© 2012 Materials Research Society
MRS BULLETIN • VOLUME 37 • NOVEMBER 2012 • www.mrs.org/bulletin
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PIEZOELECTRIC ALUMINUM
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