A surface micromachined electrostatic ultrasonic air transducer

Airborne ultrasound has many applications such as, ranging, nondestructive evaluation, gas flow measurement, and acoustic microscopy. This paper investigates the generation and detection of ultrasound in air at a few MHz. Conventional plane piston lead zirconium titanate (PZT) based transducers perform poorly for this application due to the lack of proper matching layer materials. Electrostatic, or capacitive, transducers promise higher efficiency and broader bandwidth performance. The device structure in this work consists of a capacitor where one plate is a circular silicon nitride membrane coated with gold and the other is a rigid silicon substrate. By applying a voltage between the membrane and the silicon substrate, an electrostatic force is exerted on the membrane which sets it in motion, thus generating a sound wave in air. Presented here is an electrical equivalent circuit model for electrostatic transducers which is based on the early work of Mason (1942). The electrostatic transducers were designed and constructed for operation at 1.8 and 4.6 MHz. The transducers were fabricated using standard micromachining techniques. An optical interferometer was used to measure the peak displacement of the 1.8 MHz electrostatic transducer at 230 /spl Aring//V. A transmit-receive system was built using two electrostatic transducers. The system had a signal to noise ratio of 34 dB at a transducer separation of 1 cm. Each transducer had a 3-dB bandwidth of 20%, and a one-way insertion loss of 26 dB. There is excellent agreement between the measured device performance and theoretical predictions.

[1]  C. M. Fortunko,et al.  Gas-coupled acoustic microscopy in the pulse-echo mode , 1993 .

[2]  B. Khuri-Yakub,et al.  1-3 composites for ultrasonic air transducers , 1992, IEEE 1992 Ultrasonics Symposium Proceedings.

[3]  Wouter Olthuis,et al.  Fabrication of silicon condenser microphones using single wafer technology , 1992 .

[4]  Hae-Seung Lee,et al.  An integrated air-gap-capacitor pressure sensor and digital readout with sub-100 attofarad resolution , 1992 .

[5]  J. L. Valdes,et al.  Ultrasonic time-of-flight method for on-line quantitation of semiconductor gases , 1991, IEEE 1991 Ultrasonics Symposium,.

[6]  D. Schindel,et al.  Capacitance devices for the controlled generation of ultrasonic fields in liquids , 1991, IEEE 1991 Ultrasonics Symposium,.

[7]  Henry Guckel,et al.  Surface micromachined pressure transducers , 1991 .

[8]  C Wykes,et al.  The performance of capacitive ultrasonic transducers using v-grooved backplates , 1991 .

[9]  C.-H. Chou,et al.  A new design for air transducers , 1988, IEEE 1988 Ultrasonics Symposium Proceedings..

[10]  D. Royer,et al.  Optical probing of the mechanical impulse response of a transducer , 1986 .

[11]  Tsutomu Yano,et al.  High-Frequency Ultrasonic Transducer Operating in Air , 1984 .

[12]  Robert A. Boie,et al.  Active damping of ultrasonic transducers for robotic applications , 1984, ICRA.

[13]  A. Taroni,et al.  A Temperature Compensated Ultrasonic Sensor Operating in Air for Distance and Proximity Measurements , 1982, IEEE Transactions on Industrial Electronics.

[14]  K.E. Petersen,et al.  Silicon as a mechanical material , 1982, Proceedings of the IEEE.

[15]  F. Massa,et al.  Ultrasonic transducers for use in air , 1965 .

[16]  D. Schindel,et al.  Applications of micromachined capacitance transducers in air-coupled ultrasonics and nondestructive evaluation , 1995, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[17]  C. Wykes,et al.  Diagnostic measurements in capacitive transducers , 1993 .

[18]  P. Kleinschmidt,et al.  Ultrasonic Robotic-Sensors for Exact Short Range Distance Measurement and Object Identification , 1985, IEEE 1985 Ultrasonics Symposium.

[19]  Kensall D. Wise,et al.  MOS SWITCHED-CAPACITOR READOUT AMPLIFIER FOR CAPACITIVE PRESSURE SENSORS. , 1983 .

[20]  Maria Prudenziati,et al.  A 250 KHz Piezoelectric Transducer for Operation in Air: Application to Distance and Wind Velocity Measurements , 1982 .

[21]  W. P. Mason.  Electromechanical transducers and wave filters , 1942 .