Akustica packs analog MEMS mic in 1 square mm

Akustica Inc. today will unveil what it says is the world's smallest microphone. The 1-mm² mic uses a microelectromechanical-systems 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 cell phones," said Tony Massimini, chief of technology at Semico Research Corp. (Phoenix). "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 (Itasca, Ill.) are two-chip solutions. Knowles, for instance, mounts its separate MEMS diaphragm chip, which measures about 2.5 mm², alongside an ASIC. Wire bonding feeds the diaphragm's raw output to the ASIC. The two-chip solution measures 4.72 x 3.76 mm in a package that's 17.75 mm².

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

Akustica's previous single-chip MEMS microphone was larger, in a package that measured 4 x 4 mm, because it was digital and thus needed to accommodate its built-in analog-to-digital converter circuitry. The new design jettisoned 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 and Fujitsu--have been for its digital microphones. Now the Pittsburgh company hopes to penetrate the analog microphone market as well.

Today Knowles Acoustics has several design wins in the billion-unit cell phone market. With the introduction of Akustica's analog microphone, mobile-phone makers have a second source for MEMS microphones. Sonion MEMS A/S (Roskilde, Denmark) is also designing MEMS microphones, but has yet to deliver production units to any customers, according to analyst Bourne. Infineon Tech- nologies has also announced a MEMS microphone aimed at cell phone makers, but has yet to announce any design wins.

"Knowles Acoustics was several years ahead in analog MEMS microphones, but they had a hard time convincing cell phone 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 now cell phone makers will feel more comfortable designing-in a MEMS microphone with two sources to buy them from.

"Since cell phones 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 like cell phones, 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 electromagnetic interference that can be picked up by two-chip solutions, especially those which 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 1-mm² 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."

Akustica already has an unnamed customer for its microphone, according to analysts, but the company has not yet made a formal announcement of the design win.

Currently, X-FAB Semiconductor Foundries 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.