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Digital MEMS mic put on single chip

Posted: 03 Apr 2006 ?? ?Print Version ?Bookmark and Share

Keywords:R. Colin Johnson? microphone? analog electret? microelectromechanical systems? MEMS?

Even the whizziest, most cutting-edge digital systems still use microphones that are based on 50-year-old analog electret technologies. The best candidate for a successor is microelectromechanical systems, but so far, analog MEMS microphones have been largely relegated to a single market: hearing aids.

Now, all that will change with the announcement of an all-digital MEMS microphone!the first to harness standard chip-processing techniques. The single-chip solution from Akustica Inc. offers more than digital outputs that match the digital processor and memories in PCs, PDAs, Bluetooth headsets and cellphones. It also promises to leverage the scaling power of CMOS chips to finally merge this formerly analog component with the mainstream of DSP. Akustica announced the MEMS mic at the Globalpress Electronics Summit last February.

Akustica's one-chip model AKU2000 microphone builds on five years of MEMS development of intellectual property, which the startup licensed from nearby Carnegie Mellon University. Its inventor is MEMS pioneer Kaigham Gabriel, an EE professor at Carnegie Mellon who co-founded Akustica in 2001 (see story on page 17). Gabriel researched MEMS at AT&T Bell Labs and spent five years managing MEMS development as director of the Electronics Technology Office at DARPA. He now says that Akustica's first product is the first single-chip microphone in the world to use CMOS.

"We have set ourselves apart by becoming the first company to take advantage of standard CMOS processes!most other MEMS applications use chip processes that have had to be customized for that particular product," said Gabriel, who is Akustica's chairman and chief technology officer. "We are a fabless MEMS company, but our designs can use any CMOS line anywhere. Consequently, we have robustness, lots of second sources and, thus, lots of capacity."

Akustica may need that capacity!that is, if analysts are right about the size of the market it plans to serve. From cellphones to VoIP to PDAs and PCs, industry analysts variously forecast that the microphone market could grow to nearly 1 billion units during the next decade from at least 82 million units in 2005.

For instance, Yole Development claims that the silicon microphone market was nearly 100 million units last year and will grow to 800 million units by 2010. "The winning companies deliver devices with small size and easier integration, plus they leverage the benefits of the semiconductor infrastructure in terms of manufacturing and cost structure," said Jean-Christophe Eloy, founder of Yole Development. "Companies such as Akustica are using this model to pave the way to high-volume applications."

The only other MEMS-based microphones in the market today were designed by Knowles Electronics LLC and Sonion MEMS A/S. Knowles has sold more than 82 million units of its analog MEMS mic, and Steve Cullen, an independent analyst formerly with In-Stat, said that about 8 percent of them went into high-end cellphones, a figure he predicts will rise to 20 percent this year.

But this is a two-chip solution!one chip uses a proprietary MEMS process and the other is a standard CMOS chip carrying the electronics. Knowles says it has designed a digital-output MEMS microphone that it plans to deliver later this year. It is based on the same proprietary MEMS process as its current analog MEMS microphone, plus a separate chip with the digital electronics.

Akustica's single-chip solution, on the other hand, is cast entirely in standard CMOS, requiring only a single post-CMOS processing step to free up its integral microphone diaphragm. The company said the mic has already been proven out at nine foundries using 11 different design rules!everything from an inexpensive 0.6?m process with an aluminum interconnect to a high-performance 0.18?m process using a copper interconnect.

"Our technology can be mass-produced in high volumes with the high yields and repeatability of standard CMOS IC manufacturing," said Davin Yuknis, VP of marketing at Akustica.

With the goal of hundreds of millions of units in mind, Akustica has priced its MEMS microphone at less than $4 each for 1,000 units, but prices are expected to go as low as $1.50 in high volumes. The low prices signal Akustica's aim to capture a lion's share of the near-billion-unit market looming ahead.

Akustica's design also incorporates the preamplifier and ADC on the same chip. The AKU2000 packs a fourth-order sigma-delta modulator, the output of which is a pulse-density modulated single-bit digital-output stream that is insensitive to RF and EMI. By saving the time and expense of conditioning an analog audio signal from a traditional mic and converting it to a digital format, Akustica claims to lower overall costs and eliminate noise sources as well as the need for shielded cable to route analog signals.

Analysts think Akustica may have a winning combination of price, performance and ease of integration. Because its microphone has a digital output, it will probably be designed first into existing digital appliances, from PCs to PDAs and Bluetooth headsets, and, eventually, into cellphones.

"The incorporation of a delta-sigma ADC on a single-chip microphone is both clever and limiting," said Stephan Ohr, research director of analog semiconductors at Gartner Dataquest. "In principle, Akustica should have some ready-made slots in those space-constrained applications that rely on digitized voice and audio. Cellphones, VoIP and Bluetooth headsets come readily to mind. Bluetooth headsets, for example, shipped over 140 million units in 2005 and will be more than half the cellphone market!583 million units!by 2009."

But despite the potential, Ohr said, "Akustica's success won't be immediate." Startups may embrace a new microphone technology, "since they have little to lose. But the Nokias, the Motorolas and Philips will be slower to move on something like this. Today, they pay only about 95 cents for an electret diaphragm mic, and just rely on the voice codec that is already on their baseband processor to clean up the signal." Akustica, he said, "has to convince these manufacturers that an integrated MEMS device offers a better way to design their voice-processing chain, and that could take a few years."

But by 2010, most analysts agree that Akustica and other MEMS microphones with digital outputs will be integrated not only into PCs and PDAs, but also into most cellphones.

"The most pertinent market for Akustica is obviously cellphones!a market that's closing in on a billion units per year," said Marlene Bourne, president and principal analyst at Bourne Research LLC. "But it will take a number of years for MEMS microphones to find their way into most cellphones."

A secondary category, she said, is PCs, "particularly given the growth of VoIP!suddenly, microphone quality is that much more important. Those are the key market segments. Of course, there are a myriad others, but none with the volume potential that PCs and cellphones offer."

PC microphones are currently using a four-component solution!namely, a miniature electret condenser microphone, a discrete FET, a separate operational preamplifier chip and an ADC chip. The idea of condensing all four functions into a single digital-output MEMS microphone chip holds an obvious allure. Akustica's Yuknis said that AKU2000 samples are currently in the hands of all major laptop PC, PDA and Bluetooth headset manufacturers, and that Akustica will start to announce design wins later this year.

"The overall market is in the 1 billion unit-per-year range, when you combine subsegments such as PCs and cellphone handsets," said Dean McCarron at Mercury Research. "Since virtually all of these microphones ultimately attach to ADCs, I think it's reasonable to say that the entire market is potentially available to them. But the best fit at the moment is notebook PCs, due to Akustica's ability to deal with harsh acoustic- and electrical-noise environments."

Analyst Cullen offered a two-phase analysis of what may be in store for Akustica, Knowles Electronics, Sonion MEMS and other microphone chip makers in the coming years. "Phase one of the MEMS microphone market was driven by Knowles with its analog MEMS microphone," Cullen said. "Akustica's digital MEMS microphone kicks off phase two, which will add more diverse and higher-value-added markets. One application that comes to mind is VoIP, which will benefit from higher-quality microphones for notebook and desktop PCs."

Ultimately, Cullen said, "the integration of MEMS and ICs is necessary for the long-term commercial success of MEMS. A product like this that not only integrates them, but actually makes the MEMS device compatible with standard CMOS processing is a big step forward for the MEMS industry."

- R. Colin Johnson
EE Times

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