Design, fabrication, and characterization of a MEMS dual-backplate capacitive microphone

A microphone is an instrument that measures an acoustic signal and generates an electrical output. Microphones have many common applications ranging from use in cellular phones and computers to high quality studio microphones for music recording. However, there is a less familiar application for microphones: microphones are utilized by commercial aircraft manufacturers to assist in the development of quiet aircraft. Communities surrounding airports object to the loud noises produced by approaching and departing aircraft. Therefore, strict regulations exist to limit the noise radiated by commercial aircraft. To reduce the noise radiation of airframes and jet engines, aircraft manufacturers perform rigorous testing during the development and qualification of their products. The microphones used for these measurements have specifications that differ greatly from a common audio microphone. The industry relies on expensive non-MEMS microphones for aeroacoustic measurements. To date, there have been many MEMS microphones developed; some are even successful commercial products. However, the majority are targeted for audio applications. The existing aeroacoustic MEMS microphones show promise; however, the performance must be improved to compete with existing non-MEMS microphones. The goal of this research is to design a MEMS-based microphone suitable for aeroacoustic measurements, while improving on the performance of existing devices. This study details a thorough review of previous MEMS microphones and identifies which are most suitable for aeroacoustic measurements. Furthermore, the specific opportunities for improvement are discussed. A thorough development of the theory of operation for capacitive microphones is presented. Using this theoretical framework, the design of an aeroacoustic capacitive MEMS microphone is presented. The microphone is fabricated using the SUMMiT V process at Sandia National Laboratories. Multiple microphones are tested and the results indicate the designed microphone compares favorably to previous aeroacoustic MEMS microphones.

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