Micromachined transducer design for minimized generation of surface waves

Due to their low membrane mass capacitive Micromachined Ultrasound Transducers (cMUTs) are very well matched to fluid loads. Besides the excellent transducer performance the good matching leads to an increased acoustic coupling between adjacent membranes and consequently to out of phase modes. These modes are lateral resonances of surface waves with a high quality factor. A method for suppressing the undesirable resonances has been developed and experimental results for the new transducer design are presented. The force of the electrostatic transducer increases with the square of the electric field in the gap. Due to their small gap spacing cMUTs can produce high sound pressures. Here we show how this spacing can be reduced beyond the limit of the CMOS fabrication process. On the other hand, the nonlinear force produces harmonic distortion. A procedure to find designs combining maximum sound pressure with minimum distortion has been developed. As a result, the sound pressure and according design parameters as a function of the distortion are discussed