Akustica's most original designs, since its first models in 2006, used a proprietary CMOS process to fabricate the MEMS microphone diaphragm on the same chip alongside the CMOS signal processing circuitry and analog-to-digital (A/D) converter. This patented technology, originating at nearby Carnegie Mellon University, enabled Akustica to deliver the worlds smallest MEMS microphone die. However, since its acquisition by Bosch in 2009, Akustica took advantage of the surface micro machining expertise and patent portfolio of its parent company. Using Bosch's process the MEMS diaphragm is fabricated in polysilicon on top of a silicon substrate using¬†deposition and sacrificial etching to release the mechanical diaphragm. The ASIC is then fabricated using standard CMOS manufacturing techniques¬†on a separate silicon die then wire-bonded to the MEMS die in the same package. As a result, the packages are slightly larger, but the ability to mix-and-match MEMS diaphragms with ASICs cuts costs and allows a bigger catalog to be built.
— R. Colin Johnson is the Advanced Technology Editor at EE Times, where he has written about MEMS microphones for over 10 years; in the early days of MEMS mics, his colleagues were baffled by his fascination with them, but his interest has since been vindicated.