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MEMS mics moving into mainstream

Micro-electromechanical system (MEMS) microphones were once the darlings of startups, but no more. Analog Devices (Norwood, Mass.) has announced a major thrust into the fledgling MEMS microphone market, predicted by Yole Development (Lyon, France) to sell 800 million units by 2010.

Analog Devices' entry into this potentially high-volume market follows Infineon Technologies' entry last year, though neither company has yet reported shipping production units.

“From a market perspective, Analog Devices' entry, following Infineon's entry last year, gives very high-volume customers like cell-phone makers two sources of MEMS microphones from companies that are used to supplying these kinds of high volumes,” said Marlene Bourne, founder of Bourne Research (Scottsdale, Ariz.).

“After all, the electret microphones that MEMS mics will replace in mobile phones are being delivered in the billions of units per year–those buyers will have an innate comfort level in dealing with companies like Analog Devices and Infineon.”

Two years ago, the startup Akustica (Pittsburgh) announced the world's first MEMS microphone. The company took an innovative approach using a single CMOS chip that surrounds the mechanical microphone diaphragm with its supporting electronic circuitry—including an analog-to-digital converter—enabling Akustica's digital microphone to eliminate analog noise sources and leverage CMOS economies of scale, which reduce die costs as design rules shrink.

A few months later, hearing-aid specialists Knowles Electronics (Itasca, Ill.) announced a two-chip analog MEMS microphone—one for the mechanical diaphragm wire—bonded to a second application-specific integrated circuit (ASIC) in the same package.

Shortly thereafter, another startup, Sonion MEMS A/S (originally in Roskilde, Denmark, but now part of Technitrol, Trevose, Penn.), announced a two-chip MEMS microphone.

So far, Akustica and Knowles are the only MEMS microphone makers reporting volume shipments. Akustica has been shipping its digital MEMS microphone to laptop makers, whose circuitry was already digital, while Knowles has sold its MEMS microphone into applications that required an analog MEMS mic.

Since then Akustica has announced a single-chip analog MEMS mic that downsized its die to just 1 square millimeter, and Knowles announced a two-chip digital MEMS mic that includes an analog-to-digital converter on its ASIC.

In hopes of luring high-volume cell phone makers into switching from electrets to MEMS mics with the assurance of two sources, Knowles and Akustica have also announced a cross-licensing agreement.

“Analog Devices' entry to the market will put significant pressure on Knowles and Akustica,” said Bourne. “Analog Devices and Infineon are sure to snap away significant market share as they will probably become the preferred first and second sources, with the smaller startups left to focus on niche and mid-level applications.”

Despite the tough talk from analysts about a small company's inability to manufacture in the hundreds-of-millions unit quantities that market research companies predict to be coming for MEMS microphones in the next few years, Akustica, for one, remains gung ho.

“At the end of the day, the market is so big–with billions of units up for grabs in the next five years–there needs to be more competitors out there to meet all that demand,” said Davin Yuknis, vice president of marketing and product management at Akustica. “And as far as we know we are still the only supplier with a single-chip solution, so we plan to take the high road, and not just sell commodities.”

Analog Devices' entry into the MEMS microphone market includes both analog and digital versions, using a two-chip solution housed in the same package. Common to the two versions is the MEMS microphone chip, but different ASICs are wire-bonded to the MEMS chips in the same package so OEMs can choose either analog or digital outputs.

A unique aspect of Analog Devices' design is a particle filter constructed by perforating a silicon cover over the microphone's diaphragm. The particle filter uses deep holes just 6 microns wide but 10 µm deep. The deep holes in this perforated cover are exceptionally effective at keeping dust and other contaminants from reaching the delicate MEMS diaphragm, according to Analog Devices.

“From an engineering perspective, the most unique aspect of Analog Devices' MEMS microphone is this filter that they have put over the microphone's diaphragm to keep particles from effecting its performance,” said Bourne.

Analog Devices also claims that by delaying its entry into the market by two years, compared with Akustica and Knowles, it has had time to craft a MEMS microphone with exceptional specifications. Analog Devices claims a wide frequency response, low noise level and high immunity to the mechanical and sound-pressure shocks often experienced by mobile phones and similar handheld consumer devices.

“We have the world highest signal-to-noise ratio in the industry [61 dm a-weighted], the highest immunity to mechanical and sound-pressure shocks [20 kG and 160 dB, respectively] and the highest power-supply noise rejection [50 dB and 80 dB for analog and digital versions, respectively],” said Alex Khenkin, senior acoustics engineer at Analog Devices.

Because the particle filter protects the MEMS membrane, the entire inside of the chip package serves as the microphone's back-volume, rather than just using the small volume behind the MEMS chip itself, as in designs without a particle filter. The larger back-volume enables a flat frequency response–from 100 Hz to 15,000 Hz–without the typical midrange peak, according to the company.

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