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Research reveals novel MEMS device fabrication

Posted: 30 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? lead magnesium niobate-lead titanate? piezoelectric material?

A team of researchers from the University of Wisconsin (UW)-Madison has devised what they claim as a simple, reproducible microscale fabrication technique for piezoelectric materials. According to them, they were able to integrate lead magnesium niobate-lead titanate (PMN-PT) flawlessly onto silicon.

Led by Chang-Beom Eom, a UW-Madison professor of materials science and engineering and physics, the team studied the advanced piezoelectric material PMN-PT. Such materials exhibit a 'giant' piezoelectric response that can deliver much greater mechanical displacement with the same amount of electric field as traditional piezoelectric materials. They also can act as both actuators and sensors. For example, they use electricity to deliver an ultrasound wave that penetrates deeply into the body and returns data capable of displaying high-quality 3D image. Currently, a major limitation of these advanced materials is that to incorporate them into very small-scale devices, researchers start with a bulk material and grind, cut and polish it to the size they desire. It's an imprecise, error-prone process that's intrinsically ill-suited for nanoelectromechanical systems (NEMS) or microelectromechanical systems (MEMS).

Micro-machined hyper-active cantilever structure

Counterclockwise from upper left: 1) Schematic layer structure showing the silicon base, metallic top and bottom electrodes, and active PMN-PT. 2) False color scanning electron microscope image of completed cantilever. 3) Transmission electron microscope image showing layer structure. 4) High-resolution transmission electron microscope image showing perfect atomic arrangement of the giant piezoresponse PMN-PT layer and bottom metallic electrode SrRuO3.

Applying microscale fabrication techniques such as those used in computer electronics, Eom's team has worked from the ground up to incorporate PMN-PT seamlessly onto silicon. Because of potential chemical reactions among the components, they layered materials and carefully planned the locations of individual atoms. "You have to lay down the right element first," stated Eom. Onto a silicon platform, his team added a very thin layer of strontium titanate that acts as a template and mimics the structure of silicon. Next is a layer of strontium ruthenate, an electrode Eom developed some years ago, and finally, the single-crystal PMN-PT. The researchers have characterized the material's piezoelectric response that correlates with theoretical predictions. "The properties of the single crystal we integrated on silicon are as good as the bulk single crystal," noted Eom. His team calls devices fabricated from this giant piezoelectric material 'hyper-active MEMS' for their potential to offer researchers a high level of active control. Using the material, his team also developed a process for fabricating piezoelectric MEMS.

Applied in signal processing, communications, medical imaging and nanopositioning actuators, hyperactive MEMS devices could reduce power consumption and increase actuator speed and sensor sensitivity. Additionally, through a process called energy harvesting, hyperactive MEMS devices could convert energy from sources such as mechanical vibrations into electricity that powers other small devices, for example, for wireless communication.

- Julien Happich
??EE Times

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