Comparison of piezoresistive and capacitive ultrasonic transducers

MEMS ultrasonic transducers for flaw detection have heretofore been built as capacitive diaphragm-type devices. A diaphragm forms a moveable electrode, placed at a short gap from a stationary electrode, and diaphragm movement has been detected by capacitance change. Although several research teams have successfully demonstrated that technology, the detection of capacitance change is adversely affected by stray and parasitic capacitances, limiting the sensitivity of such transducers and typically requiring relatively large diaphragm areas. We describe the design and fabrication of what to our knowledge is the first CMOS-MEMS ultrasonic phased array transducer using piezoresistive strain sensing. Piezoresistors have been patterned within the diaphragms, and diaphragm movement creates bending strain which is detected by a bridge circuit, for which conductor losses will be less significant. The prospective advantage of such piezoresistive transducers is that sufficient sensitivity may be achieved with very small diaphragms. We compare transducer response under fluid-coupled ultrasonic excitation and report the experimental gauge factor for the piezoresistors. We also discuss the phased array performance of the transducer in sensing the direction of an incoming wave.

[1]  Stephen Pessiki,et al.  Robust capacitive MEMS ultrasonics transducers for liquid immersion , 2003, IEEE Symposium on Ultrasonics, 2003.

[2]  Christofer Hierold,et al.  Surface micromachined ultrasound transducers in CMOS technology , 1996, 1996 IEEE Ultrasonics Symposium. Proceedings.

[3]  Susan C. Mantell,et al.  Calibration technique for MEMS membrane type strain sensors , 1999, Proceedings of the Thirteenth Biennial University/Government/Industry Microelectronics Symposium (Cat. No.99CH36301).

[4]  D.A. Hutchins,et al.  Micromachined ultrasonic capacitance transducers for immersion applications , 1998, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[5]  Patrick J. French,et al.  Polycrystalline Silicon Strain Sensors , 1985 .

[6]  G. Fedder,et al.  Laminated high-aspect-ratio microstructures in a conventional CMOS process , 1996, Proceedings of Ninth International Workshop on Micro Electromechanical Systems.

[7]  B. Khuri-Yakub,et al.  Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays , 2001, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[8]  D. Greve,et al.  MEMS ultrasonic transducers for the testing of solids , 2003, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[9]  David W. Greve,et al.  MEMS phased array detection in contact with solids , 2002, 2002 IEEE Ultrasonics Symposium, 2002. Proceedings..

[10]  Kevin Barraclough,et al.  I and i , 2001, BMJ : British Medical Journal.