| Mighty Microscanner
by David Pescovitz
Hyuck Choo and David Garmire are PhD candidates in the Department of Electrical Engineering and Computer Sciences.
Two UC Berkeley graduate students have fabricated a tiny microscanner, a pinhead-sized machine that can rotate a miniscule mirror back and forth 24,000 times every second with great precision. The possible applications are wide ranging, from a heads-up display that paints a video image right on your eyeball to advanced endoscopy tools outfitted with onboard CT scanners for 3D medical imaging right inside the body. Compared to similar technology currently on the market, Hyuck Choo and David Garmire's device not only performs much better, it may also be ten times cheaper when produced commercially.
"Previously, microscanners have been fabricated using complex technology involving multiple steps," says Choo, a graduate student in the Department of Electrical Engineering and Computer Sciences. "Our method is very straightforward, involves only a few processing steps, and uses only tools that are conventional for processing integrated circuits."
A scanning electron micrograph image of the microscanner.
Under the guidance of professors Richard Muller and James Demmel, Choo and Garmire designed and built their microscanner in UC Berkeley's Microfabrication Laboratory, fabrication site for the Berkeley Sensor and Actuator Center (BSAC). In 2008, BSAC will relocate to the state-of-the-art CITRIS (Center for Information Technology Research in the Interest of Society) Nanofabrication Center currently under construction.
Choo and Garmire's microscanners are quintessential examples of MEMS (micro-electromechanical systems), teensy machines with parts no bigger than the period at the end of this sentence. MEMS are mass-produced on silicon wafers through lithographically-based processes similar to those used to manufacture computer chips. Along with fabricating transistors on a wafer, the MEMS process can also build mechanical features.
A scanning electron micrograph image showing a section of the microscanner comb drive.
In the case of the microscanner, these mechanical structures form an array of little "fingers" interlocked together. One set of the fingers is fixed to the surface of the wafer while the other, elevated slightly above the surface, can move back and forth when driven by a voltage. Due to its appearance, this type of actuator is known as a comb drive. A mirror actuated by the comb drive can be positioned with incredible accuracy to scan a digital image onto your retina from a projector embedded in a pair of sunglasses, or steer a laser during eye surgery, for example. The researchers demonstrated the latter (sans eyeball) in a laboratory experiment.
"The human eye moves as often as 2,000 times every second and our scanners are fast enough to compensate for this motion," Choo says. "And while the macroscale scanners used now in refractive eye surgery might cost $1,000 or more, ours can be provided for a few dollars are less."
MEMS microscanners are not new, Choo explains. Earlier implementations typically required the fixed and moveable combs to be processed on separate wafers and then assembled together.
At top, a gray-scale image and 3D profile of the ear on a US dime as imaged by a commercial macroscale scanner. At bottom left, the 3D surface profile as imaged by the MEMS microscanner. At bottom right, the same surface after laser ablation of a small part of the ear.
"The interleaved combs are on the scale of a few microns (one centimeter = 10,000 microns) so you have to align them with the precision of one micron or less," Choo says. "And that can be very challenging."
Another previously described method to produce microscanners incorporates an annealing step that takes place in a 900 degree Celsius furnace. If the chip already incorporates any electronic components, the high temperature can easily ruin them.
"All of these steps add cost and lower yield," Choo says. "If the end product is expensive, that defeats the purpose of the MEMS approach. Our simple method lowers the production cost by an order of magnitude and results in an even higher yield."
Right now, the researchers are using their fabrication process to build MEMS-based phase-shifting interferometers, optical phase sensors that measure transient phenomena such as the diffusion of chemicals in human blood or the growth of cells. The newly designed device can make 20 to 30 measurements every second to gather data about a variety of fast changing biological processes. Already, the researchers demonstrated that their technology is 20 to 100 times faster than conventional phase-shifting interferometers.
"Making the new interferometer just demonstrates another application area for our new, diverse, and highly-useful MEMS process," Garmire says.
Hyuck Choo's home page
David Garmire's home page
Microscanner project page
Professor James Demmel's home page
Professor Richard S. Muller's home page
Berkeley Sensor and Actuator Center (BSAC)
Microfabrication Laboratory at UC Berkeley
Center for Information Technology Research in the Interest of Society
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