Spurred on by advances in MEMS CMOS processing, a low-cost, tiny single-chip microphone with high acoustic quality has moved from fantasy to reality. According to developer Akustica, it's the industry's first single-chip CMOS MEMS microphone.
The chip replaces common electretcondensermicrophone (ECM) units, a technology that's remained fundamentally unchanged for 50 years. ECMs are mechanical devices with size, manufacturability, and uniformity limitations.
"We're not only mainstreaming MEMS devices with our microphone chip, we are also helping manufacturers of laptop PCs, mobile phones, and other digital media devices to overcome the acoustic problems that have seriously limited the widespread adoption of Voice over Internet Protocol (VoIP) and other voice-based applications in the past," says Jim Rock, cofounder and CEO of fabless semiconductor manufacturer Akustica.
Microphone audio acoustics play a vital role in consumer electronics. Microsoft's Vista operating system is designed for voice-enabled applications and microphone array support. Intel's high-definition audio chip set enables enhanced voice capture through array microphones, giving users more accurate speech input. And, Skype and other VoIP applications require better acoustic performance.
"Voice input quality has never been more important," says Marcie Weinstein, Akustica's director of product marketing. She sees the industry converging behind voice-enabled applications.
FIRST IN THE FAMILY
The first MEMS microphone to arrive is the digital-output AKU2000 (Fig. 1). The surface-mountable chip comes in a 4- by 4- by 1.8-mm package. It features less than 10% total harmonic distortion at 115 dB of sound pressure level.
The AKU2000 includes the MEMS microphone, a preamplifier, and a fourth-order, 14-bit, sigma-delta modulator (Fig. 2). It's optimally designed for use in microphone array applications, which require a high level of noise immunity.
The digital chip's output is pulse-density modulated, producing a single-bit digital stream that can be decimated by a digital filter in downconversion electronics like audio codecs, DSPs, or baseband circuits for a high degree of design flexibility and freedom.
The AKU2000's design enables the use of a digital-bus architecture for the audio system of a variety of electronic devices. This not only simplifies system design, it also expedites the time-to-market.
The chip is claimed to offer 10 × lower parasitics than other-microphones. It's immune to radio-frequency-interference and electromagnetic-interference signal effects (Fig. 3). Also, it operates from 2.8 to 3.6 V. It consumes less than 75 μA in the power-down mode. In addition, it features a clock frequency of 1 to 4 MHz and a signal-to-noise ratio of 55 dB. Available in sample quantities, the AKU2000 costs $3.97 each in 1000-piece quantities.
Akustica's CMOS MEMS microphones are fabricated from the metal dielectric layers of the CMOS process and then deposited during the standard process flow. This differs markedly from other CMOS MEMS processes, where devices are fabricated in films on top of the CMOS chip and the transducer is made from proprietary materials.
"Our chip design is not just CMOS-compatible, it is made on a 'standard' CMOS process," says Kaigham (Ken) Gabriel, Akustica chairman and CTO. "The MEMS honeycomb structure we use for audio sensing is a metallization layer in the standard CMOS process."
Akustica's approach lets system engineers integrate multiple sensors and associated electronics into one common platform, instead of building each circuit discretely using a variety of process technologies and packages. The approach also eliminates chip-to-chip internal wire bonds on a traditional CMOS microphone. These bonds otherwise could contribute parasitics and decrease performance.
A GROWING FUTURE FOR CMOS MEMS
The overall microphone market comprises 1.5 billion units annually, most of which are ECMs. However, CMOS microphones continue to penetrate this market: Forecasts show growth from 32 million units in 2005 to 518 million units by 2008, with their share of the market increasing as well (Fig. 4).
Akustica foresees a vast market not only for its silicon microphones, but also for CMOS MEMS sensor technology across the board. It trademarked the term "Sensory Silicon" to describe a platform upon which monolithic analog and digital sensory systems will be built to hear, speak, and sense the world around them.
The company envisions CMOS MEMS that can deliver accelerometers, gyroscopes, RF components, signal-processing ICs, and microphones for voiceinput chips, hybrid sensors, acoustic portals to the wireless Internet, and smart microphone arrays.