Neal A. Hall

Integrated optical interferometric detection method for micromachined capacitive acoustic transducers

An integrated optical interferometric detection method for micromachined capacitive acoustic transducers is reported. The back electrode of the capacitive transducer on a transparent substrate is shaped as an optical diffraction grating and the displacement of the transducer membrane is determined with interferometric resolution by measuring the intensity of the reflected diffraction orders. By applying voltage to deflect the membrane electrostatically, the detection sensitivity is kept at the optimum level and transmission signals are generated. Initial experiments on devices microfabricated on quartz substrates show a minimum detectable displacement of 2×10−4u2009A/√Hz around 250 kHz and low frequency detection capability for microphone applications. Ultrasonic pulse-echo experiments in air at 750 kHz are also demonstrated using both a HeNe laser and a 850 nm vertical cavity surface emitting laser as the light source.

Fabrication and characterization of a micromachined acoustic sensor with integrated optical readout

Implementation and characterization of a micromachined acoustic sensor with integrated optoelectronic readout is described. The mechanical part of the sensor is surface micromachined on a quartz substrate and consists of an aluminum membrane which is electrostatically actuated by a back electrode shaped in the form of a diffraction grating. Optical detection is performed by measuring the reflected diffraction orders when the grating is illuminated through the quartz substrate. This scheme provides interferometric displacement detection sensitivity as well as a compact optical interconnect to a custom designed silicon photodetector array fabricated with n-well CMOS technology. The array also contains optical apertures formed by post-CMOS deep reactive ion etching for backside illumination. A compact hybrid packaged sensor array is formed by bonding the silicon photodetector array to the quartz substrate, resulting in an integrated acoustic sensor volume of 2.5 mm/sup 3/. Experimental characterization has been performed on integrated sensors with 200-mm-diameter, 1-mm-thick aluminum membranes. The results show a minimum detectable membrane displacement of 2.08/spl times/10/sup -4/ /spl Aring///spl radic/Hz at 20 kHz and 1.35/spl times/10/sup -4/ /spl Aring///spl radic/Hz at 100 kHz with 61-mW laser power detected on the integrated photodetector. Operation of the device with a pulsed vertical cavity surface emitting laser as the light source and differential detection of diffraction orders for noise reduction are demonstrated to show the potential for low-power, low-cost micromachined acoustic sensors.

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...

Capacitive micromachined ultrasonic transducers with diffraction-based integrated optical displacement detection

Capacitive detection limits the performance of capacitive micromachined ultrasonic transducers (CMUTs) by providing poor sensitivity below megahertz frequencies and limiting acoustic power output by imposing constraints on the membrane-substrate gap height. In this paper, an integrated optical interferometric detection method for CMUTs, which provides high displacement sensitivity independent of operation frequency and device capacitance, is reported. The method also enables optoelectronics integration in a small volume and provides optoelectronic isolation between transmit and receive electronics. Implementation of the method involves fabricating CMUTs on transparent substrates and shaping the electrode under each individual CMUT membrane in the form of an optical diffraction grating. Each CMUT membrane thus forms a phase-sensitive optical diffraction grating structure that is used to measure membrane displacements down to 2/spl times/10/sup -4/ /spl Aring///spl radic/Hz level in the dc to 2-MHz range. Test devices are fabricated on quartz substrates, and ultrasonic array imaging in air is performed using a single 4-mm square CMUT consisting of 19/spl times/19 array of membranes operating at 750 kHz.

Self-Calibrating Micromachined Microphones with Integrated Optical Displacement Detection

An optical displacement detection method for micromachined microphones is described and experimental results are presented. The microphone membrane is fabricated on a transparent substrate and the back electrode is patterned in the form of diffraction fingers. This structure forms a phase sensitive diffraction grating, providing the displacement sensitivity of an optical interferometer. The diffraction fingers are also used for electrostatic actuation, providing sensitivity adjustment and self calibration capabilities. The optical detection and electrostatic actuation capabilities are demonstrated on aluminum microphone membranes microfabricated on quartz substrates.

Modeling and design of CMUTs using higher order vibration modes [capacitive micromachined ultrasonic transducers]

Capacitive micromachined ultrasonic transducer (CMUT) design has so far relied on the first vibration mode of the CMUT membrane which most resembles piston-like motion. However, experiments show that the frequency response of the CMUT is significantly affected by the higher order vibration modes of the membrane. In this paper, we discuss the design of CMUTs considering these higher order vibration modes of the transducer membrane to tailor its frequency response for specific applications such as harmonic imaging. Based on an FEA model, we show that the vibration frequency of a particular membrane mode can be adjusted by judicious mass loading of the membrane, taking the strain energy distribution into account. Furthermore, we show that using multiple electrodes on the CMUT membrane, in conjunction with membrane mass distribution, the transmit and receive response of the CMUT can be separately optimized for harmonic imaging. An example is given to illustrate the above concepts.

Micromachined acoustic sensor array with diffraction-based optical interferometric detection

A diffraction-based interferometric optical detection method for micromachined acoustic sensors can provide better sensitivity as compared to conventional capacitance detection schemes especially at low frequency range. The optical detection method, complete with optoelectronics readout, can be integrated with a capacitive micromachined acoustic transducer. The method is utilized on a 19×19 capacitive micromachined ultrasonic transducer (cMUT) array to demonstrate ultrasonic imaging of wire targets at 750kHz in air. A silicon photodiode (PD) array is also designed and fabricated in a standard 1.3μm CMOS technology, and through-wafer etching of holes for optical interconnect is performed on the same silicon platform. Further improvement of displacement sensitivity in a resonant-cavity-enhanced (RCE) acoustic sensor is theoretically analyzed including the loss effect in the mirror, and the theoretical results are experimentally verified by measurements on devices with a thin metallic bottom mirror made of silver.

A grating-assisted resonant-cavity-enhanced optical displacement detection method for micromachined sensors

We present an integrated optical displacement sensing method for microscale sensors which is based on an asymmetric Fabry-Perot etalon structure with an embedded phase-sensitive diffraction grating. Analytical modeling of the structure shows that the etalon significantly improves the detection sensitivity as compared to a regular optical interferometer and the embedded diffraction grating enables integration of optoelectronics in a small volume. The efficacy of the method is experimentally validated on a surface micromachined diffraction-based opto-acoustic sensor fabricated on a quartz wafer. A 15 nm silver layer is used to form the bottom mirror of the etalon structure with a sensor membrane and embedded diffraction grating made of aluminum. Comparison of the results with and without the etalon shows an 8 dB increase in detection sensitivity with the etalon structure, which should be further enhanced with the use of low-loss dielectric mirrors.

Capacitive micromachined ultrasonic transducers with integrated optoelectronic readout

Capacitive detection methods impose several limitations on the performance of capacitive micromachined ultrasound transducers (cMUT) such as low sensitivity below MHz frequencies and low limits on acoustic power output. Optical interferometric detection of cMUT membrane displacement solves these problems by providing a flat frequency response down to DC and electrical isolation between transmitting and receiving electronics. A phase-sensitive optical diffraction grating structure formed by placing a grated electrode under the cMUT membranes enables integration of optoelectronic readout circuitry to cMUT arrays while allowing efficient transmit operation using capacitive actuation. This structure is used to measure membrane displacements down to 10/sup -4/ /spl Aring///spl radic/Hz level in the DC-2MHz range on cMUTs fabricated on quartz substrates. The possibility of enhanced displacement sensitivity for immersion applications by forming a multi-pass interferometer is discussed and an optical analysis of the detection method is described. The feasibility of hybrid optoelectronics integration for phased array operation is also demonstrated by experiments on cMUTs using vertical cavity surface emitting lasers and 1/spl times/8 array of photodetectors with integrated CMOS amplifiers.

Ultrasensitive directional microphone arrays for military operations in urban terrain.

Acoustic sensing systems are critical elements in detection of sniper events. The microphones developed in this project enable unique sensing systems that benefit significantly from the enhanced sensitivity and extremely compact foot-print. Surface and bulk micromachining technologies developed at Sandia have allowed the design, fabrication and characterization of these unique sensors. We have demonstrated sensitivity that is only available in 1/2 inch to 1 inch studio reference microphones--with our devices that have only 1 to 2mm diameter membranes in a volume less than 1cm{sup 3}.