Capacitive micromachined ultrasonic transducers for forward looking intravascular imaging arrays

Capacitive Micromachined Transducers (cMUTs) manufactured on silicon substrates using integrated circuit processing techniques have a great potential for fabrication of forward looking intravascular ultrasound (IVUS) imaging arrays with integrated electronics. To realize this potential for these annular arrays, in which an array element consists of a small number of cMUT membranes, the individual elements have to be modeled in detail and optimized for performance. In this paper, we use finite element analysis (FEA) to calculate the electromechanical coupling coefficient, k², using the fundamental, energy based definition. We then improve the cMUT equivalent circuit model by incorporating the FEA results in the circuit elements. Our results show that existing equivalent circuit models based on radiation impedance of a piston transducer overestimate the bandwidth by more than 40%. We show that the cMUT geometry can be optimized for maximum coupling coefficient, allowing cMUT IVUS array elements to be designed with more than 60 dBN dynamic range with 10MHz bandwidth around 20MHz. We also discuss fabrication of 64 element ring annular cMUT arrays using surface micromachining technology.