Micromachined optical microphone structures with low thermal-mechanical noise levels

Abstract

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.

Optical Microphone Structures Fabricated for Broad Bandwidth and Low Noise

“…In the 3D cross-sections, Experimentally traced interference curve using a VCSEL as the light source [11] the blue layer is a 2.25 µm thick polysilicon layer (reflector membrane), which is removed in the next image to show the details of the support arms and the grating. These features can also be seen in the micrographs which were taken from the back side of the fabricated devices.…”

A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea

“…As a result, the noise floor of the microphone is primarily influenced by its thermal noise, resulting from random impacts between the diaphragm and the surrounding air. It is well-known that the equivalent input sound pressure associated with this thermal noise is proportional to the amount of passive energy dissipation in the system ͑Gabrielson, 1993; Thompson et al, 2002;Hall et al, 2007͒. The design developed here does not require the use of a backplate electrode, which is typically the dominant cause of viscous damping in capacitive microphones ͑Thompson et al, 2002;Homentcovschi and Miles, 2004;Homentcovschi and Miles, 2005;Hall et al, 2007͒.…”

“…It is well-known that the equivalent input sound pressure associated with this thermal noise is proportional to the amount of passive energy dissipation in the system ͑Gabrielson, 1993; Thompson et al, 2002;Hall et al, 2007͒. The design developed here does not require the use of a backplate electrode, which is typically the dominant cause of viscous damping in capacitive microphones ͑Thompson et al, 2002;Homentcovschi and Miles, 2004;Homentcovschi and Miles, 2005;Hall et al, 2007͒. In addition, because the dominant motion of the diaphragm consists of rotation about its flexible supports, acoustic radiation damping and other sources of viscous damping can be kept to a minimum.…”

Towards a sub 15-dBA optical micromachined microphone

“…Since backplate capacitance is not required for signal detection, the design of highly perforated backplates with low thermal-mechanical noise is permitted. 23,36,37 …”

“…34 A microphone that uses interference through a diffraction grating has also been introduced in prior work. 23,[35][36][37][38][39][40][41] The microphone uses a backplate to hold the grating in place beneath a vibrating diaphragm and also to enable the application of electrostatic forces to the diaphragm. Since backplate capacitance is not required for signal detection, the design of highly perforated backplates with low thermal-mechanical noise is permitted.…”