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We report an array of piezoelectric monocrystalline silicon microphones for audio-range acoustic sensing. Thirteen cantilever-type diaphragm transducers make up the array, each having a closely spaced and precisely controlled resonant frequency. These overlapping resonances serve to greatly boost the sensitivity of the array when the signals are added; if the signals are individually taken, the array acts as a physical filter bank with a quality factor over 40. Such filtering would enhance the performance and the efficiency of speech-recognition systems. In the “summing mode,” the array demonstrates high response over a large bandwidth, with unamplified sensitivity greater than 2.5 mV/Pa from 240 to 6.5 kHz. Both modes of operation rely on the precise control of resonant frequencies, often a challenge with large compliant microelectromechanical-system (MEMS) structures, where residual stress causes deformation. We mitigate these ill effects through the use of stress-compensating layer thicknesses and a stress-free monocrystalline diaphragm. For determining device geometry, we develop a simple analytical method that yields excellent agreement between designed and measured resonant frequency; all devices are within 4.5%, and four are within 0.5% (just several hertz). The technique could be useful not only for microphones but also for other low-frequency MEMS transducers designed for resonance operation at a specific frequency.
Date of Publication: Feb. 2013