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Cheaper MEMS Microphones

A new way to build small microphones may be a boon for VoIP and laptops users.

Microprocessors and memory chips have been shrinking, as their performance has increased. But, for the most part, microphone technology has lagged behind. Now a Pittsburgh-based company, Akustica, is helping to bring those devices up to speed, too, by building microphones directly into integrated circuits, using a process that’s almost identical to the way chip makers mass-produce microprocessors.

The fabrication of a microphone into a chip will lead to smaller, less expensive microphones that could offer better-quality audio recorders for cell phones, cameras, and voice-over-Internet protocol (VoIP) calls, says Ken Gabriel, Akustica’s cofounder and chief technology officer.

For years, researchers have built small microphones and other devices with mechanical and electric components – gyroscopes, accelerometers, pressure sensors – using semiconductor materials. These devices are known as MEMS (for microelectrical-mechanical systems). Manufacturers of MEMS microphones make the devices by depositing layers of silicon. Then, most of them complete a separate step: bonding the microphone to a chip that contains circuitry to manipulate sound once it’s collected.

Gabriel believes his company’s approach simplifies the process by fabricating the microphone and circuitry on the same chip, at the same time, and using the same materials.

The MEMS microphone market is growing, especially for laptops, because an increasing number of people use VoIP services on their laptops, says Gabriel. And, he maintains, the traditional non-MEMS microphone in most laptops isn’t ideal for VoIP.

These conventional systems are made of Electret Condenser Microphones, or ECMs, as well as wiring and circuitry that transport the audio signal and process it. An ECM captures sound in its analogue form. This analogue signal is sent to circuitry that converts it to digital information. Along the way, however, the analogue signal is subject to interference from electric fields produced by a computer’s monitor and Wi-Fi antenna. In order to avoid degraded sound quality, an ECM must be insulated from these fields. This insulation makes the system bulky and limits its placement in a laptop; ECMs are often found near the keyboard, an awkward place to direct conversation when using VoIP or video conferencing.

Akustica’s system is more compact than traditional microphone systems because it captures sound and converts it to a digital signal on the same chip. When sound waves hit the microphone’s membrane – a thin metal mesh in the middle of the chip – it vibrates, producing a voltage that contains information about the analogue sound signal. But since the analogue signal is produced and converted to a digital signal on the same chip, it never has to experience the harsh electromagnetic environment outside the circuit. And, because interference is less of an issue, insulation isn’t needed. This allows engineers to place the microphone anywhere that a chip can fit into a laptop; multiple Akustica microphones can even fit in the bezel surrounding a laptop’s monitor Gabriel says.

To build its microphones, the company has partnered with facilities that fabricate other types of chips. As with other chips, Akustica’s are made of layers of semiconductor materials, insulating materials, and metals. To add a microphone, the engineers modified the design of a few layers to make a membrane out of the same metal used in the circuitry. After this, Gabriel says, the chip is post-processed, which includes steps to etch away some of the silicon on the chip to reveal the buried metal membrane. This final processing, he adds, can be done inexpensively and quickly.

Because it’s relatively inexpensive to mass-produce semiconductors, the Akustica technique “could become the lowest-cost method to building a microphone,” says Jonathan Bernstein, a researcher at Cambridge, MA-based Draper Laboratory, which works with small microphone systems. However, he adds, there is a tradeoff when using a semiconductor process to make a device like a microphone. A potentially higher-quality acoustic system could be made, he says, if the design weren’t limited to the same materials used in the circuit.

And while integrating a microphone into a circuit could be advantageous for some applications, it may not be necessary for others, says Levent Degertekin, professor of mechanical engineering at the Georgia Institute of Technology in Atlanta. “Hearing-aid manufacturers don’t want those capabilities built in,” he says, because they have their own digital circuitry that works well. Instead, he says, the best application right now for single-chip microphones is the laptop market.

Akustica, which spun out of a Carnegie Mellon University research project in 2001, began shipping its first microphone earlier this year. The device is currently used in some Fujitsu laptops, and last month the company announced a second product combining two microphones on one wire to capture more sound. It is expected to appear in products by year-end.

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