Meeting Review, January 10, 2006
This silicone microphone was developed to compete with the Electret Condenser Microphone (ECM) because the technology has potential for low-cost, tightly controlled performance, wide environmental stability, and a surface-mountable package. Fundamentally, it operates in the same manner as traditional condenser microphones, but the components are etched from silicon. In order to keep cost down it must be made small, which in turn constrains the sensitivity, self-noise, dynamic range, and stability.
The diaphragm compliance is the key to performance. The traditional implementation is a thin film stretched on a frame, but mimicking this in silicon has proven to be a difficult engineering challenge. Several researchers have published their results for different approaches. The
SiSonicTM uses a patented free plate approach where the diaphragm is tensioned when the electrostatic force caused by the bias pulls it over several support posts.
Two more components make up the MEMS structure: a highly-perforated back plate that uses a patented approach to minimize out-of-plane bowing, and a substrate for support and to provide the back volume. For manufacturing, surface and bulk micromachining is used in a batch process.
A CMOS circuit provides bias, buffering, and amplification. The two devices are mounted on a printed circuit board, and a wall and cover with an acoustic port completes the package. An automated test system measures frequency response, noise and current. The microphone can be supplied via tape and reel for surface mount manufacturing.
The combination of a familiar audio device with a state-of-the-art manufacturing process and the challenge of making it practical made for an engaging audio story.