MEMS Microphone
Introduction
When operating in dark murky waters, the divers or unmanned underwater
vehicles cannot depend on vision as the primary sensing modality. Acoustic
imaging, using SONAR, has been used successfully by submarines in these
situation. Traditional SONAR does not provide high resolution imaging,
except in large systems with considerable power and computational budgets.
Consider the situation where a simple hand held or head mounted acoustic "camera" with a large 2D array of ultrasonic transducers is used to image the scene in much the same way as a standard CCD camera. Subsequently, the 3D structure of the environment is reconstructed by a local processor in the users personal digital assistant (PDA). This reconstructed virtual environment can be fed directly to the divers goggles and can be superimposed on his/her visual scene, resulting in an Acoustically Enhancement Vision System (AEVS).
Our chip design aims to create a 2D array of ultrasonic transducers. The design will be the first of its kind to incorporate on chip processing. The final chip will make an ideal component for an inexpensive, low power, portable imaging system.
System Overview
The ultrasonic sensors use the availability of MicroElectroMechanical (MEMS)
structures available in some standard CMOS processes to realize ultrasonic
transducers based on the mechanical vibration of micro-plates or membranes
implemented in silicon. The plates form capacitors with the ground plane,
whose values change during the vibration [Kocis, 1996]. The deviation from
the nominal values depends on the amplitude of the incident wave, while the
phase can be extracted from the phase of the oscillations of the capacitance
value. The time of arrival of the wavefront can also be readily detected.
This approach allows considerable processing electronics to be embedded with
the detector element. Depending on the micro-plate/membrane design, the
electronics can be placed underneath the sensing regions, effectively
increasing the fill factor of the sensor to close to 100%. To facilitate the
post processing required to render the 3D model of the environment and/or to
assist in navigation, A/D conversion, temporal feature extraction and
correlation, post sensing beam steering, region of interest localization and
zooming and signal conditioning and interface standardization can be
performed on chip. This system uses reflected acoustical energy from an
external source and is subsequently a low power and low voltage device,
making it safe for underwater usage.
Structure of the acoustic pixel
Conclusion
MEMS offer a unique technology for combining acoustic sensors and processing
circuitry on the same chip. This capability can be exploited to realize highly
integrated acoustic signal processing systems that can significantly advance
the state-of-the-art of ultrasonic imaging and ranging systems. We present an
example of an integrated acoustic array that can be used for imaging in murky
waters. The approach allows dense arrays of sensors to be co-located with
processing electronics. The processing electronics can be used to implement a
variety of algorithms for improving the 3D imaging capabilities of the system.
Biologically inspired processing ideas can also be implemented in our
architecture. We plan to use this system for in acoustically enhanced vision
system (AEVS) for underwater navigation.
References
Armitage, A., N.R. Scales, P.J. Hicks, P.A. Payne, Q.X. Chen, and J.V. Hatfield, An Integrated Array Transducer Receiver For Ultrasound Imaging, Sensors And Actuators A, Vol. 47, pp. 540-544, 1995.
Baltes, H., Future of IC Microtransducers, Sensors and Actuators A, Vol. 56, pp. 179-192, 1996.
Borner, M., S. zur Horst-Meyer, M. Murphy, H. Munch, W. Schomburgand M. Vitt, Ultrasonic Measurements With Micromembranes, Sensors And Actuators A, Vol. 46, pp. 62-65, 1995
CNF, Cornell Nanofabrication Facility, Cornell University, Ithaca, NY 14853, http://www.research. cornell.edu/VPR/vpr.html, 1998.
Davidsen, R., R. Jensen and S. Smith, Two-Dimensional Random Arrays for Real Time Volumetric Imaging, Ultrasonic Imaging, Vol. 16, pp. 143-163, 1994.
Fiorillo, A., PVDF Ultrasonic Sensors For Location Of Small Objects, Sensors And Actuators A, Vol. 42, pp. 406-409, 1994.
Gallego-Juarez, J., G. Rodriguez, J. L. San Emeterio, P. T. Sanz, and J. C. Lazaro, An Acoustic Transducer System For Long-Distance Ranging Applications In Air, Sensors And Actuators A, Vol. 37-38, pp. 397-402, 1993.
M. Haller, "Micromachined Ultrasonic
Materials and Devices," Ph.D. Thesis,
Department of Electrical Engineering, Stanford University, Palo Alto, CA, 1995.
Holmberg, P., Ultrasonic Sensor Array For Position And Rotation Estimates Of Planar Surfaces, Sensors And Actuators A, Vol. 44, pp. 37-43, 1994.
Hornung, M., R. Frey, P. Brand, H. Baltes and C. Hafner, Ultrasound Barrier Based on Packaged Micromachined Membrane Resonators, Proc. IEEE Micro Electro Mechanical Systems (MEMS), Amsterdam, Netherlands, pp. 334-339, 1995.
Howe, R., B. Boser and A. Pisano, Polysilicon Integrated Microsystems: Technologies and Applications, Sensors and Actuators A, Vol. 56, pp. 167-177, 1996.
Kocis, S. and Z. Figura, Ultrasonic Measurements and Technologies, Chapman and Hall Publisher, London, 1996.
Kuratli, C. and Q. Huang, A CMOS Ultrasound Range-Finder Microsystem, ISSCC 2000 Digest of Technical Papers, February, 2000.
Lewin, P. and M. Schafer, Wideband Characterization of Ultrasonic Transducers and Materials using Time Delayed Spectrometry, Archives of Acoustics, Vol. 7, pp. 103-115, 1992.
Minhang, B. and Weiyuan Wang, Future of Microelectromechanical System (MEMS), Sensors and Actuators A, Vol. 56, pp. 135-141, 1996.
MOSIS, The Information Sciences Institute, University of Southern California, Marina del Rey, CA 90292. http://www.mosis.org, 1998.
Schlaberg, H. and J.S. Duffy, Piezoelectric Polymer Composite Arrays For Ultrasonic Medical
Imaging Applications, Sensors And Actuators A, Vol. 44, pp. 111-117, 1995.
Steinberg, B., Principles of Aperture and Array System Design, J. Wiley and Sons Publisher, NY, 1976.