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issue: September 2006 APPLIANCE Magazine

Electronics Report
Slim Sound Solution

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A single-chip microphone is said to bring high acoustic performance to laptops, mobile phones and digital video and still cameras.

Using micro-electromechanical systems (MEMS) technology, Akustica claims to have created the world’s first single-chip microphone. Besides addressing the space constraints and cost challenges consumer electronics engineers face, the microphone reportedly offers higher acoustical performance than possible with previous technologies.
According to Dr. Kaigham J. Gabriel, co-founder, chairman and chief technology officer of Pittsburgh, Pennsylvania, U.S.-based Akustica, the new AKU2000/2001 microphones are essentially “semiconductor chips that can hear.” Said to be the first MEMS devices manufactured using standard Complementary Metal Oxide Semiconductor (CMOS) processes, the microphones integrate an acoustic transducer, output amplifier and a 4th order sigma-delta modulator on a single chip. “The monolithic integration enables the signal to be digitized just microns from the microphone/transducer diaphragm, reducing parasitic capacitance, electrical leakage and susceptibility to radio frequency (RF)/electro-magnetic (EM) interference,” Gabriel explains. “This allows consumer electronics engineers to design in the digital output microphones without the cost of shielded housings or cables, reducing bill of material costs and increasing design and placement flexibility.”
The integration also means a small 4-mm by 4-mm footprint, a key feature for ultra-thin portable electronics. “Because the microphones are monolithic, not only can the individual microphone die be smaller than other approaches, but the final product can ultimately be packaged in a smaller form factor than a multi-chip module,” Gabriel notes.
Cost is reduced using the company’s patented fabrication process, which allows the chips to be manufactured by existing CMOS foundries, rather than by dedicated MEMS foundries. Gabriel says this is another industry first. “Though there are some ‘CMOS-compatible’ processes for fabrication of MEMS devices on top of CMOS wafers, these processes tend to be complicated and expensive,” he says. “However, since Akustica uses the metal-dielectric layers that are native to a standard CMOS wafer flow to form our mechanical structures, and we use industry-standard CMOS equipment for fabrication, we are easily able to manufacture monolithic devices that can be cost-effectively produced in high volumes.”
Perhaps the most important aspect of the new microphones, however, is their performance benefits. The devices are designed to replace the standard electret condenser microphone (ECM), a 50-year-old technology that Gabriel says is susceptible to RF/EM interference. The monolithic design and robust digital output of AKU2000/2001 make them less prone to noise interference, he explains, thereby enabling better acoustical performance.
The surface-mountable microphones are ideal for portable consumer electronics, including voice-enabled applications in notebook PCs and other digital media devices. Gabriel notes that consumer electronics devices must have the proper architecture to use a digital output microphone like the AKU2000/2001. “Today, most applications use an analog microphone, the output of which goes into a compressor-decompressor (CODEC), baseband chip or stand-alone analog-to-digital converter to digitize and process the acoustic signal,” he says. “The CODEC (in the case of most consumer-electronic devices) or baseband chip (in the case of mobile phones) must be specifically designed to accept the digital output…which is a single bit-stream pulse-density-modulated (PDM) output. Once a CODEC that can interface to a digital output microphone has been identified for a particular application, the integration is very standard and straightforward.”

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