Silicon’s status as the sole material for microelectromechanical systems (MEMS) is quickly being threatened by a team of students. Working with Robert Davis and Richard Vanfleet, faculty members in the Department of Physics and Astronomy, the team is discovering and publishing new methods for constructing these tiny machines.
Though often unaware, most of us enjoy using some kind of MEMS on a daily basis. Their microscopic features play a vital role in a number of everyday items, such as the sensors that set off airbags, make Wii remote controls respond, and tell camera-phones which way is up. Until now, these silicon MEMS were limited in size, sensitivity, and the type of conditions they could withstand. However, by using carbon nanotubes, Vanfleet and Davis’s research team has opened up a number of fresh possibilities.
“The reason we’re excited is that our process gives us an alternate approach to make small structures,” Vanfleet said. “Right now, we make them out of silicon. That’s about it, which means designs are limited to a few microns tall. Our process gives us ways to make MEMS taller, more sensitive. Silicon can’t do that.”
This novel method for fabricating MEMS puts to use one unique property of carbon nanotubes. Because of the molecular structure of carbon, the molecules join together and form microscopic tubes. By laying down a catalytic layer, or substrate, a forest of vertically aligned tubes can be grown at a controlled rate.
The 3-D structure is then filled in as it is sprayed with a chemical vapor, comparable to the way in which rebar reinforces a concrete wall. Silicon and silicon nitride, the traditional materials for MEMS, have been proven as adequate materials for solidifying the structure, but many other filling materials could be used. The team is very excited about the wide range of possibilities that this opens up in the field of MEMS fabrication.
“The carbon nanotubes form a framework, which then we can coat with whatever we want,” Vanfleet said. “[This new technique] allows us to make MEMS from all kinds of materials, avoiding some of the other limitations of silicon. For example, if we wanted to put a sensor in the exhaust pipe of a car, it would be too hot and too corrosive for silicon.”
Vanfleet and Davis are continuing to explore the properties and applications of carbon nanotubes in MEMS. They are currently setting up a facility on campus that will allow them to test more materials. These facilities will also serve several other department research groups as they study their own exciting applications of carbon nanotubes.
“We’re going to make lots of things with it,” Vanfleet said of the project. “We’re pushing into the materials. Looking at how strong the structures are now opens up avenues to what we can do with them.”
Vanfleet said his project is now reaching out to involve faculty from the chemistry, chemical engineering, and mechanical engineering departments, in order to make full use of the technology. The research will continue to be led by himself and Davis. He also noted that the pair collaborates on about 80 percent of their projects. He said their partnership is best described as “You made it; I look at it.”
As both professors and their mentored students continue their research as applied physicists, carbon nanotubes may soon be improving some of your favorite electronic devices.