Machine Design

Hear this: CMOS microphones on a chip

First comes single-chip microphones in silicon. Next will come a variety of inexpensive sensors made with the same basic integrated circuit technology and in high volumes.

Akustica's CMOS microphone chip is just 2.5 mm2. It is going into PCs and may soon find its way into cell phones and video cameras.

A scanning electron microscope image of the diaphragm on the Akustica microphone chip shows the serpentine pattern created in the chip's metal and oxide layers. The special structure is necessary to compensate for residual stresses caused by the metal etching process. A polymer coating goes over the surface of the chip to seal it from outside elements.

That's the plan of Akustica Inc. in Pittsburgh. Founded by researchers from Carnegie Mellon University five years ago, the company has devised a technology known as Sensory Silicon, which permits the fabrication of a microphoneon-a-chip using ordinary Complementary Metal-Oxide Semiconductor (CMOS) processes. Moreover, transistors and other circuit elements can be defined on the same chip as the microphone, during the same fabrication steps that form the microphonic transducer. This makes Sensory Silicon devices potentially inexpensive compared with similar components made with conventional MEMS ( microelectromechanical systems) techniques. The reason: MEMS transducers typically employ thin-film deposition to create some of their features. The deposition process requires temperatures significantly higher than what the electronics on a CMOS wafer can withstand. So ancillary circuitry for the transducer must reside on substrates separate from that of the MEMS chip.

Akustica's first product, the AKU2000 Microphone Chip, is already going into PCs from Fujitsu and is a candidate for use in other brands of PCs and in cell phones. It is designed as a single-chip replacement for Electret Condenser Microphones (ECM) which use polarized dielectric material as a transducer. ECMs, however, tend to pick up a lot of mechanical and ambient noise. Akustica says its Microphone Chips are less sensitive to such interference and give a better sound quality.

Akustica says its Microphone Chips can be manufactured in quantity, with guaranteed uniformity, by existing CMOS foundries rather than by dedicated MEMS foundries. This, claims the company, gives the devices compelling economics. Other kinds of high-volume transducers could come later, based on the same technology and cost structure.

The CMOS MEMS chip is a surface-mountable, automatic pick-and-place compatible, monolithic device. It integrates an acoustic transducer, output amplifier and a fourth-order sigma-delta modulator on a single chip. Akustica says it is particularly useful in microphone array applications where multiple microphones work together to cancel noise and for beam forming.

Akustica's CMOS MEMS structures are composed of the metal-dielectric structures within a standard CMOS wafer. The diaphragm is etched into the metal layers of the chip surface. But it is tough to control the thickness of the metal exactly, so residual stresses result. Consequently, the company developed a serpentine metal-and-oxide mesh structure for the diaphragm that controls the stress in the membrane. This structure also lets the company design membranes that are either flat or curved. The microphone chip measures 2.5 mm2 and the diaphragm-has a 1-mm diameter.

The company says its CMOS MEMS device fabrication has been proven in nine different foundries and 11 different CMOS technologies, ranging from a 0.6-μm three-metal process to a 0.18-μm copper interconnect process.

Akustica Inc.,

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