Ferroelectric Thin Films in Microelectromechanical Systems Applications
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D.L. Polla and L.F. Francis Si3N4 . \
Introduction Ferroelectric ceramic thin films fit naturally into the burgeoning field of microelectromechanical systems (MEMS). Microelectromechanical systems combine traditional Si integrated-circuit (IC) electronics with micromechanical sensing and actuating components.1'2 The term MEMS has become synonymous with many types of microfabricated devices such as accelerometers, infrared detectors, flow meters, pumps, motors, and mechanical components. These devices have lateral dimensions in the range of 10 yum-10 mm. Theultimate goal of MEMS is a self-contained system of interrelated sensing and actuating devices together with signal processing and control electronics on a common substrate, most often Si. Since fabrication involves methods common to the IC industry, MEMS can be mass-produced. Commercial applications for MEMS already span biomedical (e.g., bloodpressure sensors), manufacturing (e.g., microflow controllers), information processing (e.g., displays), and automotive (e.g., accelerometers) industries.3 More applications are projected in consumer electronics, manufacturing control, communications, and aerospace.4 Materials for MEMS include traditional microelectronic materials (e.g., Si, SiO2, Si3N4, polyimide, Pt, Al) as well as nontraditional ones (e.g., ferroelectric ceramics, shapememory alloys, chemical-sensing materials). The superior piezoelectric and pyroelectric properties of ferroelectric ceramics make them ideal materials for microactuators and microsensors. Ferroelectric ceramics have a host of characteristics desirable for MEMS applications. The strong piezoelectric effect
MRS BULLETIN/JULY 1996
allows electromechanical sensing and actuation. The spontaneous charge induced from mechanical strain in a ferroelectric thin film is easily sensed across a capacitor structure using a voltage or charge-sensitive amplifier. Actuators based on ferroelectrics exhibit high force generation for moderate voltage inputs. In addition ferroelectric actuators and sensors do not dissipate power in the static mode, and the ferroelectric's excellent dielectric properties result in extremely low noise during operation. Unlike nonferroelectric piezoelectric materials, such as ZnO and A1N, poling allows polycrystalline thin films to be used. Ferroelectrics also have pyroelectric properties useful for thermal detection and imaging. Among the many ferroelectric ceramics, materials in the lead lanthanum zirconate titanate family (which includes PbTiO3 and P t y Z r J V ^ O j [PZT]) are good candidates because the materials offer excellent electromechanical, dielectric, and pyroelectric properties. 5 Microelectromechanical-system devices using ferroelectric ceramic thin films have been the subject of several reports.2"10 Device designs for electromechanical applications typically include a thin, flexible structural material covered with the ferroelectric thin film sandwiched between metal electrod
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