In 2014, MEMS were a $12billion business, but has traditionally required sophisticated semiconductor fabrication facilities, which cost tens of millions of dollars to build. Potentially useful MEMS have languished in development because they don’t have markets large enough to justify the initial capital investment in production.
“The additive manufacturing we’re doing is based on low temperature and no vacuum,” said Luis Fernando Velásquez-García, a principal research scientist in MIT’s Microsystems Technology Laboratories. “The highest temperature we’ve used is probably 60°C. In a chip, you need to grow oxide, which grows at around 1000°C. And in many cases the reactors require vacuums to prevent contamination. We also make the devices very quickly. The devices we reported are made in a matter of hours from beginning to end.”
Velásquez-García has been researching manufacturing techniques that involve dense arrays of emitters that eject microscopic streams of fluid when subjected to strong electric fields. For the gas sensors, Velásquez-García and Anthony Taylor, a visiting researcher from the British company Edwards Vacuum, used so-called ‘internally fed emitters’. These are emitters with cylindrical bores that allow fluid to pass through them.
In this case, the fluid contained tiny flakes of graphene oxide. Velásquez-García and Taylor used their emitters to spray the fluid in a prescribed pattern on a silicon substrate. The fluid evaporated, leaving a coating of graphene oxide flakes only a few tens of nanometres thick.
The flakes are so thin that interaction with gas molecules changes their resistance in a measurable way, making them useful for sensing. “We ran the gas sensors head to head with a commercial product that cost hundreds of dollars,” Velásquez-García says. “What we showed is that they are as precise, and they are faster.”
Electrospray could also lead to novel biological sensors, Velásquez-García says. “It allows us to deposit materials that would not be compatible with high-temperature semiconductor manufacturing, like biological molecules,” he says.
