Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays

In this paper, we report on the characterization of 1-D arrays of capacitiv e micromachined ultrasonic transducers (cMUT). A 275- x 5600-mum 1-D CMUT a rray element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver, the transducer has a 0.28-fm/root Hz displacement sensitivity, and, as a transmitter, it produces 5 kPa/V of output pressure at the transducer surface at 3 MHz with a DC bias of 35 V, The transducer has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern of isolated single elements, as well as those of array elements ar e measured, and two major sources of acoustical cross talk are identified. A weakly dispersive non-leaky interface wave (Stoneley wave) is observed to be propagating at the silicon substrate-fluid interface at a speed close t o the speed of sound in the fluid. This wave causes internal reflections, s purious resonance, and radiation from the edges of the silicon substrate. T he large lateral component of the particle velocity generated by the membra nes at the edge of the cMUT array elements is found to be the source of thi s interface wave. Lowest order Lamb waves in the silicon substrate are also found to contribute to the cross talk between elements. These waves are ex cited at the edges of individual vibrating membranes, where they are anchor ed to the substrate. and result in a narrowing of the beam profile of the a rray elements. Several methods, such as trench isolation and wafer thinning , are proposed and implemented to modify the acoustical cross coupling betw een array elements.