A conventional capacitive micromachined ultrasonic transducer (CMUT) is composed of many cells connected in parallel. Since the plate in each CMUT cell is anchored at its perimeter, the average displacement is several times smaller than the displacement of an equivalent ideal piston transducer. In addition, the post areas, where the plates are anchored to, are non-active and, thus, do not contribute to the transduction. We propose a CMUT structure that resembles an ideal capacitive piston transducer, where the movable top plate only undergoes translation rather than deflection. Our proposed CMUT structure is composed of a rigid plate connected to a substrate using relatively long and narrow posts, providing the spring constant for the movement of the plate. Rather than the flexure of the plate as in a conventional CMUT, this device operates based on the compression of the compliant posts. For a capacitive transducer, a thin electrostatic gap is provided under the top plate. We used finite element analysis (FEA) to design and verify the structure's functionality. The simulation results show a fractional bandwidth of over 100% in immersion for all the designs. They also confirm that the average displacement of the top plate is above 90% of its peak displacement. We fabricated the first prototype based on this idea, which only requires a simple 3-mask fabrication process. In addition to 128-element 1-D arrays, we fabricated a variety of 240 μm × 240 μm, single-element transducers with different post configurations. We successfully measured the electrical input impedance of the fabricated devices and confirmed their resonant behavior in air. Further, we measured the acoustic pressure using a calibrated hydrophone at a known distance. Using this measurement, we calculated a peak-to-peak pressure of 1.5 MPa at the face of the transducer. Our results show that it is possible to fabricate CMUTs that exhibit ideal piston-like plate movement. Because of the substrate-embedded spring elements, the plate does not need to be operated in flexural mode, as in a conventional CMUT, resulting in a significantly improved fill-factor, and, thus, a more efficient device.
Keywords: CMUT; capacitive micromachined ultrasonic transducers; piston transducer; ultrasound.