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Tiny MEMS mic increases handset options

Posted: 01 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:analog MEMS microphone? digital mic? single-chip MEMS?

Akustica Inc. has unveiled what it claims to be the world's smallest microphone. The 1mm? mic uses a MEMS diaphragm and on-chip complementary CMOS analog circuitry. The integrated chip occupies about 25 percent of the die area of competing two-chip MEMS microphones.

"The small size of Akustica's analog MEMS microphone makes it very well-suited for cellphones," said Tony Massimini, chief of technology at Semico Research Corp. "And it makes sense to offer it as a die only. After all, the packaging will take up more room than this extremely small die, so offering it as a die only will allow Akustica's customers to either mount it right on their board or flexible substrate, or to do their own special multichip packaging."

Competing analog MEMS microphone solutions from companies such as Knowles Acoustics Inc. are two-chip solutions. Knowles, for instance, mounts its separate MEMS diaphragm chip, which measures about 2.5mm?, alongside an ASIC. Wire bonding feeds the diaphragm's raw output to the ASIC. The two-chip solution measures 4.72mm x 3.76mm in a package that's 17.75mm?.

"Other MEMS microphones have to be packaged, which makes them much, much larger," said Marlene Bourne, principal analyst at Bourne Research LLC. "But Akustica's microphone is a CMOS chip that can be mounted onto a PCB without the need for a traditional package, or it can be integrated into a package with the other ASICs used in a cellphone or other small electronic device."

Akustica's previous single-chip MEMS microphone was larger in a package that measured 4mm x 4mm, because it was digital and thus needed to accommodate its built-in ADC circuitry. The new design discarded the digital circuits to save die space. "We have been shrinking the analog portion down to the point that we can now announce the world's smallest analog microphone," said Akustica co-founder Ken Gabriel.

So far, all of Akustica's announced design wins!notably, laptop computers from Gateway Inc. and Fujitsu Ltd!have been for its digital microphones. Now, the Pittsburgh company also hopes to penetrate the analog microphone market.

Today, Knowles Acoustics has several design wins in the billion-unit cellphone market. With the introduction of Akustica's analog microphone, cellphone makers have a second source for MEMS microphones. Sonion MEMS A/S is also designing MEMS microphones but has yet to deliver production units to any customers, according to analyst Bourne. Infineon Technologies AG has also announced a MEMS microphone aimed at cellphone makers, but it has yet to score any design wins.

"Knowles Acoustics was several years ahead in analog MEMS microphones, but they had a hard time convincing cellphone makers to design-in their part because there were no second sources available," said Bourne. "So we see Akustica's entry into the market as being a help to both companies, since cellphone makers will now feel more comfortable designing-in a MEMS microphone with two sources to buy them from.

"Since cellphones are at the billion-unit levels, there is plenty of room in the market for Akustica, Knowles, Sonion and Infineon," Bourne said.

Avoiding noise
Although Knowles, Sonion and Infineon promote two-chip MEMS solutions as a more cost-effective option, Akustica counters that its much smaller die will be cost-competitive with any two-chip offering. In addition, it can avoid noise problems that plague the tightly packed circuitry of devices such as cellphones, the company said. In particular, switching transients can be amplified by the ASIC along with the signal from the MEMS diaphragm.

"For mobile-phone applications, our single-chip solution is much more immune to the sources of EMI that can be picked up by two-chip solutions, especially those that use wire bonding between chips!those little wires act like antennas to pick up noise," said Gabriel.

Two-chip MEMS makers also note that they can customize their second chip!the ASIC containing the CMOS circuitry!to accommodate the specific requirements of different devices. But Akustica argues that because its entire chip is CMOS, including the MEMS element, it can customize the whole chip.

"We believe that many designers using analog microphones today will be able to use our 1mm? microphone the way it is off the shelf," said Gabriel. "But since it's just another CMOS chip, it takes us no longer to modify its design for specific customers' needs than it takes to redesign an ASIC for two-chip solutions."

Currently, X-FAB Semiconductor Foundries AG makes Akustica's CMOS chips. Dalsa Corp. performs a final step that removes substrate material to free up the mechanical elements, thereby transforming them into CMOS MEMS chips.

- R. Colin Johnson
EE Times




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