The lower limit of detectable sound pressures

Tarnow, Viggo

doi:10.1121/1.395526

Cited by 25 publications

(17 citation statements)

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“…The degree of perforation for such structureswhich directly affects damping and thermal noise levels 2 -is limited by capacitance requirements. 3,12 The structure presented in Fig. 3, in contrast, consists of mostly open area and therefore very small active capacitance.…”

Section: Micromachined Optical Microphone Structuresmentioning

confidence: 94%

“…This attribute allows for the use of innovative microphone backplate designs with minimal area and low squeeze-film resistance-the dominant damping mechanism and thermalmechanical noise source in condenser microphones. 2,3 This feature may enable microphones with size characteristic of the smallest microphones demonstrated to date ͑with the essential semiconductor components contained within approximately 1.5 mm 3 volume͒, but with detection performance characteristic of the highest precision measurement microphones ͓below 20 dB͑A͒ noise levels͔. Single element microphones with this property may be useful for noninvasive instrumentation applications and audio applications where covertness is desired, while microfabricated arrays may be ideally suited for broadband sound intensity measurements and advanced speech recognition applications utilizing ambient noise suppression algorithms designed for miniature acoustic apertures.…”

Section: Introductionmentioning

confidence: 98%

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Micromachined optical microphone structures with low thermal-mechanical noise levels

Hall

Okandan

Littrell

et al. 2007

The Journal of the Acoustical Society of America

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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.

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Section: Micromachined Optical Microphone Structuresmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 98%

Micromachined optical microphone structures with low thermal-mechanical noise levels

Hall

Okandan

Littrell

et al. 2007

The Journal of the Acoustical Society of America

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“…The total rms noise of the silicon microphones has been measured at a bias voltage of 28 V and amounts to 4.1 pV(A), or an equivalent sound pressure of 43 dB(A). As expected, it is a combination of an electrical l/f noise due to the preamplifier and a white mechanical-thermal noise from the sensor element itself [24]. The electrical noise may be obtained by replacing the microphone with a capacitor or by switching off the bias voltage.…”

Section: Measured (Dashed Curve) and Calculated (Solid Curves) Sensitmentioning

confidence: 97%

Capacitive microphone with a surface micromachined backplate using electroplating technology

Bergqvist

Gobet

1994

J. Microelectromech. Syst.

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A technology for surface micromachining of freestanding metal microstructures using metd ekCtrodepOSitiOn On microphone. Electroplating technology has been used to implein copper, which serves as backplate electrode in the condenser microphone. The 1.8 x 1.8"' large microphone diaphragm etching of the substrate wafer. The realized prototypes have a measured sensitivity of 1.4 mV/Pa using a bias voltage of 28 V. The bandwidth is limited by an anti-resonance at 14 kHz which is due to the semi-rigid backplate. The resonance behavior of the backplate structure has been analyzed with finite element modeling with results in good agreement with measured data. 1911[7]. They always involve more or less laborious alignment procedures with an associated reduction in yield. The first surface a sacrificial photoresist layer has been applied to a Condenser ment a suspended and perforated 15-pm-thick microstructure micromachined structures intended for condenser microphone [ l l1-Their were presented by Hijab and diaphragm layer was in PolYsilicon With a CVD oxide as h e sacrificial layer between the diaphragm and the rigid backplate strict of the word, i.e., only one side of the wafer was processed. However, no measured results were ever reported. Lately, results On a new SiliCOn condenser microphone With a major surface micromachining step has been reported by Scheeper et al. [12]. It employs PECVD nitride for both backplate and diaphragm electrodes and a sputtered aluminium layer as a sacrificial air gap spacer. Sensitivities of 1 to 2 mVPa have been obtained at bias voltages up to 16 V. N important number of reports have been published A new interesting tool for the fabrication of surface mion capacitive sensors, such as pressure [I], ac-crostructures on silicon is metal electrodeposition combined celerometers [21 and microphones [31. some of the advantages with resist micropatterning techniques. It has been successfully of capacitive compared to, e.g., the piezoresistive applied to numerous microelectro-mechanical sensors and acsensors, are high sensitivity, low temperature coefficients, tuators [14I-[ 161. Compared to h e polysilicon technologies, and good long-temThis has been demonstrated microstructuring by electrodeposition allows for thicker layers in several of which are now corn-and features with high aspect ratios. The electrodeposition mercially available. The majority of these examples use the technology also offers a greater choice of materials, e.g., well-established technology of bulk silicon micromachining copper, nickel, gold, etc. A p~c u l a r l y useful combination in for he fabrication of the chip. However, progress has capacitive sensor design is electroplating on a sacrificial layer recently been made in surface micromachining of capacitive as Proposed by Mohr et al. [171. Underetching of the sacrificial sensors [4], [5]. The motivation for the research on surface layer leaves a free-standing electroplated structure that may micromachined is the possibility to reduce the size serve as one of the elec...

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“…4 (left) shows the measured noise and calculated thermal-mechanical noise at x = 0 (bridge center) from a bridge with l = 200 µm. The measurements are made with a spectrum analyzer, and the displacement magnitude is calibrated using Equation 3. The data is fit to the n = 0 term of Equation 12 to obtain ν 0 and τ 0 , which are then substituted into Equation 12 for the calculated thermal-mechanical noise spectrum.…”

Section: Thermal-mechanical Noisementioning

confidence: 99%

“…Thermal-mechanical noise has previously been documented in systems such as mirror galvanometers, 2 microphones, 3 and scanning force microscopy probes , 4 and results from the same physical mechanisms that give rise to Brownian motion in liquids and Johnson noise in resistors. Recent measurements have demonstrated that the thermodynamics of microscopic systems are quite distinct from their macroscopic counterparts.…”

Section: Introductionmentioning

confidence: 99%

Measurement of thermal-mechanical noise in MEMS microstructures

et al. 2003

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We report absolute measurements of thermal-mechanical noise in microelectromechanical systems. The measurements are made possible with a simple, high resolution optical technique that has a displacement resolution on the order of hundreds of femtometers per root Hz at frequencies of tens of kHz. The measured noise spectrum agrees with the calculated noise level to within 25%, a discrepancy most likely due to uncertainty in the effective dynamic mass of the vibrating bridge. These measurements demonstrate that thermal-mechanical noise can be the dominant noise source in actuated microelectromechanical devices. This noise will become even more pronounced as the size of mechanical devices continues to shrink.

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The lower limit of detectable sound pressures

Cited by 25 publications

References 0 publications

Micromachined optical microphone structures with low thermal-mechanical noise levels

Micromachined optical microphone structures with low thermal-mechanical noise levels

Capacitive microphone with a surface micromachined backplate using electroplating technology

Measurement of thermal-mechanical noise in MEMS microstructures

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