Micromachined optical microphone structures with low thermal-mechanical noise levels.
Micromachined microphones with diffraction-based optical displacement detection have been introduced previously [Hall et al., J. Acoust. Soc. Am. 118, 3000-3009 (2005)]. The approach has the advantage of providing high displacement detection resolution of the microphone diaphragm independent of device size and capacitance-creating an unconstrained design space for the mechanical structure itself. Micromachined microphone structures with 1.5-mm-diam polysilicon diaphragms and monolithically integrated diffraction grating electrodes are presented in this work with backplate architectures that deviate substantially from traditional perforated plate designs. These structures have been designed for broadband frequency response and low thermal mechanical noise levels. Rigorous experimental characterization indicates a diaphragm displacement detection resolution of 20 fm radicalHz and a thermal mechanical induced diaphragm displacement noise density of 60 fm radicalHz, corresponding to an A-weighted sound pressure level detection limit of 24 dB(A) for these structures. Measured thermal mechanical displacement noise spectra are in excellent agreement with simulations based on system parameters derived from dynamic frequency response characterization measurements, which show a diaphragm resonance limited bandwidth of approximately 20 kHz. These designs are substantial improvements over initial prototypes presented previously. The high performance-to-size ratio achievable with this technology is expected to have an impact on a variety of instrumentation and hearing applications.
Hall
Neal A
NA
Sandia National Laboratories, Albuquerque, New Mexico 87185, USA. nahall@alumni.utexas.net
Okandan
Murat
M
Littrell
Robert
R
Bicen
Baris
B
Degertekin
F Levent
FL
D000162:Acoustics; D001309:Auditory Threshold; D004566:Electrodes; D004867:Equipment Design; D006310:Hearing Aids; D006801:Humans; D008144:Loudness Perception; D009622:Noise; D055095:Optics and Photonics; D010470:Perceptual Masking; D010898:Pitch Perception; D013018:Sound Spectrography