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Fall 2008, Issue 9

Building Micromachines

Robert WhiteRobert D. White, PhD, joined the Department of Mechanical Engineering in September 2005. He earned his degrees in mechanical engineering from the Massachusetts Institute of Technology (BS and MS) and from the University of Michigan, Ann Arbor (PhD). His research centers around microelectromechanical systems (MEMS), the technology of micrometer-sized machines. He and his students develop and build these devices in the Microscale Sensors and Systems Lab. White also directs the Micro & Nano Fabrication Facility, a core facility located at 200 Boston Avenue on the Medford/Somerville campus.

Research currently underway in White’s lab includes projects on MEMS sensors for aeroacoustics, shear stress sensors for chemical mechanical planarization, shape memory alloy MEMS, and cochlear-like MEMS. For the MEMS Sensors for Aeroacoustics Project, White’s research group is developing sensors to measure properties of the turbulent boundary layer on the skin of an aircraft, which is a major source of noise inside the aircraft. “We’re building these eight by eight microphone arrays, each with 64 microphones,” says White. “Turbulence close to the skin of the aircraft generates vibrations from the outside to the inside of the cabin. There are a lot of mathematical models being used to try to characterize that boundary layer, but there’s not any experimental evidence at very short wavelengths, which requires you to have microphones very close together so you can discern if there are fluctuations of pressure over short distances. So we put all the microphones together onto a single chip.” The 64-microphone chip measures approximately ten by ten millimeters. Once White finalizes the testing and standardization of the microphone chip, his industry collaborator will use the microsensor to characterize the boundary layer of an aircraft skin, first in a wind tunnel and then on an aircraft. This new information will give the air acoustics community new data for designing walls of aircraft that transmit less noise to the cabin.

Specialized equipment for the creation and testing of micromachines is available in the Micro & Nano Fabrication Facility. “We’re a one-stop shop for everything acoustic MEMS,” says White. “We can do the computational modeling design, the fabrication, testing, and electronics.” White builds micromachines by using photolithography, a micromachining pattern-transfer process. A specialized company creates a template of the geometric pattern (called a mask because it makes some regions of the template opaque—masks them off—while leaving other regions transparent). A light-sensitive polymer is put on the surface of the material to be patterned (often a silicon wafer), then light is projected through the mask onto the polymer to expose unmasked regions, which are then dissolved away chemically. The resulting polymer-patterned silicon wafer is then put into White’s sputter tool which applies a thin metal film onto the wafer. “Gold is a metal we use a lot, but we also work with aluminum, titanium, chromium, silver, and nickel,” says White. A series of chemical treatments produces a metal-patterned silicon wafer. The process can be repeated with different masks and different metals. In this way micromachines are built up one layer at a time so they can include such things as sensors, valves, gears, mirrors, and actuators.

The Shear Stress Sensors for Chemical Mechanical Planarization (CMP) Project is a collaboration with Vincent Manno and Chris Rogers (School of Engineering faculty). CMP is a polishing process used primarily by the semiconductor and optics industries. “A lot of the mechanics that go on in that process are not well understood,” says White. “So this is a basic engineering science project to try to understand what some of the processes are that are occurring during polishing. My part of it was to do some MEMS sensing. We wanted to look at the very local forces that are happening as the [polishing] pad comes past some feature on the [silicon] wafer. There are large forces at that interaction, and if you measure just the average forces over the whole thing, you don’t see those large forces.” White’s research group created test structures that could sense the forces experienced during polishing. “We think this is the first time anyone’s measured the local forces during the actual polishing process at this sort of size scale, on the order of 50 microns across,” says White.

The Shape Memory Alloy MEMS Project is part of Barry Trimmer’s (School of Arts and Sciences, Biology Department) project on soft-bodied robots. “Some of the things that we put into the plan were MEMS devices for actuators that would act like little muscles, and some hair-like sensors” says White. The Cochlear-like MEMS Biomimetic Acoustic Sensors Project involves the design, modeling, and fabrication of MEMS sensors as external signaling mechanisms for cochlear implants, which provide a sense of hearing to individuals who are deaf or whose hearing is severely impaired. White would like to collaborate on the development and testing of these devices with a clinician who performs cochlear implants.

White is also working with Tufts Engineering faculty members Fiorenzo Omenetto and David Kaplan on a chemical communications project involving silk optics. “Fio and David have been figuring out the chemical and optical systems, and I’ve been doing the microfabrication,” says White. The micromachine must allow the user to encode information into silk and also to read the encoded message.

Nanotechnology devices are sized on the order of thousandths of a micrometer. White would like to expand the nanotechnology capabilities of the Micro & Nano Fabrication Facility. To accomplish this, he needs a device called an electron beam writer to make the masks for nanoscale projects. He is currently working on proposals to provide funding for this equipment.

For more information on Robert White, please go to http://tufts.edu/~rwhite07.
For more information on the Micro & Nano Fabrication Facility, please go to http://engineering.tufts.edu/microfab.



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