Design of a Collapse-Mode CMUT With an Embossed Membrane for Improving Output Pressure

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a competitive alternative to piezoelectric ultrasonic transducers, especially in medical ultrasound imaging and therapeutic ultrasound applications, which require high output pressure. However, as compared with piezoelectric ultrasonic transducers, the output pressure capability of CMUTs remains to be improved. In this paper, a novel structure is proposed by forming an embossed vibrating membrane on a CMUT cell operating in the collapse mode to increase the maximum output pressure. By using a beam model in undamped conditions and finite-element analysis simulations, the proposed embossed structure showed improvement on the maximum output pressure of the CMUT cell when the embossed pattern was placed on the estimated location of the peak deflection. As compared with a uniform membrane CMUT cell worked in the collapse mode, the proposed CMUT cell can yield the maximum output pressure by 51.1% and 88.1% enhancement with a single embossed pattern made of Si3N4 and nickel, respectively. The maximum output pressures were improved by 34.9% (a single Si3N4 embossed pattern) and 46.7% (a single nickel embossed pattern) with the uniform membrane when the center frequencies of both original and embossed CMUT designs were similar.

Experimental Characterization of an Embossed Capacitive Micromachined Ultrasonic Transducer Cell

“…The embossed CMUT cell achieved about 27.1% higher output pressure in comparison with the uniform CMUT cell. In accordance with the analysis in reference [20], the position of an embossed pattern played a critical role on the improvement of output pressure and the maximum value can be achieved by forming an embossed pattern at the membrane vibration center. Different to simulation, the pattern was centered at r = 12 µm on a fabricated CMUT cell and it cannot be moved.…”

“…The maximum pressure improvement of 27.1% was achieved under 170 V DC bias voltage, in which case the relative position of the embossed pattern was 0.456. This value was less than the optimum position of 0.481 that was deduced as the vibrating center in our previous paper [20] and the deviation was 5.2%, which can be ascribed to several reasons. First, the optimum value of 0.481 was deduced from a simply supported beam model with an infinitely small embossed pattern.…”

“…It was revealed that the position of the embossed pattern was critical to the output pressure improvements in [20]. In the experiment, the contact radii of the embossed CMUT were measured under different DC bias voltages.…”

“…Similar to the approach of making a thicker center membrane [8], the embossed pattern at the outer vibrating membrane can increase the membrane average displacement and therefore the output pressure is improved. Analyzed with a simply supported beam model in [20], the optimum position of the embossed pattern should be consistent with the vibration center of membrane. Since the contact radius of a collapsed membrane changes with different DC bias voltages, it is a possible way to adjust the membrane vibration center to the embossed pattern.…”

“…Some other solutions apply a DC bias voltage larger than the pull-in voltage for driving CMUTs to work from the conventional mode to other working modes, for instance, collapse mode [16,17], collapse-snapback mode [18], and deep collapse mode [19]. A method by constructing an annular embossed pattern on the membrane of a collapse mode CMUT to enhance the output pressure has been proposed in our previous works [20,21]. However, the improvement of output pressure brought by the embossed pattern has not been verified on devices yet.…”