P4M-9 Reduction of Crosstalk in CMUT Arrays by Introducing Double Periodicities

Capacitive micromachined ultrasound transducers (CMUTs) promise high transducer performance for several ultrasound applications. When making a focused ultrasound image with a 90 degree image sector we need a large number of individual elements. In off-axis beam steering neighbor elements operate at different phase. This leads to unwanted acoustic effects caused by the interaction with the fluid medium outside the array. We see high-Q resonances close to the center frequency of the array at off-axis angles, which we want to reduce. The present paper gives one approach to this. The introduction of a double periodicity, with a larger distance between elements than between CMUTs within an element is investigated. Simulations show that the resonances at frequencies closest to the center frequency are reduced, while they are increased at lower frequencies. The lowermost resonances are also shifted down in frequency by 1.5-2.5% of the center frequency. An added lossy top layer with thicknesses from 10 mum to 30 mum reduces the unwanted effects to an acceptable level, but increases the center frequency of the array. The combination of added element kerf and a lossy layer result in responses with 80-100% bandwidth and less than 1.5 dB insertion loss at 0 degrees steering angle. Reoptimizing the design might increase the bandwidth.