Capacitive Micromachined Lamb Wave Transducers
We have designed, fabricated, and characterized a new Lamb wave device. Built using capacitive micromachined ultrasonic transducers (CMUTs), the structure described uses rectangular membranes to excite and receive Lamb waves on a silicon substrate (Fig. 1) . An equivalent circuit model for the transducer is proposed that produces results, which match well with those observed by experiment. During the derivation of this model, emphasis is placed on the resistance presented to the transducer membranes by the Lamb wave modes. Finite element analysis performed in this effort shows that the dominant propagating mode in the device is the lowest order antisymmetric flexural wave (A0). Furthermore, most of the power that couples into the Lamb wave is due to energy in the vibrating membrane that is transferred to the substrate through the supporting posts of the device. The manufacturing process of the structure relies solely on fundamental IC-fabrication techniques. The resulting device has an 18-μm-thick substrate that is almost entirely made up of crystalline silicon and operates at a frequency of 2.1 MHz. We characterized this device by S-parameter and laser vibrometer measurements as well as delay-line transmission data. The insertion loss, as determined by both S-parameter and delay-line transmission measurements, is 20 dB at 2.1 MHz. When configured as a delay-line oscillator, the device functions well as a sensor with sensitivity to changes in the mass loading of its substrate. Fig. 2 shows the results of an experiment where the device was used as a humidity sensor. More details on this work can be found in [1].
FIGURE 1. Diagram of a single rectangular membrane.
References
[1] M. H. Badi, G. G. Yaralioglu, A. S. Ergun, S. T. Hansen, E. J. Wong, B. T. Khuri-Yakub, "Capacitive Micromachined Ultrasonic Lamb Wave Transducers Using Rectangular Membranes," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 50, no. 9, pp. 1191 - 1203, Sep. 2003.
Acknowledgements
This work was supported by the United States Office of Naval Research under Grant N00014-94-1-073.
