Investigation of slow evanescent waves at the surface of immersed micromachined membrane arrays

Abstract

Capacitive micromachined ultrasonic transducer (CMUT) arrays are made up of microscale (10–100µm wide) membranes with embedded electrodes for electrostatic excitation and detection of acoustic waves. The main application of CMUTs has been in medical imaging where advantages of miniaturization and electronics integration are significant. In addition to generating bulk waves in the far-field imaging medium, CMUT arrays also support dispersive evanescent surface waves. These surface waves derive their dispersive properties not only from the periodic structure of the array, but also from the membrane resonance. The CMUTs can tune the surface wave by changing the applied bias voltage to the membranes, which in effect changes the membrane stiffness. This tunability allows the possibility of CMUTs to exploit these slowly propagating evanescent waves as a means for creating subwavelength resolution fields for high-resolution ultrasound imaging and sensing in the near field. The dispersive behavior of these evanescent surface waves propagating along a CMUT array is quantified using a computationally efficient, boundary element method based model capable of adding parameter variation enabling more realistic modeling. The model is validated with experimental data obtained from a 1×16 CMUT array with a membrane resonance tunable between 5 and 6.5MHz. The effect of random variation of the CMUT properties on the surface wave characteristics is investigated. Analysis was done on transient signals from simulations and experiments in addition to using a time-frequency method to track the group velocity that varied from 1500m/s to 400m/s.