Baris Bicen

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

A miniature differential microphone is described having a low-noise floor. The sensitivity of a differential microphone suffers as the distance between the two pressure sensing locations decreases, resulting in an increase in the input sound pressure-referred noise floor. In the microphone described here, both the diaphragm thermal noise and the electronic noise are minimized by a combination of novel diaphragm design and the use of low-noise optical sensing that has been integrated into the microphone package. The differential microphone diaphragm measures 1 x 2 mm(2) and is fabricated out of polycrystalline silicon. The diaphragm design is based on the coupled directionally sensitive ears of the fly Ormia ochracea. The sound pressure input-referred noise floor of this miniature differential microphone has been measured to be less than 36 dBA.

Micromachined Accelerometers With Optical Interferometric Read-Out and Integrated Electrostatic Actuation

A micromachined accelerometer device structure with diffraction-based optical detection and integrated electrostatic actuation is introduced. The sensor consists of a bulk silicon proof mass electrode that moves vertically with respect to a rigid diffraction grating backplate electrode to provide interferometric detection resolution of the proof-mass displacement when illuminated with coherent light. The sensor architecture includes a monolithically integrated electrostatic actuation port that enables the application of precisely controlled broadband forces to the proof mass while the displacement is simultaneously and independently measured optically. This enables several useful features such as dynamic self-characterization and a variety of force-feedback modalities, including alteration of device dynamics in situ. These features are experimentally demonstrated with sensors that have been optoelectronically integrated into sub-cubic-millimeter volumes using an entirely surface-normal, rigid, and robust embodiment incorporating vertical cavity surface emitting lasers and integrated photodetector arrays. In addition to small form factor and high acceleration resolution, the ability to self-characterize and alter device dynamics in situ may be advantageous. This allows periodic calibration and in situ matching of sensor dynamics among an array of accelerometers or seismometers configured in a network.

Micromachined microphones with diffraction-based optical displacement detection

Micromachined microphones with diffraction-based optical displacement detection are introduced. The approach enables interferometric displacement detection sensitivity in a system that can be optoelectronically integrated with a multichip module into mm3 volumes without beamsplitters, focusing optics, or critical alignment problems. Prototype devices fabricated using Sandia National Laboratories’ silicon based SwIFT-Lite™ process are presented and characterized in detail. Integrated electrostatic actuation capabilities of the microphone diaphragm are used to perform dynamic characterization in vacuum and air environments to study the acoustic impedances in an equivalent circuit model of the device. The characterization results are used to predict the thermal mechanical noise spectrum, which is in excellent agreement with measurements performed in an anechoic test chamber. An A weighted displacement noise of 2.4×10−2A measured from individual prototype 2100μm×2100μm diaphragms demonstrates the potential fo...

Integrated Optical Displacement Detection and Electrostatic Actuation for Directional Optical Microphones With Micromachined Biomimetic Diaphragms

In this paper, integration and packaging of directional biomimetic microphones using a diffraction-based optical displacement detection method is described. The optical detection method senses the displacement of the microphone diaphragm by monitoring the change in the intensity of a diffracted laser beam. A detailed optical model of the integrated optical detection scheme is developed and used to guide the package design. Measurement results with microphone packages suitable for hearing aid and acoustic measurement systems show that the noise and sensitivity performances of these small-sized microphones are comparable with commercial miniature directional microphones. These microphones incorporate integrated electrostatic actuators which can be used for active feedback control. This capability is also demonstrated to improve the frequency response of the microphone without degrading its noise performance.

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.

Simulation of Thin-Film Damping and Thermal Mechanical Noise Spectra for Advanced Micromachined Microphone Structures

In many micromachined sensors the thin (2-10 thick) air film between a compliant diaphragm and backplate electrode plays a dominant role in shaping both the dynamic and thermal noise characteristics of the device. Silicon microphone structures used in grating-based optical-interference microphones have recently been introduced that employ backplates with minimal area to achieve low damping and low thermal noise levels. Finite-element based modeling procedures based on 2-D discretization of the governing Reynolds equation are ideally suited for studying thin-film dynamics in such structures which utilize relatively complex backplate geometries. In this paper, the dynamic properties of both the diaphragm and thin air film are studied using a modal projection procedure in a commonly used finite element software and the results are used to simulate the dynamic frequency response of the coupled structure to internally generated electrostatic actuation pressure. The model is also extended to simulate thermal mechanical noise spectra of these advanced sensing structures. In all cases simulations are compared with measured data and show excellent agreement - demonstrating 0.8 and 1.8 thermal force and thermal pressure noise levels, respectively, for the 1.5 mm diameter structures under study which have a fundamental diaphragm resonance-limited bandwidth near 20 kHz.

Optical Microphone Structures Fabricated for Broad Bandwidth and Low Noise

A micromachined optical microphone structure using a grating based interferometer has been presented previously and is undergoing continued development (JASA, vol. 118, pp 3000-3009, November 2005). Two advantages of the approach that have been highlighted in prior work are high displacement resolving capability of the microphone diaphragm vibration (2 pm rms over the audio bandwidth) and a flexible mechanical design space for achieving broad bandwidth and low thermal noise designs. Here, we summarize a variety of structures we are fabricating using Sandia National Laboratories silicon-based microfabrication technology to explore this versatile design space. These structures are being packaged to resemble instrumentation-type microphones with approximately 1 cm2 form factor in order to facilitate rigorous acoustic evaluation in the Micromachined Sensors and Transducers Laboratory (MiST) and anechoic test facilities at Georgia Tech.

Development of novel biologically inspired directional microphones

The development of novel directional microphones for hearing aids is described. The mechanisms underlying the design of these unusual microphones were inspired by our earlier study of the ears of the parasitoid fly Ormia ochracea [Miles, et al., J. Acoust. Soc. Am. 98, 3059–3070 (1995)]. The structure of Ormia’s ears inspired new approaches to the design of directional microphones that have the potential to be more sensitive and have lower thermal noise than typical miniature microphones. The mechanisms for directional hearing in this animal are discussed along with the engineering design concepts that they have inspired. Microphones have been fabricated out of silicon that employ either capacitive sensing or optical sensing to convert the diaphragm motion into an electronic signal. Measured results indicate that the directivity of these microphones is very similar to that of an ideal first‐order differential microphone. In addition, novel microphone diaphragms have been fabricated that posses a second‐or...

Diffraction based optical MEMS microphones and accelerometers with active electrostatic force feedback

Diffraction‐based optical displacement detection method and its use in low noise micromachined microphones have been shown earlier. [Hall et al., J. Acoust. Soc. Am. 118, 3000‐3009 (2005), Garcia et al., J. Acoust. Soc. Am. 121, 3155 (2007)]. In these devices, the integrated electrostatic port of the sensor is uncoupled from the integrated optical sensing. This structure enables one to use this port for sensitivity tuning, self characterization, and active control to adjust the device dynamics. Given that the displacement noise of integrated optical sensor is below the thermal‐mechanical noise of the mechanical structure, one can implement force feedback methods such as active Q‐control, or adjust device stiffness without adding substantial noise to the system. We implemented micromachined optical microphones and accelerometers with integrated optoelectronics integrated in a 1.5mm3 volume. We present experimental results on force feedback Q‐control of low noise omnidirectional, and biomimetic directional ...

An integrated optical microphone test‐bed for acoustic measurements

A diffraction‐based optical detection method for microphone applications has been demonstrated previously [Hall et al., J. Acoust. Soc. Am. 118, 3000–3009 (2005)]. This method, coupled with proper integration techniques can produce precision measurement microphones with 24 dBA noise levels and suitable bandwidths. Thus far, these characterization studies have been performed using experimental setups, which would disturb the acoustic field due to size and non‐symmetric features. In these regards, previous optical microphone test beds have been inadequate experimental platforms. This has motivated the development of a more robust integrated instrumentation microphone package for future testing and characterization. In order to meet the size restrictions for such an optical microphone platform, vertical cavity surface emitting lasers are used as light sources and small photodiode arrays are used to detect intensity variations in refracted orders of the optical detection method. The overall dimensions and sha...