MEMS fabrication on the cheap

MEMS fabrication on the cheap

Technology News |
By eeNews Europe

Researchers at MIT’s Microsystems Technologies Laboratories have demonstrated new ways to build MEMS on the cheap, not only enabling easy customization of the devices being produced, but also offering an alternative route to their manufacture with desktop-sized 3D printed fabs.

This new route to fabricating MEMS could yield new sensors and devices which otherwise may not have found a large-enough market to justify their full development from IP to final product using traditional processes.

The researchers’ fabrication device sidesteps many of the requirements that make conventional MEMS manufacture expensive.

"The additive manufacturing we’re doing is based on low temperature and no vacuum," says Luis Fernando Velásquez-García, a principal research scientist in MIT’s Microsystems Technology Laboratories. "The highest temperature we’ve used is probably 60 degrees Celsius. In a chip, you probably need to grow oxide, which grows at around 1,000 degrees Celsius. And in many cases the reactors require these high 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."

The actual manufacturing technique relies on the use of dense arrays of emitters that eject microscopic streams of fluid when subjected to strong electric fields. To build 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. The researchers used a fluid containing tiny flakes of graphene oxide, to be sprayed in a prescribed pattern on a silicon substrate. The fluid quickly evaporated, leaving a coating of graphene oxide flakes only a few tens of nanometers thick.

The flakes are so thin that interaction with gas molecules changes their resistance in a measurable way, making them useful for sensing.

According to Velásquez-García, the gas sensors obtained were as precise as a commercial product costing hundreds of dollars, while being faster and built for only a few cents.

In their first implementation, the electrospray emitters used by Velásquez-García and Taylor had been built using conventional semiconductor processes. But in a second study published in the December issue of the Journal of Microelectromechanical Systems, Velásquez-García reports using an affordable, high-quality 3-D printer to produce plastic electrospray emitters whose size and performance match those of the emitters that yielded the gas sensors.

Fig. 1: External row of seven emitters that are part of a 49-emitter array. The scalloping on the exterior of the emitters, due to the layer-by-layer manufacturing, is visible. Source: Anthony Taylor and Luis F Velásquez-García (edited by MIT News)


Not only were the researchers able to make the electrospray devices more cost-effective, 3-D printing allowed them to customize the devices for particular applications, improving the micro-nozzles from one iteration to the next within days.

Effectively, they were able to build new MEMS out of their custom MEMS fab desktop. Another big advantage is that the low process temperature allows sensor designers to deposit materials that would not be compatible with high-temperature semiconductor manufacturing, such as biological molecules with specific markers.

The new fabrication technique could open up new application fields for MEMS while taking more IP to viable commercial products.

"In some cases, MEMS manufacturers have to compromise between what they intended to make, based on the models, and what you can make based on the microfabrication techniques," Velásquez-García explained. "Only a few devices that fit into the description of having large markets and not having subpar performance are the ones that have made it."

Fig. 2: A completed chip with a wired graphene oxide gas sensor. The graphene oxide film is the greenish dot covering the electrode structure. Source: Anthony Taylor and Luis F Velásquez-García

Fig. 3: Optical micrograph of a fabricated conductometric graphene oxide gas sensor. The inset (top left corner) shows a close-up view of the active area of the sensor. Source: Anthony Taylor and Luis F Velásquez-García.


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