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MEMS wear sophistication well

Posted: 16 Sep 2008 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? sensor? electronic device? medical signal?

Najafi: Practical issues like packaging, testing and reliability are very important.

Engineers must take a holistic rather than a piecemeal approach when integrating MEMS into larger electronic systems.

"MEMS integration is all about bringing together the sensor, the electronics, the signal processing, the packaging and possibly a wireless back end, so that they are all working together," said Khalil Najafi, deputy director of the Wireless Integrated Microsystems (WIMS) research center of the U.S. National Science Foundation.

The question facing engineers, Najafi said, is, "How do you partition and package the system? Do you integrate the electronics monolithically, or do you use separate components?" For systems using MEMS devices, he said, "practical issues like packaging, testing and reliability are very important, and have to be built into the design process at least by the prototyping stage, making it a lot easier to then manufacture the device."

WIMS' charter is to develop test beds for MEMS components used in such applications as medical implants and handheld gas chromatography testers.

A wireless microsystem for recording neural signals from brain cortical neurons is implantable for use in neural prostheses requiring reliable chronic recording of neural signals.

Heart of the instrument
Engineers performing system design and integration need to understand the different approaches to making MEMS devices, said Najafi. For instance, a MEMS sensor can either be monolithic or can use separate MEMS and ASIC chips wire-bonded together in the same package or possibly stacked in a 3D configuration. The application will usually dictate which approach to take, but the engineer needs to be aware of the available options in order to make an intelligent choice.

A MEMS device no longer measures a single parameter, Najafi said, but forms the heart of a sophisticated instrument. "If you want to do complex measurementssay, the inertial movement with multiple degrees of freedomand need both accelerometers and gyroscopes, all with very high performance, then you are really talking about an instrument, as opposed to just a sensor," Najafi said.

That means the designer must have an understanding of interfaces, data transfer and signal processing.

Battery issue
For simple, standalone systems, the low cost of an integrated monolithic design is sometimes best. But for MEMS-based systems that need to be optimized for related applications, the modularity of separate components may work best. And many applications today are portable or wireless, making battery life an important issue.

"An increasing number of applications need to be portable, so you often need batteries, and for that you need low power," said Najafi. Because measurement accuracy requires trade-offs in power, WIMS researchers spend a lot of time trying to maximize performance per unit of energy consumed, he said.

Packaging of a MEMS device can amount to as much as 70 percent of total cost, according to Roger Grace, president of sensor consultant firm Roger Grace Associates. And Najafi said that packaging considerations must begin at the wafer scale, "because once you release the mechanical MEMS structures, you have to protect them from contamination before you even dice the wafer, or it will affect your yields. After dicing, you want them to be ready to use, so that all you have to do is maybe wire-bond to them."

This cochlear implant consists of a MEMS stimulating electrode array connected to a hermetically sealed package containing integrated electronics for signal processing and wireless power and data transfer.

Neural-controlled prostheses
WIMS' medical implant test beds include a cochlear implant and a cortical implant. The latter, designed for monitoring brain activity and controlling prostheses, must record 64 individual sites, process the signals and transmit them wirelesslyall in a very small package designed for the harsh environment of the body. Three-dimensional stacked dice pack 64 intelligent recording channels into three stacked chips measuring just 1mm x 1mm x 0.5mm.

Besides monitoring brain activity, the cortical implant could eventually be used with neural-controlled prostheses that would let amputees move prosthetic limbs simply by thinking about the action. "For a prosthetic arm, you might implant the device in the part of the brain that used to control the natural arm," said Najafi.

According to Grace, the time is long overdue for engineers to abandon piecemeal approaches to integrating MEMS components and to embrace concurrent-engineering principles.

Note the 'S'
"Engineers need to keep their eye on the 'S' in MEMS," Grace said. "The big problem today is that too many engineers spend all their time developing the MEMS portion of their device independently of the rest of the system. Only at the end do they start considering how its output can supply a usable signal."

At that point, they then have to add "amplification, filtering, analog-to-digital conversion and maybe even an embedded core for intelligence," he said. "They must consider adding communications on a network or wirelessly, and finally how to put it into a package and test everything."

Grace said the concurrent-design practices routinely used in electronic systems today would allow developers to create optimal designs that could be economically and uniformly manufactured in high volume with high yields.

- R. Colin Johnson
EE Times

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