What’s that rumbling? It’s the ever-loudening boom expected to stand the silicon microelectromechanical systems (MEMS) microphone market on its collective ears. Forecasters say the market explosion will really unfold after 2009.
Two years ago, only three or four dominant silicon MEMS microphone manufacturers existed. Knowles Acoustics had the lion’s share, followed by Akustica, Pulse Engineering’s Sonion MEMS Division, and Infineon Technologies. Now the list includes at least a dozen others (see “Representative Silicon MEMS Microphones” at www.electronicdesign.com, ED Online 20667).
They’re all preparing design and marketing plans for a major push into this field. In fact, many see it as the next “commodity” MEMS IC market with rapid growth, after accelerometers (see “Those Elusive MEMS Market Figures,” ED Online 20665).
In addition to their wide use in mobile phones and notebooks, scads of new applications for these microphones will soon emerge. These include handsets, headsets, voice recorders, camcorders, laptops for Voice-over-IP (VoIP) uses, digital cameras, MP3 players, and interactive games. Voice activation for a wide range of consumer electronic products is seen as a lucrative market. The automotive field offers potential growth in hands-free communication and navigation devices, too.
Established companies like Analog Devices, Freescale Semiconductor, NTT Dokomo, NXP Semiconductors, Omron Semiconductors, Panasonic, and STMicroelectronics—particularly those with expertise in both audio and MEMS technologies—are poised to grab a good share of the silicon MEMS microphone market. Others like GoerTek Acoustics are typical of the rising interest by China-based companies to compete in this market. Many of these firms have pre-announced some design details in anticipation of new product market introduction.
Electret condenser microphones (ECMs) are a much less expensive option than silicon MEMS types like those sold by Japan’s Hosiden Corp. The well-established ECMs use a simple structure consisting of a capacitive sensing plate and a field-effect transistor (FET) (Fig. 1).
The average selling price of silicon MEMS microphones is $1.50 to $2.00, depending on order volume and performance requirements. This is about three times greater than ECM prices. But silicon MEMS microphones offer several advantages, such as smaller size and greater integration capabilities, which enables greater design innovation. They also consume much less power (about one-half the current drains of ECMs).
Silicon MEMS microphones also offer greater immunity to radio-frequency interference (RFI) and electromagnetic interference (EMI), and they can withstand the high temperatures of a surface-mount technology (SMT) process. Although ECM manufacturers claim their designs can withstand the high temperatures of an SMT reflow process, silicon microphone makers contend that such products suffer from a shift in their performance characteristics due to the organic materials used in their construction.
Because they can be made on a batch-fabrication CMOS process, it’s possible to make silicon MEMS microphones much less expensively (even cheaper than ECMs) once the market opens up and mass production goes full throttle.
SINGLE CHIP OR MULTICHIP?
Most manufacturers use two chips in their microphone design— one for the transducer structure and one (usually an ASIC) for signal- conditioning electronics. Exceptions are Akustica, which uses a single-chip design for its CMOS microphone, and the three-chip approach taken by Pulse Engineering’s Sonion MEMS Division. Each has its advantages and disadvantages.
The single-chip approach makes it easier to integrate more functions on the same die at less cost. It’s also more compatible with a standard CMOS process in terms of process flow, packaging, and testing. It has a higher level of reliability due to the absence of wire bonds that interconnect two or more chips as well. On the other hand, it limits design flexibility for meeting different application requirements.
The smallest silicon MEMS chip comes from Akustica. Measuring 1 by 1 mm, it forms the basis of both the analog output AKU1126 Napoli and the AKU2202C Shadyside microphones (Fig. 2). Its size makes it possible to use multiple microphones, rather than just one, to provide ambient noise cancellation in products such as Bluetooth headsets or cellular handsets.
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