Joshua S. Krause

Acoustic Doppler velocity measurement system using capacitive micromachined ultrasound transducer array technology

This paper describes the design, fabrication, modeling, and characterization of a small (1 cm(2) transducer chip) acoustic Doppler velocity measurement system using microelectromechanical systems capacitive micromachined ultrasound transducer (cMUT) array technology. The cMUT sensor has a 185 kHz resonant frequency to achieve a 13° beam width for a 1 cm aperture. A model for the cMUT and the acoustic system which includes electrical, mechanical, and acoustic components is provided. Furthermore, this paper shows characterization of the cMUT sensor with a variety of testing procedures including Laser Doppler Vibrometry (LDV), beampattern measurement, reflection testing, and velocity testing. LDV measurements demonstrate that the membrane displacement at the center point is 0.4 nm/V(2) at 185 kHz. The maximum range of the sensor is 60 cm (30 cm out and 30 cm back). A velocity sled was constructed and used to demonstrate measureable Doppler shifts at velocities from 0.2 to 1.0 m/s. The Doppler shifts agree well with the expected frequency shifts over this range.

MEMS pressure sensor array for aeroacoustic analysis of the turbulent boundary layer

The second stage of the design, fabrication, and characterization of a surface micromachined, front-vented, 64 channel (8 8), capacitively sensed pressure sensor array is described. The array was fabricated using the MEMSCAP PolyMUMPs R process, a three layer polysilicon surface micromachining process with an additional fabrication step using Parylene-C. An acoustic lumped element model was used to model an individual microphone and then applied to the array as a whole. The computational results for the design, including mechanical components, environmental loading, uid damping, and other acoustic elements are detailed. Theory predicts single element sensitivity of 0.65 mV/Pa at the gain stage output in the 100-40,000 Hz band. A laser Doppler velocimetry (LDV) system has been used to map the spatial motion of the elements in response to electrostatic excitation. A strong resonance appears at 410 kHz for electrostatic excitation, in agreement with mathematical models. Static stiness measured electrostatically using an interferometer is 0.1 nm/V 2 , similar to the expected stiness.

A Microphone Array on a Chip for High Spatial Resolution Measurements of Turbulence

A microelectromechanical systems-based microphone array on a chip has been developed and applied to aeroacoustic measurements. The array is designed to measure the fluctuating pressures present under a turbulent boundary layer (TBL). Each chip measures 1 cm2 and contains 64 individually addressable capacitively sensed microphones, with a center to center pitch of ~1.25 mm. Surface topology, including the packaging, is kept to less than 0.13 mm. Element-to-element sensitivity variation in the array is less than ±2.5 dB from least to most sensitive, and phase variation is less than ±6.5° (at 1 kHz). The microphone 3-dB bandwidth is 700 Hz to 200 kHz, and the microphones are linear to better than 0.3% at sound pressure levels up to 150-dB SPL. A unique switched architecture system electronics and packaging method are employed to reduce data acquisition channel count requirements, and to maintain a low surface roughness. The array has been applied to the measurement of single point turbulence spectra under a flat plate TBL in a flow duct at Mach numbers up to 0.6 and Reynolds numbers based on plate length of 107.

MEMS Microphone Array on a Chip for Turbulent Boundary Layer Measurements

A MEMS microphone array has been designed and applied to the measurement of wall pressure spectra under the turbulent boundary layer in flow duct testing at Mach numbers from 0 to 0.6. The array was micromachined onto a single chip in the PolyMUMPS surface micromachining process, allowing high spatial resolution and low surface roughness. The chip measures 1 cm by 1 cm, and is flush mounted into the wind tunnel wall. Individual elements are 0.6 mm in diameter, with element to element spacing of 1.11 mm in the crossflow direction and 1.26 mm in the flow direction. The 64 element array has 59 working elements, 58 of which are matched to within ± 2.5 dB at 1 kHz. Phase matching between the 59 elements is ± 6.5 degrees at 1 kHz. The array has been calibrated from 100 Hz to 4 kHz in a plane wave tube. The transducer bandwidth is greater than 400 kHz as determined by laser vibrometry measurements. Sensor nonlinearity of less than 0.36% is observed at a sound pressure level of 150 dB SPL. Board level electronics allow the array to be reconfigured on the fly using computer controlled CMOS switches. Multipoint wall pressure spectra were measured in 38 array configurations at the wall of a 6 inch by 6 inch flow duct at Mach numbers from 0.0 to 0.6. The array shows excellent agreement with Kulite and Bruel & Kjaer microphone measurements in the 300 Hz to 10 kHz band, and appears to be able to measure turbulent pressure spectra at frequencies as high as 40 kHz.

Capacitive micromachined ultrasound Doppler velocity sensor.

An acoustic Doppler velocity measurement system using microelectromechanical systems (MEMS) capacitive micromachined ultrasound transducer (cMUT) array technology is introduced. This low power acoustic velocity measurement system operates in both transmit and receive modes. The device consists of 64 0.6 mm diameter polysilicon and gold diaphragms, and operates at approximately 180 kHz with a quality factor of 30. Computational predictions suggest that in transmit mode the system will deliver a sound pressure level (SPL) of approximately 66 dB SPL at 1 m from the source with an 11 deg, −3 dB beamwidth when driven with a 12 V excitation. In receive mode, the predicted sensitivity is 2.2 mV/Pa at the preamplifier output. Based on experimentally determined electronic noise densities of approximately 4×10−16 V2/Hz, resolution may be as good as 0 dB SPL in a 5 Hz band. This suggests that with good reflection, a range of approximately 8 m (4 m out and 4 m back) is achievable. Velocity resolution is expected to b...

MEMS (microelectromechanical systems) microphone array on a chip.

The design, fabrication, and characterization of a surface micro‐machined, front‐vented, 64 channel (8×8), capacitively sensed microphone array‐on‐a‐chip are described. The element pitch is 1.3‐mm center‐to‐center and is targeted at resolving the high‐wavenumber components of the pressure spectra underneath the turbulent boundary layer. Care is taken to minimize package topology to reduce flow generated self‐noise. The array was fabricated using the MEMSCAP PolyMUMPs® process, a three layer polysilicon surface micromachining process, with the addition of a Parylene‐C coating in post‐processing. An acoustic lumped element model, including mechanical components, environmental loading, fluid damping, and other acoustic elements, is detailed. Laboratory calibration indicates a sensitivity of 1 mV/Pa for each microphone over a 200–40 000 Hz band. A strong resonance occurs at 280 kHz in close accordance with modeled results. Spatial mapping of the array reveals minimal electrical and physical cross‐talk between elements. Preliminary element‐to‐element sensitivity comparisons exhibit a standard deviation of 13%, 12%, and 25% at 250 Hz, 500 Hz, and 1 kHz. Phase matching showed a 9, 10, and 15 deg standard deviation difference at the same frequencies. A more in depth analysis of acoustic sensitivity is ongoing as well as element‐to‐element variability.

Nickel on glass acoustic microsystems

An electroplated nickel-on-glass surface micromachining process has been developed and applied to fabricate condenser MEMS microphone arrays and ultrasound arrays. The major advantage of these arrays over polysilicon surface micromachined sensors is the significant reduction in stray capacitance that can be achieved by using a glass substrate. For 600 μm diameter elements with a 1.5 to 2μm sense gap, the stray capacitance per element was reduced from approximately 140–1 pF when compared to similar polysilicon devices fabricated using the PolyMUMPS process. It is important to reduce stray capacitance, since it is the driving factor for two dominant noise sources: preamplifier voltage noise and bias feedthrough noise. In addition, stray capacitance can place a limit on system bandwidth for high frequency applications such as ultrasound. In this paper, we report on cMUT ultrasound arrays for in-air Doppler applications, as well as microphone array-on-a-chip devices for aeroacoustic measurements. Characteriza...

Micromachined reconfigurable microphone array for wind tunnel testing.

A surface micromachined, front‐vented, 64 channel (8×8), capacitively sensed microphone array‐on‐a‐chip devices for aeroacoustic testing is described. The arrays are fabricated using the MEMSCAP PolyMUMPs polysilicon surface micromachining process, with a Parylene‐C passivation layer. The devices are packaged with low profile interconnects, presenting a maximum of 100 μm of surface topology. The array electronics allow the microphone outputs to be redirected to one of two channels, allowing dynamic reconfiguration of the effective transducer shape in software. Measured microphone sensitivity is 0.15 mV/Pa for an individual microphone and 8.7 mV/Pa for the entire array, in close agreement with model predictions. The microphones and electronics operate over the 200–40 000 Hz band. The dynamic range extends from 60 dB SPL in a 1 Hz band to greater than 150 dB SPL. Element variability is ±0.05 mV/Pa in sensitivity with an array yield of 95%. Off‐chip electronics provide 80 dB off isolation. Preliminary wind t...