Speaking of small …

When Alexander Graham Bell called Mr. Watson one Friday back in March 1876, he probably wanted to celebrate the invention of the telephone. But Bell’s first phone call should have been to his team of patent attorneys so they could begin preparing for the decades-long legal battles that followed.

Though this digression into telephone history may seem off topic, hold the phone. You see, Bell’s breakthrough led another American inventor, Emile Berliner, to come up with a better telephone transmitter—or, as history would have it, the first microphone. The American Bell Telephone Co. was paying attention and quickly purchased the rights to Berliner’s patent.

While microphones still come in all shapes and sizes for a wide variety of applications and needs, they too have done well in the march toward miniaturization. The Akustica Inc. MEMS microphone shown is 1mm x 1mm x 0.4mm. Silicon die image courtesy Akustica Inc.

Shortly thereafter, Thomas Edison invented the carbon microphone, which Bell Telephone would eventually own as well. Of course, carbon microphone technology, which was used in phones for the next 100 years or so, didn’t escape a patent battle of its own. Unlike the telephone patent challenges that continued in one form or another until 1897, the microphone battle pitting Bell and Berliner against Edison was resolved in Edison’s favor by 1892.

So much for my childhood memories of who invented what. Yet if this not-so-nostalgic trip into microphone history teaches us anything, it is that many people contribute to such inventions, and that technology progresses despite whatever legal battles may ensue.

Indeed, microphone patent disputes have managed to span more than a century of technological advances—from the condenser microphone in 1916 to the electret condenser microphone (ECM) in 1962 to the MEMS-based microphones that have evolved over the past two decades. Today, though, patent lawsuits surround the microelectromechanical processes involved in producing the millions of tiny MEMS-based microphones used in cell phones, smart phones, personal computers and laptops.

The legal turf battles are understandable, given that the market for MEMS microphones is forecast to grow from just under 700 million units shipped in 2010 to more than 1.7 billion units in 2014, according to iSuppli Corp., a market research firm based in El Segundo, Calif. That’s not a particularly surprising forecast considering smart phones began incorporating two MEMS microphones in 2010, in a move to suppress background noise, and are expected to incorporate even more such microphones by 2014.

“Although noise suppression has been available since 2006, the arrival of Motorola Inc.’s Droid as well as the iPhone 4 (from Apple) has caused the popularity of the technology—and of MEMS microphones—to soar,” iSuppli observed in a report accompanying its latest forecast.

What’s more, iSuppli noted that MEMS microphone makers should find it easier to compete with Knowles Electronics LLC, Itasca, Ill., thanks to a Nov. 22, 2010, U.S. International Trade Commission ruling that the company’s silicon microphone patents were invalid. Knowles, which is credited with successfully commercializing MEMS microphones in 2003, remains the category leader, and accounted for more than 80 percent of the devices shipped worldwide in 2010, according to iSuppli.

While the ITC ruling is sure to be welcome news for MEMS microphone competitors such as Analog Devices Inc., Norwood, Mass., this and similar legal challenges have done little to impede the technological progress of the MEMS microphone—particularly where size is concerned.

In fact, from 2003 to 2008 MEMS-based analog microphones dropped in size from 4mm × 6mm to about 4mm × 4mm. Then in 2008, Akustica Inc., a MEMS microphone maker based in Pittsburgh, rolled out the world’s smallest MEMS microphone at 2mm × 2mm.

These dimensions, however, refer to the overall MEMS microphone package that must adhere to standard microphone footprints, which shrink more slowly than the die inside, observed Dr. Marcie G. Weinstein, marketing strategist at Akustica.

The size of Akustica’s analog microphone die measures just 1mm × 1mm and about 0.4mm thick—a feat owing directly to an acoustic MEMS technology for which Akustica holds the exclusive patent rights. The technology allows Akustica to fabricate both the transducer and electronics in a single chip using standard complementary-metal-oxide-semiconductor (CMOS) processes.

In short, Akustica’s die “is very tiny because it is just the [moving] membrane and the circuitry on a single chip,” as opposed to one chip for the microphone and a second for the circuitry, said Weinstein.

“In the future,” she continued, “I see the size of the MEMS microphone, from an overall package point of view, as getting even smaller.” The largest part of the chipset in an analog microphone is the analog portion, she noted.

Though Akustica’s digital-output MEMS microphone package has a footprint of 3.76mm × 4.72mm, that has more to do with the industry standard than necessity. Inside the digital-output microphone is a die that is 1mm × 1mm × 0.4mm. Using the same CMOS MEMS technology, Akustica integrates an acoustic transducer, an analog amplifier and a modulator on a single chip.

“Ultimately, we believe that the industry will drive toward microphones that approach the size of the chip itself,” Weinstein suggested. “With our single-chip technology, we’ll be in the optimal position to address this market.”

Though it will take some time, Weinstein also forecast a day when digital MEMS microphones outnumber analog devices in consumer electronics. For the moment, digital MEMS microphones are mostly confined to the laptop computer market.

Computer makers wanted to move the analog microphones away from the motherboard and into the screen area of laptops. Radio-frequency interference, however, would have wreaked havoc with analog signal traces, which meant they would have had to be shielded, and that would have made them too large to fit through the hinge of the laptop.

The digital-output MEMS microphone, on the other hand, isn’t susceptible to RF interference, noted Weinstein, so that solved a huge problem for laptop makers, which began incorporating digital MEMS microphones in 2006. And the fact that two digital MEMS microphones can share the same trace was good news for laptop makers, which are now incorporating multiple digital MEMS microphones for even better voice quality communication.

Whether analog or digital, the future of MEMS microphones sounds pretty good. µ

About the author: Dennis Spaeth is electronic media editor for MICROmanufacturing and Cutting Tool Engineering. Telephone: (847) 714-0176. E-mail: dspaeth@jwr.com.