Modeling and Characterization of CMOS-Fabricated Capacitive Micromachined Ultrasound Transducers

This paper describes the fabrication, characterization, and modeling of complementary metal-oxide-semiconductor (CMOS)-compatible capacitive micromachined ultrasound transducers (CMUTs). The transducers are fabricated using the interconnect and dielectric layers from a standard CMOS fabrication process. Unlike previous efforts toward integrating CMUTs with CMOS electronics, this process adds no microelectromechanical systems-related steps to the CMOS process and requires no critical lithography steps after the CMOS process is complete. Efficient computational models of the transducers were produced through the combined use of finite-element analysis and lumped-element modeling. A method for improved computation of the electrostatic coupling and environmental loading is presented without the need for multiple finite-element computations. Through the use of laser Doppler velocimetry, transient impulse response and steady-state frequency sweep tests were performed. These measurements are compared to the results predicted by the models. The performance characteristics were compared experimentally through changes in the applied bias voltage, device diameter, and medium properties (air, vacuum, oil, and water). Sparse clusters of up to 33 elements were tested in transmit mode in a water tank, achieving a center frequency of 3.5 MHz, a fractional bandwidth of 32%-44%, and pressure amplitudes of 181-184 dB re 1 μPa_rms at 15 mm from the transducer on axis.