A Chip with Ears

Crack open a cell phone and you’ll find that, while most of the circuitry has been shrunk down and integrated onto silicon chips, the microphone is still a separate device about the size of a watch battery. A Danish electronics manufacturing consortium, Sound Holding, hopes to bring microphones into the computer age, carving them from silicon using the same techniques that make today’s transistors so small and precise.

Materials scientist Matthias Mllenborn and his team at Sound Holding create the sound-sensing component of the microphone by etching a rectangular hollow beneath a silicon chip’s surface, leaving a 500-nanometer-thick membrane of silicon on top. Sound waves hitting the membrane cause it to vibrate, generating an electrical signal that travels to an adjacent chip for processing. The team cuts manufacturing costs by using a commercially available processing chip and gluing it side by side with the sound sensor on a bigger slice of silicon. The whole package is five cubic millimeters, one-tenth the size of a cell phone microphone.

Engineers have been working on silicon microphones for almost a decade, but manufacturing difficulties have hurt the devices’ performance, preventing them from reaching the market. Sound Holding’s manufacturing method should solve that problem, but observers are not sure it will be a commercial home run. “If only performance and size counts, Sound Holding’s approach is good,” says Robert Aigner, an electrical engineer at Munich, Germany’s Infineon Technologies, which also develops silicon microphones. “The challenge will be to compete with the low prices of conventional microphones.” Sound Holding is pushing ahead, though. The consortium is equipping cell phones with prototypes and plans to start commercial production in 2003.

Thanks to automated production techniques, silicon microphones should be incredibly uniform-and outperform existing microphones, Mllenborn says. Their precision could also allow for arrays of sensors capable of determining where sounds are coming from to reduce background noise in cell phone conversations. Eventually, the minute microphones could help in everything from making hearing aids more sensitive and streamlined to monitoring the sounds of aircraft engines.